Turning Biosolids into Green Energy: Comparing ...

WEFTEC 2012

Turning Biosolids into Green Energy: Comparing Anaerobic Digestion with Combined Heat and Power to Close-coupled Gasification Marija Peric(1); Mohammad Abu-Orf(1); Beverley Stinson(1); Grant Davies(1); Terry Goss(1); Julian Chianelli(1); Robert Taylor(2); Sam Amad(2); Dale Belschner(2); Frank Hartz(2); Kevin Selock(2) ; Bob Buglass(2); Gary Grey(2) (1) *

AECOM Water, (2) Washington Suburban Sanitary Commission To whom correspondence should be addressed. Email: [email protected]

ABSTRACT The Washington Suburban Sanitary Commission performed a study comparing different biosolids management options for its Wastewater Treatment Plants with a focus on energy recovery from biosolids through the methods of drying followed by either gasification or anaerobic digestion with combined heat and power generation. The three phased approach consisting of identifying and screening technologies, performing detailed economic and noneconomic analysis of short listed options, and developing a concept design report for the selected option. Several drying and gasification alternatives were screened but only closecoupled systems were short listed for further analysis. Multiple anaerobic digestion configurations and pretreatment technologies were also screened leading to a short list of conventional mesophilic anaerobic digestion, acid-gas phase digestion and 2PAD. The thermal hydrolysis process was determined to be the most cost effective pretreatment technology to maximize digester gas production. The concept design report was based on a regional facility serving four of the five WWTPs and expandable in the future to accommodate the fifth WWTP. KEYWORDS: Biosolids, Anaerobic Digestion, Gasification, Thermal Hydrolysis, Recalcitrant Dissolved Organic Nitrogen (rDON), Solids Processing, Class A, Beneficial Reuse, Energy Recovery INTRODUCTION The Washington Suburban Sanitary Commission (WSSC) has updated its Biosolids Master Plan to assess the optimum utilization of biosolids from its five wastewater treatment plants. Currently, approximately 75% of WSSC’s biosolids are land applied and 25% are incinerated. WSSC has determined that by implementing anaerobic digestion/combined heat and power or drying/gasification, substantial environmental and economic benefits can be achieved ranging from reductions in greenhouse gas emissions, operating costs, and biosolids disposal volume. Other benefits that could be achieved by implementing one of these technologies include reducing potential for nutrient release from land applied biosolids and providing for the ability to better management of fats, oils and grease (FOG) by removing them from the sewer system thus reducing the potential for sanitary sewer overflows. Instead the FOG can be directly processed with the biosolids converting what was previously considered a nuisance into a high energy value feedstock.

Copyright ©2012 Water Environment Federation. All Rights Reserved. 4606

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The primary focus of this study was to investigate the potential to convert wastewater biomass to electricity from one or more of five WWTPs, using innovative technologies to maximize energy recovery. The technologies evaluated would enhance the environment by reducing nutrient load to waterways from land applied biosolids and reduce sanitary sewer overflows by removing grease from the sewer system. The green energy production technologies also reduce greenhouse gas emissions. Energy recovery from residuals and biosolids is one option to offset energy usage and reduce carbon footprint in public and private utilities treating domestic wastewater. Only 10% of WWTPs > 5 mgd nationwide currently recover energy. This untapped energy resource has the potential to produce 575 MW of energy nationwide, with an additional 175 MW of energy from waste Fats, Oils & Grease (FOG) nationwide. One established method of recovering energy from biosolids is to recover biogas from anaerobic digestion and produce energy using combined heat and power generation (CHP) technologies. Another method of recovering energy from biosolids is to use dried biosolids as fuel source for energy generation through the process of gasification and energy recovery. Biosolids gasification produces a synthetic gas also known as syngas with approximately one third of the energy content as compared to biogas, however, the byproduct of gasification is an inert ash with 90% less mass for disposition than a dewatered cake produced after an anaerobic digestion process. Typically, the syngas is used to dry the biosolids providing an energy neutral drying system; however, excess energy may be available depending on plant conditions. Biosolids energy recovery systems following gasification are newer technologies for the application of biosolids than is anaerobic digestion; however the demand to reduce energy costs and pressure on land application programs are paving the way for adapting dried biosolids gasification and energy recovery systems as a disposition method. The USEPA is encouraging green energy production and recovery, specifically through the use of anaerobic digestion to create digester gas that can be used in a CHP system. Using this, WSSC has obtained funding to assess the potential for installing green energy technologies at one or more of five WWTPs. Comparing energy recovery via anaerobic digestion and combined heat and power (CHP) to drying followed by gasification and energy recovery started in August 2010 for two Washington Suburban Sanitary Commission (WSSC) WWTPs. The goal was to determine the best solution(s) for beneficial use of the plant’s biosolids with a focus on tapping into the energy potential of biosolids. The study was structured to consist of a three phase approach to indentify and screen potential technologies, evaluate short listed options using both economic and non economic metrics and finally developing conceptual design reports of the selected alternative for each plant. In addition to studying individual plant approaches, regional and centralized solutions combining the sludges from more than one plant were also evaluated. This paper presents the energy recovery potential from the screened alternatives and the amount of electricity that can be produced, where applicable. The paper also presents the 20 year life cycle cost (LCC) analysis for each alternative. Non-economic analysis including carbon footprint, odor potential, noise potential, process integration, and side stream recycle impacts to enhanced nutrient removal treatment processing is also presented for each alternative.


WEFTEC 2012

BACKGROUND ON EXISTING FACILITIES WSSC operates and maintains seven wastewater treatment plants. For this study, biosolids management for the five larger plants was evaluated. A description of the five WSSC WWTPs is provided in Table 1.The combined design average flow is 4.16 m3/s (95 million gallons per day (mgd)). The design sludge loading (in dry tons per day (dtpd)) for two individual plants, one centralized and two regional biosolids facilities are provided Table 2. Table 1. Description of WSSC WWTPs WWTP Plant A 1.14 m3/s (26 mgd)

Primary NO

Secondary BNR/ENR* MLE**

Thickening Gravity Belt Thickener (GBT)

Dewatering Centrifuge

Disposal Land Application Class B Lime Stab

Plant B 1.31 m3/s (30 mgd) Plant C 0.0657 m3/s (1.5 mgd) Plant D 0.328 m3/s (7.5 mgd) Plant E 1.31 m3/s (30 mgd)

YES NO

BNR/ENR Step Feed BNR/ENR

YES

BNR/ENR

Belt Filter Press (BFP) Belt Filter Press (BFP) Centrifuge

NO

BNR/ENR 3-stage

Gravity Thickener (pre-lime) Settling at Holding Tank*** Gravity Thickener/ GBT Dissolved Air Flotation

Land Application Class B Lime Stab Land Application Class B Lime Stab Land Application Class B Lime Stab Incineration / Land Fill

Centrifuge

* BNR/ENR = Biological Nutrient Removal/Enhanced Nutrient Removal ** MLE = Modified Ludzack-Ettinger ***WAS is sent to aerated holding tank where the air is occasional turned off, the tank is allowed to settle, supernatant is decanted and solids are pumped to BFP for dewatering

Table 2. Solids production and projections for the WSSC WWTP’s

TS (dtpd) Dewatered Solids% TS (dtpd) Dewatered Solids% TS (dtpd) Dewatered Solids% TS (dtpd) Dewatered Solids% TS (dtpd) Dewatered Solids% TS (dtpd) Dewatered Solids%

Raw Dewatered Solids (UN-LIMED) Current Design Annual Average Maximum Month Annual Average Maximum Month WSSC Centralized (5 plants) 55.6 72.7 74.4 97.4 22.0% 21.9% 22.2% 22.1% WSSC Regional (2 plants) 14.6 19.1 17.8 23.3 19.0% 19.0% 19.0% 19.0% WSSC Regional (3 plants) 41.0 53.6 56.6 74.1 23.3% 23.1% 23.5% 23.3% WSSC Regional (4 plants) 34.6 48.0 42.1 59.2 20.5% 20.6% 20.5% 20.6% Plant A 13.9 18.0 17.0 22.0 19.0% 19.0% 19.0% 19.0% Plant B 15.4 22.8 18.1 26.8 22.0% 22.0% 22.0% 22.0%


WEFTEC 2012

PROJECT STUDY APPROACH The comparison of anaerobic digestion and CHP to drying/gasification and energy recovery was conducted through a three phase approach. The first phase consisted of identifying and screening potential technologies for digestion pretreatment, digestion, CHP, and technologies for drying, gasification and energy recovery. The second phase consisted of a detailed evaluation of the short listed alternatives using 20 years LCC analysis and non-economic metrics. The third and final phase consisted of generating a concept design report of the selected alternative. During the first phase, the design team established design parameters, tested the biosolids characteristics and indentified and screened potential technologies available for consideration. First the plant flows and expected growth were determined to establish design loads under current, 20-year, and design build-out conditions. Digestibility and heat value characteristics of biosolids samples were determined with bench scale and laboratory analyses. The final part of phase one focused on screening available technologies suitable for each WWTP using both energy recovery methods. The second phase of economic and non-economic evaluation was conducted to compare factors such as; 20 year lifecycle costs, carbon footprint, odor and noise potential, ability to integrate into the facility, and side stream treatment impacts. The project team formulated cost opinions for capital and operations and maintenance (O&M) to estimate the 20 year LCC for each considered alternative, which along with the non-economic factors formulates the basis for identifying the best alternative for biosolids processing and energy recovery at each treatment plant. In addition to individual plant options, combined solutions were also evaluated to compare the economy of scale impact for a regional and/or centralized biosolids facility. TECHNOLOGY EVALUATION AND SCREENING - GASIFICATION Background on Gasification Several thermal processing technologies are available to harness the energy from biosolids and offer significant mass reduction. The thermal processing alternatives to be discussed all include applying heat to the biosolids, but with varying degrees of oxygen. The three basic thermal processes are: • Gasification, limited supply of oxygen, followed by energy recovery. • Pyrolysis, in the absence of free oxygen, followed by energy recovery. • Incineration or advanced thermal oxidation, excess supply of oxygen, followed by energy recovery. Table 3 shows the operating difference between the three thermal processing technologies and the main byproducts from each. Table 3. Characteristics of Different Thermal Processing Technologies Parameter Temperature (°F) O2 Supplied By-Products

Combustion 1,650-2,000 > Stoichiometric (Excess Air) Flue Gas (CO2, H2O) and Ash

Gasification 1,100-1,800 < Stoichiometric (Limited Air) Syngas (CO, H2) and Ash

Pyrolysis 390-1,100 None Pyrolysis Gas, Oils, Tars and Char


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Since gasification is the main thermal process discussed in this paper and is a new concept for biosolids management, the basic principles of residuals gasification and requirements for practicing gasification are discussed herein. The different energy recovery methods are also described in detail. Gasification is an established process for converting organic materials to a fuel gas called synthetic gas or syngas, and has been practiced since the 1800s to generate fuel gas from coal and other biomass. Syngas is composed mainly of CO, CO2, H2 and CH4 and has a low heating value of 120-150 British Thermal Units (BTU) per cubic feet (ft3) which is approximately 25% of the heat value of biogas generated from anaerobic digestion. Although gasification is common in many industries, Environmental Protection Agency (EPA) is still considering gasification of biosolids as innovative (EPA, 2006). One of the larger differences between traditional organic materials used as the fuel source in gasification and biosolids is the higher ash content of biosolids. To effectively harness energy through gasification and energy recovery, most commercially available systems require dried biosolids with greater than 75% solids in granular form. Pelletization is not required for gasification, however, a certain degree of uniformity in the dried granular material along with low dust content is required. The required dryness depends on the technology. The energy required for drying is typically supplied by the gasification process. Energy can also be recovered from waste heat, such as a CHP system, if available onsite. Currently there are 17 installations of dryers with energy recovery worldwide with two located in North America. There are two forms of recovering energy from syngas: two stage gasification and close-coupled gasification (RBC, 2011). In two stage gasification systems, syngas is cleaned to remove impurities, mainly tar, before feeding internal combustion engines to produce electricity and recoverable heat In close-coupled gasification, syngas is thermally oxidized to produce hot flue gas that can be cooled for energy recovery to offset fossil fuel for drying and power generation. Two-Stage Gasification In two stage gasification systems, as depicted in Figure 1, the syngas produced from gasifying the dried biosolids is cleaned and the cleaned syngas can be used as a fuel source for an internal combustion engine. Cleaning the syngas is required to remove sulfur, siloxanes, and other contaminants such as tar that could damage the engine. The syngas cleaning process is not fully developed for the application of biosolids and currently considered in the innovative phase. Syngas cleaning, however, is commercially practiced in the coal industry but on a much larger scale. There is only one known facility worldwide located at Balingan Germany’s WWTP currently operating two stage gasification using biosolids; however, a second facility is currently being commissioned in Mannheim, Germany. The gasification and syngas cleaning processes at both facilities were supplied by Kopf located in Germany. The syngas conditioning technology utilized at both WWTPs is still considered innovative as defined by the EPA.


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Figure 1.Two Stage Gasificatio on System fo or Energy R Recovery Nexterra in British Columbia, C Caanada is also o currently ddeveloping a two stage gaasification system to o be used forr the applicaation of bioso olids. The pprocess is beiing developeed in conjunnction with GE Jenbacher and a a pilot deemonstration n is currentlyy in operatioon at Nexterrra’s testing facility in n Kamloops,, British Collumbia using g wood wastte as the fuell source. Nexxterra has successfu ully tested gaasification of o biosolids on o a pilot scaale but does not have any full scale installatio ons operatin ng on biosolids. All of th heir current ffull scale faccilities are opperating on wood waaste in a close coupled co onfiguration. Although h this techno ology has pottential to be a sustainablle solution foor future plaants, it was eliminateed from conssideration in n this study since there arre no full scaale installatioons in Northh America.. Close-Co oupled Gasiification Close-coupled gasificcation, show wn in Figuree 2, does not require synggas cleaningg and insteadd the syngas iss thermally oxidized. o Syn ngas oxidation generatess high tempeerature, apprroximately 1,800°F, flue gas wh hich can be used u for therm mal heat recoovery. The eenergy recovvered from thhe flue gas can c be used as the energy source to dry d to the bi osolids to thhe desired drryness and thhus minimizee or eliminatte the need for fo fossil fuells (e.g., natuural gas or fuuel oil). Thee hot flue gass can also be used u as an en nergy source for generatiing electricitty through thhe use of steaam turbines or an Organic Rankin R Cyclle engine. Electricity E geeneration witth close-couupled gasificaation is commonly practiced on other typ pes of biomaass, howeverr, this system m is not comm mon for biosolidss since it’s geenerally morre economical to use thee energy to ooffset the dryying energy requirem ment. The clo ose-coupled mode of eneergy recoverry is consideered commerrcially develloped and is cu urrently practticed in biosolids facilitiies in Buffaloo, MN (Krugger/Veolia) and in Sanfoord, FL (Max xWest). Energy and a mass balances for gaasification sy ystems show w that there could be suffficient energyy in undigesteed dewatered d sludge (depending on the t heat conntent and the percent soliids of the dewatereed cake) to dry d the materrial without the t need for any auxiliarry fuel. The gasification process is considered d a good candidate for haarnessing thee energy from m the dried biosolids annd g this energy y to dry the biosolids, b pro oducing enerrgy efficientt or an energgy neutral dryying recycling processess. For anaero obically digeested biosoliids, some off the volatile material is cconsumed annd converted d to biogas which w reducees the calorific value of the biosolidds. Since the calorific vallue of digested biosolids is lower, a hig gher dewatered solid conntent is requiired to achievve an energyy neutral drying d and gaasification prrocesses. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4611

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Figure 2.Close-Coup 2 pled Gasificcation Systeem for Enerrgy Recoverry The amount of fuel or o biosolids to t be processsed, and the amount of eexcess energgy in the hot flue gas generrated, will dictate which h form of CH HP technologgy can be praactically useed to generatte power an nd in some cases, low graade waste heeat. Anotherr factor whicch determinees the approppriate type of CHP C is the size of the faccility and thee goals for ennergy recoveery. Small too medium facilities,, with a goall of producin ng electricity y may use a ttechnology ssuch as the O Organic Rannkine Cycle (O ORC) with ~1 10-20% elecctricity efficiiency to recoover energy ffrom excess flue gas generated d. Larger faccilities with similar goalss may selectt high pressuure steam turrbines with approxim mately 15-38 8% electricall efficiency. There are prractical econnomic and noon-economicc criteria which w help deetermine thee appropriatee size and typpe of system m. Shortlistted Gasifica ation Techno ologies For the drying d follow wed by gasifiication optio ons, only cloose-coupled ggasification technology w was shortlisteed since this is the only method m curreently practicced commerccially in Norrth America. The closee coupled sy ystems offereed by MaxW West Environnmental Systtems Inc. andd I. Kruger IInc. were the screened pro ocesses for the t economic and non-ecconomic evaaluation of thhe second phhase udy. Both veendors have one full scale installatioon in the Uniited States; B Buffalo, MN N of the stu (Kruger) and Sanford d, FL (MaxW West). The two-stage t gaasification teechnologies ssuch as oness offered by b Nexterra and a Kopf weere eliminateed from conssideration sinnce they didd not have fuull scale opeerating installlations on Biosolids B in North N Ameriica. T The Krug ger BioConTM ERS systeem is a pack kage system tthat consistss of a BioConn Belt Dryerr, a reciprocaating grate fu urnace, heat recovery eq quipment andd emission control equippment. Enerrgy for the beelt drying prrocess is supp plied via thee biosolids fuurnace. Thee hot flue gass from the furnace is cooled in a heat exchan nger that traansfers heat tto the belt drrying air circculating throough the dryerr cabinet. Th he energy can n be recovered either by an air-to-airr heat exchannger, thermaal oil to air heaat exchangerr or steam bo oiler, depend ding on site rrestrictions aand desire too recover electricity y. A genericc process flow w diagram of o a Kruger B BioCon and ERS system m is providedd in Figure 3.


WEFTEC 2012

Figure 3.Generic Kruger BioCon ERS Process Flow Diagram Kruger’s reciprocating grate furnace is used to gasify and oxidize dried granular material produced from the drying process. These types of furnaces are commonly used for biomass worldwide but have been more commonly used in Europe for the application of biosolids when compared to the United States. Currently there are five installations in Europe and one in North America. The Kruger furnace operates under the principal of two stage combustion. The first stage or primary chamber occurs at the grates where under-fire or primary air is provided at 30-60% of the total air that is required for complete combustion. By starving the first stage of oxygen, gasification occurs, producing syngas. The second step occurs above the grates in the secondary chamber, where the syngas produced during the gasification step is combusted with excess air producing hot flue gas. The energy from the flue gas is recovered for the drying process and the flue gas is scrubbed before exiting out the stack. The Max West system is similar to the Kruger system in that it is a package system that operates in the close coupled configuration using the energy recovery from the gasification and thermal oxidation process for upstream drying. The Max West System however separates the gasification and thermal oxidation processes into two separate unit operations as opposed to a combined furnace like the Kruger system. The MaxWest Gasification system consists of the primary gasifier and the ash removal and storage system. The primary gasifier is a fixed bed, updraft reactor which is constructed of a refractory lined, steel unit in which primary gasification reactions take place in an induced draft environment (negative draw). Dried, processed biosolids are conveyed from the feedstock storage hopper into the gasification system feed bin located at the front end of each primary gasification cell where syngas is produced. The energy in the produced syngas is recovered through the use of a thermal oxidizer, a thermal energy diversion unit, the heat recovery system, and a cooling tower. The thermal oxidizer is a refractory lined, horizontal steel cylinder with injection ports for the admission of air to promote homogenous blending of the syngas prior to combustion. The syngas from the primary gasifier is combusted in a thermal oxidizer, which operates under an induced draft condition.


WEFTEC 2012

The energy from the hot flue gas is recovered by a heat recovery heat exchanger and heats a thermal fluid, typically thermal oil. A secondary heat exchanger or economizer can also be added on the backend of the process to recover residual energy remaining in the flue gas before it is treated by the flue gas cooling tower and scrubbing system and vented out the stack. The remaining inert, mineralized ash is removed from the bottom of the primary gasification unit through the use of a hydraulic, walking floor system. The ash is conveyed, through a hopper chute, to a drag or screw conveyance system and transferred to a disposal containment system. Each gasification system vendor offers different project delivery methods. Max West also only offers their system on a Design, Build, Own, Operate, Finance (DBOOF) model where Kruger is typically offered as either a Design, Bid, Build or Design, Build method. TECHNOLOGY EVALUATION AND SCREENING – ANAEROBIC DIGESTION In addition to gasification technologies and energy recovery systems, several anaerobic digestion process configurations and digestion pretreatment technologies were screened. Conventional and advanced anaerobic digestion processes were considered. Digestion processes that can achieve Class A biosolids were also screened. Pre-treatment methods for enhanced anaerobic digestion were evaluated. The initial screening process was based on practiced amount of volatile solids reduction and thus amount of biogas that can be generated to convert to energy. Another consideration was ability to achieve Class A now or in the future. The solids from the two plants digest differently based on bench scale digestion studies as described later. Thus different digestion processes were shortlisted for each plant. The shortlisted alternatives for Plant A included acid-gas digestion, temperature phased acid-gas digestion (2PAD), and thermal hydrolysis after combining with Plant B solids at Plant B site. The shortlisted digestion options for Plant B included conventional mesophilic anaerobic digestion (MAD), acid-gas digestion, temperature phased acid-gas digestion (2PAD). Digestion pretreatment technologies (such as Open Cel®, Crown Disintegration®, BioCrack®, etc) were also initially screened, however, due to the limited experience, the limited number of installations in North America and since the digestion facility is not constructed yet, these technologies were not considered at this time. Thermal Hydrolysis was the only digestion pretreatment technology selected for further evaluation. Since thermal hydrolysis is a digestion pretreatment technology applied to dewatered sludge cake, it is an ideal technology to consider for a regionalized or centralized digestion system. Mesophilic Anaerobic Digestion Mesophilic anaerobic digestion is one of the most proven processes for the stabilization of solids from municipal wastewater treatment. Conventional mesophilic digesters are typically operated at solids retention times of 15-25 days and at temperatures ranging from 90 to 105ºF (Metcalf & Eddy, 2003). Feed solids concentrations are typically in the range of 3-6%. Volatile solids reduction (VSR) varies between 40% and 55%. The biosolids produced in mesophilic digestion typically has ammonia concentration in the range of 600-1,000 mg/L. The biogas produced can be treated for various energy recovery uses. Figure 4 shows a generic process flow diagram for conventional mesophilic anaerobic digestion. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4614

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Figure 4. 4 Process Fllow Diagram m for Conveentional Meesophilic An naerobic Digestion as Anaerobicc Digestion Acid-Ga Acid-Gass (AG) anaerrobic digestiion consists of a highly lloaded “acidd-phase” diggester followed by a morre lightly loaaded “gas-ph hase” digesteer. The conccept is to proovide separatte, ideal environm ments for the acid-formin ng and the gaas-forming m microorganissms. The accid-phase is designed d to provide pretreatment p t (primarily biological b hyydrolysis annd acidification) and the gasphase is designed d forr maximizing g gas producction. A typ ical AG digeestion system m has an aciidphase dig gester with a detention tiime of 1-2 days followedd by a gas-phhase digesteer with a detention n time of abo out 10 days (Parry ( et al, 2004). Exp ected VSR iis in the rangge of 45-65% % with high h levels (>1,000 mg/L) of o ammonia in the recyclle stream aftter dewaterinng. Key requ uirements aree to produce a pH less th han 5.8 and vvolatile fattyy acids moree than 6,000 mg/L, wiithout using mineral acid ds in the acid d-phase. Miinimal amouunts of methaane gas, typiically less than 6 percent off the total AG G process, are a producedd in the acid--phase digestter. This gass has a lower quality q than what w is prod duced in the gas-phase g diigester and uusually contaains less thann 40% metthane. The relatively r sho ort detention n time of the acid digesteer can be maaintained by allowing for varying volumes in the digester correspondiing to variatiions in raw ssolids flow. There aree several verrsions of the AG processs in which diifferent operrating temperatures are uused for the accid and gas phases. p For example, a configuratio c n with two m mesophilic ddigesters in sseries is referreed to as AGM MM; a mesop philic acid digester d folloowed by a thhermophilic ggas digester is referred to t as AGMT T; and a therm mophilic aciid digester foollowed by a mesophilicc gas digester is referred to t as AGTM M. This proceess can produ uce Class A quality biossolids if at leeast one of thhe two phasses is operateed in the therrmophilic mode. m Figuree 5 shows a sschematic off the Acid/G Gas digestion n process. CH4, CO2

Digester Feed Solids

Acid Formerss (Mesophillic)

FVA

Methane Formers

Post Digestion D Dewatering

Figure 5. 5 Process Fllow Diagram m for Acid-gas Digestioon Copyright ©2012 Water Environment Federation. All Rights Reserved. 4615

WEFTEC 2012 M 2PADTM Anaerobic Digestion One speccific variation of the Acid-Gas Digesstion processs is the Two Phase Anaeerobic Digesstion process (2PAD) ( offerred by Infilcco Degremon nt, Inc. (IDI)). The IDI 22PAD system m consists off an acid phassed thermoph hilic digesteer and a meth hane gas phaased mesophhilic digesterr. The high temperatu ure in the thermophilic digester d desttroys pathoggens meetingg EPA requirrements for C Class A biosoliids.

In the 2P PAD process, the influen nt biosolids are a heated ass they pass thhrough a heaat recovery exchangeer. Followin ng preheating g, the residu uals are fed too the thermoophilic digesster where thhey are held for f two dayss. Temperatu ures in the th hermophilic digester aree maintainedd at 131ºF to maximize pathogen destruction. d The effluen nt from the thhermophilic digester is ffed through tthe heat reco overy exchan ngers to cooll the biosolid ds before theey are fed to the mesophhilic digester at 95ºF for ten days. Th he heat recov vered during g cooling of the biosolidds is used to ppartially heaat the biosolidss feed to the thermophilicc digester. The T 2PAD prrocess is sem mi-batch withh two to fouur sludge feeedings per day d to the sy ystem. Figurre 6 shows a schematic oof the 2PADTM process.

Figure 6. 6 Process Fllow Schema atic of a 2PA ADTM system m (From ID DI’s Literatu ure) Thermall Hydrolysiss with Meso ophilic Anaeerobic Digesstion Thermal hydrolysis (TH) ( is a slu udge conditio oning processs that preceddes anaerobiic digestion to produce a Class A prroduct and make m biosolid ds more digeestible. The TH processs disintegratees cell struccture/organicc materials and a dissolvess naturally occcurring celll polymers ((a form of protein), into an easilly digestiblee feed for anaaerobic digeestion. This pprocess subjeects dewaterred o high pressu ure and temp perature throu ugh direct stteam injectioon. The highh solids sludge to concentraation (9-10% %) reduces digestion d volu ume requirem ments to appproximately half that of conventio onal digesterrs operating at 5% solidss feed. M Tradition nally thermall hydrolysis has occurred d in a series of batch tannks (CambiTM or ThelysTTM) TM but a new wer continuo ous process (Exelys ( ) iss under deveelopment by Veolia Watter Solutionss and Technolo ogies. All th hermal hydro olysis processses use highh pressure stteam and opeerate at approxim mately 330°F F and 120-13 30 psi. The process p geneerates biogass that can bee used for electricall production and generattes a high qu uality Class A product. T There are moore than 25 installatio ons of batch h thermal hyd drolysis proccesses worlddwide but onnly one demoonstration unnit of the contin nuous process is currenttly operating g (Hillerod, D DK).


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The mostt common th hermal hydro olysis processs in operatioon today is tthe Cambi syystem shownn in Figure 7. 7 In the Cam mbi process, dewatered biosolids b aree conditionedd through thhree batch reaction steps. First dewatered biosolids b are fed to a pulpper where thhe biosolids are mixed w with flash steaam from the flash tank. The biosolid ds are pumpeed from the pulper to a rreactor wherre live steam m is injected d and the soliids are held for sufficiennt period for the hydrolyysis reaction to occur. From the reacctor, the hot,, pressurized d biosolids arre fed to a fllash tank whhere the presssure is reduceed producing g a flash steaam that is reccovered to puulp and heatt the incominng raw dewaatered cake. Fro om the flash h tank, the biiosolids are diluted d with pasteurized water to thee desired soliids content (typically 9-1 10% DS), co ooled with heeat exchangeers, and fed to the anaeroobic digestioon process.

Figure 7. 7 Process Fllow Schema atic of a Cam mbi Thermaal Hydrolyssis System The Exellys process depicted d in Figure F 8 is a newer geneeration of theermal hydrollysis currenttly under dev velopment by b Veolia. Unlike U Camb bi or Thelys, which uses a series of bbatch tanks, Exelys iss a continuou us plug flow system. Deewatered bioosolids and ssteam are conntinuously fe fed to a mixing and conden nsing system. The pressu urized sludg e is fed to a plug flow reeactor with sufficientt detention time for the hydrolysis h reeaction to occcur. From tthe plug flow w reactor, thhe solids aree cooled and d diluted with h pasteurized d water to feeed the digessters at the ddesired solid content and a temperatture. The reaaction condiitions, tempeerature and ppressure, aree similar to thhe Cambi prrocess.

Figure 8. 8 Process Fllow Schema atic of an Ex xelys Therm mal Hydrolyysis System


WEFTEC 2012

COMBINED HEAT AND POWER (CHP) OPTIONS Anaerobic digestion produces a low-BTU gas (biogas) that can be utilized in a range of fuel combustion equipment. The most optimal design to use this fuel is through the co-production of heat and power, otherwise known as a combined heat and power plant (CHP). In the CHP of this size, the biogas is typically combusted in a reciprocating internal combustion engine or in a combustion turbine. The hot exhaust gas is then passed through a heat exchanger to generate hot water or steam to be utilized in a process, such as heating anaerobic digesters. In many instances, lube oil heat and jacket water heat is also extracted to increase the recovered heat. In addition to traditional combustion technologies, fuel cells may be utilized in a CHP application. Four CHP technologies were evaluated; reciprocating engines, gas turbines, microturbines and fuel cells. Based on the projected range of gas volumes, it was determined that gas turbines are not suitable as they are mostly used for much higher gas volumes so they were eliminated from the evaluation. Fuel cells were also eliminated due to the limited number of installations and anticipated higher costs when compared to other technologies. Based on the initial screening, reciprocating engines and microturbines were recommended for further development as alternatives for the CHP system. Multiple gas cleaning and pre-treatment systems were also evaluated. The digester gas pretreatment technology selection will be coordinated with the CHP technologies chosen for further evaluation. The most appropriate technologies include iron sponge for sulfide removal, and activated carbon or chemical scrubbing for siloxane removal. Gas purification for pipeline or vehicle fueling as an alternative to CHP was not recommended in this evaluation. Reciprocating Engines Reciprocating internal combustion engines are a widespread and well-known technology. There are two basic types of reciprocating engines – spark ignition and compression ignition (EPA, 2008). Spark ignition engines for power generation use natural gas as the preferred fuel, although they can be set up to run on propane, gasoline, or landfill gas. Compression ignition engines (often called diesel engines) operate on diesel fuel or heavy oil, or they can be set up to run in a dual-fuel configuration that burns primarily natural gas with a small amount of diesel pilot fuel. Current generation natural gas engines offer low cost, fast start-up, proven reliability when properly maintained, excellent load-following characteristics, and significant heat recovery potential. Many facilities have utilized engine generators for burning digester gas. Some installations have also used digester gas in engines that directly drive equipment. Heat is recovered from the engines and the exhaust to offset heating needs. These systems typically can recover up to 45% of the thermal energy from the combustion of digester gas for use with heating applications, such as digester heating. Additionally, the engine /generator can convert approximately 35% of the thermal energy of the digester gas to the production of electrical power, yielding an overall CHP efficiency of about 80%. The U.S. Department of Energy (DOE) Advanced Reciprocating Engine Systems program (ARES) promotes engine development in the US to obtain maximum engine efficiency and low emissions from natural gas reciprocating engines for power generation. These ARES engines are also compatible with cleaned digester gas. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4618

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Microturbines Another CHP technology which has become more common during the last few years is microturbines. Microturbines are miniature industrial gas turbines. Units currently available for the application of digester gas are 30 and 200 kW units (by Capstone) and 250 kW units (by Ingersoll Rand). Electricity conversion efficiency is enhanced by use of a recuperation cycle where most of the exhaust gas heat is used to preheat the combustion air charge. There is, however, excess heat which can be recovered as hot water or steam for digester heating. Microturbines are ideally suited for distributed generation applications due to their flexibility in connection methods, ability to be stacked in parallel to serve larger loads, ability to provide stable and reliable power, and low emissions. Combined heat and power is one of its typical applications. In CHP applications, the waste heat from microturbines is used to produce hot water or steam for heat recovery applications, such as digester heating. SUMMARY OF BENCH SCALE AND LABORATORY TESTS In order to predict the performance of gasification or anaerobic digestion, samples of sludge from both plants were collected for laboratory and bench scale analysis. Sludge samples from Plant A and Plant B were shipped to Hazen Research Fuel Laboratory in Golden, CO for Proximate and Ultimate analysis. Samples were also sent to Virginia Polytechnic Institute and State University in Blacksburg, VA (Virginia Tech) to evaluate the digestibility of the biosolids. Proximate and Ultimate Analysis A proximate analysis provides the gravimetric fraction or weight fraction of moisture, volatile matter, fixed carbon and ash. An ultimate analysis is a gravimetric analysis that provides the percentage weights of specific atomic species as opposed to compounds. The information provided by the proximate and ultimate analysis are used by the gasification or combustion system manufacturer to select components suitable for optimal operating conditions and designing the fuel and ash handling systems. The proximate and ultimate analysis are provided in Table 4 and Table 5 below. Table 4. Proximate Analysis (dry basis) Source Plant A Plant B

Ash (%) 24.17 22.30

Volatile (%) 66.94 70.39

Fixed C (%) 8.89 7.31

Table 5. Ultimate Analysis Results (dry basis, wt %) Source Plant A Plant B

Carbon 41.33 43.20

Hydrogen 5.66 4.79

Nitrogen 5.57 3.29

Sulfur 0.86 1.09

Ash 24.17 22.30

Oxygen 22.41 24.33

The higher heating value (HHV) provides a measure of the heat energy available within the feed materials. The results are presented on a dry solids basis. The higher the HHV, more energy is available for release from the process. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4619

WEFTEC 2012

If gasification and drying are combined, higher HHVs would mean either less auxiliary fuel (natural gas) is required to sustain the process or there is potential to generate excess energy beyond what is required for drying. However, when looking at combined drying/ gasification systems, the efficiencies of all of the processes need to be considered (gasification, oxidation, heat transfer, dewatering, and drying efficiencies), to make this determination. Comparing the measured HHV to the calculated or theoretical values according to the modified Dulong method provides surprisingly good convergence, meaning that the two results are close to the same value. The results of this analysis are provided in Table 6 below. The HHVs for both WWTPs are in line with typical non-digested WWTP solids and allow drying and gasification to be good candidate technologies for processing the solids. Table 6. Higher Heating Value (dry basis) Source

HHV (BTU/lb)

Calculated/ Theoretical HHV (BTU/lb)

Difference (actual vs. calculated)

Plant A Plant B

7,686 7,920

7,702 8,087

-0.21% -2.1%

Digestion Potential Two sets of samples from both plants were also provided to Virginia Tech for batch digestion testing. Batch digesters were set up in triplicate using seed sludge from a local anaerobic digestion plant (Peppers Ferry WWTP). A second set of samples was provided and 75% of the contents of the first phase digesters was wasted and replaced with new sludge. The intent was that the first phase would be used both for data and to confirm that the biomass was acclimated to the sludge from WSSC. A summary of the digestion results are presented in Figure 9 below. 60

% Reduction

50 40

TS

30

VS 20

COD

10 0

Plant A Seneca

Plant B Piscataway

Figure 9. Batch Digestion Testing Results The batch tests show that the sludge from Plant A digested poorly, however, the sludge from Plant B digested fairly well. Since Plant A consists of only waste activated sludge (WAS), the result is expected and within the range of expected values for WAS. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4620

WEFTEC 2012

Plant B, however, contains primary sludge so the digestibility was significantly better; however, it was a little lower than expected for a mixture of primary and WAS. The lower digestion values for the Plant B sludge could be due to using alum in the clarification process. Available aluminum ion has been shown to bind proteins in waste water and sludge making the protein unavailable during anaerobic digestion. Batch digestion reflects a single sludge sample so it is limited in the data it can provide since sludge at a WWTP can vary from day to day and week to week. A more complete data analysis would require digestion over several detention times, typically several months of operation once the digester is at steady state. Although limited, the batch digestion data provided useful information for predicting the performance of a full scale system.

SHORT-LISTED ALTERNATIVES EVALUATION The screened drying and gasification alternatives were individual BioCon and ERS systems at each plant (Plant A and Plant B) and a combined MaxWest Facility. MaxWest did not offer individual plant systems as an option as each was a smaller system than they wanted to propose. Screened anaerobic digestion processes for economic and non-economic evaluation included conventional mesophilic anaerobic digestion (MAD) at Plant B WWTP; acid-gas phase digestion for both WWTPs; and 2PAD technology for both WWTPs. A regional digestion facility for both plants using thermal hydrolysis pretreatment was also selected for further evaluation. Based on the results of the batch digestion testing, conventional anaerobic digestion was not considered appropriate for Plant A. Energy recovery potential from all short listed technologies was performed. Evaluations have shown that electricity production potential from anaerobic digestion at each plant ranges from 600-850 kW. Drying followed by gasification showed no auxiliary energy is needed for the drying process without electrical potential for Plant A using the close-coupled method of energy recovery. For, Plant B, however, since primary sludge was produced, excess energy yielding up to 260 kW of electricity could be produced using the Organic Rankin Cycle technology. Increasing the cake solids from the existing dewatering technology at each plant from 20% to 30% using electro-dewatering to generate additional waste heat was also evaluated, however, the power costs for the enhanced dewatering equipment outweighed the electrical production potential so it was not considered for further evaluation. Furthermore combined drying/gasification by MaxWest and combined anaerobic digestion proceeded with thermal hydrolysis were considered. For the combined option using thermal hydrolysis as a biosolids pretreatment process prior to anaerobic digestion, approximately 1.3 to 1.6 MW of power could be produced. One of the largest advantages of including a combined digestion system with thermal hydrolysis pretreatment is that the solids can be hauled between WWTPs as dewatered cake as opposed to a liquid sludge which significantly minimizes the hauling costs and number of trucks required. Electrical production with a combined gasification facility was also examined; however, it was not economical or recommended by the vendor so was excluded from the evaluation. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4621

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Figure 10 shows the electrical prroduction po otential compparing indivvidual plant aalternatives tto a combined d digestion facility f with thermal hyd drolysis. Bassed on this ffigure it is evvident that a regional facility with h thermal hyd drolysis allows for the hhighest electrrical producttion potentiaal.

Figure 10. Summary of Electriccal Producttion Potenti al The outcome of the individual i pllant and com mbined plant alternative aanalysis wass the realizattion that theree may be oth her combined d plant altern natives that ccould providde WSSC a bbetter biosollids managem ment solution n. With this as a a backdro op, WSSC prrovided direection to evalluate other alternativ ves to capturre the full ran nge of poten ntial solutionns: • • • •

WSSC W Centraalized Bioso olids Facility y serving all five WWTP Ps within WS SSC service area (C Centralized Option O 1) Third T Party Centralized C Biosolids B Faccility servingg all five WW WTPs locateed outside thhe WSSC W servicce area (Centtralized Optiion 2) Two T WSSC Regional R Bio osolids Facillities, each s erving two aand three WW WTPs (R Regional Op ption 3) WSSC W Regio onal Biosolid ds Facility seerving four pplants and onne plant conttinues existinng prrocess (Regiional Option n 4)

The impaact of the pro oposed winteer ban legisllation on lannd applicationn of biosolidds was investigaated with the relative imp pact to altern natives in terrms of cost aand temporaary storage requirem ments. Marylaand Departm ment of the Environment E t (MDE) is pproposing very restrictivee winter baan limits on land applicaation as well as phosphorrus limits in the fall that effectively limit land appllication for 9-months 9 of the year. mpact Virginia,, on the otherr hand, is lesss restrictivee with regulaations that arre regional aand do not im most of the t state. Lan nd applicatio on in Virginiia appears viiable for the long term. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4622

WEFTEC 2012

Since TH H/MAD was determined to be the mo ost cost effecctive to conddition the bioosolids and maximize digester gaas production n at a combiined Plant A and Plant B facility, it w was the technolog gy used to ev valuate addiitional region nalized and ccentralized ooptions. ntial compariing regional and centraliized alternattives to the bbest Figure 11 shows elecctrical poten individuaal plant alterrnative. Baseed on these figures f it is eevident that regional andd centralizedd facilities with thermaal hydrolysiss allow for th he highest ellectrical prodduction poteential. Centralizzed Option 2 was not evaaluated for electrical e pottential since it is outside WSSC region.

Figure 11. Summary of Electriccal Producttion Potenti al

Economic Evaluatio on The life cycle c cost an nalysis (LCC C) was develloped to com mpare the treeatment alterrnatives from ma cost perspective. The analysis was w based on calculating the present w worth value for the totall cost (capital and a O&M) of o each altern native assum ming 20 year life cycle peeriod. Capital Costs C were developed d as the opinion of probable constructionn cost for eaach of the shortlisteed alternativees. The prob bable constru uction costs w were based oon preliminaary layouts oof the new facillities and ven ndor quotes for major eq quipment. Thhe constructtion costs repported here aare considereed accurate to t +/- 35%. ual O&M cost estimates for each alteernative werre calculatedd for the currrent and desiign The annu years to set s the basis for the life cycle c cost an nalysis. The O&M costs were categoorized underr the following g components; energy, trransportation n of solids, lland applicattion, chemiccal demand aand labor (inccluding both h operation and a equipment maintenannce). The O& &M costs foor the years iin between current and design weree interpolated d assuming a linear increease in cost from currennt to design.


WEFTEC 2012

The apprropriate escaalation rates were w applied d to each cattegory accorrding to the vvalues listedd in Table 7. The discoun nted rate wass applied to the escalatedd value to deetermine thee discounted values fo or each year. The presentt worth valuee for the O& &M cost wass determinedd by summinng all discounteed values forr all the yearrs from curreent to designn. Table 7. Economic Variables V Assumed A forr Analysis Economic c Variables Econom mic Variables Ele ectrical power esccalation rate Nattural gas escalation rate Fue el escalation rate Lan nd application fee escalation rate Che emical escalation n rate Lab bor escalation rate e Disscount rate Life e Cycle Period

0.030 0.030 0.030 0.030 0.050 0.030 0.030 20

Yr-1 Yr-1 Yr-1 Yr-1 Yr-1 Yr-1 Yr-1 Yr

For the options o with anaerobic diigestion, reciprocating e ngines weree selected as the preferred CHP systtem since they showed the t lower 20 year LCC w when comparred to microoturbines. Figure 12 summarizes the results of the digeester gas CH HP analysis. T The costs forr the reciprocaating enginess were used in the digestter system opptions LCC..

P Lifecycle O Options Figure 12. Summary of Digesteer Gas CHP e alternattive, includin ng CHP capiital cost, wass added to thhe present vaalue The capittal cost for each for the O&M O cost to determine th he overall prresent value for each opttion.


WEFTEC 2012

Table 8 shows LCC estimates for individual plant solutions at Plant B and Plant A along with a combined plant solution that combines the sludge from Plants A&B. Table 8 also provides a comparison of the individual plant alternatives to expanded regional and centralized alternatives. Since conventional mesophilic digestion was not considered viable at Plant A it is not shown below. Also, a solution of Baseline at Plant A and gasification at Plant B was included to show the potential for different solutions at each plant. Treatment of recycle streams was also evaluated comparing treatment in the main process vs. separate sidestream treatment with deammonification. The capital cost was assumed to be approximately $2,000 /lb nitrogen deammonified (based on experience with other local plants’ costs). Operating cost savings include 63% less aeration cost and 90% less methanol cost compared to treatment of the recycle streams in the main process. The savings in methanol and electricity provide approximately a five year payback for this sidestream treatment process. Because of the short payback period, sidestream recycle treatment with deammonification was considered in each of the combined and regional alternatives and included in capital and O&M cost analyses. Table 8. LCC Comparing Individual/Combined Alternatives to Regional and Centralized Alternatives Plant A Alternatives O&M ($/DT) CC ($/DT) Total ($/DT)

Baseline 482 73 554

A/G 346 295 641

2PAD 356 395 751

Gasification ‐ Kruger 309 268 577

A/G 215 357 573

2PAD 213 402 615

Gasification ‐ Kruger (w/ORC) 205 271 476

Plant B Alternatives O&M ($/DT) CC ($/DT) Total ($/DT)

Baseline 486 58 544

MAD 234 319 553

Combined Plants A&B Alternatives Baseline

Basline (Plant A) + TH‐Cambi MW‐min Gasification (Plant B)

O&M ($/DT)

484

217

337

CC ($/DT) Total ($/DT)

65 549

282 499

177 513

492 ‐ 492

MW‐max

MW‐ORC‐ MW‐ORC‐max min

510 ‐ 510

563 ‐ 563

584 ‐ 584

Regional & Centralized Alternatives Centralized Centralized Regional Option 1 Option 2 Option 3 Baseline

4 Train Exelys

2 Train Cambi

3 MW CHP O&M ($/DT) CC ($/DT) Total ($/DT)

405 105 510

253 190 443

250 211 461

4 Train Exelys

2 Train Cambi

Avg*

2 MW CHP w/o FOG 268 180 449

266 201 468

508 76 583

Regional Option 4

1 Train Cambi 2 Train 1 Train 3 Train in both Cambi Cambi Exelys facilities 2 MW + 670 MW CHP kW CHP 245 281 526

* Average value between 583 and 710 $/dt since tipping fee was quoted as range


248 246 495

248 230 478

251 225 476

WEFTEC 2012

The alternatives shaded in yellow in Table 8 (depicted as processing cost in $/dry ton) were determined to be the best candidates for the respective individual plant, combined plant, regional, and centralized configurations. Other criteria are considered in addition to the cost analysis; including carbon footprint and potential electricity production, which are discussed in the following sections. Individual digestion plant options were found to be the least economical. Non-Economic Evaluation In addition to the economic analysis, the systems were also evaluated and ranked based on factors such as; ease of operation and maintenance, impact of sidestreams on the Enhanced Nutrient Removal (ENR) process, sustainability, product quality, odor and noise. The operation and maintenance were quantified in terms of cost and ultimately captured in the economic analysis, however certain qualitative factors are also taken into account such as requirements for maintaining a Title V air emissions permit for drying and gasification systems or using high pressure steam for the thermal hydrolysis systems. The digestion options would place a large nutrient load back on the ENR process so additional side stream treatment processes were included in the capital costs and additional chemical costs and aeration requirements were accounted for in the LCC. The sustainability of both the digestion processes and drying and gasification processes are quantified by the greenhouse gas emission. For product quality, the digestion options produce a dewatered cake of Class A or Class B quality depending on the process configuration and inclusion of digester pretreatment technology. The drying and gasification processes produce an inert ash with significantly less mass for disposal either to a landfill or for beneficial reuse such as concrete or road production. All options should minimize odor and noise to levels that are not major deciding factors. Carbon footprint and Greenhouse gas estimates Another parameter that was important to the commission was reducing greenhouse gas (GHG) emissions and the carbon footprint of the plant. CO2-equivalent (CO2e) emissions were evaluated as an indication of sustainability for each alternative using 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories. Local Government Operations Protocol (LGOP) for the quantification and reporting of greenhouse gas emissions inventories (published in May 2010) were used in addition to IPCC guidelines. The carbon footprint calculations included emissions from electricity, natural gas, transportation, and chemical uses. The boundaries set for the carbon footprint analysis included direct emissions from the plant such as fossil fuel and digester gas combustion as well as chemical addition at the plant. The scope for the carbon footprint analysis also included indirect emissions resulting from the plant operations such as electricity purchased for running equipment, emissions associated with hauling chemicals to the plant or biosolids from the plant as well as the emissions associated with the manufacturing of chemicals. Emissions from the CHP processes and gasification process were estimated, along with the emissions from methanol addition to the wastewater for dewatering filtrate side stream treatment. However, all of these are of biogenic origins, and the IPCC excludes biogenic CO2 from the Inventory.


WEFTEC 2012

The emisssions from the t biogas used to produ uce CHP andd any emissioons from dryying/gasificaation are exclu uded since th hese emission ns are biogen nic, howeveer emissions from incom mplete CH4 combustiion were estiimated and accounted a fo or. Use of naatural gas to supplement fuel to CHP P is also inclu uded in GHG G estimates. The denitriffication reaction that occcurs in the siide stream treatmentt process is a naturally occurring o reaaction with thhe byproducct being CO2, so it is alsoo treated ass a biogenic source.

Figure 13 and Figurre 14 summaarizes the GH HG estimate s for the cenntralized, reggionalized annd the individual sludgee treatment options o evalu uated. All diigestion andd gasificationn alternativess uction in carrbon footprinnt when com mpared to thee baseline show significant saviings and redu process of o lime stabillization. Thee digestion teechnologies,, however, hhave the lowest carbon footprint and actually y provide a negative n carb bon footprinnt overall. T The low carboon footprint is realized by b the generration of elecctricity from m a biogenic ffuel source, biogas. ote that Figu ure 13 is sho owing summ marized estim mated CO2e eemissions forr two (2) Please no individuaal plants alteernatives (Baaseline, A/G, 2PAD and Gasificationn) versus com mbined two (2) plants’ allterative (Th hermal Hydrrolysis (TH))). Also note that Figure 14 does not show CO2ee emission ns for Centralized Option n 2 since thiss is considereed “outside W WSSC fencee” hence no carbon crredit for WS SSC.

Figure 13. GHG Em missions Com mparison fo or Individuaal and Com mbined Plantt Options


WEFTEC 2012

Figure 14. GHG Em mission Com mparison forr Regional aand Centrallized Biosollids Facilitiees

Estimateed Side Streeam Characcteristics, EN NR Impactss, and Proceess Recomm mendations Considerrations regarrding recent research r find dings broughht forth a pootential ENR R performanccerelated drrawback to the t application of the theermal hydrollysis pretreaatment technoology. The drawback k is that a po ortion of the organically bound nitroggen in the feeed sludge iss solubilizedd into recalcitraant dissolved d organic nittrogen (rDON N). Approxi mately 2.2% % of the feedd sludge nitroogen is converrted into rDO ON, and som me studies su uggest a conccentration off 150 mg/L w when THP iss operated at 165 C and 6 bar presssure (Dwyerr et. al., 20088). This rDO ON ends up iin the centriffuge reject waater (centratee). The prob blem is that it cannot be rremoved froom wastewatter with todaay’s readily ap pplicable tecchnology, an nd henceforth h remains inn the recalcittrant state thrroughout thee side-streaam treatmen nt and main-sstream activaated sludge pprocesses. A As such, therre is a contributtion of nitrog gen in the fin nal effluent. While theree are operatiional methodds to mitigatte the productio on of the rDO ON such as operating th he thermal hyydrolysis proocess at a low wer temperaature, this proceess still has the t potentiall to jeopardizze ENR com mpliance. Due to th hese findingss, it became prudent to confirm c that the locationn selected forr a regional oor central biiosolids faciility has the capacity c to accommodat a te the elevateed rDON contribution inn the plant efflluent flow sttream. The recommende r ed site has a more relaxeed TN permiit limit (4.0 m mg/L TN). The ENR R performancce implicatio ons were quaantified by aassuming thaat 2.2% of thhe feed sludgge nitrogen would be co onverted into o rDON. Th his would prooduce an esttimated centrrate rDON concentraation of 136 mg/L. Copyright ©2012 Water Environment Federation. All Rights Reserved. 4628

WEFTEC 2012

At the deesign annual average con nditions, the centrate rDO ON loading was estimatted at 193 lbs/day, which w rendered an increaase in the fin nal effluent rrDON by 0.777 mg/L. Thhis increase was compared d and includ ded with otheer anticipated d final efflueent nitrogen species. NH4-N and N Figure 15 illustrates this comparrison, where an effluent N NOX-N, werre respectiveely assumed at 0.5 and 1.0 mg/L based on anticip pated ENR pperformancee. In addition, effluent particulatte organic niitrogen (pON N) was assum med at 0.4 m mg/L and an rDON contrribution by tthe raw wasttewater was assumed at 0.64 0 mg/L, based b on histtorical perfoormance. Thhe rDON contributtion by the TH/AD T proceess recycle stream s adds 00.77 mg/L aand increasess the total effluent TN T from 2.5 54 mg/L to 3.31 mg/L.

Figure 15. Compariison of estim mated final effluent e nitrrogen speciaation As a resu ult of the thermal hydroly ysis-digestio on process, th the side-streaam also conttains higher NH4N concen ntrations (2,5 500 to 3,000 0 mg-N) than n conventionnal anaerobicc digestion pprocesses (Fidgore et. al, (2011 1). To furtheer minimize impacts of iincreased NH H4-N concenntrations in tthe recycle streams, a sid de-stream treeatment proccess (such ass Deammoniification) is rrecommendeed. This assu umption wass made for th he compariso on shown in Figure 15. SUMMA ARY AND PATH P FOR RWARD For indiv vidual and co ombined plan nt options, both b drying/ggasifiction/E ERS and a coombined digestion n/CHP system m with therm mal hydrolyssis appear to be the mostt favored opttions from a life cycle cosst analysis sttandpoint. Drying D follow wing by gasiification offeers the most mass reducttion and when n conducting g sensitivity analysis for the various options it apppears to be the most staable against consumables, hauling and d land appliccation factorrs. Howeverr anaerobic ddigestion witth thermal hydrolysis h haas the highesst electrical production p ppotential, annd lowest greeenhouse gass potential, which are important i faactors for WS SSC. The lim mited number of drying and gasificaation installatio ons along with air permiitting issues and impact of the new E EPA Sewagee Sludge Incinerattion regulatio ons are pointts of concern n with WSSC C.


WEFTEC 2012

Anaerobic digestion with thermal hydrolysis pretreatment, the other favored option, also contains caveat in that there are currently no thermal hydrolysis systems installed or operating in North America, although there are more than 25 systems worldwide and one is currently planned for Blue Plains. Also all thermal hydrolysis systems require high pressure steam. The centralized and regional options managing biosolids from all five WWTPs have a lower processing cost than the two individual plants or combined plant digestion options. Based on a WSSC regional facility consisting of thermal hydrolysis, FOG, mesophilic anaerobic digestion, CHP with IC engines, and land application of remaining Class A biosolids, the following benefits are projected: Environmental Benefits • Recover 1.7 MW of renewable energy from biomass • Reduced Greenhouse Gas production by 11,800 tonnes/ yr • Reduce biosolids output by more than 50,500 wet tons/yr • Reduce lime demand by 4,100 tons/yr • Reduce nutrient load to Chesapeake Bay • Reduce 5 MG/yr Grease discharge to sewers • Produce Class A Biosolids Economic Benefits • Recover > $1.5 Million/yr of renewable energy costs • Reduce biosolids disposal costs by $1.7 Million/yr • Payback of 15-18 years (compared to baseline) Community Benefits • Job creation • Mitigate increase of WSSC Customer Rates • Reduce truck traffic REFERENCES 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories, Industrial Processes and Product Use, Volume 3, Chapter 2 (Mineral Industry Emissions) and Chapter 3 (Chemical Industry Emissions) Dwyer et. al, (2008) Decreasing activated sludge thermal hydrolysis temperature reduces product colour, without decreasing degradability, Water Research 42, 4699 – 4709. Fidgore et. a.l, (2011) Deammonification of Dewatering Sidestream from Thermal HydrolysisMesophilic Anaerobic Digestion Process, WEF’s Nutrient Recovery and Management Local Government Operations Protocol (LGOP) For the quantification and reporting of greenhouse gas emissions inventories (2010) Version 1.1, May 2010 Metcalf & Eddy, Inc. (2003) Wastewater Engineering: Treatment and Reuse, Fourth Edition, McGraw-Hill Inc., New York. Parry, et. a.l, (2004) High Performance Anaerobic Digestion, WEF Residuals and Biosolids Committee Bioenergy Technology Subcommittee, White Paper US EPA (2006) Emerging Technologies for Biosolids Management US EPA (2008) Catalog of CHP Technologies Water Environment Federation (2011) Overview of Energy Recovery from Residuals and Biosolids Gasification, Residuals and Biosolids Conference, Workshop C Handout Copyright ©2012 Water Environment Federation. All Rights Reserved. 4630