POLITECNICO DI MILANO POLO TERRITORIALE DI ...

POLITECNICO DI MILANO POLO TERRITORIALE DI COMO School of Civil, Environmental and Land Management Engineering Master of Science in Environmental and Geomatic Engineering

“Optimization of the treatment and disposal of sewage sludge in the ATO of Como: options and scenarios assessment”

Supervisor: Prof. ing. Roberto Canziani Cosupervisor: ing. Roberto Di Cosmo Master Graduation Thesis by: Walter Malacrida Student ID number: 800953

A. Y. 2013/2014

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Alla mia famiglia, a cui devo tutto.

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Table of Contents 1. Introduction .............................................................................................................................................................................. 13 2. Conventional sludge management .................................................................................................................................. 15 2.1 Sludge sources, quantities and management ...................................................................................................... 15 2.2 Sludge characteristics ................................................................................................................................................... 16 2.3 Sludge treatment phases .............................................................................................................................................. 17 2.3.1 Thickening ................................................................................................................................................................. 17 2.3.2 Stabilization .............................................................................................................................................................. 18 2.3.3 Conditioning and dewatering ............................................................................................................................ 19 2.3.4 Drying .......................................................................................................................................................................... 20 2.4 Sludge disposal................................................................................................................................................................. 20 2.4.1 Agricultural reuse................................................................................................................................................... 21 2.4.2 Incineration............................................................................................................................................................... 26 2.5 Costs ..................................................................................................................................................................................... 28 2.5.1 Agricultural reuse................................................................................................................................................... 28 2.5.2 Incineration............................................................................................................................................................... 29 3. Sludge management in the ATO of Como ..................................................................................................................... 31 3.1 Background situation of the ATO of Como ........................................................................................................... 31 3.2 Sludge treatment and disposal in the 7 biggest plants .................................................................................... 34 3.2.1 Sludge treatment lines and production ......................................................................................................... 34 3.2.2 Characteristics of the sludge .............................................................................................................................. 38 3.2.3 Sludge disposal ........................................................................................................................................................ 42 3.2.4 Disposal costs ........................................................................................................................................................... 45 3.3 Sludge treatment and disposal in the small (≤ 2000 PE) plants.................................................................. 47 3.4 The future of the ATO: centralization of water services ................................................................................. 48 3.5 The future of sludge treatment and disposal ...................................................................................................... 49 4. Innovative solutions .............................................................................................................................................................. 51 4.1 The need for innovation in sludge management ............................................................................................... 51 4.2 Available databases and SYST&MS .......................................................................................................................... 54 4.3 Improvements in the sludge production and treatment ................................................................................ 57 4.3.1 Water line interventions...................................................................................................................................... 58 4.3.2 Sludge line interventions..................................................................................................................................... 60 4.4 Options in sludge disposal........................................................................................................................................... 63 4.4.1 Incineration............................................................................................................................................................... 63 4.4.2 Pyrolysis ..................................................................................................................................................................... 66 3

4.4.3 Gasification ................................................................................................................................................................ 67 4.4.4 Environmental aspects ......................................................................................................................................... 68 4.4.5 Sewage sludge ash valorization ........................................................................................................................ 70 4.5 ATO planned interventions ......................................................................................................................................... 72 5. Assessment of scenarios for the ATO of Como ........................................................................................................... 73 5.1 Scenario 0: agricultural reuse .................................................................................................................................... 73 5.1.1 Assumptions and development ........................................................................................................................ 73 5.1.2 Conclusions ............................................................................................................................................................... 76 5.2 Scenario 1: sludge-to-energy by means of incineration.................................................................................. 78 5.2.1 Assumptions and development ........................................................................................................................ 78 5.2.2 Economical and financial assessment ............................................................................................................ 82 5.2.3 Sensitivity analysis................................................................................................................................................. 84 5.2.4 Environmental aspects ......................................................................................................................................... 86 5.2.5 Conclusions ............................................................................................................................................................... 87 5.3 Scenario 2: sludge-to-energy by means of pyrolysis ........................................................................................ 88 5.3.1 Assumptions and development ........................................................................................................................ 88 5.3.2 Economical and financial assessment ............................................................................................................ 96 5.3.3 Sensitivity analysis................................................................................................................................................. 99 5.3.4 Environmental aspects .......................................................................................................................................100 5.3.5 Conclusions .............................................................................................................................................................101 5.4 Electrokinetic dewatering addition .......................................................................................................................101 5.4.1 Electrokinetic dewatering application.........................................................................................................101 5.5 Sewage sludge treatment and disposal chains for Como district..............................................................104 5.5 SWOT analysis ................................................................................................................................................................112 6. Conclusion ...............................................................................................................................................................................115 References ....................................................................................................................................................................................117 APPENDIX 1 .................................................................................................................................................................................123 APPENDIX 2 .................................................................................................................................................................................131 APPENDIX 3 .................................................................................................................................................................................138 Acknowledgements ..................................................................................................................................................................142

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List of Tables Table 1: per capita production of dry and wet sludge after water line referred to urban wastewater . 16 Table 2: characteristics of sludge coming from different treatment processes ............................................... 17 Table 3: typical thickeners performances ....................................................................................................................... 18 Table 4: advantages and disadvantages of dewatering systems ............................................................................ 20 Table 5: reference acts for the use of sewage sludge in agriculture in Europe ................................................ 22 Table 6: sewage sludge incineration facilities in Europe .......................................................................................... 28 Table 7: consistency of the WWTPs in the ATO of Como .......................................................................................... 32 Table 8: consistency of the 7 biggest plants.................................................................................................................... 33 Table 9: sewage sludge production of the biggest plants.......................................................................................... 37 Table 10: dry matter content of sewage sludge of the biggest plants .................................................................. 38 Table 11: dry matter quantities of sewage sludge of the biggest plants ............................................................. 38 Table 12: admissible concentrations of heavy metals for the sludge entering the recovery plants ....... 39 Table 13: previous admissible concentrations of heavy metals for sludge ....................................................... 39 Table 14: distinction between high quality and admissible sludge ...................................................................... 39 Table 15: heavy metals contents in sewage sludge samples of 3 plants ............................................................. 41 Table 16: total amount of sludge conferred to processing plants ......................................................................... 42 Table 17: distances from WWTPs to recovery plants in Padana plain ................................................................ 43 Table 18: surfaces available for distribution of sludge in agriculture ................................................................. 43 Table 19: costs for the disposal of dewatered sludge ................................................................................................. 45 Table 20: costs for the disposal of dried sludge ............................................................................................................ 45 Table 21: total costs for sludge disposal .......................................................................................................................... 46 Table 22: sludge production for different processes in small plants ................................................................... 47 Table 23: changes in the biggest plants due to centralization ................................................................................ 49 Table 24: energy analysis of different pretreatments for anaerobic digestion ............................................... 62 Table 25: pyrolysis products from sewage sludge ....................................................................................................... 66 Table 26: typical combustible gas composition from gasification ........................................................................ 67 Table 27: typical physical properties of sewage sludge ash .................................................................................... 71 Table 28: typical range of elemental concentrations in sewage sludge ash ...................................................... 72 Table 29: estimated emission factors for diesel heavy duty vehicles .................................................................. 75 Table 30: distances between WWTPs and processing plants ................................................................................. 75 Table 31: data for the starting year in the agriculture reuse scenario ................................................................ 75 Table 32: actual pollutants emissions due to sludge disposal ................................................................................ 76 Table 33: sewage sludge characteristics used to design the incinerator ........................................................... 79 Table 34: incineration plant technical data .................................................................................................................... 81 Table 35: quantities of residues, chemicals and energy needs ............................................................................... 82 Table 36: investments costs and electric energy produced for different plant potential............................ 82 Table 37: economic evaluation for the designed incineration plant .................................................................... 83 Table 38: percentages of sludges sent to the designed incineration .................................................................... 84 Table 39: characteristics of the sewage sludge fed to the designed pyrolysis ................................................. 89 Table 40: characteristics of dried sludge ......................................................................................................................... 89 Table 41: results for the flow rate of the heating medium ....................................................................................... 91 Table 42: energy content of heating medium with varying flow rates ................................................................ 92 Table 43: energy content of product compositions with varying percentage conversions ........................ 92 Table 44: kinetic properties from sludge pyrolysis..................................................................................................... 93 Table 45: reactor volumes with varying conversions ................................................................................................ 94 Table 46: variation of cross sectional area with fluidization velocity ................................................................. 94 5

Table 47: pyrolysis throughput values ............................................................................................................................. 95 Table 48: reactor sizing ........................................................................................................................................................... 95 Table 49: thermal and electrical energy consumptions for the designed pyrolysis plant .......................... 95 Table 50: investments costs and electric energy produced for different innovative plant potentials... 96 Table 51: management costs for different innovative plant potentials .............................................................. 96 Table 52: economic evaluation of the innovative thermal treatment plant ...................................................... 97 Table 53: percentages of sludges sent to pyrolysis ..................................................................................................... 98 Table 54: average monthly price per time slot (€/MWh)......................................................................................... 99 Table 55: dewatering and disposal costs for Como, F. M. - A. S. and Merone plants ....................................103 Table 56: SWOT analysis for agricultural reuse ..........................................................................................................113 Table 57: SWOT analysis for incineration .....................................................................................................................113 Table 58: SWOT analysis for pyrolysis ...........................................................................................................................113 Table 59: limit values in soils subject to use of sludge in agriculture ................................................................134 Table 60: limit emission values in the atmosphere ...................................................................................................138 Table 61: BAT for dust removal .........................................................................................................................................139 Table 62: BAT for acid gases removal .............................................................................................................................139 Table 63: BAT for PCDD/F removal .................................................................................................................................140 Table 64: BAT for Hg removal ............................................................................................................................................140 Table 65: BAT for nitrogen oxides removal ..................................................................................................................141

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List of Figures Figure 1: characteristics of sludge ...................................................................................................................................... 20 Figure 2: nitrates concentration in 2012 in Lombardy .............................................................................................. 24 Figure 3: simplified sludge incinerator flow diagram................................................................................................. 26 Figure 4: typical fluidized-bed incineration plant ........................................................................................................ 28 Figure 5: sludge line flow diagram – Alto Lura Bulgarograsso ............................................................................... 34 Figure 6: sludge line flow diagram – Sud Seveso Servizi Carimate ....................................................................... 35 Figure 7: sludge line flow diagram – Comodepur - Como ......................................................................................... 35 Figure 8: sludge line flow diagram – Lariana Depur – Alto Seveso – Fino Mornasco .................................... 35 Figure 9: sludge line flow diagram – Lariana Depur – Livescia – Fino Mornasco ........................................... 36 Figure 10: sludge line flow diagram – Valbe Servizi – Mariano Comense .......................................................... 36 Figure 11: sludge line flow diagram – ASIL - Merone ................................................................................................. 37 Figure 12: sludge disposal routes in the ATO of Como .............................................................................................. 42 Figure 13: budget of manure in relation to the constraints of the Nitrates Directive................................... 44 Figure 14: area available for the use of sewage sludge, net SAU used for the distribution of manure .. 44 Figure 15: Imhoff tank ............................................................................................................................................................. 47 Figure 16: architecture of SYST&MS .................................................................................................................................. 56 Figure 17: potential location for sludge cotreatments in a classical wastewater treatment plant ......... 57 Figure 18: processes occurring in electro-osmotic dewatering ............................................................................. 61 Figure 19: SNB Brabant – Moerdijk, Netherlands ........................................................................................................ 64 Figure 20: ERZ Zurich – Zurich, Switzerland .................................................................................................................. 65 Figure 21: products comparison ......................................................................................................................................... 68 Figure 22: sketch of sewage sludge fluidized bed incineration plant .................................................................. 80 Figure 23: experimental processes for P recovery from ISSA................................................................................. 87 Figure 24: pyrolysis process flowsheet ............................................................................................................................ 88 Figure 25: equivalence ratio against carbon conversion efficiency...................................................................... 90 Figure 26: innovative dewatering process description ...........................................................................................102 Figure 27: scenario 0 ..............................................................................................................................................................106 Figure 28: scenario 1 CD MP ...............................................................................................................................................107 Figure 29: scenario 2 CD MP ...............................................................................................................................................108 Figure 30; ATO-Como main plants ...................................................................................................................................109 Figure 31: ATO-Como plants and central point...........................................................................................................110 Figure 32: landspreading of sludge trough dedicated machines .........................................................................134

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List of Graphs Graph 1: sewage sludge production of the biggest plants ........................................................................................ 37 Graph 2: possible evolution of sludge produced in Como district WWTPs ....................................................... 74 Graph 3: sludge volumes and hectares trend in the agricultural reuse scenario ............................................ 77 Graph 4: greenhouse gases emissions trend in the agricultural reuse scenario ............................................. 77 Graph 5: estimation of investment costs for incinerators......................................................................................... 83 Graph 6: NPV change according to DM change .............................................................................................................. 85 Graph 7: NPV change according to slag disposal cost change ................................................................................. 85 Graph 8: NPV change according to energy needed for drying change ................................................................. 86 Graph 9: estimation of the investment cost for an innovative thermal treatment plant ............................. 97 Graph 10: estimation of the management costs for an innovative thermal treatment plant ..................... 97 Graph 11: thermal and electric output according to DM change .........................................................................100 Graph 12: NPV change according to specific management cost change ...........................................................100 Graph 13: visual comparison of costs of conventional and innovative dewatering ....................................103 Graph 14: savings due to changes in the innovative dewatering parameters................................................104 Graph 15: costs for incineration scenario with ED and the one without ED...................................................111 Graph 16: savings for incineration with ED with respect to the one without ED .........................................112 Graph 17: savings for pyrolysis with ED with respect to the one without ED ...............................................112

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Abstract Sludge produced by municipal wastewater treatment plants (WWTPs) amounts to only a few percent by volume of the processed wastewater, but its management accounts for up to 50% of total operating costs. Innovative solutions are being studied in order to minimize its production and to find new disposal ways which are more cost-effective and less impacting on the environment. In the Province of Como, sludge management is one of the main issues. In this work, the situation is described, mainly referring to the biggest plants, both in quantitative and qualitative terms. Then actual disposal routes are analyzed, which mainly concern agricultural reuse. New treatment and disposal solutions are assessed, mainly concentrating on thermal conversion processes: incineration and pyrolysis/gasification. Sludge management scenarios are simulated, taking into account chains of technologies and analyzing the final economic and environmental results. Furthermore, varying some simulation parameters, a sensitivity analysis is provided. Thermal conversion processes and electrodewatering seem to be cost-effective technologies which can be further investigated for a comprehensive project of diversification of sludge disposal routes in the ATO of Como. Finally, the problem of transparent and clear data gathering is mentioned, with the aim of developing a new WWTPs database, called SYST&MS. Keywords: sludge – disposal – incineration – pyrolysis – electrodewatering – database

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Riassunto Le quantità di fanghi prodotti da impianti di trattamento delle acque reflue municipali ammontano a solo una piccola percentuale in volume delle acque reflue trattate, ma la loro gestione può pesare fino al 50% sui costi operativi totali. Soluzioni innovative sono in corso di studio per ridurre al minimo la produzione dei fanghi e per trovare nuove modalità di smaltimento che siano più convenienti e meno impattanti sull'ambiente. Nella Provincia di Como, la gestione dei fanghi è uno dei problemi principali. In questo lavoro, la situazione è stata descritta con riferimento principalmente agli impianti più grandi, sia in termini quantitativi sia qualitativi. Le vie di smaltimento attuali sono state analizzate: esse riguardano principalmente il riutilizzo agricolo. Nuove soluzioni di trattamento e smaltimento sono state valutate, concentrandosi principalmente sui processi di conversione termica: incenerimento e pirolisi/gassificazione. Differenti scenari di gestione dei fanghi sono stati simulati, tenendo conto delle tecnologie e dell'analisi dei risultati economici e ambientali finali. Inoltre, variando alcuni parametri di simulazione, è stata fornita un'analisi di sensitività. I processi di conversione termica e di elettrodisidratazione sembrano essere economicamente vantaggiosi e possono essere il fulcro di una ricerca più approfondita per un progetto di diversificazione delle vie di smaltimento dei fanghi nell'ATO di Como. Infine, il problema della raccolta dati è menzionato, con l'obiettivo di sviluppare un nuovo database, denominato SIST&MS. Parole chiave: fanghi – smaltimento – incenerimento – pirolisi – elettrodisidratazione - database

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“Paris casts twenty-five millions of francs annually into the sea; an approximate amount given by the estimates of modern science. Science knows now that the most fertilizing and effective manures is the human manure. […]Do you know what these piles of ordure are, those carts of mud carried off at night from the streets, the frightful barrels of the nightman, and the fetid streams of subterranean mud which the pavement conceals from you? All this is flowering field, it is green grass, it is the mint and thyme and sage, it is game, it is cattle, it is the satisfied lowing of heavy kine, it is perfumed hay, it is gilded wheat, it is bread on your table, it is warm blood in your veins.” (Victor Hugo, Les Misérables, 1862)

“Historically, it was common to see schematics that showed the water treatment scheme in detail […] and an arrow at the end that simply said ‘sludge to disposal’. ” (Neyens et al., 2004)

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1. Introduction The disposal and reuse of sewage sludge have assumed increasingly importance in recent years as part of the integrated water services because of the different impacts in terms of management, economic and environmental issues related to it. The interest has also increased because of the large-scale production of rules and regulations on a European, national, and even regional level on the topic. In Lombardy, a production of around one million tonnes of sewage sludge was documented (IRER, 2010). Just over half of the sludge, after appropriate treatment in specific platforms, is designed to agricultural reuse; the remaining half is sent, in roughly equal parts, to landfill or for incineration. Although recent surveys confirm a significant reduction in sludge quantities, due to the economic and industrial crisis and the consequent decrease of process residues that end up in wastewater treatment plants, production in Lombardy is going to increase, not only because of growing consumption and population, but also because the sewage and water treatment system will improve and will lead to connect all the users to the network. The tens of millions of Euros allocated, in Lombardy, for large interventions in response to EC infringement procedure 2034/09, which imposes heavy penalties for purification and sewerage systems that will not be in accordance with limits within 2015, will lead to an improvement in the quality of rivers and the environment, but simultaneously to an increase in the quantity of sludge. With this in mind, it becomes even more pressing the necessity that authorities responsible for the protection of the environment intervene in the definition of a coherent planning framework, strategies and techniques to optimize the management of sewage sludge. Companies in the water sector, whose aim should be to identify technological systems to reduce the amount of sludge and minimize its economic and environmental impacts, claim it. The same situation is found in the Como district, where the production of sludge amounted to approximately 30,000 wet tonnes in 2013 and is going to increase because of the planned interventions to power up the WWTPs and the sewerage system. The main disposal route in the zone is agricultural reuse, to which a certain number of issues are connected, mainly related to environmental (e. g. heavy metals leaching into soils used for landspreading of sludge) and management (e. g. availability of fields for sludge distribution) problems. The key word which must be bore in mind by the authority is disposal differentiation: in this way, new disposal routes are going to be studied in order not to convey all the sludge to agriculture. More and more attention is put on thermal treatments of sludge, such as incineration, gasification and pyrolysis, which are investigated in this work, together with new treatment options and the scenarios which can develop if these disposal routes are adopted.

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2. Conventional sludge management 2.1 Sludge sources, quantities and management Both domestic and industrial wastewater treatment processes almost always involve the production of sludge, made of concentrated suspensions of the materials removed during the treatment. In fact, it is rare the case in which the purification can be obtained by gasification of the pollutants or by their solubilization in a form compatible with a proper disposal. In general, the treatment can be seen as a concentration in the sludge of substances initially in a dispersed, dissolved and suspended form and a removal by processes culminating in a phase of solid-liquid separation (sedimentation or sometimes flotation, filtration and centrifugation). As a result of the concentration step, the sludge contains a substantial portion of the pollutants originally present in the effluent; if not treated and disposed of correctly, it can then give rise to new phenomena of pollution. In urban sewage sludge, the volatile component is dominant, being formed by settled organic material, bioflocculated organic pollutants and excess biomass. Metal precipitates (hydroxides, phosphates, etc.) are present in substantial measure in the case where the cycles of purification include physicalchemical or chemical steps, such as flocculation and precipitation of phosphorus. Sludges are characterized on the basis of the chemical (pH, alkalinity, organic substance, presence of nutrients and micro-pollutants), physical (humidity, specific weight, particle size, calorific value, rheological characteristics) and biological properties that influence the treatment and disposal. Moisture and specific gravity are related to the volume; the rheological properties to the hydraulic behavior; the physical properties to the applicability of the different systems of dewatering; chemical and biological characteristics to the need for stabilization and to identify ways for proper disposal. It is observed, however, that the characterization of sludge is not subject to standardization and that in many cases there is still uncertainty about the parameters to be considered as indicators and methods of definition. Treatment and disposal of sludge are one of the major aspects of the entire cycle of water purification, with heavy economic implications: the relative costs can approach, for certain plant solutions, 40-45% of the total costs of the construction and operation of water treatment plants (Bonomo, 2008). The reduction of the quantities to be disposed of and of their reactivity in the environment plays a crucial role in the schemes of purification. A typical municipal wastewater treatment plant produces three types of sludge:  

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Primary sludge, when the initial phase of sedimentation is expected. It is made up of suspended settled material fed with wastewater, separated by simple decantation without having undergone any process of transformation. Therefore it is always characterized by high putrescibility. Secondary or biological sludge, made of biomass in excess (excess activated sludge, membranes films), including bacterial colonies and suspended, inert and volatile material, adsorbed or mechanically trapped. The level of putrescibility depends on the type of biological treatment applied. In some cases (extensive aeration treatment, trickling filters with low load) sludge may be stable enough to not require any additional treatment. Tertiary sludge, produced by phases of filtration, flocculation or precipitation downstream of the biological treatment. In the presence of simple filtration, without the addition of inorganic reagents, the nature of the sludge is similar to that of biological sludge. Otherwise chemical precipitates are present in a variable percentage in function of the process applied, but often sufficient to produce a good degree of stability.

In some cases different types of sludge may be mixed, within the same chain of water treatment. It can be foreseen, for example, the recycling of sewage sludge and tertiary ones (especially if produced by simple filtration) upstream of the primary sedimentation in order to improve the performance of the action exerted by the flocculation of excess biomass and get a better level of thickening. Other waste products in the water line (material retained by the grids and sieves, sand, fats and oils) have unique characteristics that require special disposal methods, distinct from those of the sludge. Hereafter they will not be taken into account. 15

The values reported in Table 1 are relative to the per capita production of sludge on a dry basis, with the corresponding humidity and the consequent production of wet sludge, for some biological processes for the treatment of municipal wastewater. These values are intended to refer to the flow of sludge out of the water line, before the stages of thickening and dewatering. This is a guideline, designed to provide orders of magnitude and to verify in accordance with local conditions. The quantities are reported on a dry basis in theoretical conditions of supply and resident population. Table 1: per capita production of dry and wet sludge after water line referred to urban wastewater

Plant type

per capita humidity production [g inhabitant-1 interval typical d-1] [%] [%] Primary sludge 50-55 93-96 95 Excess activated sludge 22-30 98.5-99 99 Biological sludge from low load trickling filter 13-18 93-96 95 Biological sludge from high lad trickling filter 20-25 94-97 95 Excess activated sludge in absence of primary 55-60 98.5-99 98.8 sedimentation Excess activated sludge from extensive aeration 50-55 98.5-99 98.8 Mixed primary and secondary sludge from acti70-80 96-98 97 vated sludge Mixed primary and secondary sludge from low 65-70 95-96 96 load trickling filter Mixed primary and secondary sludge from high 70-80 95-96 96.5 load trickling filter

per capita volume [l inhabitant-1 d-1] 1.00 3.00 0.33 0.47 4.50 4.00 2.50 1.30 1.70

(Source: Bonomo, 2008)

2.2 Sludge characteristics It is useful to define the typical characteristics to understand the complex behavior and nature of sludge. For a complete discussion it is possible to refer to the books available in literature, such as Sanin, Clarkson, Vesilind, Sludge Engineering, 2011. Physical characteristics of sludge are:         

specific weight, solids concentration (total solids TS, suspended solids SS, dissolved solids DS), settling (sludge volume index SVI), floc/particle size and shape, humidity and distribution of water (free, interstitial, vicinal and hydration water), filterability and dewaterability (specific resistance to filtration SRF, capillary suction time CST), rheology, floc structure and porosity, thermal conductivity.

Chemical characteristics of sludge are:     

pH, alkalinity, surface charge and hydrophobicity, nutrient and fertilizer value, heavy metal and toxic organics content. 16

Biological characteristics of sludge are:  

microbial community, surface polymers (extracellular polymeric substances EPS).

Typical characteristics of sludge originating from various treatment methods are shown in Table 2. Table 2: characteristics of sludge coming from different treatment processes

(Source: Manara and Zabaniotou, 2012)

2.3 Sludge treatment phases Sludge treatment may be composed by different stages; a typical process is summarized as follows (Metcalf & Eddy, 2003):          

preliminary treatment (screening, comminuting), primary thickening (gravity, flotation, drainage, belt, centrifuges), liquid sludge stabilization (anaerobic digestion, aerobic digestion, lime addition), secondary thickening (gravity, flotation, drainage, belt, centrifuges), conditioning (elutriation, chemical, thermal), dewatering (plate press, belt press, centrifuge, drying bed), final treatment (composting, drying, line addition, incineration, wet oxidation, pyrolysis, disinfection), storage (liquid sludge, dry sludge, compost, ash), transportation (road, pipeline, sea), final destination (landfill, agriculture/horticulture, forest, reclaimed land, land building, other uses).

Furthermore a brief explanation of the main processes is given. 2.3.1 Thickening A decrease of moisture has a relevant reduction of the flow of sludge as a result, with obvious advantages in the subsequent treatments, thereby avoiding unnecessary hydraulic overloading in the stages of stabilization, conditioning and dewatering. The passage for example from 99% to 96% of 17

moisture results in a reduction of four times in the amount of sludge. This result can be obtained through constructive and especially managerial adjustments in the operational units of both the water line and the sludge line: extension of the stay of sludge in the primary sedimentation tanks, which must then be built with hoppers of adequate capacity and depth; reduction of solid flow in secondary sedimentation tank downstream of the activated sludge tank; periodic shutdowns of the systems of mixing in the anaerobic digesters and in the tanks of aerobic stabilization, resulting in the discharge of water supernatants. One speaks in this case of simultaneous or contemporary thickening. With respect to such alternative it is common, at least in plants of medium and large size, the inclusion in the cycle of one or more specific steps which, in relation to the placement relative to the stabilization treatment, are divided into the stages of pre- and post-thickening. With pre-thickeners, the following advantages are achieved:   



Greater ease and regularity of exercise of the primary sedimentation tanks, without the use of prolonged interruptions in the operations of sludge discharge with consequent risk of clogging in extraction circuits and production of odor due to putrefactive phenomena. Containment of the area of secondary sedimentation (downstream of the activated sludge), in relation to less restrictive values for solids flux. Limitation only to the sludge line of the movement of relatively concentrated suspensions on paths typically much shorter than otherwise needed and resulting in greater ease of adoption of appropriate measures for the proper functioning (cleaning devices in the pipes, volumetric pumps or similar). Decrease of the heat transferred to the sludge if the heating is provided upstream of the stages of anaerobic digestion.

The biological transformations that occur due to the stabilization processes improve the settleability of the sludge and then make the separation of additional quantities of water possible in postthickening. In many cases these operations are carried out, at least in part, within the same stabilization reactor, with periodic shutdown of the mixing systems. The removal of the supernatant, as soon as the structure of the sludge changes, allows the reduction of the dimensions of the reactors, due to the lower volume of sludge maintained in it. The post-thickening allows the completion of the separation of the supernatants and the accumulation upstream of dewatering. The thickening is carried out by physical methods, normally by gravity, and, more rarely, by flotation, centrifugation or filtration. The use of flocculants allows an improvement of the reached moisture levels. Table 3 shows the performances of different thickening systems. Table 3: typical thickeners performances

Type of sludge

Primary clarifier

Dissolved air flotation

Gravity thickener

Total Solids Concentration after Thickening, Percent Solids RPS 5-7 8-10 WAS 3-5 1.5-2 RPS+WAS 2 4-6 4-6

Belt thickener (with conditioning)

Centrifuge

9-12 5-7 5-7

9-12 5-7 5-7

RPS: Raw Primary Sludge; WAS: Waste Activated Sludge (Source: Sanin, Clarkson, Vesilind, 2011)

2.3.2 Stabilization Many types of sludge have a high level of putrescibility and this is the case of the primary sludge from urban wastewater or biodegradable industrial products; also the sewage sludge from biological treatment is usually putrescible to varying degrees, depending on the depurative process; sufficient stability is achieved only in specific processes, such as extensive aeration and attached biomass with high surficial and volumetric loads. A stabilization phase, separated from the water line, is almost always 18

necessary to ensure proper conditions for the final disposal, for odor containment and to improve the hygienic characteristics through pathogen reduction. The putrescibility of the sludge is connected to the presence of rapidly biodegradable material, subject to anaerobic transformations in the absence of oxygen. Stabilization can be obtained according to two different modes:  

Transformation or destruction of putrescible organic matter; in the first case, aerobic or anaerobic biological processes are applied; in the second case the organic substance is completely eliminated by incineration processes. Creation of environmental conditions that prevent the activity of the bacteria and thus the onset of anaerobic degradation phenomena; possible alternatives are of chemical nature, by alkalization, or of physical nature, by drying.

2.3.3 Conditioning and dewatering The high moisture content of the sludge coming out from stabilization processes should be reduced to allow proper disposal in the environment. The characteristics of the sludge are generally not compatible with the direct application of mechanical dewatering treatments, making necessary interventions, such as preliminary conditioning, almost always of chemical nature, in some cases of thermal nature, with the aim to:   

Increase the speed of solid-liquid separation. Increase the dry matter content of dewatered (or thickened) sludge. Improve the quality of the separated liquid especially in terms of suspended solids.

For the dewatering of fresh or stabilized sludge, conditioning is almost always necessary; it can be avoided only for some limited types of industrial sludge, especially in the case of inorganic nature; if dewatering is conducted through filtration processes, the purpose of conditioning is to improve the filterability; in the case of centrifugation, to increase the size of the suspended particles, with advantages also for capture efficiency and quality of the centrifuged. The dewaterability of the sludge is conditioned by the presence of bound and interstitial water; other relevant factors are the organic content and its level of stabilization, the viscosity, the mechanical strength and especially the particle size distribution and the presence of extracellular polymers (EPS). The conditioning can be conducted by chemical (with the use of organic or inorganic additives) or physical means (thermally or with the use of sonic or electric energy). Chemical conditioning involves the addition of reagents to the sludge in order to achieve coagulation of colloidal or super-colloidal particles and their subsequent flocculation with reduction of the finely dispersed phase. Thermal conditioning involves raising the temperature of the sludge up to value of 150 - 240 °C with contact times ranging between 30 and 90 minutes, depending on the type of sludge and the characteristics of the process. Sludge mechanical dewatering conducted by means of filtration and centrifugation aims at reducing the volume and weight of sludge, through partial separation of the liquid component, in order to make it compatible with the final disposal. The dry content achievable is such as to give the sludge the appearance of a soil (the sludge is defined shovelable i.e. able to be handled by mechanical means) and characteristics necessary for its final disposal in landfills or by thermal treatments and, if they fulfill the conditions, by agronomic use. Mechanical dewatering can be made through different systems; the most common are: belt presses, filter presses and centrifuges, whose advantage and critical points are shown in Table 4.

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Table 4: advantages and disadvantages of dewatering systems

Positive aspects Belt press medium investment costs, simple to manage even if with significant time commitment, continuous operation, limited energy consumption Filter press excellent levels of dry matter achievable, efficient capture of solids, limited presence of personnel only to the phase of discharge of the panels with the possibility of conducting the filtration step outside working hours Centrifuge compact installation, continuous and automatic operation, good quality of the working environment

Critical points low levels of dry matter achievable, important consumption of water, mediocre working environments, improvable by the covering of the machines high investment cost, bulky and heavy equipment, significant commitment of personnel during the unloading phase, increase in the quantities of sludge on a dry basis in the case of conditioning with inorganic reagents, discontinuous operation need to deaden, vibration control, mediocre performance of solids capture, risk of damage by abrasion, high energy consumption

(Source: Bonomo, 2008)

2.3.4 Drying Thermal drying is a process where thermal energy is provided to the sludge to evaporate water. The process of sludge drying reduces the volume of the product, making its storage, transportation, packaging and retail easier. The process reduces the environmental impact and produces a stabilized dry granular product that is easy to store and deliver and suitable for agricultural use (Figure 1). Drying technologies can be classified in four groups, depending on the way the energy is supplied to the sludge (Arlabosse et al., 2011):    

convection or direct dryers, conduction or contact or indirect dryers, radiation dryers (solar), combined systems (convection and conduction in the same dryer) and hybrid systems (convection and conduction dryers put in series).

Figure 1: characteristics of sludge (Source: Andritz)

2.4 Sludge disposal The most common methods of disposal/utilization of sludge are:    

landfilling, reuse in agriculture after processing or composting, incineration alone or co-incineration with waste, insertion in the production of bricks, asphalt, concrete. 20

During the last decades there has been a major change in the ways sludge is disposed. Before 1998, municipal sludge was primarily disposed at seawaters or was either used as a fertilizer on agricultural land. An alternative was sludge incineration or simply landfilling. Since 1998 onwards, European legislation (UWWTD) prohibits the sea disposal of sewage sludge, in order to protect the marine environment. The agricultural use has become the principal disposal method for sewage sludge; 37% of the sludge produced is being utilized in agriculture, 11% is being incinerated, 40% is landfilled while 12% is used for forestry and land reclamation (Fytili and Zabaniotou, 2008). The latest trends in the field of sludge management, i.e. combustion, wet oxidation, pyrolysis, gasification and co-combustion of sewage sludge with other materials for further use as energy source, have generated significant scientific interest. Hereafter the focus is put on reuse in agriculture and thermal treatments. 2.4.1 Agricultural reuse At European Community level, the use of sewage sludge in agriculture is regulated by the Sewage Sludge Directive 86/278/EEC and the Urban Water Treatment Directive 91/271/EEC, along with other EU and country-specific regulations. The Sewage Sludge Directive 86/278/EEC seeks to encourage the use of sewage sludge in agriculture and to regulate its use in such a way as to prevent harmful effects on soil, vegetation, animals and man. To this end, it prohibits the use of untreated sludge on agricultural land unless it is injected or incorporated into the soil. Treated sludge is defined as having undergone "biological, chemical or heat treatment, long-term storage or any other appropriate process so as significantly to reduce its fermentability and the health hazards resulting from its use". To provide protection against potential health risks from residual pathogens, sludge must not be applied to soil in which fruit and vegetable crops are growing or grown, or less than ten months before fruit and vegetable crops are to be harvested. Grazing animals must not be allowed access to grassland or forage land less than three weeks after the application of sludge. The Directive also requires that sludge should be used in such a way that account is taken of the nutrient requirements of plants and that the quality of the soil and of the surface and groundwater is not impaired. The Directive specifies rules for the sampling and analysis of sludges and soils. It sets out requirements for the keeping of detailed records of the quantities of sludge produced, the quantities used in agriculture, the composition and properties of the sludge, the type of treatment and the sites where the sludge is used. Limit values for concentrations of heavy metals in sewage sludge intended for agricultural use and in sludge-treated soils are in Annexes I A, I B and I C of the Directive. Directive 86/278/EEC was adopted over 20 years ago with a view to encourage the application of sewage sludge in agriculture and to regulate its use, so as to present harmful effects on soil, vegetation, animals and humans. The European Commission is currently assessing whether the current Directive should be reviewed and if so, the extent of this review. For example, Directive 86/278/EEC sets limit values for seven heavy metals. Since its adoption, several Member States have enacted and implemented stricter limit values for heavy metals and set requirements for other contaminants. The European Commission had commissioned a study to the pool of companies Milieu, WRc and RPA to revise Directive 86/278/EEC; in February 2010, the report "Environmental, economic and social impacts of the use of sewage sludge on land" was published and can be easily downloaded from the net. This study allowed the European Commission to draw up an own document "Working document on sludge and Biowaste" that represents the most recent basis for discussion on the topic. The conclusions of this paper are as follows:  

There is no evidence of significant risks to the environment and human health due to the use of sludge in agriculture. It does not appear, however, clear whether this evidence comes from sufficient caution with which Directive 86/278 was set or if it is instead the result of the adoption of more stringent standards on the part of some member countries.

The implementation of the Urban Water Treatment Directive 91/271/EEC in all Member States has increased the quantities of sewage sludge requiring disposal. From an annual production of some 5.5 million tonnes of dry matter in 1992, the Community has reached nearly 9 million tonnes by the end of 2005. This increase is mainly due to the practical implementation of the Directive as well as the slow 21

but constant rise in the number of households connected to sewers and the increase in the level of treatment (up to tertiary treatment with removal of nutrients in some Member States). Table 5: reference acts for the use of sewage sludge in agriculture in Europe

Act

Entry into force

Directive 86/278/EEC Amending acts Directive 91/692/EEC Regulation 807/2003 Regulation 219/2009

18.6.1986

Deadline for transposition in the Member States 18.6.1989

23.12.1991

1.1.1993

(EC)

No

5.6.2003

(EC)

No

204.2009

In Italy, referring to Legislative Decree 152/2006, the sewage sludge is classified as "special waste" and the use in agriculture, according to the competences delegated by the State to the Regions, shall be governed by Decree No. 99/1992. In particular art. 127 of Legislative Decree 152/2006 states: 



Notwithstanding the discipline in the legislative decree January 27, 1992, n. 99, sewage sludge from wastewater treatment is subject to the discipline of waste, where applicable and at the end of the overall treatment process performed by the purification plant. The sludge must be reused whenever its reuse is appropriate. It is illegal to dispose of the sludge in fresh and brackish surface waters.

Its fate has mainly been disposal in landfills but the changes of the boundary conditions, such as the increased quantities produced as a result of the growing number of treatment plants and the most restrictive regulations on landfills (Legislative Decree no. 36/2003, transposing Directive 1999/31/EC on the landfill of waste and Minister Decree 27/9/2010, related to the waste admissible in landfills, substituting Minister Decree 3/8/2005), have forced managers to consider more and more the possibility of reuse of sludge and the use of new purification technologies which allow reducing production. The data on the use of sludge in agriculture at the national level are acquired by the Ministry of Environment and Protection of Land and Sea and are transmitted to the European Commission, in fulfillment of the obligations arising from the implementation of Directive 86/278/EEC. The national standard that defines the conditions that must be verified for the use of sludge in agriculture is the Legislative Decree n. 99 of 27 January 1992 which transposes the European Directive 86/278/EEC. The Decree sets in particular:   

The concentration limit values for heavy metals that must be met in soils and sludge. Agronomic and microbiological characteristics of sludge (the lower limits of concentration of organic carbon, phosphorus and total nitrogen, maximum values of salmonella). The maximum quantities of sludge that can be applied to land.

Besides, the hierarchy of waste management is described in the Legislative Decree 152/06. In some regions specific rules governing the matter have been enacted. In Lombardy, the D.g.r.. 1/7/2014 – no X/2031 provides the new guidelines for sewage sludge reuse in agriculture. In Lombardy, this issue is very important, both for the remarkable quantities of sewage sludge produced, both for their considerable use in agriculture, especially in the Province of Pavia; and the preparation of the Program of Protection and Use of Water (PTUA) has made it even more obvious. In PTUA only hints of the problem of sewage sludge were provided, however, not negligible in some Lombard areas, even for the availability of dispersed and sometimes sketchy information. The most advanced treatments encouraged by PTUA for wastewater (in particular dephosphatizing and filtration) in order to 22

achieve the stringent limits for discharges into water bodies required by law induce a significant increase in the quantities produced. The agronomic reuse of sludge, directly or following composting, is a viable solution to the problem of the disposal of sewage sludge and assumes considerable interest for the agronomic and economic efficacy as it replaces, in whole or in part, the chemical fertilization or other types of organic fertilization. To avoid any risky situation for the environment and the health of the population it must be properly practiced in full compliance with the regulations in particular with regard to the performance of controls on soil and sludge. However, the reuse of sewage sludge for agricultural purposes copes with social and technical obstacles. Technical problems arise due to the fact that sludge is produced all the year whereas its application on land takes place once or twice a year; consequently the sludge should be stored. Furthermore, the sludge content in specific substances should meet explicit criteria; however these norms are sometimes not clear. Another prohibiting factor of larger quantities of sewage sludge being reused in agriculture is the presence of heavy metals. In addition, it is not always accepted by the communities. The development of sludge recycling, in agriculture, is linked with the possibilities to improve the quality of sludge itself and make the public confident in the above issue. The fact that the debate on sludge disposal and recycling is constantly increasing across Europe shows that the relationship between farmers and their customers, the food industry and retailers is of vital importance for accepting sludge use in agriculture (Fytili and Zabaniotou, 2008). Finally, in the near future the legislation will be adapted to the new technical and scientific acquisitions: on the basis of those, the main regulations in Europe and North America are updated and the attention will be focused on health and hygiene requirements (pathogens such as Salmonella, E. Coli, etc.) and organic micropollutants (PAHs, PCBs, PCDD/F, etc.). In conclusion, the Nitrates Directive must be mentioned because it is strictly connected to the agricultural use of sewage sludge. The Nitrates Directive (1991) aims to protect water quality across Europe by preventing nitrates from agricultural sources polluting ground and surface waters and by promoting the use of good farming practices. The Nitrates Directive forms an integral part of the Water Framework Directive and is one of the key instruments in the protection of waters against agricultural pressures. It is implemented trough: 1. Identification of polluted water, or at risk of pollution, such as:  surface freshwaters, in particular those used or intended for the abstraction of drinking water, containing or that could contain (if no action is taken to reverse the trend) a concentration of more than 50 mg/l of nitrates  groundwater containing or that could contain (if no action is taken to reverse the trend) more than 50 mg/l of nitrates  freshwater bodies, estuaries, coastal waters and marine waters, found to be eutrophic or that could become eutrophic (if no action is taken to reverse the trend) 2. Designation as "Nitrate Vulnerable Zones"(NVZs) of:  Eutrophic river areas of land which drain into polluted waters or waters at risk of pollution and which contribute to nitrate pollution; or  Member States can also choose to apply measures to the whole territory (instead of designating NVZs). The current status of NVZ and whole territory designations can be viewed using the map viewer on the website of the Joint Research Centre. 3. Establishment of Codes of Good Agricultural Practice to be implemented by farmers on a voluntary basis. Codes should include:  measures limiting the periods when nitrogen fertilizers can be applied on land in order to target application to periods when crops require nitrogen and prevent nutrient losses to waters;  measures limiting the conditions for fertilizer application (on steeply sloping ground, frozen or snow covered ground, near water courses, etc.) to prevent nitrate losses from leaching and run-off;  requirement for a minimum storage capacity for livestock manure; 23



crop rotations, soil winter cover, and catch crops to prevent nitrate leaching and run-off during wet seasons.

4. Establishment of action programmes to be implemented by farmers within NVZs on a compulsory basis. These programmes must include:  measures already included in Codes of Good Agricultural Practice, which become mandatory in NVZs;  and other measures, such as limitation of fertilizer application (mineral and organic), taking into account crop needs, all nitrogen inputs and soil nitrogen supply, maximum amount of livestock manure to be applied (corresponding to 170 kg nitrogen/hectare/year). 5.     

National monitoring and reporting. Every four years Member States are required to report on: Nitrates concentrations in groundwater and surface waters; Eutrophication of surface waters; Assessment of the impact of action programme(s) on water quality and agricultural practices; Revision of NVZs and action programme(s); Estimation of future trends in water quality.

The Directive was transposed into Italian legislation through the legislative decree 11 May 1999, n. 152 and the Ministerial Decree of 7 April 2006. At regional level, the Law and its Implementing Regulation 37/93 had already dealt with the treatment, the maturation and the use of animal waste in compliance with the European Directive involving the companies to comply with the provisions and implementation of appropriate storage facilities. In the Lombardy Region:  

Provisions for not vulnerable areas are contained in resolution 5868/07 and its integration in December 2009. Provisions for vulnerable areas are contained in resolution IX/2208 of 14 September 2011; Vulnerable Zone Action Programme and Annex (all. 1 d.g.r. 2208, all. 1a d.g.r. 2208, all. 3 d.g.r. 2208).

Farms, with livestock or not, are called upon to respect the regional obligations, update the Plan for Agricultural Utilization and present the Operational Programme every 5 years. Figure 2 shows the situation in Lombardy.

Figure 2: nitrates concentration in 2012 in Lombardy (Source: ARPA; NVZ are nitrate vulnerable zones)

24

The strategies for the management of sewage sludge at the level of the European Community (EC) suggest the need to inspire the practice of agronomic principles for "sustainable use" and "precaution", meaning the minimization of risk through a prior hazard assessment developed on a scientific basis. Even if importance is recognized to recycling of sludge in agriculture as re-use of resources (organic substance and nutrients in particular), such a mode of use must be understood as an alternative, and therefore not exclusive to other forms of disposal. For this reason it is extremely important to define the cautions to prevent risks due to hazardous substances present in the sludge. The technical and scientific documentation of EC (European Commission, 2010) indicates the need to provide for sludge not only more limits for heavy metals, but also for other organic compounds including polychlorinated biphenyls, dioxins, benzofurans, halogenated compounds and phthalates. The use of sewage sludge in agriculture is applied by virtue of the fertilizing properties of these biomasses, with special reference to the content and the availability for plant nutrition of nitrogen and phosphorus (Boyd et al, 1980; Iakimenko et al., 1996). In addition, the sewage sludge, due to its high carbon content, contributes, both in quantitative and qualitative terms, to the budget of the humic soil (Boyd et al, 1980). Numerous studies have been conducted on changes that the use of sewage sludge on soils causes, highlighting both positive and negative outcomes. Positive effects were seen on physical characteristics of the soils (Navas et al., 1998; Sort and Alcaniz, 1999) and on chemical ones (Navas et al., 1998), as well as on its nutritional (Bramryd, 2001; Nyamangara and Mzezewa, 2001) and biological ones (Banerjee et al. 1997; Wong et al., 1998). However, sludge, being the product of the processes of water purification, contains high concentrations of organic molecules not completely degraded during the aerobic or anaerobic treatment (Schnaak et al., 1997; Réveille et al., 2003). These organic fractions may have biogenic origin (e. g. steroids) (Réveille et al., 2003) or xenobiotic (e. g. organo-chlorinated, phthalates, hydrocarbons compounds) (Schnaak et al., 1997). It follows, then, that the contribution of sewage sludge to soil should be practiced by placing particular attention to the possible accumulation of these contaminants. In fact, the prolonged treatment of the soil with sewage sludge can induce the accumulation of recalcitrant organic fractions that become an integral part of soil organic matter and in particular, of the humic fraction (e. g. humic acids) (Pacheco et al., 2003). Humic substances are colloidal, dispersed, negatively loaded polymers; their recalcitrance to biodegradation, also, determines the retention in the soil for a long time (Qualls, 2004). The humified fraction is characterized by the presence of aliphatic molecules (e. g. fatty acids, steroids, protein fractions) and protein (Réveille et al. 2003). A recent study (Adani and Tambone, 2005), which aims to assess the amending effects on a soil of several years of application of sewage sludge, has highlighted that humic acids of the sludge are able to provide a significant contribution to the characteristics of humic acids in the soil; in particular, soil humic acids were enriched in aliphatic fractions typical of sewage sludge. The use of sewage sludge in agriculture, due to their high metal content may cause the accumulation of heavy metals in soil (McBride, 1995). A recent study (Mantovi et al., 2005) reported an increase in the concentration of Zn and Cu in soils subject to twelve years application with sludge composted in mixture with organic waste. However, the accumulation of metals is to be put in relation both to their concentration in sludge used and at the doses of use, both to the characteristics of the soils, capable of influencing the accumulation and uptake by practiced crops. In summary, environmental issues that may be associated with the use of sewage sludge in agriculture mainly concern the contributions arising from:    

Heavy metals; Harmful organic compounds; Nitrogen (from which the risk nitrates); Pathogenic microorganisms.

25

2.4.2 Incineration Figure 3 shows a simplified flow diagram of a sludge incinerator.

Figure 3: simplified sludge incinerator flow diagram (Source: FHWA, 1998)

Mono-incineration technologies are: multiple hearth furnaces, fluidized bed combustors, Etagenwirbler of Lurgi, smelting furnaces, rotary kilns and cyclone furnaces (Werther and Ogada, 1999). At present, two major incineration systems are employed: multiple hearth and fluidized bed. The multiple hearth incinerator is a circular steel furnace that contains a number of solid refractory hearths and a central rotating shaft. Rabble arms that are designed to slowly rake the sludge on the hearth are attached to the rotating shaft. Dewatered sludge (approximately 20 percent solids) enters at the top and proceeds downward through the furnace from hearth to hearth, pushed along by the rabble arms. Cooling air is blown through the central column and hollow rabble arms to prevent overheating. The spent cooling air with its elevated temperature is usually recirculated and used as combustion air to save energy. Flue gases are typically routed to a wet scrubber for air pollution control. The particulates collected in the wet scrubber are usually diverted back into the sewage sludge treatment plant. Fluidized bed incinerators consist of a vertical cylindrical vessel with a grid in the lower sections to support a bed of sand. Dewatered sludge is injected into the lower section of the vessel above the sand bed and combustion air flows upward and fluidizes the mixture of hot sand and sludge. Supplemental fuel can be supplied by burning above and below the grid if the heating value of the sludge and its moisture content are insufficient to support combustion. Auxiliary fuel is normally needed to maintain the combustion process. The quantity of auxiliary fuel required depends on the heating value of the sludge solids and, primarily, on the moisture content of the incoming feed sludge. Operating temperatures can vary, depending on the type of furnace, but can be expected to range from approximately 650°C to 980°C in the incinerator combustion zone. High operating temperatures above 900°C can result in partial fusion of ash particles and the formation of clinkers, which end up in the ash stream. Lime may also be added to reduce the slagging of sludge during incineration. Incineration remains the most attractive disposal method, currently in Europe. One should have in mind that legal limitations concerning landfilling and agricultural reuse as well as that sea disposal is no longer an outlet. In that context there will be an increase in the role of incineration in the long term. The technology of incineration in terms of the process engineering, energy efficiency and compactness of plant has experienced great improvement lately. Modern fluidized bed incinerators have become more and more attractive both in terms of capital as well as operating costs, in comparison to the conventional multiple hearth type. The advantages of incineration can be summarized as follows (Fytili and Zabaniotou, 2008):

26

   

Large reduction of sludge volume; researchers have concluded that the final sludge volume after incineration is approximately 10% of that after mechanical dewatering. Thermal destruction of toxic organic compounds. The calorific value of sewage sludge is almost equal to that of brown coal; therefore incineration offers the possibility of recovering that energy content. Minimization of odor generation.

Suggested BATs for the incineration of sewage sludge are (EU Commission, 2006):  

Preferably employ the technique of the fluidized bed because of the higher combustion efficiency and lower production of flue gas than other systems; Drying of sewage sludge preferably done with the heat recovered from the incinerator.

Recent studies (Mininni et al., 2006) have shown that the integrated process of drying and incineration is much more convenient than a process without drying, both in terms of production of emissions and both in terms of consumption of conventional fuel. Nevertheless, incineration does not constitute a complete disposal method since approximately 30% of the solids remain as ash. This ash is generally landfilled and in certain cases, it is considered as highly toxic because of its metal content. One of the major constraints in the widespread use of incineration is the public concern about possible harmful emissions. However, introducing new technologies for controlling gaseous emissions can minimize the adverse effects mentioned beforehand, while the reduction in the correspondent cost gives incineration considerable advantages in future as compared to other available disposal routes. The amount of sludge being incinerated in Denmark has already reached the percentage of 24% of the sludge produced, 20% in France, 15% in Belgium, 14% in Germany while in USA and Japan the percentage has increased to 25% and 55%, respectively (Lundin et al., 2004). For example, in Japan, annual production of sewage sludge increased to 2.17 million tonnes (dry basis) in 2004 and about 70% of sewage sludge is incinerated (Muramaki et al., 2009). In a typical conventional incineration plant (Figure 4), dewatered sludge containing 80% moisture is supplied to a bubbling fluidized bed combustor fueled by a supplementary fuel such as natural gas or crude oil. Drying, volatilization, and combustion take place in the combustor. Flue gas is exhausted from a stack into the atmosphere after first passing through an air preheater, a smoke-prevention preheater, a gas cooler, a bag filter and a scrubber. Sludge incineration consumes electric power because the process utilizes many auxiliaries. A typical incinerator with a 100 t/day capacity, which is the average capacity in Japan, consumes 350 kW during steady operation. The higher heating value of the dry sludge is 16–21 MJ/kg. However, because the dewatered sludge contains about 80% moisture, the high temperature flue gas is used only for heat exchange. Additionally, the nitrogen content of the sludge is considerably higher than that of other fuels, such as coal and wood. Thus the emissions of NOx and N2O are anticipated to be high. The global warming potential of N2O is 310 times that of CO2, so the emission of N2O is a big problem. In Europe, the situation is described in Table 6: the existing facilities dedicated to the incineration of sewage sludge are 57 (Umweltbundesamt, 2001; TWG comments, 2003), compared to 467 dedicated to the incineration of municipal solid waste. In USA, the EPA conducted a study in 2011 on the incineration of sludge and 144 multiple hearth plants were found to be active and 60 fluidized bed plants (EPA, 2011). In Italy, the reference legislation is Legislative Decree 133/05 on the implementation of Directive 2000/76/EC on the incineration of waste. Sludge coming from wastewater treatment (ECW 190805) is included as non-hazardous waste which can be utilized as a fuel or for other means to generate energy in Ministerial Decree February 5, 1998 (Annex 2 - Sub-Annex 1), as amended by Ministerial Decree 186 of 04.05.2006. This type of activity, in simplified authorization system, is subject to a series of constraints for the plant, the characteristics of the sludge to be treated and the emissions.

27

Figure 4: typical fluidized-bed incineration plant (Source: Muramaki et al., 2009) Table 6: sewage sludge incineration facilities in Europe

Country

Number plants

Austria Belgium Denmark Germany Netherlands United Kingdom Switzerland Total

of Capacity (tonnes dry matter/year) 1 1 20000 5 300000 23 630000 2 190000 11 420000 14 100000 57 1660000

(Source: Umweltbundesamt, 2001; TWG comments, 2003)

2.5 Costs 2.5.1 Agricultural reuse The following considerations are taken from AA. VV., 2004, updated, based on feedbacks with Lombard reality and they are valid for medium and big WWTPs. The components of the cost of disposal of sludge handled directly by the producer consist of the following:   

The cost of disposal in relation to the stages of loading, transport (hypothesis of limited distances), distribution, plowing, operations usually made through authorized transporters; indicative average assessment: 15 €/t of sludge as it is; Administrative costs for the operational management of the relationships with farmers, periodic sludge and soil analysis, regulatory compliance; indicative average rating: 4-6 €/t of sludge as it is; Additional costs for supplementary treatment (specific for the use of sludge in agriculture), including the cost of chemicals, energy, depreciation for installing additional equipment (mainly sanitation and storage); average rating indicative for plants of a certain potential: 3 €/t of sludge as it is.

The overall average cost is reported to be about 22-24 €/t of sludge as it is. In case of implementation of a system of environmental certification (EMAS, ISO 14001), the commitments result in an increase in the cost to reach 30-35 €/t. This cost is limited and can be reached if synergy is got between the management of the purification that produces sludge and disposal activities managed in own. The market price of disposal made by third parties (including transport) which has soil as its final destination now ranges from 45 to 60 €/t of sludge as it is. These prices take into account not only the cost of proper disposal, but also management and depreciation costs of an industrial structure made specifically for this purpose. The activity can be technologically complex as the sludge comes from sev28

eral plants and any necessary treatment may have diversified management problems. In the cost components, in some cases the compensation required by farmers to make land available is not to be neglected. The average value of disposal cost in agriculture in Lombardy is 90 €/t, taking into account small WWTPs. The inputs of organic matter, nitrogen and phosphorus per hectare can be calculated in detail on the basis of the analysis of the specific sludge, compared to usable quantities that can be extremely diversified in function of the characteristics of the soil (mainly pH and CEC). For example the current legislation in force in the Lombardy Region has a range of variability between min and max 2.5 - 22.5 tDM/ha, where the highest values are permitted for sludge from the agro-food sector and soils with optimum characteristics. The conditions that occur in reality induce that rarely the doses applicable exceed 7.5 tDM/ha. These contributions allow a saving, for farmers, on the costs of fertilization of the ground, as it reduces the use of chemical fertilizers. To get a rough idea of the savings, for a contribution of nitrogen of 340 kg/ha, which corresponds to approximately 740 kg of urea, with a unit cost of 0.22 €/kg urea distributed, the savings amount to about 163 €/ha. But it must be considered that only a portion of the nitrogen content in the sludge results in the time available to the crops. Considering also the contribution of phosphorus and potassium, more limited but non-zero, the savings of 163 €/ha due to a lower or no chemical fertilization can be sufficiently reliable. Considering that one hectare of cultivated land with maize makes approximately, sold as a finished product, 1500-1700 €/ha, the saving of fertilizer made possible by the use of sludge is on the order of 10% of the value of the sold product. It should be noted that the chemical fertilizers, if exclusive, affects approximately on 13%. As regards the addition of organic matter the benefit arising out of the sludge is comparable to the use of manure. Finally, the use of a sludge having good quality characteristics is convenient, being able to allow the substitution or restriction of chemical fertilization. It is however convenient, although at a slightly lesser extent, compared to the use of manure. 2.5.2 Incineration The costs of drying treatment are of the order of 60-80 €/t of dewatered sludge conferred (AA. VV., 2004). To determine the cost of sludge combustion plant it is needed to consider the costs for:     

the transport the sludge; the storage systems; the combustion chamber; the flue gas treatment and cleaning systems and waste treatment process; the management, the personnel, the domestic consumption (for example consisting of products used as reagents, electricity) and the taxes.

Given the variability of factors that enter in the definition of the cost of a thermal treatment system, to assign a general value to these becomes difficult. An estimate of the costs of incineration of sludge says these are above 220 Euro/ton of dry material (IRER, 2007) excluding taxes (varying according to the location of the plant). This value includes the investment costs, the up-grading of existing systems and the costs of management and storage of the waste in the landfill. These costs are referred to units capable of treating from 2,000 to 5,000 tons of dry material per year or, considering the water line that is upstream, from 200,000 to 800,000 people equivalent. The investments grow for plants equipped with pre-drying and having complex gas cleaning systems. The aspect of energy recovery is taking on a fundamental role in the economic and in the environmental balance of a thermal treatment plant. Energy recovery allows reducing the use of traditional fuels, mainly with a positive return in terms of environmental performance, because, for the reduced size of a plant for combustion of sludge, it only makes little convenience from an economic point of view.

29

30

3. Sludge management in the ATO of Como 3.1 Background situation of the ATO of Como The Ambito Territoriale Ottimale (ATO) is a territory in which integrated public services are organized, such as water or waste management (see the Environmental Code, Legislative Decree 152/2006 and subsequent amendments, which repealed the L. 36/94). These areas are identified by the regions with special regional laws (in the case of the integrated water service with reference to river basins) and they act on the Autorità d’ambito, structures with legal personality that organize, control and manage the integrated services. The fundamental tool of programming, developed by the Autorità d’ambito, according to article 11 of L. 36/94, is the Piano d’ambito, through which the Authority implements, directs and controls the Sistema Idrico Integrato (Integrated Hydric System). The Piano d’ambito is the result of a series of operations which, in summary, are:      

The analysis of the existing or the recognition of works and service levels, to assess possible risks in the production capacity of the plants and assess the guaranteed service levels of the structures present on the territory of the ATO. The assessment of resource availability and demand of water services, to define the level of service deemed necessary for the satisfaction of the user. The study of the intervention plans and the investments required for the adaptation of the infrastructure to meet user demand. The identification of the critical issues and priorities for actions on infrastructures. The definition of a management model that achieves the intended aim in terms of efficiency, effectiveness and economy of the provided services. The calculation of the tariff and its possible evolution only after having the complete picture of the investments distributed in space and time.

With the Piano d’ambito it is expected to achieve adequate levels of investment in addition to new and higher standards of quality and quantity in the management of the services of water supply, sewerage and water treatment. On the basis of the intermediate and complex objectives set in the planning by the Authority, every three years it is required to prepare a detailed document (Piano Operativo Triennale) of interventions that should be achieved. As it is stated in 2010 ATO plan, in the ATO of Como there are 41 wastewater treatment plants for a total potential of 1,094,021 PE (People Equivalent) and serving 707,428 PE. The relative consistency of the different plants is described in Table 7.

31

Table 7: consistency of the WWTPs in the ATO of Como

ID

Plant Location

Served people equivalent

People equivalent potential

(Source: ATO-Como, 2010)

Among the 41 plants, only 20 have a depurative potential bigger than 2000 PE, while the others can be considered plants for small communities. Among the 20, we can focus on the 7 biggest plants. Globally these 7 plants serve 581,655 PE, the 82% of the total PE served in the ATO of Como. In 2009 these plants produced 27569.9 tonnes of sewage sludge, representing the 92% of the total production of sewage sludge from wastewater treatment plants; in fact, the document of 2012 regarding the management of waste reported a production of 29993.6 tonnes of sewage sludge in 2009 (EWC code 190805 – “sludges from treatment of urban waste water”) (Osservatorio dei Rifiuti-Como, 2012). From these considerations it can be deduced that the attention must be put on the production and the management of sewage sludge of these big plants in order to understand the situation in the ATO of Como and which possible improvements can be applied.

32

Table 8: consistency of the 7 biggest plants

Plant Bulgarograsso - Alto Lura srl Carimate - Sud Seveso Servizi spa Como - Comodepur spa Fino Mornasco - Alto Seveso - Lariana Depur spa Fino Mornasco - Livescia - Lariana Depur spa Mariano Comense - Valbe spa Merone – ASIL spa

PE served [2009] PE potential [2009] 88000 154000 73500 96000 * 167588 297217 66986 140000 24000 43300 60000 60000 101581 120000

* After the conclusion of the works in 2012 the depurative potential of Carimate plant has increased with respect to the one reported in 2010. (Source: ATO -Como, 2010)

The 2010 ATO plan expected the determination of interventions of centralization of the wastewater treatment by the aggregation of water treatment services. In the investment plan (chapter 6), the construction of non-well-defined poles of sludge drying was proposed: “The stage of disposal of the sludge produced by the treatment plants is generally a critical point for all managers, since such an operation alone sometimes is more expensive than the whole process of sludge treatment. The choice of the method of disposal determines the whole train of operations (stabilization, conditioning, dewatering, etc.) to which it is necessary to submit the sludge before disposal and involves economic and environmental assessments related in large part to the nature, composition and quality of the sludge produced and its treatment. The various stages of the sludge treatment system planned for the ATO are:     

Thickening, Digestion or conditioning, Dewatering, Heat treatments aimed at reducing the volume, Final disposal."

The Plan provided a basic and decidedly simplistic structure of the planned cycle of treatment of the sludge. From planning hypothesis and the quantification of the sludge produced in the ATO, the Plan identified two macro areas (North and South). In this context and especially in view of new legal regulations aimed to limit the use of the sludge from treatment plants in agriculture, the Plan provided two options: 1. Creation of new sludge drying plants in the two areas, aimed at the decrease in the amount of sludge to be disposed. 2. Conveyance of sludge at existing plants to enable a "widespread" management by drying sludge, with cost/benefit analysis to be carried out with the manager. Those scenarios are currently rebuttable and dated at a distance of 4 years of technological innovation and with new possibilities of management. The 2014 plan confirmed what was written in 2010 plan but underlined the problems related to sludge management, setting new important investments for the improvement of the services taking into account the new legislations developed by the Region. The main investments concern:  

Odors treatment for different plants. New dewatering unit for Mariano Comense plant for the substitution of the old belt press. 33

   

New dewatering line for Merone plant. Revamping of the anaerobic digester of Mariano Comense plant. Sludge incineration plant (7200 tonnes wet sludge/y) for Carimate and Mariano Comense plants. Revamping of biological treatment for Fino Mornasco – Alto Seveso plant.

The interventions related to sludge treatment, which are considered fundamental for the efficiency and effectiveness of the integrated hydric services, will require investments for about 14 million of euros. They have a high priority and they are the first interventions after the ones related to water supply. Now the focus will be put on the characteristics of the biggest plants.

3.2 Sludge treatment and disposal in the 7 biggest plants The sludge treatment lines of each plant are briefly described; a complete description of the plants is presented in APPENDIX 1. 3.2.1 Sludge treatment lines and production Alto Lura – Bulgarograsso The excess sludge coming from the sedimentation tank is sent to an aerobic digestion tank. The bacteria continue degrading the organic matter so that the sludge volume is reduced and there is no odor production. The remaining sludge is thickened, lifted and sent to mechanical dewatering via centrifuge. Then it is transported via cochlea to a storage tank which is periodically emptied. Excess sludge

Aerobic digestion

Post-thickening

Centrifuge

Disposal

Figure 5: sludge line flow diagram – Alto Lura Bulgarograsso

Sud Seveso Servizi – Carimate The mixed sludge from the primary sedimentation tanks is sent to pre-thickening: the pre-thickeners are 2 types of circular artifacts with a diameter of 9 m, mechanized, with a peripheral speed of scraper of 3.2 cm/s. These tanks act as accumulation for the next phase of dynamic thickening constituted by a drum of 450 ÷ 600 kgTSS/h, which allows to thicken the sludge to be sent to the next process of anaerobic digestion; before feeding the machine, the sludge is conditioned with the addition of a cationic polyelectrolyte to facilitate the separation of water from the sludge. The dynamic thickener and the lifting of the thickened sludge to digestion are carried out by mohno pumps with adjustable flow. In case of downtimes of the dynamic thickener, the sludge can be sent directly from the pre-thickeners to digestion by 3 mohno pumps with a flow rate of 5 to 17 m³/h each, whose adjustment is accomplished by the inverter. Two tanks in PRVF (plastic reinforced with glass fibers) were installed (35 m³ and 40 m³) for the dosing of nutrients (COD) to anaerobic digestion. The digestion of sludge is performed in two digesters, operating in series, the respective volume of 2,800 m³ and 1,600 m³. The mixing of the sludge is accomplished by apposite pumps (helical ducted or recirculating) and the digestion temperature is maintained approximately at 35 °C in the first digester (mesophilic) and 55 °C in the second (thermophilic) by means of two heat exchanger with surface of approximately 13 m²/each in which water circulates at the temperature of 70/80 °C, coming from a boiler of 766 kW coupled in parallel with the thermal exchange module (112 kW) of the microturbine. The containment of any odor emissions arising from the summit of the two digesters is ensured by a system called "osmogenetic barrier" that allows the atomization of a specific product. The biogas produced is sent and stored in a gasometer of 300 m³, which performs the function of storage tank and pressure stabilizer: the biogas is burnt in the microturbine (nominal capacity 33 ÷ 36 Nm³/h) that performs the function of a co-generator, producing 65 kWe and 112 kWt. Any excess biogas is burnt in a boiler which can be fueled with methane. In the case of stationary contemporary downtime of the microturbine and boiler, the biogas produced is flared. 34

The sludge from the digestion is accumulated in a tank of the post-thickening of a volume of about 140 m³, equipped with a stirrer blade. This tank is held in constant suction by a fan which conveys the flow in a suitable water scrubber: the washing and cooling of the flue gas is completed with wastewater. From the post-thickener the digested sludge is sent the mechanical dewatering that occurs through two belt presses, one reserve to the other: each machine has a width of 2.1 m and a maximum capacity of 540 kgTSS/h. Before feeding the belt press, the sludge is conditioned with the addition of cationic polyelectrolyte to facilitate the separation of water from the sludge. The air changes in the room in which the machines are located are guaranteed by 2 fans: the containment of any odor emissions is carried out by the same system as the digesters, the "osmogenetic barrier" that allows the atomization of a specific product next to the two suction points.

Primary sludge Excess sludge

Static and dynamic pre-thickening

Disposal

Mesophilic anaerobic digestion

Belt presses

Thermophilic anaerobic digestion

Post-thickening

Figure 6: sludge line flow diagram – Sud Seveso Servizi Carimate

Comodepur – Como The excess biological sludge and the tertiary sludge produced by tertiary clariflocculation and biofiltration are sent to the sludge treatment. In pre-thickening, with the densification of the sludge the volume is further reduced obtaining a concentration of about 25 – 35 g/liter. The sludge is thickened in two covered thickeners and subsequently dewatered, while the drained water is recirculated in the predenitrification tank. The sludge produced by thickening is added with polyelectrolyte and further reduced in volume by mechanical dewatering, which is done via 2 centrifuges. The dewatered sludge through augers and progressive cavity pumps (one for each line of dehydration) is sent to the storage silos, with a total capacity of 150 m3. The silos are at a sufficient height, inside a shed, to allow loading of trucks. All treatment including the loading area is closed and equipped with an air intake system; air is sent to a scrubber for the treatment. Excess sludge

Pre-thickening

Centrifuge

Disposal

Tertiary sludge Figure 7: sludge line flow diagram – Comodepur - Como

Lariana Depur – Alto Seveso – Fino Mornasco The sludge produced, already sufficiently aerobically stabilized in the step of biological treatment, is sent in tanks for the thickening and is then sent, after the addition of polyelectrolyte, to the centrifuge for dewatering. Excess sludge

Pre-thickening

Centrifuge

Disposal

Figure 8: sludge line flow diagram – Lariana Depur – Alto Seveso – Fino Mornasco

35

Lariana Depur – Livescia – Fino Mornasco The sludge produced, already sufficiently aerobically stabilized in the step of biological treatment, is sent in tanks for the thickening. Further dewatering is performed through mobile machines, Excess sludge

Thickening

Disposal

Figure 9: sludge line flow diagram – Lariana Depur – Livescia – Fino Mornasco

Valbe Servizi – Mariano Comense The sludge extracted from primary and secondary sedimentation is conveyed to the pre-thickener, where it is concentrated, then to the anaerobic digester, where anaerobic bacteria stabilize the sludge turning organic matter into inorganic substances and produce a gaseous mixture of methane and carbon dioxide that is used to maintain temperature conditions for the process (33-35 °C). The digested sludge is then further thickened through the basin of post-thickening, and finally sent to the dewatering step through the passage in a centrifuge, before the definitive storage and subsequent transport to disposal. The biogas produced in the digester is collected in a gasometer; any excess amount is burned through a special torch. Primary sludge

Pre-thickening Excess sludge

Anaerobic digestion

Disposal

Post-thickening

Centrifuge

Figure 10: sludge line flow diagram – Valbe Servizi – Mariano Comense

ASIL - Merone In pre-thickening, the excess sludge removed from the secondary decanters and primary sludge are sent in a closed circular artifact where their volume is reduced separating the water contained in them by thickening until obtaining a concentration of dry substance of about 30 g/liter. After pre-thickening, the sludge is sent to an anaerobic digester. In it the anaerobic microorganisms break down organic matter and produce minerals and biogas containing mostly methane gas (used as fuel in the boiler of the dryer). The sludge, after about 15 days of stay in the digester, is mineralized and ready for subsequent treatments. The sludge coming from the digester (with a concentration of dry substance of about 20 g/liter) is sent to post-thickening in a circular artifact, where the volume is reduced separating the water contained in it to obtain a concentration of dry substance of about 30 - 40 g/liter. The sludge coming from the post-thickening is further reduced in volume by mechanical dewatering with a centrifuge. The dewatered sludge still contains a degree of humidity equal to 75%, or a dry matter content of 25%. After dewatering the sludge is dried. The drying plant, designed to cope with the new legislation on waste management and to reduce the costs associated with the disposal, is able to reduce the volume of sludge to be disposed of, increasing the concentration of dry matter up to 90%. It consists of turbo rotary dryer using diathermic oil heated with the biogas produced by the anaerobic digesters, or with methane from the gas network. The dried sludge is loaded into silos and sent to energy plant in the nearby cement plant. In order to optimize the consumption of electricity a co-generator has been installed consisting of an engine fueled with natural gas, coupled with an alternator. The electricity produced (about 750 kWh) is capable of powering the loads of the system with a surplus that is sold to the GSE (Electrical Ser36

vices) for the distribution network. The heat from the engine is used for the heating of anaerobic digesters and for heating the office building and the chemical laboratory in winter. Primary sludge Anaerobic digestion

Pre-thickening

Excess sludge

Disposal

Post-thickening

Rotary drier

Centrifuge

Figure 11: sludge line flow diagram – ASIL - Merone

The trend of production of sewage sludge from 2009 to 2013 can be analyzed. Table 9: sewage sludge production of the biggest plants

Plants

Total disposed sludge (tonnes of wet sludge)

Bulgarograsso - Alto Lura srl Carimate - Sud Seveso Servizi spa Como - Comodepur spa Fino Mornasco – A. S. - Lariana Depur spa Fino Mornasco – Liv.- Lariana Depur spa Mariano Comense - Valbe spa Merone – dewatered sludge - ASIL spa Merone – dried sludge- ASIL spa Total dewatered sludge

2009 2010 2011 2012 2013 Mean 3195 3574 3429 3944 4421 3713 2599 3625 3270 2950 3355 3160 11030 9736 10137 10028 9921 10170 4218 4342 4508 4385 4082 4307 2866 3990 3626 848 519 2370 1761 1959 2453 2511 2182 2173 1711 1584 2973 3085 2741 2419 189 279 216 220 93 200 27380 28810 30396 27750 27221 28312 27570 29090 30612 27970 27314 28511

Total disposed sludge

tonnes/y

Graph 1: sewage sludge production of the biggest plants

31000 30000 29000 28000 27000 26000 25000 2009

2010

2011 years

2012

2013

The graph shows a reduction in the production of sewage sludge in the last years: it must be taken into account that this can be due to the period of economic crisis which affects indirectly also the production of residuals. In the future the trend will be growing (IRER, 2010) because of the increase of the population, of the consumptions and of the connections to the sewerage and because of the improvements in the depuration systems due to the change towards stricter limits for pollutants in water which will impact on sludge production. 37

After the EC infringement procedure 2034/09 for failure to comply with the obligations set out in Directive 91/271/EEC concerning urban waste-water treatment, the ATOs, the hydric system managers and the municipalities have prepared a series of measures to comply with the Directive. From 2009 to 2011 the increase in the production of sludge (approximately 5% per year) is due to these measures; so after the economic crisis, other interventions will be adopted and quantities will grow again. 3.2.2 Characteristics of the sludge The sludge has undergone several treatments and finally presents the characteristics which allow its disposal. The most important parameter is the dry matter content, because it gives information about the volume of organic material which must be moved. Hereafter we provide the dry matter content for the sludge coming from the 7 biggest plants. Table 10: dry matter content of sewage sludge of the biggest plants

Plants

Dry matter content (%) 2009 2010 2011 2012 2013 Bulgarograsso - Alto Lura srl 19.9 20.2 21.8 21.4 21.1 Carimate - Sud Seveso Servizi spa 25.7 24.3 23.7 23.5 21.51 Como - Comodepur spa 21.8 21.3 21.6 21.1 21.0 Fino Mornasco - Alto Seveso - Lariana Depur spa 20.1 20.6 21.9 21.0 23.2 Fino Mornasco - Livescia - Lariana Depur spa 3.5 3.7 3.8 22.0 22.1 Mariano Comense - Valbe spa 29.3 29.0 26.8 27.6 28.5 Merone – dewatered sludge - ASIL spa 26.3 26.6 24.6 24.1 25.4 Merone – dried sludge- ASIL spa 93.3 90.7 91.1 89.9 86.4 Weighted mean without dried sludge 20.5 19.8 20.5 22.3 22.5 Weighted mean with dried sludge 21.0 20.5 21.0 22.9 22.7 As a result, the following values of quantities of dry matter are produced. Table 11: dry matter quantities of sewage sludge of the biggest plants

Plants

Total dry sludge (tonnes of dry sludge) 2009 2010 2011 2012 2013 Mean Bulgarograsso - Alto Lura srl 636.8 720.5 746.5 843.6 931.5 775.8 Carimate - Sud Seveso Servizi spa 667.9 880.9 775.0 693.3 721.6 747.7 Como - Comodepur spa 2404.5 2073.8 2189.6 2115.9 2083.5 2173.5 Fino Mornasco - Alto Seveso - Lariana 847.8 894.5 987.3 920.9 947.0 919.5 Depur spa Fino Mornasco - Livescia - Lariana Depur 100.3 147.6 137.8 186.6 114.7 137.4 spa Mariano Comense - Valbe spa 515.9 568.2 657.4 692.9 621.9 611.3 Merone – dewatered sludge - ASIL spa 450.1 421.4 731.3 743.5 696.1 608.5 Merone – dried sludge- ASIL spa 176.8 253.4 196.9 197.5 80.4 181.0 Total dry sludge 5623.4 5706.8 6224.8 6196.5 6116.4 5973.6 Besides, agricultural reuse can be admitted if the sludge has some specific characteristics, reported in the D.g.r. 1/7/2014 – no X/2031.

38

Table 12: admissible concentrations of heavy metals for the sludge entering the recovery plants

Parameters Heavy metals Cadmium (Cd) Copper (Cu) Nickel (Ni) Lead (Pb) Zinc (Zn) Chromium (Cr tot) Mercury (Hg) Nutrients Organic Carbon Total Nitrogen

u. of m.

Admissible values

mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm

≤ 22 ≤ 1200 ≤ 330 ≤ 900 ≤ 3000 ≤ 900 ≤ 11

% DM % DM

> 10 > 1,0

(dm: dry matter) (Source: D.g.r. 1/7/2014 – no X/2031)

Previously the limits were the following. Table 13: previous admissible concentrations of heavy metals for sludge

Parameters Heavy metals Cadmium (Cd) Copper (Cu) Nickel (Ni) Lead (Pb) Zinc (Zn) Chromium (Cr tot) Mercury (Hg) Nutrients Organic Carbon Total Nitrogen

u. of m. mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm

Admissible values d.lgs. 99/92 ≤ 20 ≤ 1000 ≤ 300 ≤ 750 ≤ 2500 ≤ 10

DGR 15944/03 ≤ 20 ≤ 1000 ≤ 300 ≤ 750 ≤ 2500 ≤ 750 ≤ 10

% DM % DM

> 20 > 1,5

> 20 > 1,5

(dm: dry matter)

Then, a distinction is made between high quality and admissible sludge. Table 14: distinction between high quality and admissible sludge

Parameters pH VSS/TSS Heavy metals Cadmium Total Chromium Mercury Nickel Lead

u. of m.

Limit values high quality

%

< 60

mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm

≤5 ≤ 150 ≤5 ≤ 50 ≤ 250

admissible 5,5 < pH ≤ 11 < 65 ≤ 20 ≤ 750 ≤ 10 ≤ 300 ≤ 750 39

Parameters

u. of m.

Limit values high quality ≤ 400 ≤ 600 ≤ 10

admissible ≤ 1000 ≤ 2500 -

Copper mg/kg dm Zinc mg/kg dm Arsenic mg/kg dm Agronomic parameters Organic Carbon % DM > 20 Total Nitrogen % DM > 1,5 Total Phosphorus % DM > 0,4 Organic pollutants IPA mg/kg dm 60% (Source: D.g.r. 1/7/2014 – no X/2031)

For Bulgarograsso, Carimate and Merone plants sludge analysis are available, so that considerations on the general quality of the sludge can be made. The attention must be put mainly on the heavy metals concentrations. As it can be evinced from Table 15, all the sludges are admissible with good safety margins, but they do not have the right characteristics to be considered of high quality. This should induce to think about new solutions in order either to improve the treatment to obtain high quality sludge or to change the way the sludge is disposed. In fact the new regulations become stricter and the land availability is limited so it is expected that in the future only high quality sludge will be allowed in agricultural reuse.

40

Table 15: heavy metals contents in sewage sludge samples of 3 plants

Heavy metals limit values Copper mg/kg dm Total Chromium mg/kg dm Lead mg/kg dm Cadmium mg/kg dm Nickel mg/kg dm Mercury mg/kg dm Arsenic mg/kg dm Zinc mg/kg dm Heavy metals - Bulgarograsso Parameters u. of m.

high quality ≤ 400 ≤ 150 ≤ 250 ≤5 ≤ 50 ≤5 ≤ 10 ≤ 600 Mean value

Copper mg/kg dm Total Chromium mg/kg dm Lead mg/kg dm Cadmium mg/kg dm Nickel mg/kg dm Mercury mg/kg dm Arsenic mg/kg dm Zinc mg/kg dm Heavy metals - Carimate Parameters u. of m.

368.7 248.2 33.125 2.725 251.9 0.83 4.725 480.225

Copper mg/kg dm Total Chromium mg/kg dm Lead mg/kg dm Cadmium mg/kg dm Nickel mg/kg dm Mercury mg/kg dm Arsenic mg/kg dm Zinc mg/kg dm Heavy metals - Merone Parameters u. of m.

700.72 169.18 94.06 2.16 120.68 0.998 10.96 1289.42

Copper Total Chromium Lead Cadmium Nickel Mercury Arsenic Zinc

459.3 110.4 62.9 1.0 101.2 1.1 12.1 1439.8

mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm mg/kg dm

Mean value

Mean value

admissible ≤ 1000 ≤ 750 ≤ 750 ≤ 20 ≤ 300 ≤ 10 ≤ 2500

Standard tion 37.8 30.1 3.6 4.2 76.4 0.3 0.4 124.9

devia- Notes

Standard tion 153.1 12.6 17.7 0.4 35.7 0.3 1.5 590.1

devia- Notes

Standard tion 31.6 15.4 10.9 0.1 23.1 0.4 2.1 534.2

devia- Notes

4 samples (25-52012, 20-112012, 13-52013,12-112013)

5 samples (30-32012, 4-10-2012, 31-10-2012, 305-2013, 28-112013)

5 samples (2-12013, 26-3-2013, 21-6-2013, 25-92013, 17-122013)

(Values greater than the limit for high quality sludge are in bold)

41

3.2.3 Sludge disposal In this chapter the focus is on the destiny of sewage sludge produced by the above mentioned plants. In 2013 the 7 plants produced 27,314 tonnes of sewage sludge, 27,221 tonnes were sent to plants which process the sludge to use it in agriculture while only 93 tonnes were sent to incineration in a cement kiln, totally by the plant of Merone. According to the European Waste Catalogue and Hazardous Waste List, the sewage sludge, considered a special non-hazardous waste, is given the code EWC 190805: 19 stands for “wastes from waste management facilities, off-site waste water treatment plants and the preparation of water intended for human consumption and water for industrial use”; 08 for “wastes from waste water treatment plants not otherwise specified”; finally 05 for “sludges from treatment of urban waste water”. The processing plants are located in the Po River Valley, in which agriculture is widely developed and there is the possibility of spreading the processed sewage sludge which acts as a fertilizer for the crops. The recovery plants which actually dispose sludge on land are:    

C.R.E. spa located in Maccastorna (LO), with a recovery potential of 125,000 tonnes/year. ECODECO located in Giussago (PV), with a recovery potential of 120,000 tonnes/year. BIOAGRITALIA srl located in Corte De’ Frati (CR), with a recovery potential of 20,500 tonnes/year. V.A.R. located in Belgioioso (PV). Incineration 93 tonnes

Production 27,314 tonnes Land application 27,221 tonnes

   

CRE: 12,698 tonnes ECODECO: 9004 tonnes BIOAGRITALIA: 5000 tonnes VAR: 519 tonnes

Figure 12: sludge disposal routes in the ATO of Como

In Table 16 the total amounts of the sludge conferred to the plants are reported. Table 16: total amount of sludge conferred to processing plants

Plant C.R.E. ECODECO BIOAGRITALIA VAR

2007 2008 2009 2010 127,220 116,674 124,274 98,475 125,846 91,201 87,871 91,289 19,838 20,201 20,319 18,237 45,146 43,591 46,468 45,041

(Source: ISPRA database)

It can be easily seen that the total amount of accepted sludge is quite near to the recovery potential of the plants: in 2010, 79% of the potential for C.R.E., 76% for ECODECO, 89% for BIOAGRITALIA. Besides, it must be considered that only a part of these amounts come from Lombardy: in 2010, 55% for C.R.E, 60% for ECODECO, 53% for BIOAGRITALIA and 66% for VAR. So these plants cannot just satisfy the requests of Lombardy or Como plants and in case of increase in the amount of sludge produced, which must be disposed into agriculture, it cannot be assumed that these plants will have the capacity to accept this extra sludge. Finally, in Table 17 the distances from the WWTPs to the recovery plants are reported. These must be taken into account for any economic and environmental consideration concerning the transport of sludge.

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Table 17: distances from WWTPs to recovery plants in Padana plain

WWTPs Bulgarograsso Carimate Como F.M. A. S. Recovery plants Distance (km) C.R.E. 119 111 ECODECO 71 62 BIOAGRITALIA 164 VAR

F.M. Liv.

M.C.

105

Merone

115

83

As almost the total of sludge (99.7%) is disposed into agriculture it is better to give an explanation of how this kind of disposal is implemented. The operational phases of the process of treatment and recovery in agriculture and the soil conditions for landspreading of sludge are presented in APPENDIX 2. The surfaces used for the distribution of the sludge result from the analysis of production of livestock manure and the receptivity of the soil taking into account the constraints posed by the Nitrates Directive and the designation of vulnerable areas. In particular, the research developed by IRER in 2007 considered that, where available, the livestock manure is used and that, therefore, the land where manure is not used could also receive sewage sludge. It follows that in municipalities where the amount of livestock manure is in excess of regulatory limits envisaged by the application of the Nitrates Directive, all soils are used for effluents and, therefore, not available for the use of sludge. The surfaces used for the distribution of the sludge are therefore the difference between the used agricultural surface (Superficie Agricola Utilizzata, SAU) and the municipal area needed for distribution of the livestock effluents. Table 18: surfaces available for distribution of sludge in agriculture

(Source: IRER, 2007)

43

Figure 13: budget of manure in relation to the constraints of the Nitrates Directive (Source: IRER, 2007)

Figure 14: area available for the use of sewage sludge, net SAU used for the distribution of manure (Source: IRER, 2007)

44

3.2.4 Disposal costs The costs for the disposal of the sludge are reported in the table. Table 19: costs for the disposal of dewatered sludge

Plants

Bulgarograsso - AltoLura srl Carimate - Sud Seveso Servizi spa Como - Comodepur spa Fino Mornasco - Alto Seveso - Lariana Depur spa Fino Mornasco - Livescia - Lariana Depur spa Mariano Comense - Valbe spa Merone – dewatered sludge - ASIL spa Weighted mean

Costs for sludge disposal (€/ton of wet sludge ) 2009 2010 2011 2012 2013 85.0 70.0 72.5 71.9 54.0 72.0 71.5 72.5 73.9 56.0 74.0 74.0 65.0 65.0 65.0 84.0 70.0 64.0 64.0 55.0 16.5 16.8 28.0 145.0 145.0 82.0 71.5 72.5 73.9 57.0 152.0 116.2 72.5 73.9 57.0 76.0 66.8 63.4 71.0 60.7

For Merone plant, the costs related to the disposal of the dried sludge in the cement kiln are reported. Table 20: costs for the disposal of dried sludge

Plants

Merone – dried sludge- ASIL spa

Costs for disposal in cement kiln (€/ton of dried sludge ) 2009 2010 2011 2012 2013 64.5 70.5 75.5 76.5 73.5

For each plant the details for the disposal options can be analyzed:     

 

Bulgarograsso: in 2009, disposal in agriculture entrusted to A2A plant; from 2010 to C.R.E. plant; a change in the costs is evident, with a sharp decrease, from 2012 to 2013, with an high increase in the conferred sludge (from 3944 to 4421 tonnes); Carimate: in 2009, disposal in agriculture entrusted to ALAN/ALLEVI plant; in 2010 partly to ALLEVI, partly to C.R.E. plants; from 2011 totally to C.R.E. plant; a change in the costs is evident, with a sharp decrease, from 2012 to 2013; Como: from 2009, disposal in agriculture entrusted partly to C.R.E partly to ECODECO plants; Fino Mornasco – Alto Seveso: from 2009, disposal in agriculture entrusted to ECODECO plant; Fino Mornasco – Livescia: from 2009 to a first part of 2010, sludge sent to Alto Seveso plant for dewatering and disposal in agriculture; from a second part of 2010 to 2011, disposal as dewatered sludge in Carimate plant; from 2012, dewatering with a mobile machine and final disposal in agriculture in VAR plant; the mobile dewatering plant induced a sharp increase in the costs; Mariano Comense: from 2009, disposal in agriculture entrusted to C.R.E. plant; in 2010, partly to ALAN plant; a change in the costs is evident, with a sharp decrease, from 2012 to 2013; Merone: in 2009 a part of the sludge (1711.3 tonnes) was landfilled, a part (189.5 tonnes) was sent to HOLCIM plant for incineration in a cement kiln; in 2010 a part of the sludge (539.6 tonnes) was disposed in agriculture (C.R.E. and V.A.R. plants), a part (1044.5 tonnes) was landfilled and a part (279.4 tonnes) was sent to HOLCIM plant; from 2011, landfilling is no more an option and a part of the sludge is disposed in agriculture in C.R.E. plant and a part is incinerated in HOLCIM (EUROFUELS) plant. Passing from landfilling to recovery in agriculture induced a decrease in the costs for disposal.

45

For the disposal into agriculture, the costs are fixed according to the service contract for the collection, transportation and final disposal (recovery) of sludge arising from treatment of urban waste water (EWC 190805). For the disposal into HOLCIM plant, Merone plant stipulates a contract for sludge coincineration with HOLCIM every year. The disposal in HOLCIM plant is aimed to use sewage sludge as a source of energy in the kiln for cement clinker, in partial substitution of non-renewable fossil fuels. This can be done after verification of correspondence to specific technical requirements: the sewage sludge must be dried up to a maximum of 10% of water content and odorless. The main advantages of energy recovery in cement clinker kilns are essentially three:   

It allows a saving in the use of fossil fuels and therefore allows the reduction of greenhouse gas emissions; It avoids the accumulation, even if controlled, of pollutants, such as heavy metals, in agricultural soils; It contributes to the reduction of waste to be landfilled, in accordance with European and national regulations regarding waste management, which aim at the recovery of mass and energy.

The choice of using sewage sludge as a partial replacement of traditional fuels fits perfectly in this context, since biomass does not contribute to the increase of greenhouse gases and creates a beneficial contribution to environmental protection. HOLCIM plant is authorized to dispose 13,000 tonnes/year, but this potential has been partially exploited since now (in 2006, 2000 tonnes were disposed). The total expenditures for sludge disposal are reported in Table 21. Through a cost evaluation which has involved 4 companies (Comodepur, Lariana Depur, Sud Seveso Servizi and ASIL), the different contributions of sludge treatment and disposal on total operative costs of the plants could be analyzed:    

Disposal can contribute around 10% to the total operative costs; Energy can contribute from 1.5 to 9%; high values are due to more complex treatments (e.g. drying); Maintenance and personnel can contribute from 2.5 to 6%, variable according to the needed interventions; Chemical products can contribute around 2%.

Finally, sludge treatment and disposal can contribute from 16 to 27% to the operative costs of the plant, reaching 30% if the administrative and managerial costs are taken into account. Table 21: total costs for sludge disposal

Plants Bulgarograsso - Alto Lura srl Carimate - Sud Seveso Servizi spa Como - Comodepur spa Fino Mornasco – A. S. - Lariana Depur spa Fino Mornasco – Liv. - Lariana Depur spa Mariano Comense - Valbe spa Merone - ASIL spa

Costs for sludge disposal (€) 2009 2010 2011 271,600 250,175 248,615 187,128 259,187 237,075 816,220 720,464 658,905 354,312 303,940 288,512 47,289 66,952 101,528 144,384 140,093 177,842 272,337 203,773 231,838

2012 283,558 218,005 651,820 280,640 122,960 185,527 244,789

2013 238,730 187,870 644,889 224,510 75,255 124,379 163,051

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3.3 Sludge treatment and disposal in the small (≤ 2000 PE) plants The treatments in use in small plants produce sludge already sufficiently stable without the need for specific stabilization phases. Usually only dewatering must be performed, either on site or, when local conditions permit it, in centralized systems with the transport of the thickened liquid sludge by tankers. Table 22 shows the production of dry sludge for some processes and their volumes, before and after thickening. For septic tanks and Imhoff tanks volumes coincide as thickening occurs within them; the values for biofilm processes are relative only to the biological sludge after anaerobic stabilization (usually in Imhoff tank); to these, the contributions of primary sludge must be added. For processes requiring suspended biomass, in the absence of the primary treatments, the values correspond to the total production of the plant. As an example, for systems with extensive aeration, the production of liquid thickened sludge is in the order of 1.5 - 2.0 l/PE/day. Table 22: sludge production for different processes in small plants

Treatment

Dry matter (g dm/inh/day) Traditional septic tank 25 – 30 Primary Imhoff tank 30 – 32 Extensive aeration 40 – 50 Membrane reactor 25 – 40 Sequencing batch reac30 – 45 tor Attached biomass* 10 – 25

Volume (l/inh/day) before thickening after thickening 0.30 – 0.35 0.30 – 0.35 0.35 – 0.40 0.35 – 0.40 3.00 – 4.00 1.50 – 2.00 2.00 – 2.50 1.00 – 1.50 2.50 – 3.50 1.20 – 1.80 1.50 – 2.50

1.00 – 2.00

* limited to secondary sludge, after digestion in Imhoff tank (Source: Bonomo, 2008)

Centralized sludge dewatering is often the most appropriate solution, when transport distances are not excessive and when there is an adequate organizational structure. It requires the availability of a storage tank, with functions also of thickening and then equipped for the discharge of supernatants, dimensioned so as to ensure the rational use of the means of transport (trucks with tank capacity of 715 m3). In the case of autonomous Imhoff tanks, storage is assured from the same compartment of digestion, without the need for specific units.

Figure 15: Imhoff tank

A possible alternative to the transportation of liquid sludge may consist in the use of mobile mechanical dewatering units (generally centrifuges) mounted on vehicles and periodically localizable for short periods at different WWTPs of a single management structure. Also in this case a storage capacity is necessary where to accumulate the liquid sludge between two successive interventions. It is a solution of greater technical, economic and organizational commitment which is justified especially when the 47

dispersion of the plants makes it very burdensome for the removal of liquid sludge through transport and centralized dewatering. The majority of the plants in the ATO of Como are of small size and their sludge production is only a small percentage of the total production. The disposal route is agricultural reuse. The changes in the sludge treatment and disposal in the small plants must be discussed with the municipalities and the societies managing the WWTPs.

3.4 The future of the ATO: centralization of water services The current management of integrated hydric services (Servizio Idrico Integrato, denoted with SII) is shared between “in house” management, consortia and corporations, with a prevalence of the management carried out by individual municipalities, mainly in the areas of medium and high Lake. These separate and different forms of organization of water services consequently cause many disadvantages and create few benefits to users. It has been detected that those who manage the water services in the ATO of Como (municipalities, consortia, corporations) are 152: 127 “in house” managements and 25 external – industrial managements. Management in the areas of neighboring Provinces assumes specific characteristics. The main objectives of the SII are to remedy the imperfections of the current fragmentation and to make the benefits resulting from a more efficient, economical and effective SII available to users, in accordance with local regulations. These benefits result from a number of improvements, such as: 



 

Economies of scale: the current management units (especially at the municipal level) are too small to be economically viable in the use of personnel and resources; combinations of different operating units of an integrated service may include a larger population than the present and provide for the use of personnel, infrastructure and equipment for ad-hoc management. Economies of integration: the management, operation and maintenance of the SII require an overview of the territory and its resources, and then prepared personnel, equipment and materials that provide the same reliability for all services (water, sewerage, water treatment); their integration avoids waste of resources and duplication of efforts, inevitable if they were separated. Introduction of better technology: the aggregation of the current management units, actually larger, provides the possibility to have a better technology for an adequate service management; Better regulation of the water service: the fragmentation of the water service in many management units causes difficulties in its regulation, due to the partial view of the territory and unequal treatment for users that can be overcome through the integration.

The reorganization and the management model must therefore pursue the following objectives:      

An adequate response in terms of quality - quantity of the service to the needs of citizens. The spread over the whole territory of a management having industrial characteristics, overcoming numerous “in house” managements that currently exist. The adoption of appropriate corporate size, i.e. enabling the achievement of economies of scale and significantly improve the reliability of the service. The economic balance of the new management. A management structure capable of taking into account the differences between different areas of the Province, both from the socio-economic and organizational point of views. A process of tariff rebalancing in the territory graduated over time, in order to avoid, at the level of each municipality, significant changes in short periods of time.

In the ATO of Como, given the multiplicity of operators, it is necessary to arrive to an integrated management, with a single entity in chief, that provides a service to a group of users high enough to cover the costs of personnel, equipment and infrastructure, in order to ensure a level of profitable service, 48

complying with current legislation and financially sustainable to meet the future demands and investment therein. Given the importance of the service and the role that individuals currently involved have always played, between the various possible models of custody, the ATO of Como (Conference of Municipalities and the Province of Como) has chosen to entrust the service directly, using the "in house” formula, to a subject of new constitution owned exclusively with public participation. Of course, given the actual extreme fragmentation, a step by step procedure has been chosen, distinguishing two main phases:  

Transitory period: actual larger managerial entities will enlarge their responsibility with the management of a larger territory in order to diminish the number of small “in house” managements; Stationary period; a single managerial entity will be constituted and with the objective to manage all the administrative, commercial and engineering activities, leaving operation and frontoffice tasks to localized managers.

The transitory period can be developed through the centralization scheme proposed in the 2010 ATO plan, with the new situation of the enlarged central plants of our interest highlighted in the following table. Fino Mornasco – Livescia will convey its flowrate to Bulgarograsso plant. Table 23: changes in the biggest plants due to centralization

Plant Bulgarograsso Carimate Como Fino Mornasco - Alto Seveso Mariano Comense Merone

Number of new connections 1 16 8 1 1 0

Actual PE 88000 73500 167588 75300 60171 114974

Final PE 154000 80500 297200 140000 60171 114974

(Source: ATO -Como, 2010)

3.5 The future of sludge treatment and disposal The problem of sludge management can be framed at a general level, highlighting the following points:   

To deal with the problem of disposing/recycling of sludge, which is taking on an increasing weight, it is necessary to act simultaneously in several directions, all equally important and effective; Following the basic principles of European legislation on waste, such directions can be identified in a scale of priority interventions aimed at minimizing the production of sludge, interventions aimed at recovery (matter and energy) and finally disposal in safety; As regards the minimization of the production, there are interesting prospects in technological field: some processes are already commercially available and many promising systems are in the testing phase. The reliability, the cost-effectiveness, the disadvantages and indirect damages resulting from the application of these processes (e. g. changes in the characteristics of effluent quality, adverse effects on biomass, etc.) should be evaluated very carefully on a case by case basis. In particular, some of the technologies appear to be interesting and intended to be conveniently applied in the near future (there are already signs in this direction through the first applications, e. g.: thermolysis), working in the "sludge line", as they appear "safer" (as no risk to the purification process is present) than those that operate in the "water line." It should also be noted that even within the treatments already present on the plants there is certainly room for improvement that can be achieved through careful management, which would lead to a reduction of the production of sludge; 49







As regards the recovery of material it is essentially referred to the use of sludge in agriculture (possibly after composting). In this case, there are many initiatives launched primarily by control or standardization bodies, designed to test the effects of this practice and to "renew" the existing rules, to ensure that the recovery of the resource material can be conducted to minimize the potential risks to humans and the environment; In the field of thermal treatment, alternatives seem interesting to allow energy recovery, as on the one hand ensure the control of noxious emissions, and on the other allow the exploitation of the resource. Also in this area, next to conventional processes, such as incineration, and applications that use dried sludge as alternative fuel in industrial furnaces (e. g. cement plants), the study of alternative systems continues (gasification, pyrolysis, thermocatalysis etc..) and there are also important industrial applications; Finally, with regard to the disposal in landfills, in the regional scenario, and also with a view to recent standards, it will necessarily be limited to the residues from the treatment.

Under the current state of the system it is clear that the use of traditional technologies such as liquid sludge reuse in agriculture after appropriate pre-treatment to obtain qualitative characteristics that comply with the regulations could still be a viable alternative, albeit with a progressive increase in the the cost of disposal. The regional guidelines for the sludge provide most stringent requirements with regard to the quality of the sludge and thus a lower availability of areas ready to receive them. In addition, the delta between the minimum and maximum cost for disposal is still a strong impact on the economies in general. The analysis of the quantities of sludge produced in recent years outline the scenario of storage and subsequent use in agriculture as a predominant fate after mechanical dewatering of sludge inside the plants. As said so far, and for the greater difficulty of checking the sludge for agricultural use, it can definitely be said that the sludge disposal as agriculture is no longer the only hypothesis plausible for the future. As shown previously, it seems desirable to determine the kinds of treatment that are innovative within the Integrated Water Service, in principle built and managed by the single Manager as a result of the agreement signed with the Autorità d’ambito for the ATO of reference. The management of sludge produced from the treatment plants of medium-small size results in common practice greatly simplified, for obvious reasons of a technical-economic development. The most common form of disposal is the conferment to companies that operate the reuse in agriculture. It is not considered appropriate, at least at first, to "force" the small plants to procedures of management different from those of today.

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4. Innovative solutions 4.1 The need for innovation in sludge management The current global climate change drives societies to think about more sustainable ways of using resources and waste management. To achieve effective sustainability, three elements are of fundamental importance and cannot be separately considered; they are (Adams, 2006):   

The environmental element (environmentally robust, supported by applicable normative); The economic element (economically affordable, technologically feasible, operationally viable); The social element (socially desirable, culturally acceptable, psychologically nurturing).

Sewage sludge management is greatly related to environmental, economic and social aspects in order to meet the goal of sustainable management. Decision makers should combine in an optimum way the available alternative sludge handling routes considering all available information on technical, economic and environmental issues. The selection of the most appropriate sludge treatment technology is a key factor in the application of an integrated sewage sludge management system (UNEP, 2009). In combination with economic and social considerations, this approach would help sludge managers to design more sustainable management systems. Sludge characteristics are the most important parameter to be taken into account for the selection of the appropriate technology. A plausible solution in wastewater management should include collective management of sewage sludge and implementation of the 3R (Reduce, Reuse and Recycle) policies and strategies (Samolada and Zabaniotou, 2012). In Europe different FP7 projects (http://ec.europa.eu/research/fp7/index_en.cfm) have been developed to find possible improvements for sludge management; among them, we can cite:   

Routes: novel processing routes for effective sewage sludge management (http://www.euroutes.org/); P-REX: more efficient use of phosphorus and opportunities for recycling (http://www.prex.eu/); END-O-SLUDG: sludge production, treatment, product development, recycling and the environment as an integrated system (http://www.end-o-sludg.eu/).

Routes project is addressed to the development of innovative techniques for:    

Production of sludge suitable for agricultural utilization: well stabilized, low presence of contaminants, hygienized Sludge minimization (metabolic uncoupling methods, microbial Fuel Cells, Sequencing Batch Biofilter Granular Reactor) Materials and energy recovery from anaerobic digestion ((NH4)2SO4, Biopolymers, Biogas) Sludge disposal (integrated process of wet oxidation including the liquid phase treatment).

P-REX concerns phosphorus recovery through:      

Phosphorus recovery from ash Phosphorus recovery from sludge processes Phosphorus recovery through biosolids valorization on arable land Environmental, economic and risk assessment of P recovery options Europe-wide implementation of phosphorus recovery Management, dissemination and training.

END-O-SLUDG main challenges are: 

Source control of pollutants 51

  

Material reuse or disposal Environmental and health impacts of sludge management practice Regulatory compliance and public perception.

The national legislation (Ministerial Decree 27/9/2010) does not allow, with some exceptions, the specific destination of sludge in landfills both for the limits imposed on the organic matter and the limits for the dry content and calorific value of the incoming waste. The more widespread practice is reuse for agriculture which, however, has a growing need for stringent rules that discipline landspreading. Unwilling and unable to address the place of the legislator, the WWTPs managers should think about the diversification of disposal that should provide an improvement in the sludge management. Besides, the Piano d’Ambito of 2014 underlined the difficulties encountered in the disposal of sewage sludge, because of the new connections to WWTPs, which will have to treat more wastewater and thus produce more sludge, and the new regional legislation. In the same document, several investments are planned to improve the sludge management of different WWTPs:  

 

Merone: introduce a new line in the sludge dewatering to make up for downtime during periods of maintenance of the only available machine and to cope with the foreseeable increase in the production of sludge as a result of new revamping works. Mariano Comense: restore functionality of the section of the anaerobic digestion of sludge. It is necessary to completely replace all existing equipment, including biogas lines, heat exchangers, etc. with the increase of the treatment capacity by new dynamic sludge thickener. The project will also improve the final quality of the sludge, with lower odors emissions. Carimate: sludge incineration. Joint Intervention with Sud Seveso Servizi - Valbe Servizi S.p.A. Fino Mornasco – Alto Seveso: realization of an aerobic digester.

These investments give a clear idea about where the improvements should be made: dewatering line, stabilization (aerobic and anaerobic digestion) and diversification of disposal, for example through thermal destruction and valorization (heat and electric energy production). Below a list of problems/issues that require an investigation is given, for the definition, by the ATO, of a strategy of optimal intervention for sludge management. In some cases practical suggestions are provided, in other cases it necessarily refers to following studies: 



 



In general, it is believed that the optimal strategy should differentiate at the most the alternative of recovery/disposal, leaving open many alternative routes. This would allow, in relation to the evolution of all those factors that are difficult to predict, for adaptation of the system to the technical, environmental, economic and social changes. The adoption of appropriate protocols for verification of functionality of the various compartments of the sludge line of purification plants allows optimizing the operation of existing facilities, thereby achieving, already in the system, the maximum energy production (in case of presence of anaerobic digestion ) and a decrease in the production of sludge to be disposed of. A level of quality improvements could be achieved with a more careful control of industrial discharges into the sewer system. Probably the adoption of little-used (such as thermal drying and incineration) or unconventional (such as systems of minimization of sludge production) technologies should be promoted, at least where they have been determined to be of technical and economic feasibility. In order to make sure the practice of agricultural reuse, measures at various levels could be put in place: monitoring protocols for the characterization of sludge upstream of its contribution to centralized processing platforms; adaptation of processing centers and sludge treatment aimed at re-use in agriculture; definition of criteria for implementing a monitoring of soils and crops; preparation of guides to good practice for the reuse of biomass, etc.. Having to make new works/installations, it is necessary to start from recognition of the status quo. Furthermore, it is necessary to consider the initiatives already in progress and consolidated, in some situations, for the recovery/disposal of sludge. 52





 

The analysis of the spatial distribution and size of wastewater treatment plants, recovery centers, land suitable for agricultural reuse is an important element of evaluation. Indeed, it is necessary to take into account the mass flows and therefore the transport of the various alternatives. Many areas are involved in the socio-economic problem of the management of sludge and it can strongly influence the scenarios. The evolution in the near future characterizing these areas can then determine the addresses to be taken. Among the factors most directly involved, the following can be remembered: agriculture in the broadest sense, the legislation (not just environmental), the waste sector, the market recovery and disposal systems. The effects of the implementation of the Nitrates Directive determines, in many areas of Lombardy, a surplus of manure compared to the receptivity of the soil. The social problems were not taken into account in this study. It is known, however, that they may pose a serious constraint to the implementation of environmental interventions. An intensive campaign for the characterization of sludge from plants to be able to define quality in relation to the treatment processes could represent a useful knowledge base for guiding the choices of planners.

The results of this research are therefore to be understood, ultimately, as "preparatory" to a feasibility study in which it is possible to define the scene of action, make the necessary investigations at the regional level and detail the costs of intervention. The innovative treatment technologies can be grouped into three groups (ISPRA, 2009): • • •

Group A: treatments performed on thickened sludge aimed at reducing the dry matter produced, increasing the recovery of biodegradable carbon substance and improving the characteristics of quality and tractability of the sludge; Group B: treatments performed on the dried sludge, aimed at the valorization of the substances contained in the sludge through the production of marketable products; Group C: treatments performed on the ash produced by the incineration of the sludge or by coincineration of sludge and solid waste, at the end of their development in the construction industry.

The following processes belong to Group A of the innovative technologies: • • • • •

thermal hydrolysis in acidic environment; biological and/or chemical decontamination; wet oxidation with oxygen (wet air oxidation); advanced oxidation by ozone; mechanical disintegration of the bacterial cells.

The following processes belong to Group B: • thermal conversion of organic matter into liquid fuel; • thermal conversion of organic matter into fuel gas; • sintering the dried sludge mixed with clay; • aerobic fermentation (composting of sludge only). The following processes belong to Group C: • •

thermal sintering after appropriate pre-treatment; solidification (vitrification) after appropriate thermal pre-treatment.

Because of the importance of the sludge quantities the choices of possible treatment alternatives must be cost-effective or at least comparable with the current. Finally, the Deliberation n ° X/649 of the 09/06/2013 of the Lombardy Region highlighted the difficulty of finding information on the status of implementation of the interventions, the great difficulty of 53

ensuring a flow of information between offices and managers and the absence of a systematic and integrated management of information collected by different agencies (AATO, Province, managing bodies, Departments and competent ARPAL). To present the current situation, the available databases are described, together with a new database currently developed by Politecnico di Milano.

4.2 Available databases and SYST&MS Lombardy Region has established the Osservatorio Regionale Rifiuti (Regional Waste Observatory) with the Law n. 37 of 28 June 1988 and it was subsequently confirmed by Law n. 21 of July 1, 1991 and Law n. 26 of 12 December 2003 and subsequent amendments. The Regional Waste Observatory is a structure that coordinates the Provincial Observatories, with the task of processing and dissemination of data pertaining to the production and management of municipal solid waste and waste collection. The data and information are provided by the municipalities to the Provinces and, once verified, sent to ARPA (the agency for the protection of the environment) for the preparation of the Annual Report. The proper management of databases in the field of waste allows the representation and monitoring of the regional reality on the production and management of waste (municipal and industrial) and effectively supports the planning and design of future activities by the local regulatory authorities. ARPA Lombardy arranges the collection, reclamation (as a correction of the found errors) and processing of all data required by the regulations. Available databases, managed by ARPA Lombardy, consist of: 







O.R.SO. (Superregional Waste Observatory) is a web-based application that collects production and solid waste management data of the Lombard municipalities (1,544 subjects) and waste treatment plants located in the region (approximately 3,000). There are information about the production, management, municipal waste streams and data relating to the recovery and disposal of waste in the plants; MUD database: collection of data on hazardous waste takes place through the MUD statements that are presented each year at the Chamber of Commerce of the territorial jurisdiction by the parties responsible to the presentation (art. 189 of Legislative Decree no. 152/2006). The Chamber of Commerce shall subsequently submit the statement of the region to the Regional Section of the Land Registry at ARPA (this is approximately 70,000 annual statements). The data contained in MUD are not immediately used, but require a substantial remediation work (as a correction of the errors) to eliminate and/or reduce the principal errors often due to the use of paper forms; production data of special waste result from the processing of this information; CGR-WEB - Georeferenced Waste Land Registry is a web-based application that contains all the administrative and technical information on waste treatment plants in the Lombardy Region regardless of the procedure by which they were allowed (ordinary, simplified or AIA); operating since the beginning of 2013, it is updated by the Province and the Region which are the competent authorities for the issue of authorizations; Inventory of equipments containing PCBs required by Legislative Decree no. 209/1999, contains information about the equipment contaminated with PCBs (polychlorinated biphenyls, such as transformers and capacitors) and their disposal, surveyed through biannual declarations made by the holders of such equipment.

SYST&MS is a database developed by the Hydro Informatics Lab of Politecnico di Milano, in the Como campus, under the supervision of Prof. Roberto Canziani. The database comes from the need to "take a picture" of the current situation of the purification of the Province of Como. Its main aims are:  

to help the decision makers (WWTPs managers, control and planning authorities) taking their choices, gathering the information about the actual managerial, technological and planning situation; to inform the users (citizens) so that the system is transparent and can let the users participate and know which decisions are taken.

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The keywords of the system are: • •

•

update: census and mapping of the technologies, information transparency; rationalize and innovate: proposals for innovative solutions already on the market (water line: loop-control, short cut nitrification, MBR optimization, Anammox on supernatant; sludge line: electro-osmotic dewatering, incineration and phosphorus recovery) with pilot tests and simulations in accordance to the managers and their development policies; valorize: public acceptance and economical, energetic and environmental sustainability, to fix the appropriate fee, shared by the users.

The idea is based on the UWWTD database developed by the EEA, whose drawback is to have limited information; so the starting point for SYST&MS is to extend the information about the ATO of Como. The architecture of the system is shown in Figure 16. The architecture of the database represented in the figure simply shows the possibility to interact with the webclient DB created by CMS Drupal. The structure in PostgreSQL and PostGIS allows technicians to add content in a user-friendly way, to show the spatial data in OpenLayer through the WMS or WFS calls to a server. The layer can be constructed by directly editing the contents of the DB through appropriate forms of CMS. More details for GIS components: • •

• • • • •

Drupal is a free and open-source content management framework written in PHP and distributed under the GNU General Public License. PostgreSQL, often simply "Postgres", is an object-relational database management system (ORDBMS) with an emphasis on extensibility and standards-compliance. As a database server, its primary function is to store data, securely and supporting best practices, and retrieve it later, as requested by other software applications, be it those on the same computer or those running on another computer across a network (including the Internet). It can handle workloads ranging from small single-machine applications to large Internet-facing applications with many concurrent users. PostGIS is an open source software program that adds support for geographic objects to the PostgreSQL object-relational database. PostGIS follows the Simple Features for SQL specification from the Open Geospatial Consortium (OGC). OpenLayers is an open source JavaScript library for displaying map data in web browsers. It provides an API for building rich web-based geographic applications similar to Google Maps and Bing Maps. A Web Map Service (WMS) is a standard protocol for serving georeferenced map images over the Internet that are generated by a map server using data from a GIS database. A Web Feature Service Interface (WFS) provides an interface allowing requests for geographical features across the web using platform-independent calls. IstSOS is an OGC SOS server implementation written in Python. IstSOS allows for managing and dispatch observations from monitoring sensors according to the Sensor Observation Service standard. The project provides also a Graphical user Interface that allows for easing the daily operations and a RESTFull Web API for automatizing administration procedures.

55

Figure 16: architecture of SYST&MS

So, the goal is to start from the current information, updating them by analyzing the available DBs, the authorizations for discharges, DBs of the Province and especially the useful contributions of the leading operators of wastewater treatment plants in the area (working with Politecnico di Milano), to build a new database with smart access that can be replicated in other national and international systems and can be a starting point for identifying the problems and the potentials of the plants and the locations of new innovative solutions for the disposal of sludge or new treatment processes. In this way, SYST&MS will act as a decision support system. A decision support system is a computer system that assists decision-makers in choosing between alternatives or actions by applying knowledge about the decision domain to arrive at recommendations for the various options (Poch et al., 2004). It incorporates an explicit decision procedure based on a set of theoretical principles that justify the “rationality” of this procedure. Thus, a DSS is an intelligent information system that reduces the time in which decisions are made in an environmental domain, and improves the consistency and quality of those decisions. SYST&MS will help studying the actual situation and taking decisions by: •

Geolocalizing: o o o

•

the treatment plants in the Province, the companies for sludge disposal, future scenarios and alternative sludge disposal, depending on the planning decisions.

Displaying: o o o o o

the reference normative limits for plants, the data of discharges (monitoring and self-control of water), the technological characteristics of the systems and processes, the data and sludge disposal costs incurred by the major operators, the data about energy consumption. 56

•

Implementing: o o o

•

queries on WWTPs (possible data: P.E., flowrate, loads, etc.), queries on data, geospatial query: distances between towns, water bodies pollution levels, distances for sludge disposal.

Evaluating dynamic indices of environmental and energy efficiency aspects.

4.3 Improvements in the sludge production and treatment The techniques for reducing sludge production can be considered as a preventive intervention to contain the costs of disposal. More generally, in this context also those techniques that enhance the quality of sludge (e. g. reducing the content of heavy metals) can be taken into account, thereby enabling, for example, the agronomic reuse. There are established and non-conventional systems that can be applied to the water line or to the sludge line. These techniques may be of interest to the biological or chemical/physical compartments and be themselves biological or chemical/physical. The choice of the most appropriate technology must take into account many factors, mainly technical and economic, with a careful evaluation of the advantages and disadvantages that may result from their application, either direct (i. e. related to the sludge production) and indirect (i. e. affecting other aspects and/or other stages of the system). In the next chapters, the interventions adoptable in the water line are divided from those applicable to the sludge line. Figure 17 shows potential locations for cotreatment in a classical urban wastewater treatment plant. These pretreatment methods have improved recently in popularity due to a number of factors, including (Carrère et al., 2010):   

A trend towards lower nitrogen limits, which is driving up sludge ages and decreasing degradability of activated sludge streams. Increased final handling costs (especially for final destruction options like incineration). Increased legislative requirements for stabilization performance and pathogen removal.

Figure 17: potential location for sludge cotreatments in a classical wastewater treatment plant (Source: Carrère et al., 2010) T1: Cotreatment on activated sludge process. T2: Cotreatment on the activated sludge recirculation loop. T3: Pretreatment of primary sludge before anaerobic digestion. T4: Pretreatment of waste activated sludge before anaerobic digestion. T5: Pretreatment of mixed sludge before anaerobic digestion. T6: Cotreatment on the anaerobic digester recirculation loop.

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4.3.1 Water line interventions Interventions in the chemical/physical treatments Among the steps of chemical/physical and mechanical treatment of a wastewater treatment plant that can most affect the production of sludge, it was considered important to focus on the dosage of chemical flocculants (with various objectives), activated carbon and on the micro screening compartment. The chemical reactants with coagulant/flocculant function may be used for several reasons, for example to deal with overload (dosage in the primary phase), to improve the settleability of the secondary sludge and to remove the phosphorus. The choice of the reagents and the definition of the conditions of treatment should be assessed carefully, considering the size (weight and volume) of the flakes that form, to be assessed on the basis of the effective hydraulic conditions (upward velocity). The dosage of the reagents, while determining the advantages described above, determines an increase in the production of sludge which is not indifferent. In general the additional "chemical" sludge production ranges from 20% to 120% of the total, in function of the reagent dosage and the alkalinity of the water (Colombo, 2003). Even in the case of a dosage of powdered activated carbon (directly in the biological phase and possibly also at the level of pre-treatment) it is appropriate to take into account the consequences for sludge production (increase owing to the presence of the PAC itself, decrease due to the effect of the adsorption of biodegradable organic compounds, increase due to the increment of the biological activity following the elimination of inhibitory substances, etc.). With the adoption of microgrids the ability to remove suspended solids can reach 5 – 15% depending on the characteristics of the slurry. This can lead to a proportional reduction of the production of sludge, with an increase, however, in the screening material up to 60% (2 - 11 kg/PE/year) compared to the use of fine grids (2 - 4 kg/PE/year) (Masotti, 1996). Interventions in the biological compartment Among the main factors which are known to influence the production of biological sludge, the sludge age and the availability of dissolved oxygen should be mentioned. Clearly, the definition of "correct" values of these parameters in the design of systems has to take into account first of all the process requirements (e. g. nitrification) and economic aspects. The extended aeration systems find their own origin in the need to simplify the sludge line (as regards the sector of stabilization). In the management, however, the main constraints are constituted by the structural limits of the system (volume of the reactors, the potential of the oxygen supply system, possible problems of sedimentation of the sludge (pinpoint) etc.). The non-conventional technologies proposed to reduce the production of excess sludge in a plant can be classified in two main categories: physical/chemical and biological. Within these two groupings exist both methods that, as innovative, already counted some applications to real scale, and methods recently introduced that have been studied so far only in laboratory or pilot scale. Chemical treatments offer several advantages, including easy process setup, a stable efficiency, flexible management parameters which can be easily modified according to the needs. Among the aspects that limit their applicability to large scale, one of the main items consists of the operating costs for the purchase of equipment and chemical reagents that can be high. These costs of investment and management must be naturally evaluated in function of the saving obtainable following the reduction of the amount of sludge to be treated and to send to final disposal. Some treatments are applied directly to the activated sludge in the reactor; others are instead applied on the recirculation line of the sludge or on a part of this. The methods of biological type are applied in order to intervene in the cellular metabolism and on the mechanisms of endogenous respiration and cell lysis. There is a consumption of substrate for the metabolism only, i. e. which does not result in the synthesis of new biomass, but only in the production of energy. The metabolism is associated with the mechanisms of cell lysis, and the production of sludge is inversely related to this type of activity. To reduce the volumes of excess sludge is then needed to disturb the balance between anabolism, i. e. cellular synthesis, and catabolism or energy production, in favor of catabolic reactions. 58

Among the treatment we can cite: 

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 

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Chemical oxidation of the recirculated sludge: this process involves the treatment of the recirculated sludge with an oxidant (e. g. ozone, hydrogen peroxide): a part of the sludge is mineralized, while another part is solubilized into biodegradable organic compounds that can be oxidized in the reactor by the activated sludge; in fact, the use of strong chemical oxidants induces a significant cell lysis. Thermo-chemical treatment of the recirculated sludge: the heat treatment together with the chemical one (pH change) allows obtaining the hydrolysis of the components of the sludge, both as regards the cell lysis and for degradation of organic substances. Anaerobic/anoxic treatment of recirculated sludge: OSA (Oxic-Settling-Anaerobic) treatment is a variant of the conventional activated sludge process, obtained by inserting an anaerobic reactor in the recirculation. This process consists of aerobic steps interspersed with phases of anaerobiosis, corresponding to an alternating respectively of "feasting" (rich of nutrients) and "fasting" (poor of substrate) phases. The alternation of aerobic and anaerobic conditions tends to dissociate anabolism to catabolism and stimulates the catabolic reactions. Alternate cycles processes: they are processes that arise for the removal of nutrients but that are seen to be also effective to minimize the production of sewage sludge. In this case, the duration of the aerobic/anoxic phases is established on the basis of the measured values of dissolved oxygen and redox potential in the tank, which are then compared with the values in the database of the system. Thermophilic aerobic treatment: this system consists in treating a flow of sludge taken from the bioreactor through a thermophilic aerobic process and then re-enter it in the activated sludge tank.. The alternation of mesophilic and thermophilic conditions would seem to favor the processes of hydrolysis of the organic substance and an increase of the energy needs of mesophilic bacteria for their maintenance and their reconstitution. “Metabolic uncouplers" process: in presence of certain substances called “metabolic uncouplers”, energy derived from the oxidation of organic substance is dissipated in the form of heat instead of being stored as ATP. Development of protozoa/metazoa for bacteria predation: it is known that the protozoa and metazoa act as predators in the activated sludge ecosystem and they should fulfill a control function of the bacterial growth itself. The development of predators in the activated sludge process reduces the presence of bacteria and thus the production of sludge. Using other electron acceptors such as oxygen substitutes: this technique relies on the principle of reducing the yield of cell growth, for example by oxygen substitutes, such as nitrates, which provide a coefficient of specific heterotrophic cell growth lower than that obtainable in the presence of oxygen. Ultrasonication: ultrasounds are able to break up the flakes promoting the lysis of the particulate material, for effect of both mechanical and dynamic mechanisms. MBR (Membrane Biological Reactors) systems: the expected production of sludge in a MBR system is usually lower than in the traditional systems, due to the high concentration of biomass in the tank (7 to 20 gTSS/L), a low F/M (Food/Microorganism) and a high sludge age. These conditions do not favor the growth of cells because of the increase in the demand for energy needed by bacteria to survive. Granular sludge: the granular sludge systems, based on the immobilization of microorganisms that treat the wastewater, allow getting some advantages when compared with conventional activated sludge: high concentrations of biomass in the reactor (15 - 60 gTSS/L), high organic loads (up to 10 kg COD/m3/d) and very low production of sludge. Treatments using electric field pulses: the PEF (Pulsed Electric Field) technology has been proposed for the treatment of the sludge, since it causes the rupture of microbial cells. A PEF device discharges a high-voltage electrical pulse in the sludge (> 20 kV) for thousands of times per second. The strong electric field attacks phospholipids and peptidoglycan, the main constituent of the cell membrane and of cell walls, which have a negative charge, therefore, sensi59

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tive to the action of the electric field. The consequent opening of the pores in the membranes and the cell walls causes the breaking of the cells and their lysis. Mechanical disintegration: the mechanical disintegration is a process aimed at increasing the solubilization of the sludge. The solubilization is obtained through the destruction of the cells and the breakdown of flocs. In general, at low intensity the disintegration of the flakes is obtained, while for high values of energy the cells are destructed. The lysate obtained from the destruction of the sludge is recirculated to the head of the activated sludge tank. Heat treatment of the sludge: the application of a heat treatment of the sludge (by heating it) causes the breakdown of sludge flocs, a high level of solubilization, cell lysis and the release of intracellular water. The major limitation for the diffusion of this process resides in the high costs associated with heating the secondary sludge, characterized by a low concentration of solids. One possible solution is the application of "low temperature" (< 100 °C) heat treatment, in order to reduce the energy required for heating and the need to operate at lower pressures.

4.3.2 Sludge line interventions Interventions in the chemical/physical treatments With regard to conventional techniques (thickening, mechanical dewatering and thermal drying), considerations about the latest technological developments are provided: 

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Thickening: the latest innovations include the use of dynamic thickening, adopting machines derived from those originally designed for the mechanical dehydration. Among the examples that may be mentioned, some consolidated technologies are centrifuge decanter, gravitational table, cylindrical sieve. Mechanical dewatering: companies are developing innovative technologies with higher removal efficiencies. Thermal drying: as part of the conventional schemes, the general goal seems to make improvements, acting on specific phases of treatment. In particular, new devices are adopted for improving systems for handling sludge and that allow the exhaust gas recirculation and energy recovery, with the advantage of simplifying the line of air purification (at least for the step of deodorization) and reduce the consumption of energy. Another objective of manufacturers of systems is to improve the sludge/heating medium contact inside dryers (especially in indirect ones), through specific conformations of the internal devices.

Regarding unconventional interventions on the sludge line, the following is a list and brief description of those recently proposed/applied (at different scales): 

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Thermal hydrolysis and thermo-chemical treatment: it is often associated with the use of acid reactants which involve some drawbacks among which corrosion problems, the need for a post-neutralization, solubilization of heavy metals and phosphates. The thermo-alkaline treatment can be an alternative to thermo-acid treatment and allows overcoming some limitations on the application of the latter. Ultrasonication: the application of sonication at low frequencies (20 to 40 kHz) for the reduction of the sludge is a consolidated technology. Mechanical disruption: the mechanical disintegration of the sludge results in a release of organic substances that can be easily biodegraded. The mechanically treated sludge may therefore constitute a valid substrate for the denitrification process, improve the performance of anaerobic digestion or allow the recovery of products such as nitrogen and phosphorus. Chemical oxidation: among the chemical oxidation processes, ozonation (preferable when the subsequent digestion of the sludge is aerobic) and the Fenton treatment are cited. Anoxic Gas Flotation (AGF): this process uses an anoxic gas to float, concentrate and re-enter the bacteria, enzymes, organic acids, proteins, and the substrate in the anaerobic reactor. This is done in order to accelerate and complete the conversion of sludge into biogas. Electrokinetic dewatering: electrodewatering is a process in which a low direct current (DC) electric field is applied through the sludge segment to cause an electro-osmotic phenomenon; 60

that is, fluid flow in a charged particle matrix. Electro-osmotic flow enhances extra water removal from sludge, resulting in an increase in solids content of the final sludge cake. Additional benefits are (Tuan et al., 2012): o

o o

The water content of the treated sludge remains a critical factor affecting the choice of transportation vehicles, transportation costs, and suitability of sludge for incineration. The additional decrease in water content obtained by application of electricity denotes a significant reduction in sludge volume and increase in energy content. Evaluation of simultaneous electrodewatering and disinfection of sewage sludge has shown that a high DS content can be achieved while removing fecal coliforms. One more advantage of electrodewatering over conventional mechanical dewatering is the prevention of filter fouling, which has resulted in significant improvements in industrial waste sludge treatment using a gravity-driven thickening belt.

Figure 18: processes occurring in electro-osmotic dewatering

Interventions in the biological compartment Pre-treatments before anaerobic digestion If combined to sludge anaerobic digestion, the pretreatment objective is to not only reduce the final amount of sludge to be disposed of, but also to increase methane production. Various methods may have the effect of:  

Increasing degradability extent, leading to higher energy recovery and lower residual digested sludge. Increasing degradation kinetics, making it possible to decrease sludge retention time in the digester, therefore making it possible to reduce digester volume or increase the organic load rate of a given digester. The experimental results confirmed that shortening of SRT (increase OLR) lead to increases in gas production rate and volumetric methane productivity like-wise a decrease in VS destruction efficiency (Nges and Liu, 2010). 61

Mechanical and low intensity processes such as biological pretreatment, sonication, and high pressure treatment generally increase rate, while high intensity processes such as thermal hydrolysis increase extent and rate. Mechanical processes have low energy requirements, as electricity. Low intensity thermal phased pretreatment has higher energy requirements, but as thermal energy, which is generally available at a lower cost. Thermal hydrolysis has a high energy requirement, as thermal energy. Overall performance and feasibility depends on a wide range of factors, not just encompassed by energy use and performance. For instance, sludge dewaterability is a key parameter, and the polymer dose and total solids content in the sludge cake can be improved or degraded depending on cotreatment conditions. As a result of cotreatments, a fraction of nitrogen, phosphorus and refractory COD is released into the dewatered sludge liquid phase. If this phase is returned to the main activated sludge process, the degradation of such additional load may increase aeration costs. However the recovery of phosphorus and a part of nitrogen by struvite crystallisation is a sustainable option made possible by digestion. The basis of comparison for pretreatment methods can thus be divided into a number of different components including (Carrère et al., 2010):    

Whether the treatment method is aimed at activated or primary sludges. Treatment effectiveness, whether it increases just degradation rate, or increases the overall amount of material available (bioavailability). Cost of treatment, particularly energy cost, and secondary costs caused by nutrient release or generation of byproducts Chemical consumption, particularly for oxidative or chemical treatment.

Muller et al. considered a 250,000 PE virtual WWTP to compare stirred ball milling, ozonation, lysate centrifugation and sonication. The authors provided several classifications of pretreatments according to (Muller et al., 2004):     

Energy demand: lysate centrifuge < stirred ball mill < sonication < ozonation. Increase of sludge degradation: ozonation > stirred ball mill > sonication > lysate centrifuge. Increase in polymer demand for dewatering: lysate centrifuge < stirred ball mill < sonication < ozonation. Increase in polymer demand: lysate centrifuge < stirred ball mill < sonication < ozonation. Increase in soluble COD and ammonia concentrations in supernatant after dewatering: sonication < lysate centrifuge < stirred ball mill < ozonation.

A summary of performance and energy outcomes for the major digester options is given in Table 24. Table 24: energy analysis of different pretreatments for anaerobic digestion

(Source: Carrère et al., 2010)

Anaerobic digestion can be mesophilic, if it takes place in a temperature range between 30 and 38 °C, or thermophilic if temperature varies in the range 50 - 57 °C. Ratings carried out on the energy balance 62

between the energy produced by the process and the one used for heating the sludge have identified in mesophilic digestion the most advantageous process for civil installations of average-high potential. Another improvement in the anaerobic digestion process consists in a pretreatment under thermophilic conditions. The reduction of volatile suspended solids in the digesters operating both under thermophilic and mesophilic conditions is around 30% (Foladori et al., 2010). Pre-treatments before aerobic stabilization An interesting process, proposed in the past, but which did not find significant application, is the autothermal thermophilic aerobic digestion (known by the acronym ATAD), in which the reaction temperature is maintained naturally due to the exothermicity of the reactions and the coverage (not necessarily sealed) of the tank. This process, in its typical configuration, consists of two reactors placed in series in which the second works at higher temperatures than the ones in the first (usually the first reactor operates in mesophilic condition and the second in thermophilic conditions) (Sanin et al., 2008; Foladori et al., 2010). The higher removal yields of volatile solids can be achieved if reactors operate in thermophilic conditions, with the further advantage of obtaining a sanitized sludge. Anaerobic/aerobic stabilization The sequential anaerobic/aerobic sludge digestion is attracting more and more interest and the first studies are showing higher achievable performance with this solution compared to the conventional treatment of digestion (Kumar et al., 2006a, 2006b; Parravicini et al., 2008; Zupancic and Ros, 2008; Tomei et al., 2011). Alternate cycles processes The considerable consumption of electric energy linked to the use of systems of aerobic stabilization can be reduced alternating aerobic and anoxic (or anaerobic) phases. The alternate cycles process consists of a reactor that can operate in different cycles, depending on the objective which it is intended: the elimination of nutrients contained in the recirculation to the head of the plant, the selection of the kind of biomass to support to the biological processes of the water line or the minimization of the production of sewage sludge. In the latter case, the reactor is fed with biological excess sludge and sewage sludge. Theoretically, the reduction of the amount of sludge is obtained as the heterotrophic growth rate assumes low values in anoxic conditions (0.30 - 0.36 gVSS/gCOD), when compared with that under aerobic conditions (0.45 gVSS/gCOD). The obtainable reduction of sludge does not seem to be very high since the bacterial biomass represents only 15 - 30% of the total suspended solids of the sludge. The obtainable reduction of the VSS is in the order of that of the traditional aerobic digestion with the advantage that, the alternation of phases reduces energy consumption.

4.4 Options in sludge disposal 4.4.1 Incineration Incineration has already been previously described in chapter 2.4.2. Changes in the sludge incineration should focus on the self-sufficiency of the plant, through thermal recovery, electricity production and energy efficiency, and on environmental protection, abating pollutants emissions. Examples of sewage sludge incineration plants can be found in Netherlands (SNB Brabant – Moerdijk) and Switzerland (ERZ Zurich – Zurich), whose configuration and operational parameters are shown in Figure 19 and 20.

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Figure 19: SNB Brabant – Moerdijk, Netherlands (Source: Outotec)

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Figure 20: ERZ Zurich – Zurich, Switzerland (Source: Outotec)

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4.4.2 Pyrolysis Pyrolysis is a thermo-chemical decomposition of organic materials in the absence of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible. During pyrolysis, large complex hydrocarbon molecules of biomass break down to smaller and simple molecules of gas, liquid, and char:   

Gases are non-condensable vapors, CO, CO2, H2, CH4, C2H4, C2H2, C2H6, etc. and cannot be used in regular natural gas lines and need to be combusted onsite for heat and electricity generation. Liquids are condensable vapors containing valuable chemicals which can be easily stored and transported. Solids are char and ash (good source of nutrients as fertilizer and activated carbon).

The summary of results of sewage sludge pyrolysis is shown in Table 25. Table 25: pyrolysis products from sewage sludge

(Source: Kim et al., 2008)

In general, the gaseous products represent from 15% to 30% by weight of the initial product, with a percentage incidence increasing with the temperature of the process, and consist essentially of hydrogen, carbon monoxide, carbon dioxide, light hydrocarbons (methane, ethane, ethylene and acetylene) and other minor constituents. The residual liquid obtained by the condensation of the vapor phase is, on average, 50 to 60% by weight of the starting material; it contains significant levels of moisture (up to 60 - 80%) and consists of complex organic substances such as alcohols, ketones and condensable hydrocarbons of various natures. The solid residues represent approximately on 20 - 30% by weight of starting material, and have an average calorific value of between 5,000 and 6,000 kcal/kg: they are constituted by carbonaceous substances, similar to bituminous coal, at low temperature pyrolysis (400-500 °C), and to anthracite at higher temperatures (800-900 °C). The pyrolysis products may have various uses, depending on the type of material treated, although for the treatment of waste the most frequent use is fuel for energy production. The characteristics of the obtained materials and their relative amounts depend on the type of material treated, the operating conditions with which the pyrolysis is conducted, in particular the temperature and time of exposure of the material to such process. Long exposure at moderate temperatures favors the production of char, while limited exposure to medium - high temperatures maximizes the production of liquid fractions. 66

For example, with very short exposure times (less than 1 second) at temperatures of 500 °C it is possible to obtain a yield in liquids up to 80% of the input material; to do this it is necessary to "freeze" the reactions and promote the condensation of the gaseous fractions formed through a sudden cooling (quenching) which allows to avoid the formation of lighter compounds that remain in the gaseous state at chamber temperature. If instead the main purpose is the formation of a gas, it is possible to obtain a good fuel calorific value (typically between 3,500 and 5,000 kcal/Nm3). The different operating conditions through which the pyrolysis process is conducted are mainly identified by the residence time of the material in the treatment conditions. This allows classifying the process into the following categories:   

slow pyrolysis or carbonization, characterized by low reaction rate and limited temperature (300-500 °C), so as to maximize the yield of solid products (char); conventional pyrolysis, able to provide gaseous, solid and liquid products, in variable quantities depending largely on the operating temperature; fast or flash pyrolysis, aimed at maximizing the production of light compounds (gaseous or liquid), which may undergo subsequent treatments for use as a fuel or raw material for the chemical industry.

Pyrolysis equipment are more or less the same used for the gasification (fixed bed, moving bed, fluid bed, rotating drum); the most significant experiences, however, have focused on the use of the rotating drum with indirect heating. 4.4.3 Gasification Biomass gasification is the conversion of a feedstock by partial oxidation into a gaseous product. The resultant gas (called syngas) is primarily composed of hydrogen, carbon monoxide, carbon dioxide, water, methane and nitrogen with trace amounts of other higher hydrocarbons. The partial oxidation is performed at temperatures in the range of 500-1400°C using an oxidant such as air, pure oxygen, steam or a combination of these gases. The choice of oxidant is instrumental in determining the composition of the product gas. Hydrogen and carbon monoxide production can be maximized by using oxygen or steam oxidants. Gasification reactions are mostly exothermic allowing the reactor to operate in a self-sustaining fashion. The gasification process is a cycle where hot char residue is in contact with moist sludge followed by drying and gasification. Through this process, nearly all of the organic carbon in the biomass feed can be converted into combustible gases and mineral residues. Considerations such as feedstock surface area, size, shape and compositions are important in determining the quality and quantity of syngas produced. The operation of a gasifier is highly dependent on its classification: fixed-bed updraft, fixedbed downdraft, bubbling fluidized-bed and circulating fluidized bed. From a performance to capital cost benefit analysis of the three different types of gasifiers, fluidized bed gasifiers are the most cost effective option. This comparison includes factors such as thermal and engine output, net power and efficiency against the overall capital cost. Table 26: typical combustible gas composition from gasification

(Source: Fytili and Zabaniotou, 2008)

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4.4.4 Environmental aspects A summary of the products of the different processes is provided in the figure below.

Figure 21: products comparison

Given the composition of sewage sludge, the thermal treatment can lead, if not properly managed, to the problem of pollutants that may cause significant environmental impacts, in particular pollution of air, soil, surface water and deep water. The technologies available for treating flue gases are able to meet the requirements of the most restrictive regulations; the introduction of processes increasingly refined has allowed increasing the removal yields, limiting the consumption of reagents and of energy. Significant advances have also been obtained in preventive phase (minimization of generation of some pollutants). As regards the analysis of the health risk associated with the emission of pollutants, the effects of emissions of air pollutants on the health of the population exposed constitute a basic element in the debate on environmental compatibility with many human activities; such effects must be taken into account in a study of the acceptability and in the choice of the location of installations of thermal destruction of waste and therefore in the specific case of sludge. The application of this methodology to modern plants allows highlighting how the impact on health can be maintained below the limits laid down in the international arena (e. g. WHO), or less than one chance in a million for the onset of a tumor-like effect due to the presence of the plant for the person most exposed. The environmental impact of a drying plant has to be assessed considering: the emissions into the atmosphere, which, though limited, are always present; liquid effluents, first of all the water used to wash the exhaust air; the quality of dried sludge; the noise of the system. Atmospheric emissions are closely linked to the nature of the sewage sludge treated in the purification of origin. The pollutants of greatest weight from a quantitative point of view are volatile organic compounds, ammonia and powders. Among the volatile organic substances phenols, organic nitrogen compounds and to a lesser extent aldehydes are present. The presence of ammonia is due to the efficiency of nitrificationdenitrification section in water line. The composition of the powders is closely linked to that of the sludge. The presence of dioxins and furans in the sludge fed to the plant of drying can lead to their release during the thermal process (volatilization). In this regard, however, reference data lack. The presence of phenol and ammonia in certain concentrations can cause bad odors in correspondence of the unloading area of the emitted gaseous flow. A device which is often adopted in the management phase to limit odors is to control the process so that the sludge is not degraded but is dried evenly until the desired degree. Through the treatment of the gases produced by drying it is possible to observe the limits on emissions imposed by law. Another source of emission is constituted by the exhaust fumes of the burners, variables depending on the fuel used (diesel, fuel oil, natural gas, LPG, biogas, etc.). The liquid 68

wastes produced (variable depending on the type of dryer and the condensation system adopted from 1-2 to 10 m3/h for a dryer of average potential) can be sent to the treatment plant, with consequent economic advantage compared to other type of treatment. Among the main features of these substances there is a high content of suspended solids and COD. Since during the drying any combustion does not take place, these plants do not have the problems associated with emissions from combustion, such as the volatilization of heavy metals. In drying plants, in relation to the energy used in the boiler, carbon dioxide, sulfur dioxide and nitrogen oxides can nonetheless be emitted. Also during the operations of drying, it has an inevitable loss of ammonia, which in the case of installations with wet scrubber for the emissions treatment, for example, is recovered with the water of condensation. It should however be pointed out that these emissions are present both in the case of treatment of sludge of domestic and industrial source. The pollutants associated with combustion, as well as to any other thermal treatment of sludge are related to:   

release into the atmosphere of heavy metals; emissions of dioxins and furans, NOx, SO2, HCl, HF and IPA; solid residues (slag and fly ash in particular).

The presence of heavy metals in sludge, in the form of hydroxides, carbonates, phosphates, silicates and sulfates, is mainly due to industrial activities. Various tests were carried out at laboratory level in order to decrease the heavy metals in the sludge using acid solutions (in particular HCl, HNO3, H2SO4), in which the heavy metals dissolve. However, the large-scale main method available to reduce heavy metals in the sludge lies in their source reduction. The metals are not destroyed by combustion processes, but they are both collected in the residue of the process and emitted into the atmosphere. Dioxins and furans are unwanted by-products. The content in untreated sewage sludge can vary between a few hundred and a few units of ng/kg of dried sludge. The concentration in the flue gas emitted from the other hand, after purification, is approximately 0.1% compared to that of the sludge entering. It should be noted that in the cooling sections of the plants (primarily boiler and dedusting systems), dioxins and furans, however, can reform. This reaction, which occurs at temperatures between 300 °C and 350 °C, takes the name of "de novo synthesis", imposing to take appropriate actions, which however allow minimizing the presence of these compounds compared to the most restrictive legal limits. As the organic chlorine is present in the sludge treatment only in modest concentrations, the problem related to emissions of dioxins and furans is reduced compared to what happens in the processes of treatment of municipal waste. The high production of fly ash from the sludge can lead to high concentrations of ash in the outgoing gas. These particles can become receptors of substances and gases present in the combustion fumes, following the phenomena of absorption, adsorption or condensation. The particulate can then be composed of inert, metal oxides or by a condensate of organic substances partially unburned, including PAH and also tends to adsorb metals such as copper (which exerts a catalytic action in reformation of dioxins from chlorinated precursors), mercury, arsenic, cadmium, chromium and lead, which are toxic. Emissions of NO, SO2, HCl, HF, N2O and CO are influenced in particular by the concentration of sulfur, nitrogen, chlorine, etc. in the sludge. Regarding nitrogen oxides formation, mechanisms are basically of two types: the first, thermal oxidation of the nitrogen present in the combustion air, resulting in the formation of so-called "thermal NOx"; the second, conversion of nitrogen that may be present in fuel that leads to the formation of so-called "conversion NOx." The use of technology of combustion of the sludge involves, in addition to gaseous emissions, the production of various types of residues: the slag separated on bottom of the combustion chamber and residues of purification processes. From an environmental point of view the mechanisms of enrichment, which develop during the combustion process, constitute the most important factor in determining the quality of the fractions produced. This phenomenon of enrichment causes the preferential concentration of some metal elements in fly ash and particulate matter emitted in the stack; it is thus necessary to recourse to a treatment that will effectively fix the pollutants. It must be still emphasized how the production of the slag, in the case of combustion of sludge, is less than that which would be feeding in the combustion of municipal waste. 69

The use of technology of pyrolysis or gasification allows containing the environmental problems, by any innocuous solid residues (it is possible to obtain the vitrified residual with a good degree of inerting against the release of harmful elements in the environment) and the substantial reduction of the flow of gas to be treated. The use of oxygen in the gasification process determines fairly limited production of gases, but requires the supply or on-site production of oxygen, whose costs and energy consumption are not negligible. Using a gasification process it is possible to produce hydrogen, also if not as the only resulting gas. The synthesis gas obtained (having a calorific power of an average of 4 MJ/Nm3 and containing H2, CO, CO2, CH4, N2 and other gases in percentages lower than 0.5%), may have a 10-11% H2, which in the final analysis could be used in fuel cells, known to be interesting from the point of view of environmental impact. The pyrolysis process leads to the production of coal, tar and gas; in particular pyrolysis produces a solid fraction in which about 20% of the organic substance of departure remains. Almost 80% of the volatile substance is transformed into "light" gas (mainly CO, CO2 and water, while only 1-2% is consists of hydrocarbons); substances that remain liquid are in the order of 1%. The synthesis gas produced in a process of pyrolysis or gasification must necessarily be purified. This gas needs in fact to a treatment of removal of some contaminants, such as dust, vaporized heavy metals, hydrochloric acid. The cleaning system of the syngas is configured so as to obtain an efficient cleaning of the same. The advantage of the pyrolysis process is to concentrate heavy metals (with the exception of mercury and cadmium) in the final residue. The percolation of these metals is reduced in the case of the slag obtained by the treatment of pyrolysis, in contrast to that of the waste incinerator. Very often a primary treatment of pyrolysis is followed by a cracking of volatiles in a secondary reactor. From an energy point of view, the sludge is comparable to fossil fuels, or a material containing oxidizable elements (mainly carbon and hydrogen) which can release energy as quantified calorific value. Such energy can be used for:   

Production of heat only; Only producing electricity (or mechanical power); Combined production of electricity and heat (cogeneration).

Energy recovery allows reducing the use of traditional fuels, with a positive return in terms of environmental performance. For the production of energy, in fact, these systems reduce the emission of air pollutants associated with the use of conventional fuels. Another advantage comes from the decrease of greenhouse gas emissions compared to landfill disposal; in fact, landfilling would produce carbon dioxide and methane; methane is about 25 times more impacting than CO2. 4.4.5 Sewage sludge ash valorization Sewage sludge ash is the by-product produced during the combustion of dewatered sewage sludge in an incinerator. Approximately one third of the solids content of sewage sludge consists of inorganic matter which forms incinerated sewage sludge ash (ISSA) during combustion. Typical removal efficiencies of ash particles suspended in the flue gas are 95–99%. This translates to an estimated global ISSA production of 1.7 million tonnes per year, mainly from the USA, the EU and Japan, which are the main regions operating sewage sludge incinerators (Cyr et al., 2007; Murakami et al., 2009). The general characteristics of ISSA have been reported in the literature (Cyr et al., 2007) and this shows that the major elements in ISSA are Si, Al, Ca, Fe and P (Table 28). Crystalline forms of these elements are invariably quartz (SiO2), whitlockite (Ca3(PO4)2) and hematite (Fe2O3). Sewage sludge ash is primarily a silty material with some sand-size particles. The specific size range and properties (Table 27) of the sludge ash depend to a great extent on the type of incineration system and the chemical additives introduced in the wastewater treatment process.

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Table 27: typical physical properties of sewage sludge ash

Physical properties Gradation (% passing) 4.76 mm (No. 4 sieve) 2.38 mm (No. 8 sieve) 2.00 mm (No. 10 sieve) 2.00 mm (No. 10 sieve) 0.85 mm (No. 20 sieve) 0.42 mm (No. 40 sieve) 0.21mm (No. 80 sieve) 0.149 mm (No. 100 sieve) 0.074 mm (No. 200 sieve) - (0.0902 mm) 0.02 mm 0.005 mm >0.001 mm Loss on Ignition (%) Moisture Content (% by Total Weight) Absorption (%) Specific Gravity

Values Wegman 99 99 99 85 66 10-13 -

Khanbiluardi 100 98 73 53 38 -

Bulk Specific Gravity Plasticity Index Permeability (ASTM D2434 - cm/sec)

Waste Commission 100 100 100 100 100 98 83 56 20 12 2 1.4 0.28

Gray 100 100 100 47-93 2-13 -

1.6 2.44 - 2.99 1.27 – 1.82 Nonplastic 1 x 10-4 - 4 x 10-4

(Source: FHWA, User Guidelines for Waste and Byproduct Materials in Pavement Construction, 1998)

A wide variety of potential reuse applications have been reported in the literature for ISSA (Donatello and Cheeseman, 2013): 

   

Phosphate recovery o Recovery of P by acid leaching o Recycling of acid insoluble ISSA residue o Thermal methods of P recovery from ISSA Additive to Portland cement; Lightweight aerated cementitious materials; Glass–ceramics; Sintered materials containing ISSA: o Bricks, tiles and pavers o Manufacture of lightweight aggregates

71

Table 28: typical range of elemental concentrations in sewage sludge ash

Element

Silicon (Si) Calcium (Ca) Iron (Fe) Aluminum (Al)

Concentration% Oxide Reported as Elemental ConcentraReported as Oxides tion Ref. (2) Ref. (6) Ref.(10,16) Ref.(15) (SiO2) 5.6 - 25.7 20 27.0 14.4 - 57.7 (CaO) 1.4 - 42.9 8 21.0 8.9 - 36.9 (Fe2O3) 1.0 - 16.4 4 8.2 2.6 - 24.4 (Al2O3) 1.1 - 8.5 7 14.4 4.6 - 22.1

Magnesium (Mg) Sodium (Na) Potassium (K)

(MgO)

0.6 - 1.9

2

3.2

0.8 - 2.2

(Na2O) (K2O)

0.1 - 0.8 0.3 - 1.6

0.3 0.5

0.5 0.6

0.1 - 0.7 0.07 - 0.7

Phosphorus Sulfur (S) Carbon (C)

(P2O5) (SO3) -

1.2 - 4.4 0.3 - 1.2 0.6 - 2.2

6 -

20.2 0.9 -

3.9 - 15.4 0.01 - 3.4 -

(Source: FHWA, User Guidelines for Waste and Byproduct Materials in Pavement Construction, 1998; where ref. (2), (6), (10), (15) and (16) are listed. Website: http://isddc.dot.gov/OLPFiles/FHWA/009686.pdf)

It is likely that these processes will become more attractive as both phosphate prices and ISSA disposal costs continue to increase. Acid washing to recover P requires the recycling potential of the acidinsoluble ISSA to be considered. When milled, this acid-insoluble residue has promise as a partial cement replacement. ISSA can be successfully recycled via a number of different routes and the main reason why this material is sent to landfill is lack of industrial scale examples of the recycling applications already demonstrated at the laboratory scale.

4.5 ATO planned interventions Recently, different investments have been planned in the ATO:  





the modification of the dewatering line of Merone plant; the ATO should invest 220,000 euros (ATO-Como, 2014); the revamping of the anaerobic digestion line of Mariano Comense plant. Possible technological improvements can be applied, according to the new research developed for obtaining better results in anaerobic digestion, mainly in terms of energy production. The ATO should invest 1,100,000 euros (ATO-Como, 2014). Carimate and Mariano Comense plant agreed in 2011 to develop studies on the construction of an incineration plant for 7200 tonnes of sludge per year. The ATO should invest 4,600,000 euros for this project (ATO-Como, 2014), which represents the most considered alternative to agricultural reuse of sludge in the ATO of Como; an engineering society developed a technical report in 2007 about the possible construction and operation of a disposal platform for biological sludge, located in Como, based on the technology of pyrolitic process. The system should have had 30,000 tonnes of sludge as input, with the final production of electric energy through a gas turbine. The project was not developed.

Other interventions are consolidated; Carimate plant started a test to apply BioCrack® (Vogelsang) technology on sludges, an electrokinetic disintegration process that increases the efficiency of biogas plants. BioCrack enhances the exposure of the nutrients in biosuspensions to the fermenting bacteria. The result: increased gas yield and better utilization of the substrates.

72

5. Assessment of scenarios for the ATO of Como 5.1 Scenario 0: agricultural reuse 5.1.1 Assumptions and development The intermediate treatment facilities are functional to the needs of farms. The Lombard system necessarily involves the creation of centers of control and treatment of biological sludge with specific functions:   

to ensure the needs of the crop cycles through appropriate storage and to manage timeperiods of prohibition of the use of organic fertilizers while giving continuity to the public utility service; to ensure compliance through chemical-microbiological controls of homogeneous lots of sludge; to perform treatments of stabilization and sanitation complying with the more stringent limits imposed by the Lombardy Region.

The agricultural recovery of sludge has found fertile land in some areas like Lodi, Low Milan and Pavia and in particular in Lomellina area. The top reasons are:    

strong agricultural character; vast cultivated areas (farms up to 1,000 ha in the area) with limited presence of residential/production settlements; poor/no presence of animal waste and consequent high demand for organic matter.

The interaction among innovative sectorial regulations compared to the national ones, a particularly receptive and suitable territory and professionals with decades of experience has created a specialist district with specific plant facilities and technical expertise supporting local farms, not found on the national territory. In the Lombardy Region there are areas with excess of nutrients from manure and territories with deficiency. Overall, the region is deficient and annually imports about 800,000 tons of fertilizers corresponding to a value of 200 million euro. The biological sludge that meets the regulatory requirements, if successfully used for conditions of use and period of application, is a valuable contribution of organic matter and nutrient content. A desirable scenario in the medium term reckons on the facts that:  

the agricultural recovery of biological sludge will have continuity and appreciation in a way directly proportional to the quality that the product in the future will be able to offer and guarantee. the adaptation of national legislation to the new technical and scientific acquisitions is fundamental: on the basis of the main trends in Europe and North America, attention should be focused on sanitation requirements (pathogens such as Salmonella, E. Coli, etc.) and organic pollutants (PAHs, PCBs, PCDD/F, etc.).

According to what has been studied in the previous chapters, some critical points must be underlined: 



the availability of land for agricultural application of sewage sludge is not sure for what concerns the WWTPs of the ATO of Como; in fact the processing plants to which the sludge is conferred receive sludge also from WWTPs outside the ATO of Como and outside the Region; due to the increase in the production of sewage sludge, the available land for the plants may not be enough for the total disposal of the future quantities of sludge; the application of sludge on the soil has the positive impact of improving the characteristics for the growth of crops; on the other hand, there is the serious impact linked to metals and mi73







cropollutants accumulation into soils; according to some simulations (IRER, 2007), the limits for metals into soils will be overcome in 10-25 years; thanks to Nitrate Directive, more and more attention is put on nitrogen balances into soils in order to avoid problems linked to it; sludge disposed into soils increases the amounts of nitrogen in them; in Lombardy Region some areas have already had problem related to nitrogen and so more and more concern should be put on it; because of the previous reasons, it is supposed that in the next years the price of disposal into agriculture will rise, either for the search of new areas for the disposal of sludge, either for the construction and operation of new processing plants, which will have the duty to store the sludge and process it to apply it in the appropriate way onto soils in the right periods of the year preventing the accumulation of nitrogen and metals; an important issue is related to the future characterization of sludge in order to determine the metals and micropollutants content before the application of it; due to more stringent legislation, this will be necessary before any other operation in this disposal route.

The assumptions used for the development of the scenario are the following:  

sludge is totally disposed of by landspreading for agricultural reuse; based on the trend observed between 2009 and 2011, sludge production will increase by 5%/year till 2023 due to a wider application of tertiary treatments to all WWTPs in the district; then a further increase of 2%/year till 2030 due to completion of the finishing treatments even at smaller plants; finally, they will remain stable till 2048. The final possible trend is shown in graph 2;

Tonnes of wet sludge

60000 50000

40000 30000 20000 10000

2047

2045

2043

2041

2039

2037

2035

2033

2031

2029

2027

2025

2023

2021

2019

2017

2015

2013

0 Years Graph 2: possible evolution of sludge produced in Como district WWTPs

  

Land application of sludge for agricultural reuse is limited to a maximum amount of 7.5 tDM/ha/year; specific disposal cost will increase by 10% every two years due to the need of upgrading the processing plants because of new stricter regulations; then by 1% when the situation is stabilized, after 15 years; for the calculations of the volumes of sludge, it is assumed that water has a specific weight of 1 ton per cubic meter while dry matter has a specific weight of 1.5 ton per cubic meter; then each contribution from a specific plant has its dry matter content for which a specific volume is given; 74

 

the transport of the sludge to processing plants is done by trucks (diesel heavy duty vehicles) of 10 m3 carrying capacity; their pollution potential in terms of emitted compounds is provided in Table 29; the impact of nitrogen leaching into the soil has not been evaluated; this will constitute a problem in the zones where pollution related to nitrogen excess is detected and it will be stated directly by municipalities through particular plans;

Table 29: estimated emission factors for diesel heavy duty vehicles

NOx Total g/km g/kg fuel g/MJ

10.4 42.3 0.995

CH4 0.06 0.243 0.006

VOC 2.01 8.16 0.192

CO

N2O

8.98 36.4 0.857

0.03 0.122 0.003

CO2 774 3138 73.8

(Source: EEA, 2013)



the distances between WWTPs are provided in Table 30.

Table 30: distances between WWTPs and processing plants

WWTPs Bulgarograsso Carimate Como F.M. A. S. F.M. Liv. Recovery plants Distance (km) C.R.E. 119 111 ECODECO 71 62 BIOAGRITALIA 164 VAR 83

M.C.

105

Merone

115

The data for 2013, which is the starting year, are the following: Table 31: data for the starting year in the agriculture reuse scenario

Bulgarograsso - Alto Lura srl Carimate - Sud Seveso Servizi spa Como - Comodepur spa Fino Mornasco - Alto Seveso - Lariana Depur spa Fino Mornasco - Livescia - Lariana Depur spa Mariano Comense - Valbe spa Merone - dewatered - ASIL spa Merone - dried - ASIL spa

Total disposed Total dry Disposal sludge (t/a) % DM sludge (t/a) cost (€/t) 4421 21.1 931 54.0 3355 21.5 722 56.0 9921 21.0 2083 65.0 4082

23.2

947

55.0

519 2182 2741 93

22.1 28.5 25.4 86.4

115 622 696 80

145.0 57.0 57.0 73.5

Totally, 27,314 tonnes of sludge are disposed, corresponding to 25,250 m3, with a cost of approximately 1,700,000 € on around 830 hectares. Actual pollutants emissions due to truck transport are the following:

75

Table 32: actual pollutants emissions due to sludge disposal

Pollutant Nitrogen oxides NOx Methane CH4 Volatile Organic Compounds VOC Carbon monoxide CO Nitrous oxide N2O Carbon dioxide CO2

Emissions [tonnes] 2.72 0.02 0.53 2.35 0.01 202.71

5.1.2 Conclusions The final scenario, simulated till year 2030, is the following:   

total disposal costs will increase by around 275,000 €/year; surfaces needed for landspreading will increase by around 45 ha/year; total number of trucks will increase by around 136 vehicles/year and related CO2 emissions will increase by 11 tonnes/year; it must be highlighted that an increase of the emissions of the pollutants has a different impact in terms of global warming; for example, methane has 20-yr global warming potential of 72, while for nitrous oxide it is 289;

The graph shows the increasing trend for volumes and hectares. In 2030, around 47,000 m3 of sludge will have to be disposed, on around 1546 hectares. In 2030, the emissions are constituted by:      

5.10 tonnes of NOx; 0.03 tonnes of CH4; 0.98 tonnes of VOC; 4.40 tonnes of CO; 0.01 tonnes of N2O; 379.3 tonnes of CO2.

The estimated quantities should be considered as the maximum value of sludge production and consequent emissions. It is reasonable to assume that real quantities should be lower due to the application of new technologies for sludge production minimization.

76

Graph 3: sludge volumes and hectares trend in the agricultural reuse scenario 50,000.00

1800.0

45,000.00

1600.0

40,000.00

1400.0

1200.0

30,000.00

1000.0

25,000.00 800.0

20,000.00

Hectares

Volumes [m3]

35,000.00

600.0

15,000.00

sludge volumes

400.0

10,000.00 hectares for landspreading

5,000.00

200.0 2047

2045

2043

2041

2039

2037

2035

2033

2031

2029

2027

2025

2023

2021

2019

2017

2015

0.0 2013

-

Years

A further analysis of the trend of major greenhouse gases is given in the following graph. Graph 4: greenhouse gases emissions trend in the agricultural reuse scenario 0.04

400.00

350.00

0.03 0.02

250.00 0.02

Tonnes of CO2

300.00

200.00

0.01

150.00

0.01 Methane

Nitrous oxide

Carbon dioxide 2047

2045

2043

2041

2039

2037

2035

2033

2031

2029

2027

2025

2023

2021

2019

2017

100.00 2015

2013

Tonnes of CO2 equivalent (20 years)

0.03

Years

77

5.2 Scenario 1: sludge-to-energy by means of incineration 5.2.1 Assumptions and development To develop any detailed consideration about incineration, some critical issues should be taken into account: 

 

the sludge conferred to the new plant must have appropriate characteristics so that the monocombustion process can be self-sufficient and efficient (autogenous combustion); the necessity of pre-drying must be assessed in order to obtain the required calorific value for the nonconventional fuel fed to the plant; a fundamental issue is the disposal of ashes obtained as final product of the thermal oxidation; new routes of valorization of this product should be assessed, to find the most convenient price in the economical balance; pollutants emissions must be quantified and characterized; the final environmental balance must be evaluated, with particular reference to global warming potential and health issues.

The assumptions used for the development of the scenario are the following:      

the increase of sludge quantities and of specific disposal costs is the same as in scenario 0; economic and environmental aspects will be evaluated for one thermal treatment plant (e.g.: fluidized-bed incinerator) serving the entire district; the characteristics of the sludge conferred to the plant are assumed to be those listed in Table 33; sludges are mixed before being burnt in the combustion chamber; according to the different quantities conferred by each plant and their different characteristics (mainly dry matter and moisture content), the final characteristics of the sludge incinerated are presented in Table 33; the according to energy balances the Lower Heating Value (LHV) of the sludge must be sufficient to sustain an autogenous combustion; as dewatered sludge does not allow self-sufficient combustion, it must be dried in a preliminary step to achieve a sufficient LHV; environmental regulations require a minimum free oxygen in the flue gas, and excess air must be provided consequently.

In BREF WI, the Joint Research Center (European Commission, 2006) states which are the best available techniques for waste incineration in general and for sewage sludge incineration in particular (chapter 5.1 and 5.5). For sewage sludge incineration it is in generally considered that:  

the use of fluidized bed technology may generally be the BAT because of the higher combustion efficiency and lower flue-gas volumes that generally result from such systems. There may be a risk of bed clogging with some sewage sludge compositions; the drying of the sewage sludge uses heat recovered from the incineration, to the extent that additional combustion support fuels are not generally required for the normal operation of the installation; in this case, normal operation excludes startup, shut-down and the occasional use of support fuels for maintaining combustion temperatures.

78

Table 33: sewage sludge characteristics used to design the incinerator

dry matter% g/l volatile matter% DM C% VM H% VM O% VM N% VM S% VM C/N P% DM Cl% DM K% DM Al% DM Ca% DM Fe% DM Mg% DM Calorific value kWh/t

mixed sludge digested sludge MIXING 25 30 26.52 72 50 65.31 51 49 50.39 7.4 7.7 7.49 33 35 33.61 7.1 6.2 6.83 1.5 2.1 1.68 7.2 7.9 7.41 2 2 2.00 0.8 0.8 0.80 0.3 0.3 0.30 0.2 2 0.75 10 10 10.00 2 2 2.00 0.6 0.6 0.60 4600 3000 4113.46

C% H% O% N% S% P% Cl% K% Al% Ca% Fe% Mg% Others% tot

9.18 1.33 5.94 1.28 0.27 0.50 0.20 0.08 0.05 2.50 0.50 0.15 3.03 25.00

7.35 1.16 5.25 0.93 0.32 0.60 0.24 0.09 0.60 3.00 0.60 0.18 9.69 30.00

8.73 1.30 5.82 1.18 0.29 0.53 0.21 0.08 0.20 2.65 0.53 0.16 4.84 26.52

VM% Inerts% U%

18.00 7.00 75.00

15.00 15.00 70.00

17.32 9.20 73.48

In the following paragraph the main characteristics of an incineration plant for sludge management are presented and will be used for the evaluation. It will comply with the requirements of safety and reduction of pollutants, as stated by the European Directive 2000/76/EC. The size and potential must satisfy the load of produced sludge ensuring continuous operation 24 hours a day, 7 days a week. A sketch of the plant is given in Figure 22.

79

Figure 22: sketch of sewage sludge fluidized bed incineration plant

The sludge will be stored in silos or bunkers and then fed to the fluidized bed furnace. The fluidized bed furnace is constituted by a cylinder of refractory material. In the lower part of the combustion chamber a grid required for the distribution of air of fluidization is installed. Above this grid we find the sand bed (fluidized bed) and the area of post-combustion. The sludge in input must be screened to eliminate any foreign bodies, stones, glass, etc., and if necessary ground into particles of a maximum size of 50 mm x 50 mm x 50 mm. The sludge is completely combusted within the combustion chamber. The inorganic components are pulverized to ash and leave the combustion chamber together with the gaseous flow passing through the top of the same. The gas passes from the incinerator to a heat recovery system consisting of heat exchangers, steam and water collection tanks, recirculation pumps and a steam turbine connected to a current generator. The flue gas treatment system is constituted by:   

electrostatic precipitator for dust removal; stages of dry absorption (with PAC dosage) and wet absorption (wet scrubbers) for the removal of heavy metals acid gases; fan for sending flow in the chimney.

The principal characteristics of the plant at its maximum capacity are summarized in Table 34.

80

Table 34: incineration plant technical data

Sludge conferment and storage Wet sludge flowrate (27% DM) ton/year 52,615.41 Wet sludge flowrate (40% DM)

ton/year

Dry matter flowrate

ton/year

Potential

PE

35,000.00 9,282.15 320,000.00

Drying LHV activated sludge LHV digested sludge Drying temperature Evaporated water LHV dried sludge Dry matter content

kJ/kg kJ/kg °C ton/h kJ/kg %

2102.74 1482.41 105.00 2.01 4250.70 40

Combustion Wet sludge flowrate Dry matter flowrate Temperature Retention time Oxygen conc. in flue gas Combustion chamber volume Excess air Air flowrate Flue gas flowrate Thermal power

ton/hour ton/hour °C s % m3 % Nm3/hour Nm3/hour MW

Thermal recovery and EE generation Temperature of gas before boiler °C Temperature of gas after boiler °C Steam flowrate kg/hour Steam temperature °C Steam pressure bar Net electric energy production * kW

4.00 1.59 850 7.6 8 27.17 62 9216.47 12877.33 4.72 850 250 4380.41 350 40 751.6

* if all the steam is used for electricity production

The combustion produces solid residues, known as slags and ashes, whose removal needs chemical compounds which must be dosed in different steps. Besides, the phase which requires the highest energy is drying. These facts are summarized in Table 35.

81

Table 35: quantities of residues, chemicals and energy needs

Slags and ashes Slag production ton/d 10.61 Ash production ton/d 2.65 3 Ash conc. in flue gas g/Nm 8.58 3 Limit concentration g/Nm 0.01 Removal efficiency % 99.88 Chemicals NaOH 47% kg/hour 250 NH3 25% kg/hour to be fixed PAC 15% kg/hour 20 Energy consumption Energy needed for drying kW 2010.89 5.2.2 Economical and financial assessment The investment costs include the following technical components: section for the reception/pretreatment of the waste, the device for thermal treatment (fluidized bed incinerator), the boiler, the section for electricity generation, the flue gas cleaning, the chimney stack, the bottom ash extraction system, the electric control system, the auxiliary services, the site acquisition and the civil works. A study developed by the Greater London Authority in January 2008 (Greater London Autorithy, 2008) and adapted in the doctoral thesis of Panepinto in 2011 has provided an analysis of the investment costs of the plants for thermal treatment of wastes built from the 90s distinguishing between conventional incinerators and innovative technology plants (gasifiers and pyrolyzers). Table 36 shows the different investment costs for various plant potentials in terms of wet sludge. Table 36: investments costs and electric energy produced for different plant potential

Plant potential (t/year) 100,000 – 115,000 150,000 170,000 – 200,000

Net electric energy produced (MWel) 6-7 9 11 – 13

Investment costs (M€) 40 – 50 55 – 80 65 – 100

Based on these data, a simple interpolation can be performed in order to obtain a rough estimation of the investment costs for any plant potentiality. The result is shown in Graph 5. Three linear functions are obtained: highest cost, average cost, and lowest cost lines. The investment cost of the plant of 320,000 PE (35,000 tonnes of wet sludge) as previously described is approximately 14 million euros. This cost refers to a plant consisting of a single line of treatment and includes the cost of the civil works. Finally, the ancillary works will amount to approximately 10% of the sum of the cost of installation and civil works, so 1.4 million euros. The total amounts to 15.4 million euros. Then, the management costs to be considered are the costs of maintenance (assumed to be 2% of the investment cost), personnel costs (15 workers, 30,000 €/year each), the cost of chemicals (assumed to be equal to 15 €/t, 2,160 t/year) and the cost for the disposal of the slags (110 €/t). It is assumed to have no economic returns due to energy recovery. In fact, analyzing the results obtained from the balances carried out (in this case by the thermal balance), the obtainable recovery of energy and electricity is only sufficient to ensure self-sufficiency of the system itself, also with need of methane and electricity for integration. In Table 37 the results of the economic evaluation for the plant are given. To assess the value of the investment it is necessary to decide how much sludge will be sent to incineration. It is assumed that the plant will start full operation from 2018. Two different collection poles can be considered in the ATO: the first pole will collect the sludge coming from Bulgarograsso, Como 82

and Fino Mornasco plants, while the second will collect the sludge coming from Carimate, Mariano Comense and Merone plant. This distinction is due to geographical and logistic reasons. In addition it should be considered that the sludge produced by the first pole plants is aerobically digested while the one from the second is usually anaerobically digested. In this case the percentages shown in Table 38 are assumed. Graph 5: estimation of investment costs for incinerators 120 y = 0.0005x

100

y = 0.0004x 80 Investment 60 costs [millions €]

y = 0.0003x

40

20 0 0

50000

100000

150000

200000

250000

Plant potential [tonnes of wet sludge] (Source: adapted from Panepinto, 2011) Table 37: economic evaluation for the designed incineration plant

Investment costs Plant and civil works

Result

14,000,000.00 €

Ancillary works Total Interest rate Plant lifetime Amortization Annual installment

Management costs Maintenance Personnel (24 h, 8000 h/y, 15 people) Chemicals (270 kg/h, 8000 h/y) Slag disposal Electric energy Methane

u. of. m.

1,400,000.00 € 15,400,000.00 5% 30 6.5% 1,001,792.10

Result

€ % years % €/year

u. of. m. 308,000.00 €/year

450,000.00 32,400.00 varying every year varying every year varying every year

€/year €/year €/year €/year €/year

83

Table 38: percentages of sludges sent to the designed incineration

period 2018-2022 2023-2027 2028-2032 2033-2037 2038-2042 2043-2048

pole 1 90% 90% 85% 85% 85% 85%

pole 2 90% 90% 100% 100% 100% 85%

In this way, after a first transient period, the potential of the plant will be always exploited around 80%, reaching full potential in the last two decades. For the financial assessment the Net Present Value (NPV) technique is used. The NPV is a methodology by which the present value of a series of expected cash flows is defined not only by adding them but discounting them on the basis of the interest rate. The first cash flow is equal to the disbursement of the year 0 (- sign) equal to the initial investment. The residual value of the plant at the end of its useful life is considered null; this is certainly permissible if it is assumed that the initiative has duration equal to that of the civil works (usually 20-30 years). If NPV> 0, the investment is profitable; if zero it is indifferent to the use of the initial capital with any other initiative with interest rate equal to the barrier rate; if less than zero it is not profitable. The rate for which the NPV is equal to 0 can be calculated: this rate is called the internal rate of return (IRR) and the difference between the IRR and the adopted interest rate measures the return on investment compared to the use of capital used as a reference. The positive cash flows are the savings due to the fact that the sludge incinerated is not sent to agriculture; the negative cash flows are the management costs of the incineration plant. The adopted interest rate is 5% and the expected lifetime of the plant is 30 years. The result is that:     

the NPV is approximately 27,700,000 €; the IRR is 13.5%; the profitability is 8.5%; the investment is recovered in 11 years; the highest investment which can be recovered is about 43,000,000 €.

5.2.3 Sensitivity analysis Sensitivity analysis is very useful when attempting to determine the impact the actual outcome of a particular variable will have if it differs from what was previously assumed. By creating a given set of scenarios, the analyst can determine how changes in one variable will impact the target variable. Changes are made in the following variables:   

dry matter entering the incineration plant, because it can be decided to apply new dewatering technologies to reach an higher dry matter content if the gain in the incineration phase is high, as, if the dry content is more, there is less need of water evaporation and energy consumption; slag disposal cost, because there could be ways to valorize slags obtained after the incineration of sludge, in this way decreasing the costs of their elimination; energy consumption of the drying phase, because it can be assessed the possibility to apply a different drying technology to attain a better gain in the whole assessment.

The target value is the financial variable, the NPV. It results that: 

higher dewatering, resulting in higher dry matter content, previous to incineration is the main goal to be reached because a too low dry matter alone can make the investment unprofitable 84

 

because of its various consequences on the plant management; the gain will be around 750,000 € each percentage point of DM; slag disposal costs changes are not so affecting the whole assessment, but a way to valorize them can permit high gains; the gain will be around 50,000 € each euro of disposal cost; lowering energy consumptions in the drying step has a gain if the energy is monetized, accounting for energy certificates; if not, the energy is just wasted without any economic return.

The following graphs are simple way to visualize the previous considerations, with the designed case represented with a different color.

Milion

Graph 6: NPV change according to DM change 35.00 30.00 25.00 20.00 NPV [€] 15.00 10.00 5.00 16.6

18.6

20.6

22.6

24.6

26.6

28.6

30.6

% DM entering

Milion

Graph 7: NPV change according to slag disposal cost change 35.00 30.00 25.00 20.00 NPV [€] 15.00 10.00 5.00 60

80

110

130

150

170

slag disposal cost [€/ton]

85

Milion

Graph 8: NPV change according to energy needed for drying change 40.00 35.00 30.00 25.00 NPV 20.00 [€] 15.00 10.00 5.00 400

600

800

1000

1200

1400

energy need for drying [kWh/ton water]

5.2.4 Environmental aspects A brief analysis of the Italian legislation about pollutants from combustion activities and best available technologies is provided in APPENDIX 3. For the designed plant, the focus is the dust emissions, as particulates can represent a carrier for heavy metals. Assuming that 20% of the inert solids leave the combustion chamber as fly ashes so the 80% of inert solid remain in the combustion chamber as bottom ashes, at full operating conditions, it results that:   

2.65 tonnes of fly ashes are produced every day; the final dust concentration in the flue gas will be 8.58 g/Nm3. As stated by the law, the average limit value is 10 mg/Nm3, so to reach this value, 0.11 tonnes per hour must be removed by the dust removal system, with an efficiency of 99.88%; 10.61 tonnes of slags are produced every day.

The possibility of solid waste reuse/recycling is primarily determined by their characteristics in terms of organic matter content and leaching of metals and salts. To obtain solid residues with best features direct process control techniques are first applied in order to facilitate a complete burnout of organic substances and then get a very low content of unburned in the slag and ashes. The level of unburned is still depending on the characteristics of the waste and it is lower if the sludge is pretreated or homogeneous. One of the most promising opportunities to valorize bottom ashes is to recover phosphate from ISSA. There are various options for recovering P from sewage sludge but the disadvantages are the relatively high water and organic matter contents, which increase the processing capacities required. The fact that ISSA is a dry and free flowing powder greatly simplifies processing operations for subsequent phosphate extraction when compared to either phosphate rock or liquid and dilute sewage sludge. Some experimental processes for P recovery from ISSA are consolidated and reported in the literature, and a scheme of them is given in the following figure (Donatello and Cheeseman, 2013).

86

Figure 23: experimental processes for P recovery from ISSA (Source: Donatello and Cheeseman, 2013; (a) Takahashi, 2001, (b) Franz, 2008 and (c) Petzet, 2011)

5.2.5 Conclusions It can be summarized that:   

the financial assessment is positive, with a positive NPV and with a good profitability of the investment; BATs for the removal and control of the pollutants are available to minimize emissions in flue gas and fly ashes; moreover, phosphorus recovery from solid residues will contribute to their valorization and will decrease the final disposal cost; the variability of the characteristics of the feed waste, such as humidity, calorific value and metal content, affects the economic evaluation.

87

5.3 Scenario 2: sludge-to-energy by means of pyrolysis 5.3.1 Assumptions and development The objective is to provide a preliminary design of a pyrolysis plant in Como district. The required throughput cannot be higher than 3.5 MW of thermal energy by supplying approximately 1 t/h of sewage sludge. A specific attention has been paid to maximize conversion efficiency and minimize environmental emissions. The basic process flowsheet is presented in the following figure.

Figure 24: pyrolysis process flowsheet

Ultimately, energy will be produced from the scrubbed syngas. Various technologies exist for the conversion of stored chemical energy to electric power, classified as either being engines or turbines. It was decided to opt for gas engines, as they usually operate under higher efficiencies. General pretreatment will usually involve drying and pulverizing. This will increase the calorific value of the feed and decrease handling costs. The chemical reactor is where the pyrolytic/gasification process takes place, yielding high value products from low value feeds. Its design is based on mass and energy balances, which involve:   

Fuel feed rate and its LHV (i.e. pretreated sludge). Flow rate of the heating medium. Product flow rate.

The plant potential is 30,000 wet tonnes (27% DM) per year, 23.1 dry tonnes (95%) per day or 1 dry ton per hour. For what concerns the feed fuel (sewage sludge) the following characteristics are assumed.

88

Table 39: characteristics of the sewage sludge fed to the designed pyrolysis

MIXING dry matter% g/l volatile matter%DM C% VM H% VM O% VM N% VM S% VM C/N P% DM Cl% DM K% DM Al% DM Ca% DM Fe% DM Mg% DM Calorific value kWh/t

26.52 65.31 50.39 7.49 33.61 6.83 1.68 7.41 2.00 0.80 0.30 0.75 10.00 2.00 0.60 4113.46

C% H% O% N% S% P% Cl% K% Al% Ca% Fe% Mg% Others% tot VM% Inert% U%

8.73 1.30 5.82 1.18 0.29 0.53 0.21 0.08 0.20 2.65 0.53 0.16 4.84 26.52 17.32 9.20 73.48

Sludges must have been dried to a minimum 95% DM before any pyrolysis process (Arlabosse et al., 2011). After drying, the sludge will present the following parameters: Table 40: characteristics of dried sludge

dry matter% g/l volatile matter%DM C% VM H% VM O% VM N% VM S% VM C/N P% DM Cl% DM K% DM Al% DM Ca% DM Fe% DM Mg% DM

94.3 65.3 51.0 7.4 33.0 7.1 1.5 7.2 2.0 0.8 0.3 0.2 10.0 2.0 0.6

C% H% O% N% S% P% Cl% K% Al% Ca% Fe% Mg% Others% tot VM% Inert% U%

31.41 4.56 20.33 4.37 0.92 1.89 0.75 0.28 0.19 9.43 1.89 0.57 17.72 94.31 61.6 32.7 5.7

The mass balance calculation requires the calculation of the equivalence ratio (E.R.), which is the actual air-fuel to the stoichiometric air-fuel ratio. This term is essential in air deficient systems, such as pyrolytic reactors. Pyrolysis takes place in the absence of air, hence the E.R. is zero. However, a complete89

ly inert environment is practically never achieved and the actual E.R. will be greater than zero. The figure below depicts the effects of E.R. and carbon conversion (Basu, 2010).

Figure 25: equivalence ratio against carbon conversion efficiency (Source: Basu, 2010)

A lower E.R. value tends to increase tar production, but a higher E.R. value tends to emit more products of complete combustion (i.e. CO2, etc.). For this balance, an E.R. value of 0.25 was used, corresponding to a carbon conversion efficiency of 90%. From this, the flow rate of the heating medium (usually steam), according to Basu, 2010, is given as: 𝑀𝑓(𝑎) = 𝐸. 𝑅 × 𝑀𝑓

(Equation 1)

Where: Mf (a): Flow rate of the heating medium (kg/s); E.R.: Equivalence ratio; Mf: Wood Feed Rate (kg/s)

The calculated heating medium flowrate is 0.067 kg/s. The volume flow rate of the product gas from a desired net heating value is found by: 𝑄

𝑉(𝑔) = 𝐿𝐻𝑉(𝑔)

(Equation 2)

Where: V(g): volume flow rate of the gas produced (Nm3/s); Q: Reactor’s required power output (MW); LHV(g): Net heating value (MJ/m3)

For the reason that the volume of gases changes with temperature or pressure, it is necessary to specify temperature and pressure of the flow rate. However, equation 2 assumes standard conditions of temperature and pressure (i.e. 1 atmosphere and 20 °C). For this balance, the LHV(g) is unknown. LHV(g) values for typical gasification systems using steam ranges from 10 MJ/Nm3 to 18 MJ/Nm3 (Basu, 2010). Taking the reactor’s required thermal output power as 3.5 MW, and syngas density as 0.95 kg/Nm3, the mass flow of the product gas can be resolved. Hence the mass flow of char/tar can be resolved from summing up the mass flow of the feed and heating medium and subtracting the mass flow of the product gas. The table below shows the values for the volumetric flow rate of the gas produced and hence the mass flow rate of the gas produced and the char/tar/bio oil (M(c/t/b)) byproducts with varying LHV(g) values:

90

Table 41: results for the flow rate of the heating medium

LHV gas MJ/Nm3 V gas Nm3/s M gas kg/s M (tar/char/bio oil) kg/s 10 0.35 0.33 0.002 11 0.32 0.30 0.032 12 0.29 0.28 0.057 13 0.27 0.26 0.079 14 0.25 0.24 0.097 15 0.23 0.22 0.113 16 0.22 0.21 0.127 17 0.21 0.20 0.139 18 0.19 0.18 0.150 Most pyrolytic/gasification reactions are predominantly endothermic. This implies heat must be supplied to the reactor for these reactions to take place at the designed temperature. The amount of external heat supplied to the reactor depends on the heat requirements of the endothermic reactions as well as the pyrolysis temperature. The pyrolysis temperature is set at 550 °C. The general energy balance equation is given by: 𝑄 = 𝑚 × 𝐶𝑝 × ∆𝑇

(Equation 3)

Where: Q: Energy; m: mass flow rate; Cp: kJ/(kg K); ∆T: Temperature Change (°C)

The first step of this energy balance involves resolving the heat energy content of sewage sludge supplied to the reactor. According to Arlabosse, 2005, the specific heat per kg of wet sludge is given by:

C p (W ) 

W 1 C pw  C pDM 1W 1W

(Equation 4)

Where: W is the water content (kg/kg), Cpw and CpDM are the specific heat of the water and the dry sludge.

The specific heat of the dry sludge is measured using a calorimeter for a range of temperature between 35 and 105°C. A linear expression may be used:

C pDM  1434  3.29T

(Equation 5)

Where: T is the temperature (°C).

After drying, the sludge is at 105 °C. Obtaining a specific heat value of approximately 1.9 kJ/kg/°C, the energy content of the sludge after drying (from 15 to 105 °C) is approximately: 𝑄 = 0.268 (𝑘𝑔/𝑠) × 1.9(𝑘𝐽/𝑘𝑔/°𝐶) × (550 − 15 °𝐶) = 274 kW

(Equation 6)

Heating requirements for the reactor are supplied via fossil fuel (usually methane). The specific heat and temperature change of the heating medium are known to be 1.017 kJ/kg/°C and 300 °C. Hence from application of the energy balance equation, the energy content of the heating medium, with varying mass flow rates are shown in the table below:

91

Table 42: energy content of heating medium with varying flow rates

Q heating medium kg/s 0.054 0.056 0.059 0.062 0.064 0.067 0.070 0.072 0.075 0.078 0.080

Q heating medium kW 16.32 17.14 17.96 18.77 19.59 20.40 21.22 22.04 22.85 23.67 24.49

Q products kW 290.60 291.41 292.23 293.05 293.86 294.68 295.49 296.31 297.13 297.94 298.76

The energy content in the product stream is basically the sum of energy content in sewage sludge and the heating medium. The energy content in the syngas produced is essentially a percentage of the energy content in the product stream. Taking the average value for the energy content of the product stream as 294.68 kW, the energy content of syngas and char/tar/bio oil produced with varying percentage conversions are shown in the table below: Table 43: energy content of product compositions with varying percentage conversions

percentage conversion % 10 20 30 40 50 60 70 80 90

Q syngas kW 29.5 58.9 88.4 117.9 147.3 176.8 206.3 235.7 265.2

Q char/tar/bio oil kW 265.2 235.7 206.3 176.8 147.3 117.9 88.4 58.9 29.5

To the assumed percentage conversion efficiency of 90% it corresponds:     

an E. R. equal to 0.25; an heating medium flow rate equal to 0.067 kg/s; an average energy content of the products equal to 294.68 kW; a syngas energy content equal to 265.2 kW, an average char/tar/bio oil production equal to 0.09 kg/s.

Then, the geometric configuration and preliminary sizing of the reactor is resolved. In order for the volume to be resolved, the pyrolysis kinetics will have to be determined. This will involve determination of the sludge pyrolysis reaction rate constant. The temperature dependence of the reaction rate constant, and hence the rate of the chemical reaction can be determined by the Arrhenius equation: 92

𝐸𝑎

𝑘 = 𝐴𝑒 −𝑅𝑇

(Equation 7)

Where: k: rate constant for chemical reactions (s-1); A: Pre-exponential factor (s-1); e: Exponential function; Ea: Activation energy (J/mol); R: Gas constant (J/mol/K); T: Absolute Temperature (K).

The pyrolysis reaction occurring within the reactor is at 550 °C, implying the absolute temperature will be 823.15 Kelvin. The gas constant is 8.314 J/mol/K. A summary of the kinetic properties from sludge pyrolysis is given below (Ji et al., 2010): Table 44: kinetic properties from sludge pyrolysis

Fuel

Ea (kJ/mol)

A (min-1)

sewage sludge

82.3 – 109.2

7.7 × 106 – 2.8 × 109

(Source: Ji et al., 2010)

Inserting these kinetic data into the Arrhenius equation gives a rate constant of 0.015 s-1. To determine the volume, a mass balance will have to be carried out on the reactor. The general mass balance for any individual reactant or product is: 𝐴𝑐𝑐𝑢𝑚𝑢𝑙𝑎𝑡𝑖𝑜𝑛(1) = 𝑖𝑛(2) − 𝑜𝑢𝑡(3) − 𝑟𝑒𝑎𝑐𝑡𝑒𝑑(4)

(Equation 8)

Since the reactor in discussion is to be designed as a Continuous Stirred Tank Reactor (CSTR), the general assumptions are:   

Steady State No temperature/concentration gradients Continuous flow

Applying these assumptions to the general material balance above, term (1) becomes zero since operation is at steady state. Terms (2) and (3) are essentially flows in and out of the reactor and term (4) will just be the rate of the reaction. Substituting mathematical expressions and rearranging it gives the volume of the reactor as:

𝑉=

𝑣𝑡 𝑘

×[

1 1−𝑥𝑎

− 1]

(Equation 9)

Where: V: Volume of the reactor (m3); vt: volumetric flow rate (m3/s); k: rate constant (s-1); xa: percentage conversion.

The volumetric flow rate can be determined by dividing the mass flow rate of the feed (sludge) by the density of sludge. Taking the average density of sludge as approximately1460 kg/m3, the volumetric flow rate is 1.83 × 10-4 m3/s. Applying the equation above, the table below shows a list of volumes needed for sludge in the pyrolysis reactor and residence times with varying percentage conversion:

93

Table 45: reactor volumes with varying conversions

percentage volume residence conversion % m3 time s 10 0.001 7.4 20 0.003 16.6 30 0.005 28.4 40 0.008 44.2 50 0.012 66.3 60 0.018 99.4 70 0.028 154.7 80 0.049 265.1 90 0.109 596.6 For this reactor design, a percentage conversion of sludge into products as 90% is assumed. This implies the volume of the reactor to be designed is approximately 0.109 m3 (109 liters), with a residence time of 596.6 seconds, approximately 10 minutes. The total height of the shell may be given by the sum of the height of the bed and the freeboard height. The height of the bed, in which the reaction takes place, is given by:

𝐻𝑏𝑒𝑑 =

𝑉 𝐴𝑏

(Equation 10)

Where: Hbed: Height of the bed (m); V: Volume of the bed (m3); Ab: Cross sectional area of the bed (m2)

The cross sectional area of the bed is given by:

𝐴𝑏 =

𝑉(𝑔) 𝑈(𝑔)

(Equation 11)

Where: V(g): volume flow rate of the gas produced (m3/s); U(g): Fluidization velocity (m/s)

Typical fluidization velocity varies between 3 - 5 m/s. The average volumetric flow rate of the gas produced is approximately 0.26 m3/s. From this and applying the equation above, the table below shows variation of cross sectional areas and bed heights with fluidization velocity. Table 46: variation of cross sectional area with fluidization velocity U gas m/s 3 3.4 3.8 4.2 4.6 4.8

Ab m2 H bed m 0.09 1.27 0.08 1.44 0.07 1.61 0.06 1.77 0.06 1.94 0.05 2.03

Adding a freeboard height of 10% of the bed height, the total height of shell is given. The contact medium provides the heat to sustain the reaction; it can be sand or any other special physical medium with suitable characteristics for heat exchange and high thermal inertia. Pyrolysis configurations with steel balls are proposed by some specialized companies. In the pyrolysis reactor, in addition to the volume needed for the transformation of sludge, there must be space for the contact medium. 94

As a rough example, for 100 kg of sludge, 0.833 m3 of steel balls are required; for the designed sewage sludge input, a volume of 8.02 m3 is needed, with an additional reactor having a volume of 0.802 m3 (10% of the total) for the steel balls preheating. A summary of the design is given below:  

a pyrolitic fluidized bed reactor for conventional pyrolysis process (residence time: 5 – 30 minutes; temperature: 450 – 600 °C) is designed; fixing a thermal output of 3.5 MWth and assuming an electricity conversion efficiency of 40%, an equivalence ratio of 0.25, to which a syngas conversion of 90% corresponds, a fluidization velocity of 3 m/s the following parameters are obtained:

Table 47: pyrolysis throughput values

Throughoutput mass flow rate of the heating medium energy content in the heating medium average energy content in the product stream average energy content of the syngas average char/tar/bio oil production

0.067 kg/s 21.22 kW 294.68 kW 265.21 kW 0.088 kg/s

Table 48: reactor sizing

Reactor specifications temperature volume needed for the sludge residence time shell height cross sectional area

550 0.11 596.6 1.27 0.09

°C m3 s m m2

Table 49: thermal and electrical energy consumptions for the designed pyrolysis plant

Plant energy balance power needed for drying thermal power needed for drying electric power needed for drying plant thermal power net thermal power electricity conversion electricity produced net electricity functioning net electric energy

2461.63 2215.47 246.16 3.5 1.28 40 0.51 267.65 8000 2,141,206.05

kW kW kW MW MW % MW kW h/y kWh/y

Finally, pyrolysis products can be introduced in the post-gasification chamber, if present, where they get in contact with a controlled amount of air and are gasified. The gas produced is mixed with pyrogas and extracted to be sent to turbines. The products which are not gasified are collected and sent to the combustion phase, where they are completely burnt. Slags are produced, collected, stored and sent to disposal. 95

To simply dimension the combustion chamber needed to burn the residual solid material, the total production of char/tar/bio oil is calculated, assuming an average production of 0.09 kg/s, obtaining 2790 tonnes as result. These materials have an average heating value of 35 MJ/m3, and the combustion chamber can sustain a heating rate of 625 MJ/h/m3. As a consequence, the combustion chamber is approximately of 18 m3. 5.3.2 Economical and financial assessment The abovementioned study developed by the Greater London Authority in January 2008 (Greater London Autorithy, 2008), and adapted in the doctoral thesis of Panepinto in 2011, provides data for the economical assessment for innovative thermal treatment plants. In this case the large variability of the investment cost is attributable to the pretreatment of the waste, generally required in the innovative thermal treatment plants, at least to reduce the sizes of the waste and humidity; in some cases, pretreatment is carried out at other plants and therefore costs are not included in the cost of investment. With increasing plant size (above 100,000 to 150,000 tonnes/year) the unit cost for plants with innovative technology is subject to reductions with respect to conventional incinerators that are smaller than expected. This is largely due to the fact that, in general, the total capacity of the system involves the use of modular units, unlike the incineration plants, that can be built in larger units with smaller unit costs. The data in the table below show the average investment costs and the expected quantities of net electricity, for various sizes of plants with innovative technology that realize energy recovery by cogeneration cycle with a steam turbine. A plant with innovative technology could be competitive with potential of less than 100,000 t/y, due to the fact that for such small size incinerators are not economically viable and, as a matter of fact, small incineration plants are uncommon. Table 50: investments costs and electric energy produced for different innovative plant potentials

Plant potential (tonnes/year) 50,000 100,000 – 115,000 150,000 170,000 – 200,000

Net electric energy produced (MWel) 5 7 8 - 10

Investment costs (M€) 27 - 38 40 - 65 50 - 95 65 - 110

(Source: Panepinto, 2011)

The same study provides, for innovative technologies, the range of costs for different sizes. Table 51: management costs for different innovative plant potentials

Plant potential (tonnes/year) 45,000 150,000 170,000 – 200,000

Management costs (€/ton) 55 - 75 50 - 65 43 - 60

(Source: Panepinto, 2011)

These are costs that include depreciation rate, cost of personnel, maintenance, material consumption, waste disposal, but do not take into account the cost of financing the project, of the profit of enterprise and revenue from the sale of energy. A point estimate of the investment needed for the construction of these facilities and related costs can only be done downstream of an in-depth analysis, in which all the variables will be assessed in detail (size, location, type of treatment, principal environmental technologies adopted, energy recovery, electricity market, local conditions). Here a very rough estimate of the management costs can be calculated. For the previous data, a simple interpolation can be performed in order to obtain a rough estimation of the investment and management costs for each plant potential. The results are shown in the graphs. 96

Graph 9: estimation of the investment cost for an innovative thermal treatment plant 140 120 y = 0.0006x 100 y = 0.0005x

Investment 80 costs (million €) 60

y = 0.0003x

40 20 0 0

50000

100000

150000

200000

250000

Plant potential (tonnes of wet sludge) (Source: adapted from Panepinto, 2011) Graph 10: estimation of the management costs for an innovative thermal treatment plant 80 70

y = -1E-04x + 79.377

60

y = -8E-05x + 69.15

50

y = -7E-05x + 58.923

Management 40 costs (€/ton) 30 20 10 0 0

50000

100000

150000

200000

250000

Plant potential (tonnes of treated sludge) (Source: adapted from Panepinto, 2011)

In the following table the results of the economic evaluation for the plant are given. Table 52: economic evaluation of the innovative thermal treatment plant

Investment costs Plant and civil works Ancillary works Total Interest rate Plant lifetime Amortization Annual installment

Result

u. of. m. 4,300,000.00 € 430,000.00 € 4,430,000.00 € 5 30 6.5% 280,000.10

% years % €/year 97

Management costs Specific management cost

Result

Annual cost

u. of. m. 66.75 €/wet ton varying every year €/year

As previously done for the incineration scenario, to assess the value of the investment it is necessary to decide how much sludge will be sent to pyrolysis. It is assumed that the plant will start full operation from 2018. Table 53: percentages of sludges sent to pyrolysis

period 2018-2022 2023-2027 2028-2032 2033-2037 2038-2042 2043-2048

pole 1 70% 60% 55% 55% 55% 55%

pole 2 70% 60% 60% 60% 60% 55%

In this way, the potential of the plant will be always exploited above 90%. Besides, the amount of sludge sent to agriculture will reach the actual value just at the end of the simulation period (2048). For the financial assessment the Net Present Value (NPV) technique is used. In addition to the consideration of the savings and the management costs in the cash flows, the energy sold is taken into account, also connected to the concept of Green Certificates. Green Certificates (GCs) are tradable instruments that GSE (Gestore Servizi Energetici) grants to qualified renewable-energy power plants (IAFR qualification). The number of certificates issued is proportional to the electricity generated by the plant/system and varies depending on the type of renewable source used and of project (new, reactivated, upgraded, renovated system/plant). The GC support scheme is based on the legislation which requires producers and importers of non-renewable electricity to inject a minimum quota of renewable electricity into the power system every year. GCs represent proof of compliance with the renewable quota obligation: each GC is conventionally worth 1 MWh of renewable electricity. GCs are valid for three years: those issued in respect of electricity generation in a given year (reference year) may be used towards compliance with the obligation also in the following two years. To fulfill their obligation, producers and importers may inject renewable electricity into the grid or purchase an equivalent number of GCs from green electricity producers. For the calculation of green certificates for plants that entered into operation after 31 December 2007, the GSE issues Green Certificates for 15 years, by multiplying the net energy recognized for the constants, differentiated by source (Table 1 of the Finance Act 2008, as amended by Law 99 of 23/07/2009): for sludge which is biodegradable waste, biomass other than those referred in the act, the coefficient is 1.30. The reference price for the market of GC for the year 2014, pursuant to the provisions of Article 2, paragraph 148, of Law no. 244 of 24 December 2007, is 114.46 €/CV (or 114.46 €/MWh). Besides, the plant can sell the electric energy produced by sending it to the grid; the national Authority guarantees a minimum price for the energy produced till 1,500,000 kWh, which is equal to 38.9 €/MWh. After this quota of energy, the excess can be sold basing on the price of the local market and according to the different electricity consumption bands (F1, F2, F3). GSE reported the following prices in August 2014 for northern Italy:

98

Table 54: average monthly price per time slot (€/MWh)

Band Mean price €/MWh

F1 38.62

F2 43.74

F3 36.03

F1 are the peak hours: 8 a.m. – 7 p.m. on weekdays (Monday – Friday); F2 are the intermediate hours: 7 a.m. – 8 a.m. and 7 p.m. – 11 p.m. on weekdays, 7 a.m. – 11 p.m. on Saturday; F3 are the off-peak hours: 0 a.m. – 7 a.m. and 11p.m. – 12 p.m. on weekdays and Saturday, every hour on Sunday and public holidays.

So fixing the GC value to 114.46 €/MWh, available till 2032, with a multiplying coefficient of 1.3., the minimum energy selling price to 38.9 €/MWh till 1,500,000 kWh, then to 36.78 €/MWh (local market value for F1) for the excess sold and adopting an interest rate of 5% and a plant lifetime of 30 years, the result is:     

the NPV is approximately 16,800,000 €; the IRR is 20%; the profitability is 15%; the investment is recovered in 7 years; the highest investment which can be recovered is approximately 21,000,000 €.

5.3.3 Sensitivity analysis Changes are made in the following variables:   

dry matter entering the pyrolysis plant, because it can be decided to apply new dewatering technologies to reach an higher dry matter content if the gain in the pyrolysis phase is high, as, if the dry content is more, there is less need of water evaporation and energy consumption; management costs, because there could be ways to valorize slags obtained after the final combustion of the products of pyrolysis, in this way decreasing the costs of their elimination; energy consumption of the drying phase, because it can be assessed the possibility to apply a different drying technology to attain a better gain in the whole assessment.

The target value is the financial variable, the NPV. It results that:   

higher dewatering, resulting in higher dry matter content, previous to pyrolysis, can help to improve the financial assessment, because an higher thermal power output can be obtained and so more energy is produced; management costs changes are heavily affecting the whole assessment, so a way to make them low and stable can permit high gains; lowering energy consumptions in the drying step has a gain if the energy is monetized, accounting for energy certificates; if not, the energy is just wasted without any economic return.

The following graphs show these considerations, with the designed case marked differently.

99

Graph 11: thermal and electric output according to DM change 500.00

3.80

450.00

3.70

net electric energy produced

400.00

3.60 output thermal power

350.00

3.50

300.00

3.40

kWel 250.00

3.30 MWth

200.00

3.20

150.00

3.10

100.00

3.00

50.00

2.90

-

2.80 16.6

18.6

20.6

22.6

24.6

26.6

28.6

30.6

% DM in the raw sludge feed entering the plant

Milion

Graph 12: NPV change according to specific management cost change 25.00

20.00 15.00

NPV [€]

10.00 5.00 50

60

66

70

80

90

100

management costs [€/t wet sludge]

5.3.4 Environmental aspects The system is quite complex and it is difficult to assess the environmental impacts because different processes are involved (pyrolysis, gasification, combustion) which have different environmental implications. The main issue is related to the emission of heavy metals, which can be fixed in the slags and leave the plant through them, or volatilize and exit with the flue gas and acid gases. Besides, when the syngas must be cleaned, pollutants and dusts are removed from the stream, and these constitute a potential pollution. Obviously BAT for pollution prevention and control exist for this system and they must be investigated and designed in order to comply with the emission limits. The clean-up technologies include:   

Wet & dry scrubbing technologies Cyclones Electro-static precipitators 100

The pyrolysis process produces a coal (pyrocoke) as a solid residue, of black color, odorless and with discrete LHV, still having a discrete carbon content (about 30%). Quantitatively, the coal output amounts to 30-40% of the incoming waste (sometimes even 50%, depending on the processes and the quality of the incoming waste) and represents a product for which it is needed to find the right disposal route; options can be:  

inerting, through a subsequent step of combustion/gasification, with further plant complication; the use as a fuel as an alternative to coal (for example in cement factories, in brick kilns, blast furnace), but the condition of the market for that product should be checked.

It is noted that the heat treatment systems that provide for the vitrification of waste produces a solid residue which is substantially an inert material and as such can be disposed of in landfills for inert waste in accordance with DM 3/8/2005 - Definition of eligibility criteria for waste to landfill (Art. 5 Table 2: Concentration limits in the eluate for acceptability in landfills for inert waste). 5.3.5 Conclusions It can be summarized that:   

the financial assessment is positive, with a positive VAN and with a very good profitability of the investment; there are BAT for removal and control of the pollutants, in order to minimize the emissions; however, the production of solid residue is an important issue, difficult to model; its production depends on the features of the feed so further investigations should be made on it; the variability of the characteristics of the feed waste, such as humidity, calorific value and metal content, affects the economic evaluation.

5.4 Electrokinetic dewatering addition 5.4.1 Electrokinetic dewatering application Actually, most of dewatering of sewage sludge in the ATO of Como is done through mechanical devices, mainly centrifuges and belt presses. A new emerging technology in this field is electro-enhanced dewatering. Within the combination of mechanical and electrodewatering techniques, almost all the equipment is based on belt press structure. The filtration membrane in the belt press can provide an integral contact with a conductive material so as to serve as a first electrode. Such fact, together with the presence of the second electrode in the form of the second sheet, allows a voltage to be applied across the material to be dewatered. The resultant electro-osmotic effect acts with the drainage effect produced by the mechanical pressure to enhance the dewatering procedure. The electro-osmotic dewatering effect may be produced simultaneously with belt press dewatering. The optimal way for utilizing the electrokinetics for dewatering is a combination of conventional mechanical methods like centrifuge and belt press with electro-osmosis. By such way the advantage of mechanical could be kept as mechanical method still owns the function for processing the major partition of sludge dewatering. The electro-osmosis could be a rational supplement for an advanced dewatering procedure for the treatment of hard-to-dewatered liquid. The studied configuration for the set-up of an innovative and cost-effective dewatering method comprehends:   

a first thickening stage which aims to bring the sludge from 2.5% to 6.5% DM content; a second mechanical dewatering stage based on pressure application to bring the sludge from 6.5 to around 11% DM content, varying according to the pressure gradient applied; a third electrodewatering stage to reach a final dry matter content around 30%.

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Thickening

Mechanical dewatering

Electro-dewatering

Figure 26: innovative dewatering process description

The energy consumption is:  

0.25 kWh per ton of water treated for the thickening step; varying according to the pressure applied for the mechanical dewatering step, basing on the formula:

P

gQH (Equation 11) 

Where: P is the needed power; ρ is the sludge density; g is the gravity acceleration; Q is the flow rate; ΔH is the pressure gradient applied; η is the machine efficiency.



90 kWh per ton of water treated for electro-dewatering; this value is an average between the initial and final energy consumption of the device, because as the DM content increases, it is more difficult the extract water.

The advantages of the application of this method are: 

 

high volume reduction with a strong impact to reduce the total disposal cost in agriculture, because of the presence of less volumes to be transported and disposed, or in thermal destruction, because of the less need of drying of sludge to have a calorific value compatible with autogenous combustion; reported pathogen and odor reduction; self-containment of the structure.

The disadvantages are: 

    

few vendors: commercial full-scale equipment for sludge electrodewatering are, for example, the CINETIK linear electrodewatering system (Eimco Water Technologies), electro-osmosis dehydrator (ELODE; ACE Korea Incorporation), EDW (Water Technologies of Australia),and Electrokinetic (Electrokinetic Limited, UK); emerging technologies; relative high power consumption; need of high feed solids; low throughput; high capital cost.

The actual dewatering line and disposal costs for Como, Fino Mornasco – Alto Seveso and Merone plants are summarized in table.

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Table 55: dewatering and disposal costs for Como, F. M. - A. S. and Merone plants

Como Power Functioning time Inlet flowrate Centrate flowrate Outlet flowrate Inlet DM content Outlet DM content Centrate DM content Sludge density Dewatered sludge

kW h/year m3/h m3/h m3/h % % % kg/m3 tonnes/year

Energy consumed

kWh

Costs Electric energy specific cost Agriculture disposal cost Overall electric energy cost Overall disposal cost Total cost

€/kWh €/ton € € €

37.0 3800 20.5 18.0 2.4 2.5 21.0 0.01 1075.3 9921

Fino Mornasco Alto Seveso Merone 14.6 30.0 3800 3800 9.2 6.7 8.2 6.1 1.0 0.7 2.5 2.5 23.2 25.4 0.01 0.01 1083.8 1092.5 4082 2741

140,600.00 0.16 65 22,496.00 644,865.00 667,361.00

55,480.00 0.16 55 8,876.80 224,510.00 233,386.80

114,000.00 0.16 57 18,240.00 156,237.00 174,477.00

The costs for agricultural disposal and the savings due to the application of electrodewatering can be shown in the following graphs, in which the Como plant case is presented. The total costs of conventional disposal in agriculture are given by the sum of the cost of electric energy for dewatering and the cost for disposal in agriculture, while the total costs of innovative disposal in agriculture are given by the sum of electric energy for electrodewatering and the cost for disposal in agriculture.

Milion €

Graph 13: visual comparison of costs of conventional and innovative dewatering 1.20 1.00 0.80

Agriculture disposal cost with ED Actual EE consumption

0.60

EE consuption with ED Actual total disposal cost

0.40

Total disposal cost with ED

0.20 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Further improvements in the method can be obtained by changing some parameters of the process, such as the DM content of the sludge entering electro-dewatering, by applying a higher pressure in the mechanical dewatering stage, which will consume more energy. 103

Thousand €/year

Graph 14: savings due to changes in the innovative dewatering parameters 80.00 70.00 60.00 savings with 11 % DM to ED

50.00 40.00

savings with 12 % DM to ED

30.00

savings with 13 % DM to ED

20.00

10.00 24

26

28

30

32

5.5 Sewage sludge treatment and disposal chains for Como district To describe the theoretical combinations of sewage sludge treatment and disposal options, different flow diagrams have been constructed. In each diagram the quantities of sludge and the outcomes of the disposal routes in economic and environmental terms are presented. Then, an analysis of the geography of the ATO is done, in order to understand which can be the best localization for the disposal plants. In fact the fluxes of sewage sludge are different from zone to zone and it can be easily seen that the biggest plants are situated in the southern part of the district. For the development of a project it must be decided if the project must involve only the major plants or the big and small plants. In fact, in the second case it must be assessed in advance if it is convenient for the small plants to convey their sludge to the pre-processing and disposal plant located near the central point, which can be Como, with respect to the agricultural reuse option. The convenience is related to the cost of transport, because the distance between the small plants and Como can reach 60 km (Sorico case). The following disposal configurations can be developed: 0. agricultural reuse – business as usual 1. incineration with (CD) conventional dewatering or (ED) electro-dewatering, (MP) for the main plants or (ALL) for all the plants; 2. pyrolysis with (CD) conventional dewatering or (ED) electro-dewatering, (MP) for the main plants or (ALL) for all the plants. Electrodewatering machines can be added in the big plants, where possible, while in the small plants machines exploiting solar energy can be an option. The focus with a complete description is put on:   

scenario 0: agricultural reuse scenario 1 CD MP: incineration with conventional dewatering for the main plants scenario 2 CD MP: pyrolysis with conventional dewatering for the main plants.

Other possible scenarios are mentioned and the focus is put only on the main issues:    

scenario 0 ED: agricultural reuse with electro-dewatering scenario 1 CD ALL: incineration with conventional dewatering for all the plants scenario 1 ED MP: incineration with electrodewatering for the main plants scenario 1 ED ALL: incineration with electrodewatering for all the plants 104

  

scenario 2 CD ALL: pyrolysis with conventional dewatering for all the plants scenario 2 ED MP: pyrolysis with electrodewatering for the main plants scenario 2 ED ALL: pyrolysis with electrodewatering for all the plants.

Finally, there could be choices on dewatering and drying treatments: 

incineration preceded by: o conventional dewatering and electrodewatering till 40%; o conventional dewatering and drying; o electrodewatering till 30/35/40% and drying; o conventional dewatering, electrodewatering and drying.

The same ones can be assessed for pyrolysis.

105

In 2013: 27,314 tonnes of wet sludge In 2048: 51,107 tonnes of wet sludge Transported volumes Used trucks (10 m3 capacity)

Transport

In 2013: 2525 trucks In 2048: 4724 trucks

In 2013: 25,250 m3 In 2048: 47,242 m3

Figure 27: scenario 0

WWTPs sludge production

CO2 emissions In 2013: 202.7 tonnes of CO2 In 2048: 379.3 tonnes of CO2 Mean CO2 emitted (2013 – 2048) Mileage

339.2 tonnes of CO2/year Agricultural reuse

In 2013: 262,000 km In 2048: 490,000 km

Used land

Mean disposal cost

In 2013: 826 ha In 2048: 1546 ha

In 2013: 57.3 €/ton of wet sludge In 2048: 133.9 €/ton of wet sludge

Mean disposal cost (2013 - 2048): 102.4 €/ton of wet sludge

106

WWTPs sludge production

64.55 tonnes of CO2/year

In 2013: 27,314 tonnes of wet sludge

Mean number of trucks (2013 – 2048)

In 2048: 51,107 tonnes of wet sludge

819 trucks/year Transport

Mean volumes (2013 – 2048)

Mean conferred wet sludge (2018 – 2048)

8193 m3/year

41,660 tonnes/year

Mean mileage (2013 – 2048)

Agricultural reuse

Drying

83,400 km/year Mean used land (2013 – 2048) 264 ha/year

4 tonnes/hour 27.1 m3 reactor 68% boiler efficiency 1.1 kg steam/kg sludge

163 kW/year Mean net thermal power consumption (2018 – 2048) 867 kW/year

Figure 28: scenario 1 CD MP

Mean CO2 emitted (2013 – 2048)

Mean electric power consumption (2018 – 2048)

Combustion in FB reactor

Mean residues production (2018 – 2048) 3138 tonnes slag/year 33 tonnes fly ash/year

Mean disposal cost (2018 - 2048): Mean disposal cost (2013 - 2048): 102.4 €/ton of wet sludge

without the investment 48.3 €/ton of wet sludge with the investment 69.3 €/ton wet sludge

Mean disposal cost (2018 - 2048): without the investment 40.8 €/ton of incinerated wet sludge with the investment 65.0 €/ton of incinerated wet sludge

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Mean CO2 emitted (2013 – 2048)

In 2013: 27,314 tonnes of wet sludge

246 kW/year

190.30 tonnes of CO2/year Mean number of trucks (2013 – 2048)

In 2048: 51,107 tonnes of wet sludge

2380 trucks/year Transport

Agricultural reuse

Mean electric power consumption (2018 – 2048)

Drying

Mean volumes (2013 – 2048)

Mean conferred wet sludge (2018 – 2048)

23,800 m3/year

28,000 tonnes/year

Mean mileage (2013 – 2048) 245,900 km/year Mean used land (2013 – 2048) 647 ha/year

1 ton/hour 90% conversion efficiency 18 m3 combustion chamber 3.5 MW thermal output

Mean net thermal power consumption (2018 – 2048) 2215 kW/year

Figure 29: scenario 2 CD MP

WWTPs sludge production

Pyrolysis

Mean char/tar/bio oil production (2018 – 2048) 2800 tonnes/year

Mean disposal cost (2018 - 2048): Mean disposal cost (2013 - 2048): 102.4 €/ton of wet sludge

without the investment 81.7 €/ton of wet sludge with the investment 87.4 €/ton wet sludge

Mean disposal cost (2018 - 2048): without investment 58.2 €/ton of wet sludge with investment 68.0 €/ton of wet sludge

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3

7 1

4 5

2 6

Figure 30; ATO-Como main plants

1: Bulgarograsso – AltuLura; 2: Carimate – Sud Seveso Servizi; 3: Como – Comodepur; 4: Fino Mornasco – Alto Seveso – Lariana Depur; 5: Fino Mornasco - Livescia – Lariana Depur; 6: Mariano Comense – Valbe Servizi; 7: Merone - ASIL 109

Como – Sorico 60 km

Como – Mariano C. 60 km

Figure 31: ATO-Como plants and central point

If the scenario is addressed to all the plants, the total disposed sludge quantity must be increased by 10%: this does not cause particular interventions in the thermal process plant, but the transport cost for each small plant is the important variable; so for assessing the scenarios marked with ALL, this should be taken into account. If electrodewatering is added, the assessment of the possibility of adding the machine in the plant is the main issue; in fact the new method based on electro-osmosis can work only on some sludge and has its capital and management costs (mainly energy consumption) which have their consequences on the economical balance. For what concerns the localization of the new thermal conversion plant, Como will be the choice which optimizes the costs, because it is central with respect to the WWTPs of the North and the South of the district; however, the construction of such a plant in that zone will have to face strong public opposition: in fact in Como area a big wastewater treatment plant and a municipal solid waste incinerator (Albate zone) are already present. So the positioning of the new plant should be in another zone, for example near Bulgarograsso, Fino Mornasco – Alto Seveso or Merone plants, which, after Como, are the biggest plants of the district. The solutions can also be thought to be modular, expandable (mainly for pyrolysis), meaning that the plant can be enlarged depending on the conferment choice, and on site, so very near to the one of the WWTPs, so that it can cover the energy consumption of it through energy produced from the thermal conversion of the waste and minimizing the costs of transport. 110

For the application of electrodewatering the following considerations can be done: 





if every big plants decides to adopt electrodewatering, its total cost of disposal in agriculture decreases and even an investment related to thermal processing of sludge results less profitable; in fact the latter counts on the fact that the delta between annual disposal costs in agriculture and management costs for the thermal treatment plant will increase year by year; with the decrease of quantities of sludge due to electrodewatering, the delta is not so high; in the incineration scenario, electrodewatering allows an high reduction of the costs compared to incineration without it, and the annual savings can arrive till 43%, due to no need of energy for drying; the graphs show the costs in the two cases and the range of savings; after 2030, the potential of the plant is fully exploited and so the total savings for the disposal start to decrease because there is no additional space to incinerate other sludge which must be sent to agriculture. for pyrolysis the situation with electrodewatering can be even more positive, allowing to have years where the gains (in terms of GCs and sold electric energy) overcome the expenses for the disposal in agriculture of the sludge which is not thermally treated, reaching a maximum of 1.4 million euros spared; after this maximum in 2032, Green Certificates are no more recognized because their release lasts for 15 years and so the savings have a steep decrease and remain quite stable.

Milion

Graph 15: costs for incineration scenario with ED and the one without ED 1.80 1.60 1.40 1.20 1.00 Savings [€] 0.80

costs without ED costs with ED

0.60 0.40 0.20 2048

2046

2044

2042

2040

2038

2036

2034

2032

2030

2028

2026

2024

2022

2020

2018

0.00

Years

111

45%

50000

40%

45000

35%

40000 35000

Savings

30%

30000

25%

25000 20%

20000

15%

15000

10%

Sludge quantities [wet tonnes]

Graph 16: savings for incineration with ED with respect to the one without ED

10000

5%

5000 0 2048

2046

2044

2042

2040

2038

2036

landspread sludge 2034

2032

2030

2028

2024

2022

2020

2018

0%

2026

incinerated sludge

Years

Milion

Graph 17: savings for pyrolysis with ED with respect to the one without ED 1.60 1.40 1.20 1.00 Savings 0.80 [€] 0.60 0.40 0.20 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030 2031 2032 2033 2034 2035 2036 2037 2038 2039 2040 2041 2042 2043 2044 2045 2046 2047 2048

-

Years

5.5 SWOT analysis To provide a general final comparative assessment of the three scenarios (agricultural reuse, incineration, pyrolysis) a SWOT analysis is performed. A SWOT analysis is a structured planning method used to evaluate the strengths, weaknesses, opportunities and threats involved in a project. Identification of SWOTs is important because they can inform later steps in planning to achieve the objective:    

Strengths: characteristics of project that give it an advantage over others. Weaknesses: characteristics that place the project at a disadvantage relative to others Opportunities: elements that the project could exploit to its advantage Threats: elements in the environment that could cause trouble for the project

Strengths and weakness are internal to the project, opportunities and threads are external. 112

A qualitative evaluation of the treatment methods was followed also on the basis of the following selected criteria incorporating financial, social and environmental aspects (UNEP, 2009; Samolada, Zabaniotou, 2014):    

Does the method resolve the problem? Does the method decrease GHG emissions? Is the technology mature? Does the legislation support the method?

Table 56: SWOT analysis for agricultural reuse

AGRICULTURAL REUSE Strengths Improvement of the agronomic characteristics of the soil Consolidated activity, guaranteed by the regional system Actual cheapest disposal method Opportunities New positive perception of the farmers

Weaknesses Metal and micropollutants leaching into the soil Sludges need to be stabilized Processing plants located far from WWTPs GHGs emissions due to trucks transporting sludge Threads Stricter limits for admissible sludges Increase in disposal costs

Table 57: SWOT analysis for incineration

INCINERATION Strengths High reduction of sludge volumes Possible utilization of the obtained ashes Existing BATs for emissions control systems Well-developed legislative frame Opportunities Less energy needs if coupled with electrodewatering

Weaknesses Partial solution to the problem Air pollution (NOx , SO2 , CO2 , chlorinated compounds) Drying is required Heavy metals in slags and fly ashes Low energy efficiency and energy balance Threads Public opposition Ash disposal

Table 58: SWOT analysis for pyrolysis

PYROLYSIS Strengths High reduction of sludge volumes Possible utilization of the produced compounds (char/tar/bio oil) High efficiency and energy self sufficiency Existing BATS for emissions control systems Compact and modular plants Opportunities Extensive expertise Turn the waste into a valuable raw material

Weaknesses Partial solution to the problem Air pollution Drying is required Heavy metals in slags and fly ashes Threads Viability is proven only in large scale plants, not for sewage sludge

113

114

6. Conclusion From the previously studied scenarios for the whole district, it can be stated that: 

pyrolysis is more profitable than incineration, in terms of IRR;



electrokinetic dewatering is a way to increase the profitability of the disposal, because decreases sludge volumes which must be transported and drying needs, which are highly impacting on the economical balance;



the disposal costs, averaged on the total time horizon of the scenario analysis, are: o o o



102.4 €/ton of wet sludge for the agricultural reuse – business as usual scenario; 69.3 €/ton of wet sludge for the incineration with conventional dewatering for the main plants scenario; 87.4 €/ton of wet sludge for the pyrolysis with conventional dewatering for the main plants scenario;

the CO2 emissions due to trucks transport, averaged on the total time horizon of the scenario analysis, are: o o o

339.20 tonnes of CO2/year for the agricultural reuse – business as usual scenario; 64.55 tonnes of CO2/year for the incineration with conventional dewatering for the main plants scenario; 190.30 tonnes of CO2/year for the pyrolysis with conventional dewatering for the main plants scenario;



the best alternative seems to be fluidized bed incineration, having minimum disposal costs and CO2 emissions; this is due to the fact that an incineration plant can treat more sludge with respect to a single pyrolysis plant; the best localization and the characteristics of the sewage sludge, which is the feed fuel, should be the next subjects for feasibility studies adopting this technology;



the best alternative should be compared to the choice of building more than one pyrolysis plant, because the latter can be adapted to the wastewater treatment plants and cover the energy needs of them, improving their economical balance and partially solving the sludge disposal problem, with an high profitability of the investment;



the thermal conversion plant should be localized near one of the biggest plants in the southern part of the district, excluding Como for limited land availability and for the already existing waste-to-energy plant. Any other option should be attentively evaluated, paying special attention to public acceptance, environmental impact assessment issues, human health risk assessment, energy and carbon foot prints.

Evaluations reported in the previous chapters for incineration and pyrolysis are merely a preliminary hypothesis. To compile a pre-feasibility study, a complete characterization of the fluxes of sludge and of sludge composition should be done, to develop a precise quantitative and qualitative analysis and to outline all the possible treatment technology configurations. The recently developed database SYST&MS can help in providing updated information that can be made available to all stakeholders for a participated decisional procedure. Through the collaboration between research institutions, regulatory agencies, public administrations and water service companies, the database can be the first step to build a decision support tool to simulate different scenarios for the district and find the best solution.

115

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APPENDIX 1 The Province of Como is bordered to the north by Switzerland, with which it shares important road infrastructures, which constitute an essential means of communication between central Europe and the industrial areas of northern Italy; it is also connected by highways with the two Milan airports (Linate and Malpensa), from which it is respectively 40 and 20 km far. The local economic apparatus has a consistency of about 46,000 enterprises, distributed to 38% in the secondary sector, 57% in the tertiary sector and only 5% in agriculture: a company every 12 inhabitants. Como is therefore an industrial province, but with businesses in the service sector that are evolving rapidly. Among the historically productive activities the manufacturing industry stands, with “pillars” in the three sectors of textiles, industrial machines and furniture, which themselves take up around 76% of the employees in the sector (textiles 32%, machines 29% and furniture 15%). There are, however, other productions ranging from food to chemical industry, from footwear to paper, from rubber to plastic materials and they are characterized by the high presence of small and medium businesses and for the large number of artisans. An important place is occupied by the sector of transport and related services, to signify the strategic position that the Province of Como has for trade between northern and southern Europe. In the service sector there are about 26,000 companies among which trade activities and those of tourism. For what concerns the wastewater treatment facilities, there are 43 active installations among the 58 present for the purification of urban waste water. Much of the north of the province is not yet covered by integrated water treatment services. Most of the industrial waste is purified at the public facilities. This means that 50% of the daily load is of industrial type, with peaks up to 90% load. The 7 main plants are presented from the next page.

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Bulgarograsso Wastewater Treatment Plant – Alto Lura srl

GENERAL DATA      

Located in: Bulgarograsso Year of construction: 1979 People Equivalent: 154000 Type: activated sludge treatment Final receptor: Lura torrent Coordinates (WGS84): 45°44'33.23"N, 9° 0'57.22"E

DESCRIPTION OF THETREATMENT LINE Water treatment line          

Coarse screening; Initial lift; Fine screening; Aerated grit removal; Oil removal; Denitrification; Oxidation and nitrification (2 lines in parallel); Final sedimentation (2 lines in parallel); Sand filtration; Ozonation.

Sludge treatment line     

Sludge recirculation; Thickening; Aerobic digestion; Post-thickening; Mechanical dewatering by centrifuge.

MAIN PLANNED INTERVENTIONS (PIANO D’AMBITO 2014)   

Construction of four new overflow channels: 300,000 € Plant power up: 12,780,000 € Tertiary treatment adjustment: 203,625 €.

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Carimate Wastewater Treatment Plant – Sud Seveso Servizi spa

GENERAL DATA      

Located in: Carimate Year of construction: 1987 People Equivalent: 96000 Type: activated sludge treatment Final receptor: Seveso torrent Coordinates (WGS84): 45°41'23.43"N, 9° 7'21.60"E

DESCRIPTION OF THETREATMENT LINE Water treatment line            

General by-pass; Coarse screening; Initial lift; Fine screening; Grit removal; Oil removal; Primary sedimentation; Denitrification Oxidation and nitrification; Final sedimentation; Sand filtration; Disinfection.

Sludge treatment line     

Sludge recirculation; Thickening; Anaerobic digestion; Post-thickening; Mechanical dewatering by belt press.

MAIN PLANNED INTERVENTIONS (PIANO D’AMBITO 2014)  Storage tank / phytodepuration: 200,000 €  Disk filtration: 300,000 €  UV disinfection: 200,000 €  Surfactants and color removal: 5,500,000 €  Substitution of belt press with centrifuge: 200,000 € 125  Sludge incineration plant – 7200 tonnes/year - (with Mariano C.): 4,600,000 €.

Como Wastewater Treatment Plant – Comodepur spa

GENERAL DATA

     

Located in: Como Year of construction: 1970 People Equivalent: 297000 Type: activated sludge treatment Final receptor: Cosia torrent Coordinates (WGS84): 45°48'21.54"N, 9° 4'38.61"E

DESCRIPTION OF THETREATMENT LINE Water treatment line

    

      

Fine screening; Initial lift; Grit removal; Coagulation and flocculation; Division of the flow on three lines: o complete water line; o storage tank releasing wastewater into pre-denitrification tank; o biofiltration for carbon removal, nitrification, denitrification, releasing into sand filters; Pre-denitrification; Oxidation and nitrification; Post-denitrification; Final sedimentation; Tertiary coagulation-flocculation; Sand filtration; Disinfection.

Sludge treatment line

  

Sludge recirculation; Thickening; Mechanical dewatering by centrifuge or belt press in case of emergency.

MAIN PLANNED INTERVENTIONS (PIANO D’AMBITO 2014)  

Maintenance: 2,573,724 € Tertiary treatment adjustment: 203,625 €.

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Fino Mornasco Wastewater Treatment Plant –

Alto Seveso – Lariana Depur spa

GENERAL DATA

     

Located in: Fino Mornasco Year of construction: 1978 People Equivalent: 140000 Type: activated sludge treatment Final receptor: Seveso torrent Coordinates (WGS84): 45°44'58.64"N, 9° 3'42.67"E

DESCRIPTION OF THETREATMENT LINE Water treatment line         

Coarse screening; Initial lift; Fine screening; Grit removal and oil removal; Denitrification; Oxidation and nitrification; Final sedimentation; pH correction, flocculation, sedimentation; Disinfection.

Sludge treatment line   

Sludge recirculation; Thickening; Mechanical dewatering by centrifuge (with the use of polyelectrolytes).

MAIN PLANNED INTERVENTIONS (PIANO D’AMBITO 2014)   

Biological treatment aeration system adjustment – substitution of flow jet system with microbubbles system: 1,300,000 € Tertiary treatment adjustment: 203,625 € Biological treatment adjustment with aerobic digester for future MBR application: 18,083,000 € 127

Fino Mornasco Wastewater Treatment Plant – Livescia – Lariana Depur spa

GENERAL DATA

     

Located in: Fino Mornasco Year of construction: 1980 People Equivalent: 43300 Type: activated sludge treatment Final receptor: Livescia torrent Coordinates (WGS84): 45°44'22.79"N, 9° 2'20.17"E

DESCRIPTION OF THETREATMENT LINE Water treatment line

       

Coarse screening; Fine screening; Homogenization; Initial lift; Denitrification; Oxidation and nitrification; Final sedimentation; Disinfection.

Sludge treatment line

 

Sludge recirculation; Mobile dewatering.

MAIN PLANNED INTERVENTIONS (PIANO D’AMBITO 2014)   

Construction of storage tank: 1,600,000 € Plant adjustment: 2,300,200 € Tertiary treatment adjustment: 203,625 €. 128

Mariano Comense Wastewater Treatment Plant – Valbe Servizi spa

GENERAL DATA

     

Located in: Mariano Comense Year of construction: 1973 People Equivalent: 60000 Type: activated sludge treatment Final receptor: Terrò torrent Coordinates (WGS84): 45°41'12.20"N, 9°10'0.25"E

DESCRIPTION OF THETREATMENT LINE Water treatment line

          

Coarse screening; Initial lift; Fine screening; Aerated grit removal; Oil removal; Coagulation-flocculation (not used); Primary sedimentation (2 lines in parallel); Denitrification; Oxidation and nitrification; Final sedimentation (2 lines in parallel); Disinfection segment currently not in use.

Sludge treatment line

  

 

Sludge recirculation; Thickening; Anaerobic digestion with cold operation (heat exchanger, gasometer and torch are not used); Post-thickening; Mechanical dewatering by centrifuge (with the use of polyelectrolytes) and belt press in case of emergency.

MAIN PLANNED INTERVENTIONS (PIANO D’AMBITO 2014)  Maintenance and initial lift station power up: 500,000 €  Construction of upstream concrete storage tank: 10,000,000 €  Odor emissions treatment: 650,000 €  Anaerobic digester functionality restoration: 1,100,000 €  Sludge incineration plant – 7200 tonnes/year - (with Carimate): 4,600,000 €. 129 

Merone Wastewater Treatment Plant – ASIL spa

GENERAL DATA      

Located in: Merone Year of construction: 1985 People Equivalent: 125000 Type: activated sludge treatment Final receptor: Lambro torrent Coordinates (WGS84): 45°46'3.53"N, 9°14'36.10"E

DESCRIPTION OF THETREATMENT LINE Water treatment line            

Coarse screening; Initial lift; Fine screening; Grit removal; Oil removal; Coagulation-flocculation (not used); Primary sedimentation; Denitrification; Oxidation and nitrification; Final sedimentation; Disinfection with sodium hypochlorite (not used); Treatment of odor abatement (not used).

Sludge treatment line     



Sludge recirculation; Pre-thickening; Anaerobic digestion; Post-thickening; Mechanical dewatering by centrifuge and belt press in case of emergency; Thermal drying.

MAIN PLANNED INTERVENTIONS (PIANO D’AMBITO 2014)  Maintenance: 2,025,000 €  Installation of new centrifuge: 220,000 €  Tertiary treatment adjustment: 203,625 €.

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APPENDIX 2 The operational phases of the process of treatment and recovery in agriculture 1. Conferment Waste suitable for the withdrawal shall be conferred in shovelable state by authorized transporters through suitable trucks, equipped with coverage: in particular, regarding to odorous waste, loads can be uncovered only after their entry into the system. If the operators detect a failure to comply with this rule by the bestower, they must report the matter to the Technical Director, who will send a formal reporting. The logistics and the methods of management of the center and the average characteristics of the sludge conferred allow defining a sequence of steps that can be summarized as follows:  



Check the weekly schedule for delivery, the form for identification, the sludge conferred proving the suitability of the sludge to the treatment provided in the facility with the taking of a sample of sludge as it is for each load; In the case the sludge conferred satisfies entirety the above requirements, the process will proceed to the storage and processing in anticipation of the most appropriate period to proceed for spreading; in case of sludge already stabilized and sanitized by the manufacturer, the retention after treatment will be performed in accordance with Legislative Decree no. 99/92, Art.12, paragraph 4. In case the sludge does not meet the specifications provided in the preliminary approval, the same is returned to the sender indicating the fact to the Authorities within 24 hours. 2. Storage

According to Legislative Decree 152/2006, R13 operations are performed at this stage. The typical operations of the conferment are:    

  

  

Vehicle arrival at the center; Load testing with the schedule of contributions; Administrative control of the accompanying documentation: identification form for the waste; Taking samples for verification of heavy metals and/or parameters at risk: o at the discretion of the recovery plant, checks are carried out on a sample with the same procedure for approval; o on loads coming from defined and consistent production processes or systems, samples are taken every three months according to the equivalent population, with sampling directly on the conferred sludge; Weighing the load; Unloading in the specific storage section; Olfactory and visual inspection of the product: visual verification means that the sludge coming from the same treatment plant has the same color and the same compactness; olfactory verification means that the load should not have odors not conform to the type of product (a biological sludge must have a faint smell of fermenting organic matter, any kind of "chemical" smell is suspect); Weighing the empty vehicle and registration of the quantity discharged; Certification of the conferment and formal acceptance of the cargo; Empty vehicle can leave the plant.

Outgoing vehicles must be subject to washing at the appropriate internal section, in order to remove any dirt and deposits that were formed on the wheels. On site there is the constant presence of skilled personnel that oversees the smooth running of the authorized process; the same staff monitors and intervenes in all stages of the treatment where operations require manual work. There is also the con131

stant presence of technicians, who define the specific mode of treatment, oversee and control that the process is executed and pursued with the best technical efficiency. The sludge, within 72 hours from its arrival, is sent to the conditioning process in the following ways. 3. Mixing The purpose of this phase is to make the special waste qualitatively constant in time to have a biomass easily manageable during the stages of fertilization of soils. The sludge which is not stabilized and/or not sanitized is sent, by means of the wheel loader, in the loading hopper and from here, by auger screw, to the mixer. In a mixer intimate homogenization of the sludge occurs in order to obtain a physically and chemically uniform sludge. The sludge already stabilized and sanitized by the manufacturer is loaded in the adjacent storage area, going to form a single output material together with the sludge coming out from the mixer (conditioning as defined by Legislative Decree no. 99/92 Art. 12.4). Periodically, verifications are done on the sludge stored in the output from the mixer, to comply with the legislative requirements (daily for pH and fortnightly for heavy metals and microbiology). 4. Stabilization and sanitation treatments According to Legislative Decree 152/2006, R12 (former R3) operations are performed at this stage. These treatments can be: 

 

biological: prolonged aerobic stabilization, mesophilic anaerobic digestion at minimum 35°C with a retention time of minimum 30 days, thermophilic anaerobic digestion at minimum 55°C with a retention time of minimum 20 days, thermophilic aerobic stabilization at minimum 55°C with a retention time of minimum 20 days; chemical: calcium oxide, ammonia; physical: thermal drying.

As an example the case of chemical treatment is shown, because it is widely applied in the plants where the sludge is conferred by the WWTPs taken into consideration The waste to be treated is inserted into the loading hopper and sent to the machine intended to mix it with lime oxide. The lime oxide is contained in a silo and sent to the mixer by means of a metering auger. The sludge to be treated and the lime oxide are intimately mixed and subsequently discharged by an auger in the temporary storage area where the sludge is left for the time necessary to the completion of the chemical reaction (1 - 2 hours max). Verification on the sludge is carried out daily, by determination of pH and if the analytic response is positive, this is placed in the storage area. In case of negative analytic response, the sludge is subject to treatment. From d.g.r. 1/7/2014 – no X/2031, for a stabilized sludge: "It should be verified that the ratio of the SSV/SST sludge to be used must be