Review Articles
Constructed Wetlands
Review Article with Case Studies
Constructed Wetlands for the Treatment of Organic Pollutants Raimund Haberl1, Stefano Grego2, Günter Langergraber 1 (*), Robert H. Kadlec3, Anna-Rita Cicalini4, Susete Martins Dias5, Julio M. Novais5, Sylvie Aubert6, Andre Gerth7, Hartmut Thomas7 and Anja Hebner7 1
IWGA-SIG - Department for Sanitary Engineering and Water Pollution Control, BOKU – University of Natural Resources and Applied Life Sciences, Vienna, Muthgasse 18, A-1190 Vienna, Austria 2 Dipartimento di Agrobiologica ed Agrochimica, Università della Tuscia, Via S. G. Decollato 1, I-01100 Viterbo, Italy 3 University of Michigan and Wetland Management Services, Chelsea, Michigan, USA 4 Dipartimento Biologica e Ambiente, ISRIM, I-05100 Terni, Italy 5 Instituto Superior Técnico, Centre for Biological and Chemical Engineering, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal 6 Laboratory for Environmental Biotechnology (LBE), Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland 7 BioPlanta GmbH, Benndhorfer Landstrasse 2, D-04509 Delitzsch, Germany * Corresponding author: Günter Langergraber (
[email protected])
DOI: http://dx.doi.org/10.1065/jss2003.03.70 Abstract
Background. Constructed wetlands (wetland treatment systems) are wetlands designed to improve water quality. They use the same processes that occur in natural wetlands but have the flexibility of being constructed. As in natural wetlands vegetation, soil and hydrology are the major components. Different soil types and plant species are used in constructed wetlands. Regarding hydrology surface flow and subsurface flow constructed wetlands are the main types. Subsurface flow constructed wetlands are further subdivided into horizontal or vertical flow. Many constructed wetlands deal with domestic wastewater where BOD and COD (Biochemical and Chemical Oxygen Demand respectively) are used as a sum parameter for organic matter. However, also special organic compounds can be removed. Objective. The objectives are to summarise the state-of-the-art on constructed wetlands for treatment of specific organic compounds, to the present the lack of knowledge, and to derive future research needs. Methods. Case studies in combination with a literature review are used to summarise the available knowledge on removal processes for specific organic compounds. Results and Discussion. Case studies are presented for the treatment of wastewaters contaminated with aromatic organic compounds, and sulphonated anthraquinones, olive mill wastewater, landfill leachate, and groundwater contaminated with hydrocarbons, cyanides, chlorinated volatile organics, and explosives. In general the removal efficiency for organic contaminants is high in all presented studies. Conclusion. Constructed wetlands are an effective and low cost way to treat water polluted with organic compounds. There is a lack of knowledge on the detailed removal pathways for most of the contaminants. Removal rates as well as optimal plant species are substance-specific, and also typically not available. If a constructed wetland provides different environmental conditions and uses different plant species the treatment efficiency can be improved. Recommendations and Outlook. There is a great need to lighten the black box 'constructed wetland' to obtain performance data for both microbial activity and the contribution of the plants to the overall removal process. Also genetic modified plants should be considered to enhance the treatment performance of constructed wetlands for specific compounds. Keywords: Constructed wetlands; groundwater; organic con-
taminants; wastewater; water treatment
Introduction
Wetlands are transitional environments between dry land and open water. In an ecological context, wetlands are intermediate between terrestrial and aquatic ecosystems. Wetlands can be defined as ecosystems that depending on constant or recurrent, shallow inundation or saturation at or near the surface of the substrate (Lewis 1995, in: Vymazal et al. 1998). The major wetland components are: • Vegetation. The prevalent vegetation consists of macrophytes that are typically adapted to areas heaving hydrologic and soil conditions described in the definition. • Soil. Soils are present and have been classified as hydric, or they posses characteristics that are associated with reducing soil conditions. • Hydrology. The area is inundated either permanently or periodically at mean water depth of ≤2 m, or the soil is saturated to the surface at some time during the growing season of the prevalent vegetation. Compared with the vegetation of well-drained soils, wetland plants have a world-wide similarity which over-rides climate and is imposed by the common characteristics of a free water supply and the abnormally hostile chemical environment which plant roots must endure (Etherington 1983, in: Vymazal et al. 1998). Wetland plants have elaborated structural mechanisms to avoid root anoxia. The main strategy has been the evolution of air spaces (aerenchyma) in roots and stems that allow the diffusion of oxygen from the aerial portions of the plant into the roots (e.g. Armstrong and Armstrong 1990). The magnitude of oxygen diffusion through many wetland plants into the roots is apparently large enough not only to supply the roots but also to diffuse out and oxidize the adjacent soil (Teal and Kanwisher 1966, and Howes et al. 1981, in: Vymazal et al. 1998). Four groups of aquatic macrophytes can be distinguished on a basis of morphology and physiology (Wetzel 1983, in: Vymazal et al. 1998): 1. Emergent macrophytes grow on water saturated or submersed soil from where the water table is about 0.5 m below the soil surface to where the sediment is covered with approximately 1.5 m of water (e.g. Acorus calamus, Carex rostrata, Phragmites australis, Scirpus lacustris, Typha latifolia).
JSS – J Soils & Sediments 3 (2) 109 – 124 (2003) © ecomed publishers, D-86899 Landsberg, Germany and Ft. Worth/TX • Tokyo • Mumbai • Seoul • Victoria • Paris
109
Constructed Wetlands 2. Floating-leaved macrophytes are rooted in submersed sediments in water depths of approximately 0.5 to 3 m and posses either floating or slightly aerial leaves (e.g. Nymphaea odorata, Nuphar lutea). 3. Submerged macrophytes occur at all depths within the photic zone. Vascular angiosperms (e.g. Myriophyllum spicatum, Ceratophyllum demersum) can occur in water up to 10 m deep (1 atm hydrostatic pressure) but non vascular macro-algae can occur to the lower limit of the photic zone (up to 200 m, e.g. Rhodophyceaered algae). 4. Freely floating macrophytes are not rooted to the substratum; they float freely on or in the water column and are usually restricted to nonturbulent, protected areas (e.g. Lemna minor, Spirodela polyrhiza, Eichhornia crassipes). 1
Wetlands for Water Treatment (Constructed wetlands, Wetland treatment systems)
Natural wetlands have been used for wastewater treatment for centuries. In many cases, however, the reasoning behind this use was disposal, rather than treatment and the wetland simply served as a convenient recipient that was closer than the nearest river or other waterway (Reddy and Smith 1987). Naturals wetlands are still used for wastewater treatment (e.g. Kadlec and Tiltion 1979), but since 10 to 20 years the use of constructed wetlands has become more popular and effective around the world (e.g. Reddy and Smith 1987; Kadlec and Knight 1996; Cooper et al. 1996). Constructed wetland treatment systems are engineered systems designed and constructed to utilize the natural processes involving wetland vegetation, soils, and their associated microbial assemblages to assist in treating wastewater. They are designed to take advantage of many of the same processes that occur in natural wetlands, but do so within a more controlled environment. Constructed wetlands for wastewater treatment may be classified according to Brix (1994) to the life form of the dominating macrophytes into 1. Free-floating macrophyte-based systems, 2. Submerged macrophyte-based systems, and 3. Rooted emergent macrophyte-based systems.
and according to the water flow different rooted emergent systems are distinguished into • • •
surface flow systems, horizontal subsurface flow systems, and vertical subsurface flow systems.
Surface flow wetlands (SF) are densely vegetated by a variety of plant species and typically have water depths less than 0.4 m. Open water areas can be incorporated into a design to provide for the optimisation of hydraulics and for wildlife habitat enhancement. Typical hydraulic loading rates are between 0.7 and 5.0 cm.d–1 (between 2 and 14 ha.1000 m–3.d–1) in constructed surface flow treatment wetlands. The subsurface flow wetland (SSF) technology is based on the work of Seidel (1967). Since then the technology has grown in many European countries and is nowadays worldwide applied. Subsurface flow wetlands use a bed of soil or gravel as a substrate for the growth of rooted emergent wetland
110
Review Articles plants. Mechanically pre-treated wastewater flows by gravity, horizontally or vertically, through the bed substrate, where it contacts a mixture of facultative microbes living in association with the substrate and plant roots. The bed depth in SSF wetlands is typically between 0.6 and 1.0 m, and the bottom of the bed is sloped to minimize overland water flow. 1.1
Elimination principles
1.1.1 General
The elimination principles are similar for all systems. Raw or pre-treated waste water flows through the constructed wetland. The elimination processes take place during this passage; they are based on various complex physical, chemical and biological processes within the association of substrate, macrophytes and microorganisms. The removal mechanisms principally depend on: • • • •
hydraulic conductivity of the substrate, types and number of microorganisms, oxygen supply for the microorganisms, and chemical conditions of the substrate.
Numerous mechanisms occur in constructed wetlands to improve water quality: • • • • • •
settling of suspended particulate matter, filtration and chemical precipitation, chemical transformation, adsorption and ion exchange on surfaces of plants, substrate, and litter, breakdown, and transformation and uptake of pollutants and nutrients by micro-organisms and plants, and predation and natural die-off of pathogens.
Wetland treatment systems are effective in treating organic matter, nitrogen, phosphorus, and additionally for decreasing the concentrations of heavy metals, organic chemicals, and pathogens. 1.1.2 The role of plants
Macrophytes growing in constructed wetlands have several properties in relation to the treatment process. This makes plants an essential part of the design of constructed wetlands. The most important effects of the macrophytes in relation to the treatment process are physical effects. The roots provide surface area for attached micro-organisms, and root growth maintains the hydraulic properties of the substrate. The vegetation cover protects the surface from erosion and shading prevents algae growth. Litter provides an insulation layer on the wetland surface (especially for operation during winter). As far as known the effect of plant uptake plays a minor role for common wastewater parameters compared to the degradation processes caused by micro-organisms. For other pollutants like heavy metals and special organic compounds the selection of different plant species can however play a major role to enhance the treatment efficiency. Both the plant productivity and the pollutant removal efficiency are relevant in finding an appropriate plant for a given application.
JSS – J Soils & Sediments 3 (2) 2003
Review Articles
Constructed Wetlands
If the wetland is not harvested the substances incorporated in the plant will be returned to the system during decomposition. When dead plants are degraded the organic constituents can act as an additional carbon source for denitrification. Some plants also release organic compounds, which can also be used for denitrification. Oxygen release from roots into the rhizosphere is well documented (Armstrong and Armstrong 1990) but in situ still a matter of controversy. Compared to the amount of oxygen brought into the system from the atmosphere via convection and diffusion, the release of oxygen from plant roots can be neglected. 1.2
Design, operation and maintenance
1.2.1 Design
The design of surface flow constructed wetlands is relatively simple, assuming that there is enough area available. Several simple equations can be used that describe the elimination processes in constructed wetlands for different parameters on the basis of the influent and effluent concentrations: • Regression models: If one correlates the mean values of the influent and effluent concentrations the result is a input/output regression model. • First-order models with background concentration: Calculating the mass balance, assuming plug-flow conditions and introducing a background concentration c* one gets the following equation to calculate the concentration c at a standardised flow distance y (inflow: y = 0; outflow: y = 1):
c −c* k = − ⋅ y ln c − c * q in
(1)
where cin = influent concentration; k = first-order areal rate constant [m/d]; and q = flow [m/d]. The change in concentration via flow distance is given by
q⋅
dc = −k ⋅ (c − c *) dy
(2)
Kadlec and Knight (1996) give values for c* and k for several parameters. •
Irreversible Models. If c* = 0 the first-order models are called irreversible models.
Kadlec (2000) summarises that first-order models are inadequate for the design of treatment wetlands. This is the conclusion drawn from results of test wetlands that showed that the overall rate constants and background concentrations are strongly dependent on both the hydraulic loading and the inlet concentration. Simple first-order models are also not suitable to handle short-cut flows in constructed wetlands due to the conceptual assumption of plug flow.
JSS – J Soils & Sediments 3 (2) 2003
Depending on the required effluent standards either horizontal or vertical flow subsurface constructed wetlands have to be used. An efficient pre-treatment regarding suspended solids removal is essential for a long term operation of subsurface flow constructed wetlands in general. If low ammonia effluent concentrations are required only vertical flow constructed wetlands with intermittent feeding can guarantee a good nitrification. Sufficient oxygen supply is the main factor for a good performance of vertical flow constructed wetlands. Several design guidelines used are based on 'rules of thumb' providing a specific area per people equivalent (e.g. the Austrian standard ÖNORM B 2505, 1997, or the German standard ATV-A 262, 1997). Design recommendations based on calculations on oxygen demand and oxygen supply are given by Platzer (1999). Theoretical considerations on long term operation of vertical flow constructed wetlands are given by Luckner et al. (1998), in particular on pore size reduction due to influent inorganic suspended solids. To check and optimise existing design guidelines, and to get a better understanding of the transformation processes inside the black box 'constructed wetland' a simulation tool for subsurface flow constructed wetland was developed (Langergraber, 2001). Up to now only the calibration of the flow model of the simulation tool was possible mainly due to the lack of measurement methods for the parameters of the reactive transport model (Langergraber, 2002). However, only a fully calibrated model can be used for optimising existing design guidelines. 1.2.2 Costs
It is well-known that when applying CWs the costs strongly depend on the specific demands and circumstances of the site, such as topography, distance to the receiving water, existing devices, availability of necessary area, etc. Additionally it has to be taken into account, whether or not the owner of the plant himself is participating in construction and/or operation. In each case the costs have to be estimated with respect to the individual circumstances. Therefore the costs given in Table 1 are not of general validity, they shall only give a range of order for vertical flow CW for wastewater treatment. The construction costs for experimental plants in Upper Austria amounted to approximately 2'200 EUR.p.e.–1. For pilot or experimental systems in general, construction costs are higher than for practical plants due to high construction Table 1: Specific costs for some constructed wetland components (including material and labour)
Component
Specific costs
Settling tank
1550
EUR / unit (5−10 p.e.)
Buffering tank
1300
EUR / unit (5−10 p.e.)
Plastic liner + geo textile
30
EUR / m2
Soil material (sand,gravel)
70
EUR / m3
Inlet pipe system
50
EUR / m
Drainage pipe system
30
EUR / m
Pumping/valve device
540
EUR / unit
111
Constructed Wetlands standard and to scientific measurement devices. The costs of practical systems are about 1000 EUR.p.e.–1. Therefore for a single-house-CW 4000–7500 EUR seem to be a realistic range for construction costs. The costs for the vegetated stage including inlet- and outlet devices, sealing, substrate, plants amount to about 100 to 150 EUR.m–2. In comparison with a conventional WWTP the construction costs for a CW are in the same range, not taking into account area requirement. However, operation/maintenance (O/M)-costs are lower for CW compared to conventional systems due to less energy demand and technical devices used. Usually the O/M-costs increase with the decreasing size of a plant (for Austria the O/M-costs are estimated to be about 300, 200 and 150 EUR.p.e.–1.a–1 for 5, 10, and 20 p.e., respectively). 1.2.3 Problems in operation and maintenance
It has to be distinguished whether or not a CW has been designed and constructed properly. CWs that have not been properly constructed suffer from various problems, like clogging, surface run-off, short-circuiting, bad plant development. As a consequence they show bad performance. In opposition to that, properly constructed CWs usually perform well. Only unpredictable problems which are common also with conventional systems occur, like failures of installation devices (i.e. pumps or valves). In addition to that a severe problem is low temperature, which does not influence the performance of the CW itself but – as an example – may affect the inflow distribution device of a VF bed which is situated above bed-surface. In temperate climates this fact has to be taken into account. 2
Application of constructed wetlands
2.1
Overview
In general constructed wetlands are a cost-effective and technically feasible approach for several reasons: • • • • • • • • • •
wetlands can often be less expensive to build than other treatment options, wetlands can be built and operated simply, wetlands utilize natural processes, operation and maintenance expenses (energy and supplies) are low, operation and maintenance require only periodic, rather than continuous, on-site labour, wetlands are able to tolerate fluctuations in flow, wetlands are able to treat wastewaters with very different constituents and concentration, they are characterized by a high process stability (buffering capacity), they are characterized by low excess sludge production, and they facilitate water reuse and recycling.
In addition: • • • •
they provide habitat for many wetland organisms, they can be built to fit harmoniously into the landscape, they provide numerous benefits in addition to water quality improvement, such as wildlife habitat and the aesthetic enhancement of open spaces, and they are an environmentally sensitive approach that is viewed with favour by the general public.
112
Review Articles The applications of constructed wetlands for treatment of water with organic contaminants are wide spread and include treatment of wastewater such as stormwater, domestic, agricultural and industrial wastewater as well as polluted groundwater and surface water. Domestic wastewater. Constructed wetlands for domestic wastewater treatment are generally applied as the main or tertiary treatment stage. For use as a main treatment stage subsurface flow constructed wetlands are used only. For tertiary treatment both surface and subsurface flow constructed wetland can be used. For constructed wetlands as the main treatment stage either horizontal flow or vertical flow constructed wetlands can be utilised depending on the desired effluent quality. A good pre-treatment is necessary to reduce the loading of suspended solids. If low ammonia effluent concentrations are required only vertical flow constructed wetlands with intermittent feeding can guarantee good nitrification. Sufficient oxygen supply is the main factor for a good performance of vertical flow constructed wetlands. Design recommendations can be based on calculations on oxygen demand and oxygen supply. Considerations on long term operation of vertical flow constructed wetlands should in particular consider pore size reduction due to influent inorganic suspended solids. To improve denitrification rates, combined systems with different types of constructed wetlands are also in use (e.g. Laber et al. 1997, and Cooper et al. 1999). Surface flow constructed wetlands are widely used as an advanced treatment stage (Kadlec et al. 2000). Horizontal and vertical flow constructed wetlands for tertiary treatment of a conventional treatment plant effluent were applied e.g. by Green et al. (1999) and Schönerklee et al. (1997), respectively. Agricultural wastewater. Constructed wetlands were used to treat agricultural wastewater from farms with animal production (e.g. Sun et al. 1999, Junsan et al. 2000). Further, crop runoff can be controlled by using constructed wetlands (Kadlec et al. 2000). Pesticides auch as Atrazine have also been effectively treated in pilot-scale tests (Kao and Wu 2000). Food wastes. Food processing wastes usually have high organic loads, which are easily biodegradable. Constructed wetlands have been used to treat potato processing water (Burgoon et al. 1999), dairy wastewater (Karpiscak et al. 1999; Geary and Moore, 1999; Pucci et al. 2000), olive oil mill wastewater (Herold et al. 2000) and slaughterhouse effluent (Ide et al. 2000). Industrial wastewater. Constructed wetlands can be used to treat several kinds of industrial wastewater: coal and metal mining waters, metal processing wastewater, hydrocarbon effluents from the petroleum industry, oil-sand processing water, and wastewater produced by pulp and paper industries. Specific examples of the various literature on this topic, for coal mining and natural gas wastewaters, are Tyrell et al. (1997), and Johnson et al. (1999), respectively. Other organic compounds produced as waste by industrial and military activity, such as chlorinated hydrocarbons, and the explosives 2,4,6-trinitrotoluene (TNT) and hexahydro-
JSS – J Soils & Sediments 3 (2) 2003
Review Articles
Constructed Wetlands
1,3,5-trinitro-1,3,5-trizine (RDX), have also been successfully removed or retained by constructed wetland systems (Wolverton and MacDonald 1986, Best 1997, 1999, 2000). The use of constructed wetlands to treat industrial wastewater is cost effective in many cases, compared with traditional 'pump and treat' remedies, as shown in the case of chlorinated alkenes (Ferraro et al. 2000). Uses of constructed wetlands technology currently under development are the removal of sulphonated aromatic compounds produced wastewater from die manufacture and removal of a wide range of nitro-aromatic compounds in industrial effluents (see case studies). 2.2
Application to specific organic compounds
Many constructed wetlands deal with domestic wastewater where BOD and COD are used as a sum parameter for organic matter. However, even special organic compounds can be removed using constructed wetlands, apart from the nitrogen and phosphorus removal normally associated with their use. Major removal mechanisms are volatilisation, photochemical oxidation, sedimentation, sorption, and microbial degradation by fermentation, aerobic and anaerobic respiration. Bioaugmentation of the sediment and sorption by Macrophytes is of particular importance. Duarte-Davidson and Jones (1996)
give a summary of which organic compounds are taken up by plants in relevant quantities. In general there is a lack of knowledge on the detailed removal pathways for organic compounds. Table 2 gives plant types that have been successfully used for specific organic applications. Experience exists with wastewater from the petroleum industry, different food processing industries, leachate, pulp and paper wastewater, and waters containing surfactants, solvents, antifreeze compounds, mineral oils and pesticides (Kadlec and Knight 1996, Behrends et al. 2001, Ji et al. 2002). Some other specific compounds that have been successfully treated are MTBE (methyl tertiary-butyl ether), trichloroethane, BTEX (benzene, toluene, ethylbenzene and xylene) and cyanide (see case studies) and explosives like TNT and RDX. A problem encountered with large organic molecules as suspended solids (SS) is the potential for clogging the pores of the substrate, particularly in subsurface flow wetlands. An account of this in the case of a constructed wetland receiving wastewater from a dairy farm, over a five year period, is given by Tanner et al. (1998). Organic matter (OM) is shown to accumulate over the duration of the study, leading to a reduction in hydraulic retention time of up to 50% so that 'pre-settling of SS in suitable settling tanks or surface-flow wetlands, which are less susceptible to clogging and more amenable to periodic desludging, is advised'. An alternative
Table 2: Plant types used for specific organic pollutions
Pollutant type
Plant used
Comments
Reference
Hydrocarbons
Phragmites spp.
Petrochemical wastewater application
Lakatos 2000
Phragmites australis
Comparative study of 11 species, in which all others died within a year Laboratory tests of straight-chain alkanes, in the range C10 to C26
Simi 2000
Typhia spp. Scirpus californicus
Oil and Grease Mineral oils Chlorinated volatiles Aromatics
Typhia spp. Phragmites spp. Typha latifolia Phragmites spp. Rumex hydrolapatum
Refinery effluent used in comparison of 3 species. Scirpus showed highest densities. Dairy effluent application, comparison study of 3 species on oils and BOD etc. Treatment of heavy-oil produced water Schilling Farm for removal of trichloroethylene from groundwater plume For removal of aniline, nitrobenzene, nitopenols and sulphonilic acid Sulphonated anthraquinones
Glycols
Phragmites spp.
Casper Phytoremediation project, to remove BTEX, MTBE and hydrocarbons For benzoic acid, used as representative compound for nonhalogenated organics Used in latter stage of airport-runoff treatment system
Atrazine
Typha latifolia
Used as part of bioaugmentation tests
TNT
Heteranthera dubia
RDX
Scirpus cyperinus
Comparison of 7 species, emergent and submerged. Choice based on removal rate Comparison study as above. Wool grass expected to be most effective/wetland area For reduction of TSS and COD in dairy application
Schoenoplectus spp. & Salix spp. Scirpus validus
General organics
Phragmites australis Brachiaria arrecta and B. mutica hybrid Phragmites spp. Monochoria vaginalis Presl
JSS – J Soils & Sediments 3 (2) 2003
Slaughterhouse effluent use. Effective in reduction of BOD, COD, N and P levels, and in promoting biomass growth Effective for Olive Mill Wastewater Pig farm application for reduction of BOD, COD, SS, S
2–
Revitt and Omari et al. 2000 Campagna and Marques 2000 Perdomo et al. 2000 Ji et al. 2002 this paper this paper this paper this paper Zachritz et al. 1996 Revitt and Warell et al. 2000 Runes et al. 2001 Best et al. 1999 Best et al. 1999 Pucci et al. 2000 Ide at al. 2000 Herold et al. 2000 Junsan et al. 2000
113
Constructed Wetlands
Review Articles
approach is given by Dobelamnn and Müller (2000) for effluent from the winery industry, which also has high organic content and particle load. They suggest using pre-treatment of waste by anaerobic digestion, before treatment using the constructed wetland. A particular study of the removal of organic compounds from wastewater contaminated by Lignite Pyrolysis, includes an analysis of the molecular size range over which the wetland under test was effective (Wießner et al.1999). The results showed that removal rates for molecules between 1 kDa and 0.3 ¼m increased with time, but that overall removal rates actually decreased, with decolourisation thus less effective over time. Molecules outside this size range were only filtered to a limited extent at any stage. The Wetland plants may themselves also contribute significantly to the OM. Clogging of the substratum should be avoided by selection of plants with low levels of refractory compounds in their litter (Tanner et al. 1998). Clogging is clear to increase with hydraulic loading for a given wastewater type, as well as the plant life suffering if exposed to a high pollutant density. Optimal loading conditions for particular waste can however be raised by selection of appropriate plant species or a good choice of system design. Examples of this are provided by Best et al. (1999) for the effect of varied TNT and RDX concentrations on different plant species, and by Zachritz et al. (1996) for the effect of system design on Benzoic acid removal. 3
Process Description (Fig. 1): Settlement tank (3,0 m³) as pre-treatment, followed by a feeding tank (2,7 m³) and a 42 m² vertical flow bed; System was designed at 5 m²/pe. The intermittent feeding is done by an automatic valve which opens 4 times a day. Since 1998 the wastewater is fed intermittently by a mechanical device without electric power. The outlet pipe is adjustable at different heights in the outlet manhole. Dimensions: 1 bed: 6,5 m x 6,5 m and 0,8 m deep. Substrate: 0–8 mm sand/gravel. Liner: 2 mm PE plastic liner. Plants: common reed (Phragmites australis). Inlet distribution: Above ground PVC pipes with 8 mm holes; below the holes plates are situated to prevent erosion. Outlet collection: Perforated drainage pipes (100 mm diameter) in 20 cm gravel layer (16–32 mm grain size).
Case Studies
3.1
Fig. 1: Scheme of CW farmhouse Schillhuber
Domestic wastewater
Effluent standard: 25 mg/l BOD5, 90 mg/l COD, 30 mg/l TSS, 10 mg/l NH4-N (at wastewater temperatures >12°C in the outlet).
3.1.1 Secondary treatment with vertical flow systems
by Raimund Haberl & Günter Langergraber Case Study: Wolfern/farmhouse Schillhuber, Upper Austria, Austria. Description: The farmhouse Schillhuber is situated in the hilly region of Upper Austria. It is too far away from the village Wolfern to be connected to the sewer line. Therefore it was selected within a pilot project to be an example for the many other farms in the area. The CW was designed by the IWGA-SIG of the University of Agricultural Sciences Vienna (BOKU). Constructed: Spring/Summer 1991. Operational: September 1991 to date.
Performance: Table 3 shows the performance results from 1992– 1997. As can be seen the Austrian effluent standards are being met easily. The phosphorus elimination decreased from 72% to about 40–50% within the operation time due to limited adsorption potential of the substrate. During 1995 an experiment to increase the total nitrogen (TN) elimination was undertaken. A recirculation pump was installed in the effluent which pumped the nitrified effluent into the settlement tank of the pre-treatment. An 80% recirculation rate increased the TN elimination to 72% (from originally 40% without recirculation).
Table 3: Performance of the Wolfern/farmhouse Schillhuber vertical flow constructed wetland from 1992 to 1997 (given figures are yearly averages) Year
Hydrolic
BOD5
Removal
load (mm/d)
In mg/l
Out mg/l
1992
24
143
11
1993
32
186
1994
27
1995
30
1996
COD
Removal
(%)
In mg/l
Out mg/l
92
378
54
9
95
533
139
3
98
120
3
98
37
157
