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Constructed Wetlands for Wastewater TreatmentM. Sundaravadivel a & S. Vigneswaran ba Graduate School of the Environment, Macquarie University, Sydney, Australiab Faculty of Engineering, University of Technology, Sydney, Australia
Available online: 03 Jun 2010
To cite this article: M. Sundaravadivel & S. Vigneswaran (2001): Constructed Wetlands for Wastewater Treatment, CriticalReviews in Environmental Science and Technology, 31:4, 351-409
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Critical Reviews in Environmental Science and Technology, 31(4):351–409 (2001)
1064-3389/01/$.50© 2001 by CRC Press LLC
Constructed Wetlands for WastewaterTreatmentM. SundaravadivelGraduate School of the Environment, Macquarie University, Sydney, Australia
S. Vigneswaran*
Faculty of Engineering, University of Technology, Sydney, Australia
ABSTRACT: In the field of wastewater treatment, energy-intensive and highly mechanized tech-nologies are giving way to nature-based technologies that utilize solar energy and living organisms.Constructed treatment wetland (CTW) technology has played an important role in bringing about thechange. Wetland technology can provide cheap and effective wastewater treatment in both temperateand tropical climates, and are suitable for adoption in both industrialized as well as developingnations. Currently, CTWs are being utilized for removal of a range of pollutants and a broad varietyof wastewaters worldwide. The objective of this article is to provide a comprehensive review of theCTW technology and to present the pollutant removal performance experiences gathered through theapplication of this technology around the world.
* Dr . S. Vigneswaran, Professor of Environmental Engineering, Faculty of Engineering, University of Tech-nology, Sydney, PO Box 123, Broadway, NSW 2007, AUSTRALIA. Tel: 61 2 9514 2641; Fax: 61 2 95142633; Email: [email protected]
Table of ContentsI. Background .............................................................................................. 352
A. Natural Systems for Wastewater Treatment .................................. 353B. Natural Wetland Systems ............................................................... 354
II. Constructed Wetlands .............................................................................. 355A. Constructed Habitat Wetlands ........................................................ 356B. Constructed Flood Control Wetlands ............................................. 356C. Constructed Aquaculture Wetlands ................................................ 357
II. Constructed Wetlands for Wastewater Treatment .................................. 357A. Components of Constructed Treatment Wetland (CTW) .............. 358
1. Wetland Vegetation ................................................................ 3582. Soil or Substrate (Media) Supporting Vegetation ................. 3623. Water Column (In and Above the Substrate) ........................ 3644. Living Organisms ................................................................... 364
B. Types of Constructed Treatment Wetlands .................................... 364IV. Pollutant Removal Mechanisms .............................................................. 367
A. Physical Processes .......................................................................... 368B. Chemical Processes ......................................................................... 368
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C. Biological Processes ....................................................................... 368D. Limiting Factors of Wetland Processes ......................................... 369
V. Design of Constructed Wetlands............................................................. 370A. The UK Model for Design of Constructed Wetlands .................... 371B. The First-Order Plug-Flow Biokinetic Model ............................... 371C. Design Procedure ............................................................................ 373D. Design Example .............................................................................. 374
VI. Construction of Treatment Wetlands ...................................................... 375A. Site Selection .................................................................................. 375B. Pretreatment .................................................................................... 375C. Configuration .................................................................................. 377D. Inlet Structure, Feeding Arrangements, and Outlet Structures ...... 379E. Reed (Wetland Plant) Beds ............................................................ 384F. Vegetation Planting......................................................................... 384
1. Use of Seeds ........................................................................... 3852. Use of Propagated Seedlings ................................................. 3853. Use of Transplantation ........................................................... 3864. Use of Rhizomes .................................................................... 386
G. Establishment of Vegetation ........................................................... 386VII. Operation and Maintenance .................................................................... 387VIII. Performance of Constructed Wetlands ................................................... 388
A. Organic Substances ......................................................................... 3891. Organic Pollutant Removal Performance .............................. 391
B. Nutrients Removal .......................................................................... 3921. Nutrient Removal Performance ............................................. 396
C. Pathogen Removal .......................................................................... 3971. Pathogen Removal Performance ............................................ 398
D. Heavy Metals Removal .................................................................. 4021. Heavy Metals Removal Performance .................................... 403
IX. Concluding Remarks ............................................................................... 403References ................................................................................................ 404
I. BACKGROUND
For the last more than 5 decades efforts toward controlling water pollution dueto municipal wastewater in industrialized countries have almost exclusively fo-cused on implementation of expensive centralized collection and treatment pro-cesses. Because of the highly technical and mechanical nature of conventional‘concrete and steel’ treatment units, the service life of these facilities is less than25 to 30 years. Most of the treatment facilities in these countries are already in needof overhaul or complete replacement, and there is a burgeoning dilemma on howto address the funding needs for this cause (Grove and Silberman, 1995).
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In contrast, in developing countries, where still an estimated 2 billion peopledo not have access to ‘adequate’ sanitation facilities, the main cause of waterpollution is disposal of untreated domestic wastewater and its most importanteffect is on human health. Rivers of Asia are among the most polluted in the worldand contain 10 times as many bacteria from human wastes as compared to thedeveloped countries (Saxena and Frost, 1992). It is clear that the course of actiontaken by the industrialized nations is not a viable alternative for developingcountries. The high cost of infrastructure investment, continual replacement andongoing operation costs of conventional wastewater management facilities, takethese technologies beyond the financial grasp of most developing countries. Thus,there is a critical need for cost-effective, long-term, wastewater treatment tech-nologies to deliver public health and environmental protection in both the devel-oped and developing countries.
A. Natural Systems for Wastewater Treatment
Conventionally, wastewater treatment is accomplished by physical, chemical,and biological processes. Typically, these processes are supported by naturalcomponents such as microbial organisms, but in a complex array of energy-intensive mechanical equipment. Conventional treatment systems, therefore, con-tribute to (1) depletion of nonrenewable fossil fuel sources, and (2) environmentaldegradation that occur due to extraction of nonrenewable resources, and also dueto the byproducts/final products of these technologies, such as biosolids andsludge. Therefore, attempts for developing cost-effective treatment approach al-ways revolved around using only the natural components devoid of any mechanicalrequirements that use up energy. Using plants to purify wastewater has alwaysfascinated researchers and holds intuitive appeal to the general public as well.Consequently, many natural systems that use the ability of plant species in uptaking
FIGURE 1. Wetland zone formation on landscapes (Source: Hammer, 1990).
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or degrading the pollutants were developed. The natural treatment systems thathave been developed so far can be categorized into three major categories (Reedet al., 1995):
1. Aquatic or pond/ lagoon systems,2. Terrestrial or land application systems, and3. Wetland systems
The aquatic natural treatment systems involve impounding wastewater inponds or lagoons for sufficient period so that pollutants and pathogens in waste-water are removed through natural biological degradation processes. Floating plantspecies such as algae, duckweed, and water hyacinth may also present in thesesystems to support the biological processes. Oxidation ponds, facultative lagoons,and waste stabilization ponds are some of the examples for aquatic treatmentsystems.
Terrestrial or land application-based natural wastewater treatment systemsinvolve utilizing unsaturated soil layer to provide either direct filtration and assimi-lation of pollutants or a rooting medium for plant growth that aid in filtration anduptake of pollutants from wastewater. On-site water filtration systems, low-rateand high-rate land application systems and overland flow systems are examples ofterrestrial treatment systems.
Wetland treatment systems use either the natural wetlands or constructedwetlands for treatment of wastewater. Wetlands have been used as conve-nient wastewater discharge sites for more 100 years in some regions. Whenmonitoring was initiated at some of the existing wetland discharges, anawareness of the water purification potential of wetlands began to emerge.Significant advances have since been made in the engineering knowledge ofcreating constructed wetlands that can closely imitate the specialized treat-ment functions that occur in the natural wetland ecosystems. Among thenatural treatment systems, the many advantages such as simplicity of designand lower costs of installation, operation, and maintenance offered by con-structed wetlands make them an appropriate alternative for both developedand developing countries.
B. Natural Wetland Systems
Natural wetlands are the areas of transition between terrestrial and aquaticsystems (Figure 1). These are land areas that are wet during part or all of the yearbecause of their location in the landscape. Wetlands may be called swamps,sloughs, marshes, bogs or ecotones depending on the types of plants in these areasas also their geographic locations.
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While there is no single definition that can correctly describe wetlands of alltypes and for all purposes, the Clean Water Act of the US Government defineswetlands as:
areas that are inundated or saturated by surface or groundwater at a frequency andduration sufficient to support, and that under normal circumstances do support, a preva-lence of vegetation typically adapted for life in saturated soil conditions (Mitsch andGosselink 1993).
Another definition of wetlands for scientific purposes was proposed by theU.S. Fish and Wildlife Services, which describes wetlands as the transition areasbetween terrestrial and aquatic systems where water is the dominant factor deter-mining development of soils and associated biological communities. The defini-tion specifies that wetlands need, at least periodically, to fulfill one or more of thefollowing four requirements (Hammer 1990):
• Areas where the water table is at or near the surface or the land is coveredby shallow water
• Areas supporting predominantly hydrophytes (water-tolerant plant species)• Areas with predominantly undrained hydric soils. Hydric soils are those that
are sufficiently wet for long enough to produce anaerobic conditions, therebylimiting the types of plants that can grow on them
• Areas with non-soil substrate such as rock or gravel that are saturated orcovered by shallow water at some time during the growing season of plants.
Wetlands that are dominated by water tolerant woody plants are generallycalled as swamps; those with soft-stemmed plant species as marshes; and thosewith mosses as bogs. Swamps and marshes can be of either salt water or freshwatertype. Saltwater swamps are popularly known as mangroves.
Wetlands have high rate of biological activity and hence high rate of vegetativegrowth as well as zooplanktons. Wetlands along the shores of seas, lakes andriverbanks play a valuable role in their stabilization and protection from erosivetides, waves, storms, floods and winds. They also function as groundwater re-charge areas and sometimes as discharge areas where the water table touches thesurface level. Because of their ability to transform and store organic matter andnutrients, wetlands are often described as the ‘kidneys of the landscape’.
II. CONSTRUCTED WETLANDS
Constructed wetlands, as the term suggests, are man-made wetlands artificiallydeveloped in areas where they do not occur naturally. Constructed wetlands maybe developed for one or more of the following reasons:
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• To create wetland habitats to compensate and offset the rate of conversionof natural wetlands resulting from agricultural and urban development, andhence to conserve native flora and fauna including aquatic plants, fish, waterbirds, reptiles, amphibians and invertebrates (Constructed habitat wetlands);
• To act as a flood control facility (Constructed flood control wetland);• To be used for production of food and fiber (Constructed aquaculture
wetlands); and• To be a wastewater treatment system and to improve water quality (Con-
structed treatment wetlands).
Although constructed wetlands are being developed in many parts of the worldfor various functions, their wastewater treatment capabilities have attracted re-search efforts for a wide range of treatment applications including domesticwastewater, urban stormwater, industrial/agricultural flows, landfill leachates, acidmine drainage, etc.
A. Constructed Habitat Wetlands
Wetlands constructed primarily to develop a wildlife habitat are referred to asconstructed habitat wetlands. In addition to emergent wetland plants, most con-structed wetlands have adequate supply of water, either in the subsurface environ-ment or as surface water. These two components provide the essential basis of anecological habitat. Habitat wetlands are being constructed in all four major catego-ries of wetlands, namely, salt marshes, saltwater swamps, freshwater marshes, andfreshwater swamps. Saltwater swamps and marshes are constructed close to estua-rine waters so as to provide water in the correct salinity range to encourage theestablished species. Freshwater swamps and marshes are constructed near anyupland location with the principal consideration being the availability of a predict-able source of water for creating appropriate environmental conditions for thewetland plant species. There is a growing consideration for utilization of treatedwastewater as the water source for freshwater-constructed habitat wetlands. Impor-tant examples of such systems include Arcata Wildlife Sanctuary in California, theOrlando Wilderness Park and Greenwood Urban Park in Florida, and the WhangereiTreatment wetlands in New Zealand (Knight, 1997).
B. Constructed Flood Control Wetlands
Flood control systems that include significant areas of wetlands vegetation arecalled constructed flood control wetlands. In Massachusetts, riverine floodplain wet-lands were deemed so effective for flood control by the U. S. Army Corps of Engineersthat they purchased the wetlands rather than build expensive flood control structures
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(TPWD, 2001). Varying sizes of impoundments are being used to store naturalfloodwaters to offset their losses due to urban and agricultural development. Emergentwetlands plants either naturally or artificially colonize most of such systems. Thesesystems are engineered to provide specific hydraulic functions for water storage and forbleed-off intervals following storms. Constructed flood control wetlands are sitedgenerally at low elevations in the landscape to allow gravity inflows from the adjacentupland system generating runoff. In some cases, they may have to be located infloodplain areas, resulting in wetland functions in addition to flood control. Fluctuatingwater levels in these type of constructed wetlands may limit their use for any secondaryfunctions such as a habitat or water quality improvement.
C. Constructed Aquaculture Wetlands
Historically, wetland systems have been used for husbandry of aquatic foodspecies such as crayfish, shrimps, and prawns. Water-tolerant food plant species suchas rice, cranberries, and water chestnuts are technically wetland plants, hence thereis potential for compatible aquaculture within wetlands constructed for water qualitymanagement or for flood control. This potential is being explored for rice culture inBrazil and China. Increased fish and wildlife productivity in wetlands receivingelevated nutrients in wastewater inflows can be harvested for economic gains.
III. CONSTRUCTED WETLANDS FOR WASTEWATER TREATMENT
In 1953, Dr. Seidel of Max Planck Institute in Plon, Germany, first reportedabout the possibility to lessen the overfertilization, pollution, and silting up of inlandwaters through appropriate plants (Brix 1994a). Since its initial ‘discovery’, effortsto harness and develop the natural treatment ability of wetland systems have beenundertaken by both government and private research interests around the world.
Constructed treatment wetlands offer effective, reliable treatment to wastewa-ter in a simple and inexpensive manner. Major advantages of CTWs include:
• They operate on ambient solar energy and require low external energy input;• They achieve high levels of treatment with little or no maintenance, making
them especially appropriate in locations where no infrastructure supportexists;
• They are relatively tolerant to shock hydraulic and pollutant loads thatensures the reliability of treated wastewater quality;
• Unlike the conventional treatment systems, no specific design life period isgenerally prescribed for CTWs and as such they tend to have increasedtreatment capacity over time, by setting up feedback loops that result in self-repairing systems;
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• Wetland vegetation generate oxygen and consume carbon dioxide, therebyhelp improving air quality and fight global warming; and
• Wetland vegetation provide indirect benefits such as green space, wildlifehabitats, and recreational and educational areas.
Some of the limitations of CTWs include:
• They require large land area for the same level of treatment by conventionalsystems making them unsuitable for centralised treatment for sources thatgenerate large quantities of wastewater, such as large cities;
• They require long period, typically two or three growing seasons, for thevegetation before optimal treatment efficiencies are achieved;
• The process dynamics of the CTW systems are yet to be clearly understoodleading to imprecise design and operating criteria;
• These systems typically lie outdoor and spread over large area, their perfor-mance is susceptible to storm, wind, and floods;
• There are possibilities of problems due to mosquitoes and other pests andinsects that may use these systems as their breeding ground; and
• Steep topography and high water table may also limit the adoption of thesesystems for wastewater treatment.
A. Components of Constructed Treatment Wetland
1. Wetland Vegetation
Originally, the basis for employing constructed wetlands for wastewater treatmentis the ability of water plants to translocate oxygen to their roots, and the surroundingwater (wastewater, in case of treatment wetlands) environment. Although a number ofother pollutant removal processes have been identified, the wetland plants play a majorrole in the occurrence of most of these processes. Within the water column, the stemsand leaves of the wetland plants significantly increase surface area for biofilm devel-opment. Plant tissues, moreover, are colonised by photosynthetic algae as well as bybacteria and protozoa. Likewise, the roots and rhizomes that are buried in the wetlandsubstrate provide for attached growth microorganisms (Brix, 1997). Major roles ofvegetation in constructed treatment wetlands are summarized in Table 1.
a. Wetland Plant Species
A wide variety of aquatic plants can be used in constructed wetland systemsdesigned for wastewater treatment. Commonly, however, constructed treatmentwetlands are planned as marsh-type wetlands and are planted with emergent
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macrophytes (rooted plants that anchor to the substrate media) that are adapted towater-dominated environment. Frequently used macrophytes species are cattails(Typha sp.), reeds (Phragmites sp.), bulrushes (Scirpus sp.), and sedges (Carexsp.). The general requirements of plants suitable for use in constructed wetlandwastewater treatment systems include (Tanner, 1996):
1. Ecological acceptability, that is, no significant weed or disease risks or dangerto the ecological or genetic integrity of surrounding natural ecosystems;
2. Tolerance of local climatic conditions, pests and diseases;3. Tolerance of pollutants and hypertrophic water-logged conditions;4. Ready propagation, and rapid establishment, spread and growth; and5. High pollutant removal capacity, either through direct assimilation or storage,
or indirectly by enhancement of microbial transformations.
Specific requirements will vary depending on the functional role of wetlandplants in the treatment systems. This will be related to the type of wetland design
TABLE 1Major Roles of Macrophytes in Constructed Wetlands
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and its mode of operation (continuous or batch), loading rate, and wastewatercharacteristics. Other ancillary objectives (such as ecological, aesthetic, recre-ational, and economic) of wetland developments may also affect the choice of theplants.
b. Oxygen Supply
Oxygen can be considered to cycle within wetlands. Oxygen enters via inflowsor by diffusion at the water surface when the surface is turbulent (for example, dueto wind mixing). Oxygen is also produced within the water column during photo-synthesis. It is well documented that aquatic macrophytes release oxygen fromroots which influences the biogeochemical cycles in the sediments due to theeffects on the redox status of the sediments (Barko et al., 1991; Sorrel and Boon1992). Qualitatively this is visualized by the reddish color associated with oxidizedforms of iron on the surface of the roots.
Wetland plants have not evolved new and novel structures for dealing with theenvironment where there is reduced oxygen availability for respiration, and lowerlight penetration and scarcity of carbon dioxide for photosynthesis. Rather theysurvive the harsh environment through many structural and physiological adapta-tions (Guntenspergen et al., 1990). Formation of lacunae and/or arenchyma tissueis a characteristic feature of non-woody wetland plants that are commonly adoptedin treatment wetlands. The lacunae or gas filled permeable arenchyma allowsoxygen from the atmosphere to be provided for root metabolism.
In anaerobic soils, oxygen is transferred to the roots primarily for plantrespiration (Kadlec 1995) and only excess oxygen is leaked to the micro-zonearound the root (rizhosphere). Oxygen release is primarily at the root tip to detoxifyand oxidize potentially harmful substances in the rhizosphere (Armstrong andArmstrong, 1990). In this zone, oxidation reactions can take place, while anaerobicreactions can occur only microns away (Figure 2).
However, there are also some controversial claims about the magnitude ofoxygen supply by plants to the wastewater in constructed wetland systems.Armstrong and Armstrong (1988) found little oxygen actually escaping from rootzone, and the small amounts of this surplus oxygen varies with the plant species.Lawson (1985) calculated that up to 4.3 g.m–2.day–1 of oxygen flux is possible fromthe roots of Phragmites sp. Brix and Schierup (1990) found that of a total oxygeninflux of 5.86 g.m–2.day–1 to a wetland planted with Phragmites, 3.76 g come fromthe atmosphere directly to the water column. The remaining 2.1 g oxygen influxwas through the plant tissues, of which 2.08 g gets transferred to the rizhomesystem for root respiration. Only 0.02 g of oxygen that was in excess of plantrequirement was leaked by the roots into the water environment. Gries et al. (1990)measured root oxygen release to be in the range of 1 to 2 g.m–2.day–1. Buchanan(1987) found that in the short term, there were no differences between the treat-
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ment performance of filter beds planted with reeds and plastic substitutes in surfaceflow wetlands, and the beds with plants performed badly under high organicloading rates.
c. Physical Effects
The physical presence of vegetation in wetlands distributes and reduces thecurrent velocities of the water, which creates better conditions for sedimentationof suspended solids. Light attenuation by the wetland plants hinders the productionof algae in water below the vegetation cover. The vegetation cover in a wetland canbe regarded as a thick biofilm located between the atmosphere and the wetland soilor water surface in which significant gradients in different environmental param-eters occur. Wind velocities are reduced near the soil or water surface comparedto the velocities above the vegetation, which reduces resuspension of settledmaterial and thereby improves the removal of suspended solids by sedimentation.In temperate areas, the plant cover provides insulation during winter and helpskeep the substrate free of frost (Smith et al., 1997).
d. Hydraulic Conductivity
In treatment wetlands, wastewater flow is largely intended to be below the surfacethrough channels created by living and dead roots as well as through the pore space of
FIGURE 2. Oxygen transfer through root zone.
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the substrate medium. As roots and rhizomes grow, they disturb and loosen the soil.Further more, when the roots and rhizomes die and decay, they may leave behind tubularpores and channel, which can improve the hydraulic conductivity of the substrate. Thismay be largely true with gravel medium based substrate. On the contrary, the hydraulicconductivity of soil-based systems often decrease (Marsteiner et al., 1996). Data onhydraulic conductivity in soil-based reed beds in Austria, Denmark, and in the UK alsodo not support the increase in hydraulic conductivity due to wetland plants in soil-basedsystems (Conely, Dick, and Lion, 1991; Haberl and Perfler, 1990).
e. Nutrient Uptake
Nutrient requirement for growth of wetland macrophytes, mainly the nitrogen andphosphorus, are taken up primarily through their root systems. Marginal uptake occursalso through immersed stems and leaves from the surrounding water (Gumbricht,1993). Thus, the vegetation may be helpful in removal of nutrients from wastewater.Shaver and Mellio (1984) have shown that nutrient uptake is maximum during theinitial period of establishment of the plants in constructed wetlands, and the efficiencytends to decrease as available nutrient input rises, that is, when the nutrient loading ratesincreases, the uptake of nutrient by plants decreases. Tanner (1996) found that duringthe first 2-year period of operation, plant uptake of nutrients could only account for 6to 10% for nitrogen, and 6 to 13% of phosphorus removal from wetlands.
f. Organic and Antibiotic Excretion
Root systems of wetland plants also release substances other than oxygen.Early experiments of the Max Planck Institute in Germany showed that the bulrushSchoenoplectus released antibiotics from its roots. It is also known that a range ofsubmerged macrophytes release compounds that affect the growth of other species.However, the role of these compounds in wetland treatment processes has not yetbeen experimentally verified. Plants also release a wide range of organic com-pounds through their roots (Rovira, 1969; Barber and Martin, 1976). Reportedvalues of these organic compounds are in the range of 5 to 25% of the photosyn-thetically fixed carbon (Brix, 1997). The organic carbon so excreted may act as acarbon source for denitrification process, and hence enhance the nutrient removalprocess in constructed wetlands (Platzer, 1996).
2. Soil or Substrate (Media) Supporting Vegetation
The media that physically supports vegetation in a constructed wetland is vitalas it forms an integral link in treatment processes that occur in the wetland. While
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soil will generally be the support media in natural wetlands, constructed treatmentwetlands more often rely on coarse and fine gravel. Apart from supporting vegeta-tion, the substrates also act as the principal storage of all biotic and abioticcomponents that exist in a wetland. In addition, coarse sand and gravel substratesprovide surface area for attached growth microorganisms and promote filtrationand settling of suspended solids.
Hydraulic conductivity of the substrate is a major factor in constructed treat-ment wetlands. Maintenance of hydraulic conductivity is required to stabilize thehydraulic retention time of the wetland system. Wetland systems with fine and soilbased substrate will have low hydraulic conductivity, while coarse sand and gravelbased medium display higher conductivity. Soils will have a hydraulic conductiv-ity of 10–5 ms–1 or less, whereas a uniform gravel in the range of 3 to 6 mm or 5to 10 mm will have an initial values in the order of 10–2 ms–1 or higher (Chen etal. 1993). Characteristics of various types of media and their hydraulic conductiv-ity are presented in Table 2.
A common problem in constructed wetland operation is clogging. Severalstudies with soil based treatment wetlands have reported problems of clogging andcausing overflows of wastewater resulting in bed erosion and poor plant growth(Cooper and Hobson, 1990). Infilling and occlusion of interstitial spaces by solidswill reduce the effective volume available within the substrate, leading to increas-ing flow velocities, decreasing hydraulic retention times, and short-circuiting.Many cases of surface flow problems investigated by USEPA (1993) were attrib-uted to inadequate hydraulic design, or introduction of fine inorganic sedimentsduring construction and planting, which clogged void spaces in the bed. Thisphenomenon leads to bypassing and hence reduced treatment performance. There-fore, soil is usually not recommended as substrate for wastewater treatment wet-lands, and gravel has been used in reed bed systems in several countries. Gravelallows through-flow of water from the start, and, if the bed gradually starts to clogwith solids, this can be counterbalanced by the growth of rhizomes and rootsopening up the bed. The rate of clogging in gravel-bed wetlands will initiallydepend only on influent solids loading and efficiency of retention. In the longerterm, factors such as the degradable fraction of the suspensoids and their rate of
TABLE 2Media Characteristics
a
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microbial and chemical degradation under the wetland environment will determinethe solids accumulation and hence the clogging rate.
Beauchamp et al. (1988) calculated the theoretical service life of a hypotheti-cal gravel-bed constructed wetland in relation to clogging by organic and inor-ganic wastewater solids and microbial detritus (but ignoring plant litter contri-butions) to be in the order of 100 years. Reed and Brown (1992) estimated solidsaccumulation during the first 18 months operation of a coarse gravel wetland tobe less than 1% of the available pore space of the media. Measurements ofhydraulic gradients in gravel-bed wetlands treating domestic wastewater at Rich-mond, Australia (Fisher, 1990) showed major reductions in substrate hydraulicconductivity at the head of the wetlands during the first year of operation. Thedownstream permeability remained relatively stable over the 2 1/2 years moni-toring period, with no indication of advancement solids accumulation along thelength of the bed.
3. Water Column (In and Above the Substrate)
Maintaining the water column is an important requirement of constructedwetlands since the water level governs the major ecological functions occurring inthe system. Water provides the environment for biochemical reactions to occur andacts as a transport medium to carry the end-products such as gases, organic acidsetc., from one reaction site to another reaction site.
4. Living Organisms
A variety of beneficial micro- and macroorganisms is an integral part ofwetland ecosystem. While the presence of vertebrates and invertebrates (higherlevel animals) may not be essential for the functioning of CTWs, microbial formsof organisms play a critical role. Microorganisms that are naturally found inwater and wastewater, such as bacteria, fungi, protozoa, etc., thrive in wetlands,which provide suitable environmental conditions for their survival and prolifera-tion.
B. Types of CTWs
Depending on the level of water column with respect to the substrate bed,CTWs are classified into two broad categories as:
1. Surface flow (SF) wetlands, and2. Sub-surface flow (SSF) wetlands.
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In SF wetlands (Figure 3), the substrate bed is densely vegetated and the watercolumn will be above the surface of the bed. The SF systems are flooded andexpose water surface in the system to the atmosphere. Plants predominantly growon soil bed in these systems and the depth of water column is typically less than0.4 m.
In SSF wetlands, the water level is maintained below the surface of thesubstrate bed. The substrate medium in SSF wetlands is usually made of gravel toprovide high void space to enable wastewater loaded on the bed to quickly seepthorough the bed. Soil-based SSF wetlands are also found in northern Europe(Kadlec 1995).
Depending on the direction of flow of applied wastewater, SSF wetlands canbe either horizontal flow type or vertical flow type. In horizontal SSF systems(Figure 4), the substrate is maintained water-saturated through continuous applica-tion of wastewater. The bed depth of horizontal SSF wetlands is typically less than0.6 m and the bottom of the bed is sloped to minimize flow above the surface.
In vertical SSF wetlands, wastewater is applied through different arrangementof wastewater feeding and collection mechanisms to maintain a vertical directionof flow. This is achieved either by intermittent wastewater application or byburying inlet pipes into the bed at a depth of 60 to 100 cm (Figure 5). The totaldepth of bed is in the range of 2 to 3 m. Since the wastewater infiltrates throughthe substrate bed, this type of wetlands are also called ‘infiltration wetlands’.
Constructed wetlands are also classified on the basis of how macrophytes(plants) grow in the system. Thus, wetlands can be
1. Floating macrophye systems;2. Submerged macrophyte systems; or3. Rooted emergent macrophyte systems.
Floating macrophytes are plant species that float on the surface of the water,and do not require a substrate for their growth. Alage, duckweed (Lemna sp.),water hyacinth (Eichornia crassipes),
M. aquaticum and Salvania sp. are some of the floating macrophytes adaptedfor wastewater treatment.
FIGURE 3. Surface flow CTW.
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Submerged macrophyte systems have plants species that are submerged in thewater column and do protrude beyond the water surface. Isoetes lacustris, Lobeliadortmanna, Egeria densa, and Elodea canadensis are among the submerged aquaticplant species.
Rooted emergent macrophytes are plants that are generally attached to thesubstrate in the wetland with leaves extending above the water surface. Reeds(Phragmites sp.), cattails (Typha sp.), bulrushes (Scirpus sp.) and sedges(Carex sp.) are common among the emergent aquatic plant species used intreatment wetlands. Unless specifically mentioned otherwise, wetland treat-ment systems mean constructed wetlands planted with rooted emergent macro-phyte species.
FIGURE 4. Horizontal flow subsurface CTW.
FIGURE 5. Vertical flow sub-surface CTW.
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IV. POLLUTANT REMOVAL MECHANISMS
Removal of pollutants in a constructed treatment wetland is believed to beaccomplished in the following ways:
• Direct uptake of pollutants by the plants• Plants and substrate media provide large surface area for proliferation of
microorganisms that degrade pollutants• Sedimentation of solids due to the decreasing velocity of flow through
CTWs• Filtering of large particles occurs through root and reed masses• Adsorption of nutrients (such as nitrates and phosphates) by soil and sub-
strate media• Wetland detention time allowing for natural die-off of pathogens• UV radiation and excretion of antibiotics by plants to destroy pathogens
Pollutant removal processes occur by interaction with wetland vegetation, thewater column, and the wetland substrate. Table 3 sets out details of process typesand the pollutants removed. The processes may be physical, chemical, or biologi-cal.
TABLE 3Pollutant Removal Processes
a
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A. Physical Processes
Physical pollutant removal processes in wetland systems occur due to thepresence of plant biomass and the substrate media. Plants physically retard thepathways of wastewater enhancing sedimentation of suspended solids. The media(soil or gravel) acts as filter beds as in filtration processes, thereby aiding physicalremoval of suspended solids through straining.
B. Chemical Processes
Chemical reactions between substances, especially metals, can lead to theirprecipitation from the water column as insoluble compounds. Exposure to atmo-spheric gases and sunlight can lead to breakdown of organic pesticides and destruc-tion of pathogens. Antibiotic chemicals excreted by plants can also play a role inremoval of pathogens present in wastewater.
C. Biological Processes
Constructed wetland systems are biological systems in which biological pro-cesses play a major role in removal of pollutants. Six major biological reactionshave been identified as aiding the pollutant removal performance of wetlandsystems (Mitchell, 1996b). These are: photosynthesis, respiration, fermentation,nitrification, denitrification, and phosphorus removal.
Photosynthesis is performed by wetland plants, which results in removal oforganic carbon from the water column and addition of oxygen. Oxygen exhaled byplant leaves during photosynthesis may increase the partial pressure of oxygen inthe atmosphere close to water surface, and hence enhance diffusion into water(Bedford et al., 1991). If floating or submerged plants are present, they will directlyexhale oxygen into water. Oxygen while aiding biological degradation of organicpollutants, in combination of carbon, it drives the nitrification process.
During respiration, oxygen leaking out through the root hairs of the plants intothe surrounding water environment may create an oxygen-rich area around the rootzone (rhizosphere) of plants. Such oxygen leaks helps to maintain partial aerobicconditions in the water column (Hiley, 1995).
Fermentation refers to the decomposition of organic carbon in the absence ofoxygen into energy-rich compounds like methane, alcohol, and volatile fatty acids.Fermentation is activated by the metabolic activities of microbial organisms presentin the water column and the substrate media.
Nitrification/denitrification processes also are mediated by the microorgan-isms and result in removal of nitrogen from wastewater. While aiding biologicaldegradation or organic pollutants, oxygen in combination with carbon, drives the
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nitrification process. The physical process of volatilisation is also contributes tonitrogen removal in treatment wetlands. The matrix of aerobic and anaerobicenvironments that develop in the wetlands help achieve nitrification and at thesame time denitrification (Armstrong and Armstrong, 1988).
Biological phosphorus removal occurs within biofilms that develop on thesubstrate and microorganisms present in sediments.
Plants take up dissolved nutrients and other pollutants from water and convertthem into additional plant biomass. The nutrients and pollutants then move throughthe plant body to underground storage organs. When the plants age and die, theseare deposited into the sediments as litter and peat.
Microorganisms present in the wetland system, including bacteria, fungi,coagulate colloid material, stabilize and remove dissolved and colloidal organicmatter by converting them into various gases and new cell tissues. Many of themicroorganisms in wetlands are the same as those occur in conventional biologicaltreatment systems.
D. Limiting Factors of Wetland Processes
As with any other biological wastewater treatment systems, the process ratesare dependent upon various environmental factors such as temperature, pH, oxy-gen availability, hydraulic and pollutant loads (DLWC 1998a). The chemical andbiological processes are specifically prone to changes in environmental factors.Under some environmental conditions, process rates may slow down cease alto-gether, or even reverse, releasing pollutants. Biological activities in the rhizosphere
TABLE 4Effects of Pollutant Overloading on Wetland Processes
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and in the biofilms are particularly sensitive. In general, effectiveness of pollutantremoval processes that rely on biological activities may be reduced due to decreasein metabolic activities caused by low temperature. Many metabolic and chemicalactivities are also pH dependent, and are less effective if pH is too high or too low.
The treatment capacity of constructed wetland is limited also by the hydraulicand pollutant loading rates. Hydraulic overloading occurs when the flow exceedsthe design capacity, thus reducing the actual hydraulic retention time. Pollutantoverload occurs when the influent pollutant loads exceed the process removal ratesof the system. Other environmental factors, including excessive organic matter,nutrient or toxins, or lack of oxygen, also have effects on wetland processes. Table4 summarizes the effects of pollutant overloads.
Depending on the size and design of wetland, the salinity of water withinwetlands can increase as the water levels drop, and the pollutants may becomeconcentrated. Subsequent high flows may flush pollutants from the system trans-porting them to the discharging water bodies.
V. DESIGN OF CONSTRUCTED WETLANDS
Constructed wetlands are getting wide recognition as effective alternative forwastewater treatment at reduced costs. The process of designing and predictingperformance of wetlands is improving rapidly, as more experience is gained withthe operation of these systems. Designing constructed wetlands entails:
• Sizing for a particular wastewater flow rate, pollutant loading, and desiredremoval of a given pollutant;
• Inlet and outlet structures for water level control, recycling, flow splittingand distribution;
• Flow path configuration for cells in parallel and/or series;• Depth variation within and between cells for habitat diversity (if and when
required), better flow distribution, and pollutant removal;• Planting details, including species selection, planting density, range of
species; and• An operation and maintenance plan.
Different sets of guidelines for design of constructed wetlands have been devel-oped. Kadlec and Knight (1996) pointed to the exponential growth of new informa-tion in the field of constructed wetlands and warn against the blanket use of simplisticdesign guidelines for all situations. The approaches currently used to design con-structed wetlands are not significantly different from the approaches used in conven-tional biological treatment systems. Constructed wetlands are commonly designed asattached growth biological reactors. In attached growth processes, sufficient contactwith biofilms on such substrates as gravel, plant stems, roots, and sediment layers is
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important, because much of the pollutant removal is mediated by microbial activity.To maximize pollutant removal by this process, wetland design aims to optimize thetheoretical hydraulic retention time (HRT), and then ensure that the actual HRT isas close as practicable to the theoretical HRT.
A. The UK Model for the Design of Constructed Wetlands
The simplest model for treatment of domestic wastewater is adopted in UK,where treatment wetland is mostly designed as subsurface flow systems. Forhorizontal flow systems, the surface area Ah is calculated using the followingmodel (Cooper and Green 1998):
AQ C C
khd o e
BOD
== −−(ln ln )(1)
where Qd = average daily flow rate of wastewater, m3.day–1, Co = average BODof the influent, mg.L–1, Ce= average design BOD of the effluent, mg.L–1, kBOD =reaction rate constant, m.day–1.
The rate constant kBOD has been measured as 0.06 for systems used for second-ary treatment of wastewater, and as 0.31 for tertiary treatment.
For secondary treatment of sewage, assuming 250 lpcd (liters per capita perday) and BOD in the range of 150 to 300 mg.L–1, the area is calculated as about2.5 to 5.0 m2 per person. For tertiary treatment, the area requirement is arrived as0.5 to 1 m2 per person. For systems treating combined sewer overflow is consid-ered as equivalent to tertiary treatment.
B. The First-Order Plug-Flow Biokinetic Model
Organic degradation (BOD, COD, TOC), nitrification, adsorption, disinfection(pathogen removal) in biologically driven processes generally follow first-order kinet-ics (Metcalf and Eddy Inc., 1991). Accordingly, performance of attached-growthbiological reactors is described as first-order kinetic reactions model assuming that plugflow and steady state conditions prevail in the reactor. First-order reactions are said tooccur when the rate of reaction is directly proportional to first power of the concentra-tion of the reactants (in case of treatment systems, the pollutants). Thus, pollutantremoval in treatment wetland can be expressed as follows:
e
o
tCC e Tk== −− (2)
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where, Co = average influent BOD concentration, mg.L–1, Ce= average design ef-fluent BOD concentration, mg.L–1, kT = temperature dependent first-order reac-tion rate constant, day–1, t = hydraulic retention time, day.Hydraulic retention time t is expressed as:
tL WD
Q== η
(3)
where L = length of wetland, meter, W = width of the wetland, meter, D = depthof water column, meter, η = porosity of the substrate medium (percentage ex-pressed as fraction), Q = average flow rate, m3.day–1.Substituting the value expression for ‘t’ from Equation 3 in Equation 2, andconverting into linear format, the following expression is obtained.
(ln ln )C C kLWD
Qe o T−− == −− η(4)
Rearranging the terms to obtain the area (m2) of subsurface flow wetland required,
A LWQ C C
k Dso e
T
== == −−(ln ln )
η(5)
The value of the rate constant kT is estimated using the following equation:
k kTT== −−
20 2020θ ( ) (6)
where θ20 is the temperature coefficient for rate constant. The values of θ20 and k20
depend on the type of pollutants encountered in surface and subsurface flowsystems. Values for common pollutants are presented in Table 5.
Assuming that laminar flow prevails and that Darcy’s law applies, the flow ratein a constructed wetland can be determined as:
Q A k Sc s== (7)
where Ac = cross-sectional area of flow, m2, ks = hydraulic conductivity of thesubstrate medium, m.day–1, S = hydraulic gradient (dimensionless).In turn, the width of the wetland is then determined as:
WA
D
Q
k S Dc
s
== == (8)
Once the width is determined, the length of the constructed wetland can beobtained from the surface of the wetland obtained from Equation 4.
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C. Design Procedure
Based on the kinetic and hydraulic model outlined in the previous section, thecurrently recommended design procedure is
• Selection of plant species and determination of bed depth. Several authorssuggest that bed depth has to be used in calculating surface area requirementfor both the surface flow and subsurface flow wetland. However, in case ofSF wetlands this obviously means the depth of water column. SSF wetlandsare to be designed and operated in such a way that water level lies below thetop surface of the bed. However, more often than not it is experienced thatsurface flow occurs in SSF systems after few months of operation. In all theSSF systems with BOD loading rate higher than 0.2 kg.m–2.day–1 in UK andUSA, surface flow was reported (Crites, 1994). For this reason, we havesuggested here that for design, the depth of total water column both in andabove the substrate (instead of only the depth of bed), is to be consideredfor the term ‘D’ in the model. That is, D = Db + Df , where Db is the depthof substrate media and Df is the depth of surface flow.
• Selection of the bed slope and defining hydraulic gradient will be the nextstep. A review by Conely et al. (1991) indicates that slope varied from 0 to10%, with most designs occurring within the 0 to 3% range. As discussedin Section VII.E, only a minimum slope to pass required flow rate isrecommended, and in any case, it should not exceed 1% (Cooper, 1990).
• Selection of substrate media, in case of subsurface flow wetlands, anddefining hydraulic conductivity (ks) and porosity (η).
• Determination of cross sectional area of flow (Ac) using Equation 7.• Determination of bed width (W), using Equation 8.• Calculation of seasonal kT (for both winter and summer conditions, in
regions where the fluctuation is very high) according to Equation 6.• Determination of required surface area (As) (for summer and winter loading,
where required), using Equation 5.• Determination of bed length (L) from the area requirement.
TABLE 5Temperature Coefficients and Rate Constants
a
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D. Design Example
This section provides an example of the designing of a constructed wetlandto treat domestic wastewater from a small towns of 6000 population. Wastewatergeneration is at 160 L per capita per day. The initial BOD of the primary treatedwastewater is 180 mg/L and the treated wastewater needs to conform to a BODstandard of 30 mg/L. Fine- and medium-sized gravel are as substrate material forwetland bed of depth 0.6 m. Further assumptions are porosity of 40% andhydraulic conductivity of 10.2 × 10–2 m/sec. Annual average temperature ofwater is 27oC.
Average wastewater flow rate = 6000 × 160 = 960,000 L = 960 m3/dayInfluent BOD concentration Co = 180 mg/LDesign effluent BOD concentration Ce = 30 mg/L
Depth of bed (Db) is given (0.6 m), which is the average depth of rizhosphere(root zone) for common reed (Phragmites australis) and most other com-monly used aquatic macrophytes. Assume the depth of surface flow (Df) asnot more than 0.15 m, when the system is under stabilized operationalcondition. Hence, the total depth of water column (D) will be 0.6 + 0.15 =0.75 m.
Let the slope of the bed be 0.5%Porosity (η) = 40% = 0.4Hydraulic conductivity (ks ) = 10.2 × 10–2 m.sec–1 = 8812.8 m.day–1
Cross-sectional area of flow (using Equation 7)
AQ
k S xmc
s
== == ==960
8812 8 0 00521 8 2
. ..
Width of wetland bed (W) = Ac / D = 21.8 / 0.75 = 29 m, say 30 m.Reaction rate constant k27 can be calculated using values given in Table 4 for
subsurface flow wetlands and Equation 6. Then, the area requirement for SSFwetland is estimated.
k x day2727 20 11 104 1 06 1 66== ==−− −−. ( . ) .
Ax x
ms == −− ==960 180 30
1 66 0 75 0 43454 2[ln ( ) ln ( )]
. . .
Therefore, the length of the bed = 3454 / 30 = 115 m.
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VI. CONSTRUCTION OF TREATMENT WETLANDS
A. Site Selection
Once a constructed wetland is decided as the treatment alternative for waste-water management, many siting considerations need to be evaluated to arrive atoptimum design, construction and operation of the facility. The degree of siteevaluation will differ according to the magnitude of the project. The site selectionconsiderations can be classified into four categories (Brodie, 1990):
1. Land use and general considerations: Availability of adequate land area andaccessibility of the site is almost always an issue in selection of sites fortreatment wetlands. Required land area is dictated by the design size of thewetland, flow control structures, associated civil structures, buffer zones, etc.The mode of disposal of treated wastewater will be an important considerationfor selection of site. Use and values of the site and adjacent land should alsobe evaluated with respect to community views about potential odours, mosqui-toes, aesthetic impacts, and other environmental impacts.
2. Hydrology: Hydrologic considerations in wetland site selection includecharacterising the surface and groundwater flow patterns, quantity and quality,depth of groundwater table, and existing and potential uses.
3. Geology: Geologic considerations include characterization of surface materi-als and soils, bedrock depth, topography, availability of construction materialsetc. Soil and surface materials should be characterised at candidate sites forthickness and depth, classification and composition, use as construction mate-rial, drainage characteristics, erosion potential, and variability. Bedrock depthoften eliminates a site from consideration for constructed wetlands. Shallowbedrock sites require either blasting or ripping and disposal of large quantitiesof rock or importing large amounts of soil. Topography affects cut and fillrequirements. The site should be ideally flat to gently sloping. Steeper slopesrequire maximum earth moving activities and increases the cost of the treat-ment system.
4. Regulatory consideration: Environmental regulatory considerations are im-portant so that general concerns about the performance of the treatment systemis evaluated prior to siting of wetlands. This will minimise delays or cessationsdue to unanticipated environmental issues or public perceptions.
B. Pretreatment
Initially, it was perceived that CTWs would not require pretreatment, andscreened and unscreened wastewater could be applied to the system. However,experience over the last two decades have shown that appropriate pretreatment,
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could contribute to simpler operation and maintenance of wetland system. There-fore, current recommendations are to precede treatment wetlands with preliminaryand primary treatment systems.
‘Preliminary treatment’ involves screening and/or comminution (where appli-cable) to remove coarse solids. ‘Primary treatment’ is to remove heavier solids andto reduce organic loads. Primary treatment can be achieved by the use of imhofftanks, septic tanks, primary sedimentation tanks, or stabilization ponds. While‘preliminary treatment’ is now considered as a minimum, often it may not besufficient. Therefore, wherever possible, treatment wetlands should be preceded byboth ‘preliminary’ and ‘primary’ treatment (Figure 6). For smaller systems, screen-ing followed by a well-designed imhoff tank/ septic tank may be adequate. Formedium and large systems, stabilization ponds will be useful as pretreatmentsystems, if land is not a limitation. Where land area is limited, primary settlingtanks, are recommended.
Figure 6. Pretreatment options with constructed wetlands.
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C. Configuration
The configuration of a CTW affects the hydrologic factors controlling pollut-ant removal process. It is important, therefore, that the configuration seeks tominimize short-circuiting in order to maximize wastewater contact with the entiresurface area of an SF system, and the cross sectional area of an SSF system. Theratio of length to the width (L/W) is a key design factor to achieve this condition.For a SF system, and L/W ratio of at least 10 is required. Performance studiesindicate that SF systems with high L/W ratios (as high as 75) consistently outper-formed systems with lower W/W ratio (Steiner and Freeman Jr., 1990). For SSFsystems, on the other hand, L/W ratio of as low as one will be adequate, wheresolids have to be distributed over a greater portion of the bed.
Configuration of wetlands also depends on the degree of pretreatment given tothe wastewater, required treatment area, shape and grading of the available land,etc. General requirements for wetland cell configuration are (DLWC 1998a):
• Graded bed slopes and associated structures to assist water movement andto enable self-draining when required (typically 0.1 to 1.0%). Lower slopesare more appropriate for SF systems, and higher slopes for SSF systemsbecause of their higher hydraulic resistance;
• Capability to vary operational water depth, particularly during planting andcommissioning periods, and also during operation. This is essential becauseit is likely to be the sole degree of operational control in constructed wetlandsystems;
• Minimization of stagnation zones, short-circuiting and encouragement ofmixing by designing for plug-flow;
• Appropriate length to width (L/W) ratio; and• Adequate arrangement of flow distribution devices at the inlet section, and
multiple collection devices at the outlet section of each cell.
Alternative configurations of CTWs (Figure 7) include a single cell, parallelcells, series cells, and a combination of series and parallel cells. Single rectangularcell is the simplest design and least expensive to build. However, its operationalflexibility is limited. Hence, single cell is recommended for small flows, wherewastewater disposal after only primary treatment during short maintenance periodsof the system is not objectionable.
The likely problems of single cell CTW system can be avoided by adoptingconfigurations containing at least two parallel cells. The flow is split and simulta-neously fed to cells in operation. This will increase the operational and mainte-nance flexibility. One cell can be operated when the other is drained for mainte-nance. Possibility for mosquito control is enhanced by draining and each cell atregular intervals. Depending on site conditions, care may be taken during construc-tion to ensure appropriate flow splitting in parallel systems. Simple weir structures
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similar to splitter boxes would normally suffice for flow splitting in small systems.Splitter boxes, internal flow divider dikes, and flow distribution boxes, etc. in-crease the cost of the treatment system.
When the CTW system contains more than one cell, wetland cells can beoperated also in series wherein the flow moves sequentially from one cell toanother. The main operational advantage of cells in series is minimization of short-circuiting, leading to overall better performance. Another advantage is the opera-
Figure 7. Configurations for CTWs.
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tional flexibility. Cells in series provide opportunity to recirculate wastewaterbetween cells, and physically separate treatment zones for various pollutants. Forexample, while organic removal takes place in first (or first and second) cell,nitrogen removal can be enhanced in the subsequent cell(s) by maintaining specialoperating conditions in these cell(s). During construction, it is important to includethe ability to bypass each cell individually. If one cell from a set of cells in seriesrequires repair or maintenance, the design of the system should allow for bypassingonly this cell, so that the performance of the system is affected to a minimum.
Combining parallel and series flow options gives both operational flexibilityof cells in parallel, and the improved treatment capacity for different pollutants.However, these advantages may have to be offset against the additional costs ofearth works, inlet and outlet structures, and flow distribution structures.
An open water pond system between consecutive wetland cells could providepollutant removal and operational benefits. The pond will enhance ammoniareduction and nutrient removal through algal uptake, increased pH, and nitrifica-tion. A common configuration of CTW system with ponds is a SF cell and a SSFcell preceding and succeeding the pond, respectively (Figure 8d). The pond caninterrupt short-circuiting in the upstream cell and reestablish uniform flow distri-bution in the downstream cell. Also, for mosquito control, Gambusia fish can bestocked, which will migrate between pond and SF cells. The pond can facilitateproviding prior aeration (natural or using mechanical aerators) to wastewaterentering the downstream SSF cell to enhance nitrification.
In some instances, it may be desirable to recirculate the flow back through thewetland in order to:
• Improve the treatment efficiency of the wetland, notably for nitrogen re-moval through denitrification;
• Effectively dilute the influent and therefore decreasing peak loading andavoiding localized overloading; and
• Return poorly treated effluent to the system.
While recycling within the system may not change the overall system HRT, itmay change local velocities and HRTs. Therefore, if recirculation is planned,hydraulics and control systems may need to be designed and installed accordingly.
D. Inlet Structure, Feeding Arrangements, and Outlet Structures
Inlet structures play an important role in treatment wetlands by aiding effectiveflow distribution across the full width of wetland cells. A number of different inletdesigns have been used (Figure 8). Inlet and outlet structure for surface flowsystem is typically simple. Subsurface flow systems rely more on inlet and outletstructures for uniform flow distribution, particularly if the L/W ratio is smaller.
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FIGURE 8. Inlet distribution systems (Source: Adapted from Cooper 1993; DLWC1998b).
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Inlets above the bed allow adjustment of flow distribution and maintenance,preclude clogging and back-pressure problems, and aerate the wastewater.
The simplest methods for distribution of flow into the wetland include sawtoothweirs. However, the weir system is often not recommended for flow distributionsince they are expensive to construct and maldistribution of flow is often causedby screenable material collecting on the edges (Cooper, 1993). Also, the channeltends to act as a sedimentation tank and collects sludge and grit. Some of therecommended feed arrangements to effectively utilize the wetland area in serpen-tine systems and those with large L/W ratio are presented in Figure 9. Theminimum recommended interval between two feed streams along the width is 3 to5 m. Smaller intervals are better for flow distribution.
FIGURE 9. Feeding arrangements for CTWs (Adapted from Steiner and Freeman Jr. 1990).
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With large L/W ratio, piping is less and inlet construction is simplified.However, to distribute solids in the influent across a greater portion of the wetland,the influent may need to be fed at different lengths of the cell or the effluent mayhave to be recirculated to dilute the influent pollutant solids concentration.
In general, outlet collectors will be similar in construction to the inlet struc-tures. The functions of outlet structures are to:
• Collect the effluent water without creating new dead zones in the wetland;• Control the depth of water in the wetland;• Assist in prevention of clogging; and• Provide access for sampling and flow monitoring.
Constructed wetlands using soil as substrate will require a stone collector withan open-jointed pipe running across the width of the cell at the base (Figure 10).It is essential to provide outlet structures in such a way to adjust the depth of water
FIGURE 10. Simple outlet structures (Source: Adapted from DLWC 1998b).
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column in the bed. The water level control helps to maximise the flow path ofwastewater through the reed bed. A broad range of outlet structures are availableto achieve this requirement:
1. The simplest and the cheapest method is to use a 90° elbow with O-ring seals,which allow the end of the pipe to be raised or lowered by twisting the elbow.
2. The vertical outlet pipe can comprise a series of sockets sections or telescopicsections, which can be adjusted increase or drop the water level.
3. Drop board structure (Figure 11) that consists of a number of boards, usually100 mm height, can be set into the embankment. By adding or removing thedrop boards, the water level can be varied. Though constructed on-site, it canalso be pre-cast and fixed at site. Wooden drop boards are prone to shrinkingand swelling due to soaking and drying, hence may pose difficulties whileremoving or replacing. Boards made of HDPE may help to overcome suchproblems. Boards need to be stored safely when removed. The drop boardstructure is expensive compared to other arrangements.
Nutrient-rich algal growth can develop in open water areas of the wetland andcan be expected to be flushed during high flow conditions. A final filtering usingfine screens to remove such biomass is desirable. This will be an importantrequirement for systems designed for phosphorus removal to reduce export ofphosphorus through biomass (White et al., 1996). Two types of outlet screens canbe considered. The first is a mesh-type screen on orifice water level control
FIGURE 11. Drop board for water level control (Source: Adapted from DLWC 1998b)
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structure to reduce the incidence of blockage by plants, debris or other materials.The screen may house the entire outlet structure. The second type is a solid screenfor weir type outlet structures. This type can prevent floatable matters such asalgae.
E. Reed (Wetland Plant) Beds
Construction of reed bed is a crucial part of building treatment wetlands.Where groundwater contamination is a concern, the in situ soil needs to be wellcompacted (hydraulic conductivity less than 10–5 mm/sec). Alternately, an imper-meable liner below the substrate may be required. Bentonite clay, asphalt, syn-thetic butyl rubber or plastic membranes can be used for this purpose. The linermust be thick, strong and smooth to prevent root penetration and attachment. Whengravel with sharp edges is used, a layer of fine sand should be placed on thesynthetic membrane to prevent puncture during placement of gravel. In Denmark,0.5- to 1.0-mm-thick polyethylene liners have been successfully used (Brix, 1987).River gravel and crushed limestone are the common gravel substrates used inEuropean gravel bed wetlands.
Depth of bed influences the HRT in subsurface flow wetlands. A 0.6 m deepbed is common, although more shallow beds (down to 0.3 m) have been alsoconstructed in many locations (Crites, 1994). The depth of bed is to be decided sothat it is compatible with the type of plant selected. When placing the gravel, it willbe advantageous to have coarse (large size) upper layers, and finer bottom layers,as it will allow easier and quicker percolation of wastewater facilitating the use ofentire cross section of the bed, thereby minimizing short-circuiting through surfaceflows. Another advantage will be the avoidance of clogging of pores of larger sizegravel layer that occurs due to placement of finer materials on top of it. The gravelmay need to be washed before placement in the bed.
The top level of the bed does not need to be sloped. Many of the early wetlandconstructions were done with surface slope. With surface slopes, it is impossibleto adopt flooding to effect weed control. Therefore, it is commonly recommendedto use a level bed surface to allow weed control, and minimum slope necessary forthe base to allow the water to pass through the bed.
F. Vegetation Planting
Establishment of vegetation is another important factor in the construction ofCTWs, because the wetlands may not reach peak performance until the vegetationis well established. The selection of plant species depends on the source andcharacteristics of wastewater to be treated. It is desirable to select plants with highgrowth rate that would grow year around. Ideally, the selected plant should grow
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densely, spread rapidly, and have an extensive horizontal and vertical root system.The planting period of vegetation needs to be attuned to the annual cycle of thespecies. In general, early spring growing season plantings have been found to bemost successful for the widely accepted constructed wetland plant species ofPhragmites and Typha (Tanner, 1996). Plant propagules used for treatment wet-lands can be seeds, rhizome sections, tissue cultures on agar slants, greenhouse-grown potted seedlings, or field-harvested stocks.
1. Use of Seeds
It is possible to establish plants in the wetland directly from seeds. Manyspecies of wetland plants have seeds that can be field harvested in very largenumbers, whereas some species have few seeds. For example, cattails (Typha sp.)carry thousands of individual seeds while bulrushes (Scirpus sp.) contain only 20to 30 seeds each. Seeds can be obtained from existing natural or constructedwetlands nearby. Using seeds may be the cheapest option for planting. However,there are two potential problems in this method (Chambers and McComb, 1994).First, it takes quite a long period to establish a viable plant cover in the bed.Second, prior to maturation weed plants that are difficult to control may overtakethe wetland. Another common result of using seeds for establishing vegetation isthe development of bed areas that either do not have any plants. Use of seeds,therefore, is recommended only in conditions where other methods are not pos-sible.
2. Use of Propagated Seedlings
Direct planting of propagated seedlings often been chosen as the most practicaland cost-effective means of establishing plants in CTWs (Parr, 1990; Surrency,1993). Seedlings are young nursery plants. Depending on the size of the bed, thetreatment wetland may need a large number of seedlings, and their availability inrequired numbers may be an issue. In several regions of Europe and the US,propagation and sale of wetland plant species has become a big business, wherethere are wetland nurseries that can supply thousands of plants for use in con-structed wetlands (Kadlec and Knight, 1996). However, in regions where wetlandplant nurseries are not present, and where it is not possible to acquire in requirednumbers from existing/natural beds, part of the task of construction of treatmentwetland would be propagation of seedlings. The construction schedule may haveto be accordingly progressed so that the propagules can be planted in the bed atappropriate season.
Seedlings are easily planted using planting tubes, a tool specifically designedfor rapid hand planting. The tool consists of a hollow tube with a mechanical jaw
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that is pushed into the substrate. When the jaw is opened, the plant slides down thetube into the hole. Not all plant species are amenable for this method of planting,and other appropriate tools such as shovels, dibbling sticks or spikes may berequired. The survival rate of seedlings is significantly higher than for field-germinated plants using seeds (Kadlec and Knight, 1996). Therefore, seedlingsmake it easier to arrive at a healthy plant cover appropriate density. This, in turn,will be helpful in substantially reducing the system start-up time for water qualitycontrol.
3. Use of Transplantations
In region where wetland plants are abundant, transplantation is an option tovegetate constructed wetlands. Field harvesting the transplants requires digging toscoop the plants from the ground and spread them in open. Transplanting matureplants may be more difficult than planting seedlings on account of the size. Largerand deeper holes have to be made on the bed to allow the rhizome or root materialto be completely buried. A major advantage of this method is that the plant coveris readily established once the planting is completed, and hence require very littleor no start up period. One of the disadvantages that has been reported from theexperiences in Europe is that the reeds tended not to spread outward to fill in thegaps as quickly as other methods of planting (Cooper, 1993).
4. Use of Rhizomes
Use of rhizomes for plant establishment in treatment wetlands is another wayof utilizing field-harvested plants. Instead of using the whole of the plant, only therhizome part is planted in the bed. Because herbaceous wetland plants store mostof their growth reserves in their roots and rhizomes, this method can produceshoots and mature plants faster than the use of seedlings (Grace, 1993). A carefullyplanted rhizome can produce numerous daughter plants within few weeks (Kadlecand Knight, 1996). It is recommended to plant rhizome segments with at least twonodes. Rhizomes need to be planted obliquely at a 45° angle with some portionabove the water level. The recommended density is two per square meter.
G. Establishment of Vegetation
The key requirements to establish a healthy wetland plant are water, nutrient, andlight. While the first two requirements can be controlled, the third is provided bynature. In treatment wetlands, water level constitutes a critical aspect of plantsurvival during the first season after planting. A common mistake is to assume that
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since the plant is of wetland origin, fluctuating water availability is easily tolerated.Frequently, too much (too deep) of water creates more problems in establishing thevegetation cover than too little water (Guntenspergen et al., 1990). A deeper watercolumn retards availability of oxygen at their roots. This is particularly a problemduring the first growth season. Water level management during the early growthperiod of vegetation is, therefore, important (DLWC, 1998b). Planting is best doneon a wet substrate, maintaining the water level so that it just about saturates thesubstrate between watering. As the plants grow in height, they will have an increasedability to transport oxygen to the root zone. This allows the water level to be raisedaccordingly. Flooding conditions, however, should be avoided until the plants growhigh enough to generate stem with leaves that protrude above the water level.
While not much is known about nutrient requirements of wetland macro-phytes, general fertility concepts are often applied. Nutritional requirements pre-scribed at the levels equal to cultivation of on-land grass crop (Tanner, 1994).Maintaining nutrient conditions in soil substrates is a relatively easier task than ingravel beds. However, in case of treatment of nutrient-enriched wastewater, thismay not be a problem.
VII. OPERATION AND MAINTENANCE
Constructed wetlands are designed to be ‘passive’ and low-maintenance sys-tems not requiring continual upkeep. However, they being dynamic ecosystemswith many variables, problems may occur during their operation. The main objec-tives of operation and maintenance of wetland systems, therefore, are to:
1. Ensure the wetland to operate as per design objectives under all scenarios,2. Maximize treatment efficiency and capacity,3. Save costs by providing early warning mechanisms through detection of
problems at early stages, and4. Extend the active life span of the system, thereby delaying the need for major
retrofitting, and hence the long-term viability of the system.
Major operation and maintenance requirements for CTWs include maintainingthe water level, vegetation cover, and the inlet and outlet structures (DLWC1998b). It is essential that reeds have access to water at rhizome level at all times.The system is to be inspected, at least on a weekly basis, for the initial two or threegrowing seasons, to check the functioning of the flow distributors and the positionof the outlet flow control mechanism. Monitoring may include flow measurement,influent, and effluent sample collection for characterization.
Maintaining the vegetation cover is often experienced to be the major main-tenance exercise in CTWs. Potential factors that create problems in maintaining thevegetation cover, and the corrective measures are (Kadlec and Knight, 1996):
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1. Water stress: This occurs due to too low levels of water. Sometimes, waterlevels are reduced to promote root penetration. However, this reduction shouldbe controlled so that it does not exceed the growth rate of the roots and shouldgenerally be less than about 1 cm/day. The situation can be corrected by raisingthe outlet weir, adding more water to maintain adequate moisture of thesubstrate.
2. Flood stress: The wetland macrophytes have an upper limit of tolerance to thedepth of water column, due to the dissolved oxygen limiting conditions suchdepths create. The flood tolerance may be 0.6 m for wastewater with high flowrate or pollution load, and may be only 0.3 m in stagnant water with highpollutant loads. The condition can be corrected by lowering the outlet weirs orreducing the flow.
3. Nutrient stress: Adequate supply of nutrients is essential to optimize the plantgrowth and hence maintaining the vegetation cover. If macronutrients such asnitrogen (N), potassium (K) and phosphorus (P) are below minimal levels foradequate plant growth, they may have to be added. In addition, plants alsorequire numerous micronutrients such as Iron (Fe), Manganese (Mn), Molyb-denum (Mo), etc. Adding fertilisers and micronutrients will help to promotehealthy plant growth.
4. Dissolved oxygen stress: Physical and chemical factors lead to low to very lowdissolved oxygen conditions. The physical factors may be solids clogging themedia void spaces, deposit of litter and decaying organic materials that createanaerobic conditions. Some chemical constituents dissolved or suspended inthe water column that make a demand on oxygen also cause low dissolvedoxygen conditions. Lowering water levels, reducing solids and organic loadingrates will be needed to correct this condition.
In view of the fact that only the new growth in macrophytes takes up pollut-ants/nutrients, older growth must be harvested in order to maintain design treatedwater quality. Harvesting is also important on the account of suggestions made bymany studies of such systems that vegetation acts only as a temporary storage poolfor the pollutants and allowing older vegetation to decay within the wetland willresult in reentry of pollutants, affecting the treated water quality (Brix, 1994;DLWC, 1998b; Faulkner and Richardson, 1993; Verhoeven and Meuleman, 1999).
VIII. PERFORMANCE OF CONSTRUCTED WETLANDS
Constructed wetland systems have emerged as a viable wastewater treatmenttechnology. There are more than 1000 municipal, stormwater, and industrialtreatment wetlands in North America (Knight, 1997). Denmark, Germany, andUnited Kingdom each have more than 200 operating treatment wetlands (Brix,1994a). Constructed wetlands can remove a range of pollutants, including biode-
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gradable organic substances (measured as suspended solids, BOD, COD, andTOC), nutrients (nitrogen and phosphorus), pathogens, trace heavy metals, traceorganics and other toxic or hazardous pollutants. This section presents a review ofremoval process models and performance in experimental, pilot-scale, and field-scale constructed wetland systems.
A. Organic Substances
A majority of operating constructed wetland systems in the world are fortreatment of municipal wastewater in which organic substances constitute theprominent pollutants. Hence, a vast information bank is available on organicremoval performance of constructed wetland systems. Wetlands are efficient usersof external carbon sources, manifested by excellent reductions in BOD and COD.Settleable organics are rapidly removed by quiescent conditions, deposition, andfiltration. Degradable carbon compounds are rapidly utilized in wetland metabolicprocesses.
Wetlands are excellent sediment traps and remove suspended solids from theincoming wastewater usually within tens of meters. Gearheart et al. (1983) re-ported that solids removal mostly occurred in the initial 12 to 20% of the cell areaof a pilot-scale system studied at Arcata, California. Physical processes responsiblefor removal of suspended solids from wastewater include sedimentation, filtration,adsorption onto biofilms and flocculation/precipitation.
Sedimentation of suspended solids is based on flow retardation that leads togravity settling of solids (Fennessey et al., 1994; Pettecrew and Kalff, 1992). Therate of settling will depend on the size, shape and charge of the particles. Sus-pended solids, during their settling process can react and bind with various pollut-ants including organic substances, nutrients, heavy metals and pathogens, therebyaiding removal of these pollutants from the water column.
Rough surface area of the stem of the plants and the substrate media (gravel)can trap and filter the solids, or adsorb on the surfaces. Fine colloidal solids of samecharge that repel each other and remain in suspension is thought to be removed byflocculation and precipitation with the help of organic flocculants released bywetland plants.
While constructed wetlands are effective in reducing suspended solids, sedi-ments can show a large cycle of sedimentation, resuspension within the wetland(DLWC 1998). A combination of several factors, including water shear, gasfloatation, and bioturbation, induce this process. Water shear is caused by turbulentmovement of water due to strong wind action and high flow velocity resulting inmixing action. Once these water movements exceed the critical shear forcesrequired, settled particles are brought into suspension from the substrate (White etal. 1996). Methane and other gases that are produced during anaerobic decompo-sition of organic materials in wetlands that rise as bubble can cause re-entry of the
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settled solid particles into suspension. Resuspension of solids due to the activitiesof animals, including fishes, is called bioturbation.
The sequence of resuspension, lateral movement, and resettling occurs repeat-edly for a given particle, which is referred to as the concept of ‘sediment spiralling’in wetlands (Kadlec, 1995). Thus, the solids exiting the wetland may not be thesame as those entering the system.
Biodegradation of organic compounds involves hydrolysis and catabolic ac-tivities of both autotrophic and heterotrophic microorganisms. Both aerobic andanaerobic degradation of soluble organic substances are responsible for BODremoval. The biodegradation by aerobic heterotrophic organisms are according tothe following reaction:
(CH2O) + O2 → CO2 + H2O (9)
The autotrophic group of bacteria degrade organic compound that containnitrogen under aerobic conditions (nitrifiers). Thus, although both the groups ofbacteria consume organics, the faster metabolic rate of heterotrophs means thatthey are responsible for the reduction in the BOD from wastewater (Cooper et al.,1996). According to Reddy and Burgoon (1996), microbial respiration is fre-quently limited by electron acceptor (oxygen) rather than electron donor (carbonavailability). Reduced oxygen availability will therefore influence aerobic degra-dation of organic matter.
Anaerobic degradation of organics also occurs in constructed wetlands in theabsence of availability of dissolved oxygen. This is a multistep process carried outby either facultative or obligate anaerobic heterotrophic microorganisms.
First step is the fermentation process carried out by facultative microorganismsleading to the formation of end products such as acetic, butyric and lactic acids,alcohols and gases such as CO2 and H2 (Vymazal, 1995).
C6 H12O6 → 3 CH3COOH (Acetic acid) (10)
C6H12O6 →2 CH2CHOHCOOH + H2 (Lactic acid) (11)
C6H12O6 → 2 CO2 + 2 C2H5OH (Ethanol) (12)
In the second step, strictly anaerobic microorganisms utilize the end-productsof fermentation process and convert them into either hydrogen sulfide or methane.
CH3COOH + H2SO4 → 2 CO2 + 2 H2O + H2S (13)
CH3COOH + 4 H2 → 2 CH4 + 2H2O (14)
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1. Organic Pollutant Removal Performance
The North American database information on BOD removal performance ofoperating wetland systems (USEPA, 1993) indicates that typical mass BOD removalefficiencies are about 70% or more at mass loading rates of up to 280 kg.ha–1.day–1.Figure 12 illustrates the relationship between the mass loading of BOD in awetland and the mass of BOD removed.
Austria has 293 CTW systems, of which 160 are in operating conditions. Acomparison of performance of 30 of these systems indicated an average BODremoval efficiency of 96% for vertical flow type wetlands and 93% for horizontalsubsurface flow wetlands (Haberl et al., 1998).
In Australia, a pilot scale study of gravel bed wetland conducted at Frankston,Victoria, over a period of 4 years, found that once fully developed, the suspendedsolids and BOD removal efficiencies were consistently higher than 90% (Davis etal., 1996). A simultaneous evaluation of sand bed system gave slightly lowerremoval efficiency at 80%.
In Belgium, an average rate of BOD removal efficiency of 83% is reportedfrom 20 and odd systems operating in the country. According to Cadelli et al.(1998), although the high removal efficiency is due to dilution effects caused byhigh rainfall rates in Belgium, it is pertinent to consider that the removal isachieved under high hydraulic loading rates that are typically more than 250% ofthe design hydraulic loading rates.
The design organic loading rates of 60 CTWs in Czech Republic varied from50 to 240 kg BOD.ha–1.day–1 and most of the systems achieved the BOD levels thatare well below the prescribed standards (45 mg.L–1) in the country. Reported
FIGURE 12. BOD mass loading and removal rates in treatment wetlands (Source:Adapted from Knight et al., 1993).
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suspended solids removal also was good and the systems could consistentlyachieve suspended solids less than the required statutory levels (40 mg.L–1)(Vymazal, 1998).
In Denmark, from 134 CTW system performance details with BOD loadingrates ranging from 1 to 240 kg.ha–1 .day–1, 98% of the systems reported outlet BODlevel of less than 20 mg/L (Brix, 1998). The suspended solids removal was alsohigh with 92% of the systems reporting less than 20 mg.L–1.
A wetland pilot project initiated in 1996 to introduce CTW concept fordomestic wastewater treatment in the tropical conditions of Masaya, Nicaragua,reported BOD levels of less than 5 mg.L–1 for treated effluent after 17 months ofobservation period (Platzer and Ramirez, 1998).
In India, a constructed wetland installed with a hydraulic loading rate of5 cm.m–2.day–1 in Bhubaneshwar reported the BOD removal efficiency in the rangeof 78 to 91% (Juwarkar et al. 1995). A pilot-scale study using gravel filled reed bedtreating domestic wastewater from an University Campus in Indore, India, reportedBOD and suspended solids removal in the range of 65% and 78%, respectively(Billore et al., 1999). A first full-scale wetland treatment system in Kathmandu,Nepal constructed to treat domestic wastewater reports BOD removal efficiency ashigh as 97 to 99% (Shreshta et al., 2000).
B. Nutrient Removal
Removal of macronutrients, namely, nitrogen and phosphorus by constructedwetlands is a complex cyclic process believed to be involving a number of‘conceptual compartments’ including the water column, sediments, plant roots,biofilms, plant stems and leaves. In a stabilized constructed wetland, the propor-tions of the total wetland nutrient load held in these compartments are (Faulknerand Richardson, 1990):
• Soils, sediment and litter/peat: 80%• Water column : 15 to 20%• Plants and other biota : 5%
A simplified wetland nitrogen cycle is presented in Figure 13. The process ofbiological nitrogen removal follows the sequence of nitrification of ammonium-nitrogen into nitrite and nitrate nitrogen, and denitrification of nitrate-nitrogen intogaseous nitrogen. Nitrification takes place first, generally in the rhizosphere and inbiofilms. This is an aerobic process. Denitrification follows under anaerobic con-ditions.
Nitrification, the biological oxidation of ammonium is a two-step processcatalysed by Nitrosomonas and Nitrobacter microorganisms. The nitrifying bac-teria derive energy from the oxidation of ammonia and/or nitrite and carbon
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FIGURE 13. Simplified nitrogen cycle (Adapted from Kadlec and Knight 1996).
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dioxide is used as a carbon source to synthesize new cells. In the first step,ammonia is oxidized to nitrite in an aerobic reaction catalyzed by nitrosomonasbacteria.
NH4+ + O2 ‡ NO2
– + 2H+ + H2 (18)
The nitrite is then oxidized aerobically by nitrobacter into nitrate.
2 NO2 – + O2 → 2 NO3
– (19)
The nitrifying microorganisms have to compete with other heterotrophic mi-croorganisms involved in biodegradation of organic material (thus involved inBOD removal). In order to perform nitrification, the BOD in wastewater must beless than around 20 mg.L–1 (Reed et al., 1995). Some other environmental factorsthat can impede nitrification is the availability of dissolved oxygen, and tempera-ture and pH of the wastewater.
During denitrification, the nitrates formed are reduced under anaerobic condi-tions and in the presence of readily biodegradable organic matter being used as acarbon source. The reaction is catalyzed by facultative microorganisms such asPseudomonas sp., Aeromonas sp. and Vibrio.
NO3 – + Organic C → N2 (NO and N2O) + CO2 + H2O (20)
Some of the major environmental factors that affect denitrification includeabsence of oxygen, temperature, pH, soil moisture, and availability of carbonsource. Apart from bacterial activities, nitrogen removal can also occur due touptake by plants. The uptake capacity of emergent macrophytes in constructedwetlands can vary from 200 to 2500 kg.ha–1.year–1 (Brix, 1997). The potential rateof nutrient uptake is limited by the growth rate of the plants and the concentrationof nutrients in the plant tissues. Thus, from the point of view of removal of nitrogenfrom wastewater, plants that have rapid growth rates and capability to attain a highstanding crop (biomass per unit area) (Reddy and Debusk, 1987) can influence therate of removal.
Phosphorus is typically present in wastewater as orthophosphate and organicphosphorus. It accumulates in wetland sediments/ litter. The sediment/litter com-partment is the major pool for phosphorus (more than 95%) in the cases of naturalwetlands (Faulkner and Richardson, 1990). A simplified phosphorus cycle inwetlands is presented in Figure 14.
Phosphorus removal mechanisms in constructed wetlands include adsorption,plant absorption, complexation, sedimentation, and precipitation (Watson et al.,1990). Bavor and Addock (1994) describe phosphorus removal in CTWs as acombination of both short-term and long-term processes. Uptake by plants andmicrobial organisms in substrate media and sediments, are the short-term pro-
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cesses. Absorption and accretion into the sediments is a long-term removal pro-cess.
Adsorption is considered the most significant mechanism for phosphorusremoval (Bavor and Addock, 1994; Mann, 1995). The adsorption of phosphorusoccurs mainly because of complexation and precipitation reactions with iron,calcium, magnesium, and aluminium minerals in the sediment (Moshiri, 1993).Adsorption and precipitation can initially only be considered as short-term storage.However, once this material is buried with a layer of peat, it becomes part of thelong-term ‘sink’ storage (Richardson and Craft, 1993). Sedimentation is a physicalprocess whereby particulate matter containing phosphorus in the water columnsettles and accumulates on the wetland floor. If the particulate matters remaininsoluble, the phosphorus removal by this mechanism is permanent. However, ifthe particulate materials are biodegradable organics, after degradation, phosphoruswill be released back to the water column.
Uptake of phosphorus by plants through their roots is for the growth of tissues.Many studies found that there is little uptake of phosphates from water column orsoil by emergent wetland plants (Faulkner and Richardson, 1993; Mann, 1995;Vymazal, 1995). The plant uptake by emergent macrophytes is reported to bevarying from 30 to 150 kg.ha–1.year–1 (Brix, 1996). Another important role theplants play in phosphorus removal is to extract water from the soil through its roots.This creates a driving force for water to move from the water column to thesediment, increasing the contact with binding sites, which in turn enhances adsorp-
FIGURE 14. Phosphorus cycle in wetlands (Source: Adapted from Kadlec and Knight 1996).
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tion. Microorganisms also play a definite role in the wetland phosphorus cycle.Bacteria can assimilate phosphorus in their cell structures the same way that itoccurs in conventional biological treatment plants.
As such, plant uptake and substrate media adsorption will not effectively‘remove’ phosphorus from wetland systems. It has been estimated that there is aseasonal return of approximately 35 to 75% of phosphorus to the system throughplant decomposition (Richardson and Craft, 1993). To effectively ‘remove’ phos-phorus over an extended period, it is necessary to harvest plants and/or dredge thesubstrate media. Otherwise, in the same way as nitrogen, phosphorus also will bereleased back to the system with decaying plant materials (Verhoeven and Meuleman,1999). However, some researchers while not favoring the harvesting of plants fornutrient removal, support the same only to maintain hydraulic capacity, promoteactive growth and to avoid mosquito problem (Reed et al., 1995).
1. Nutrient Removal Performance
A survey of literature on the removal efficiencies for nutrients achieved in con-structed wetland systems indicates extreme variations. Literature indicates that phos-phorus removal efficiency is strongly dependent on loading rate with 65 to 95%removal at loading rates less than 5 g.m–2.yr–1. However, removal efficiency decreasesto 30 to 40% or less when phosphorus loadings are greater than 10-15 g.m–2.yr–1
(Faulkner and Richardson, 1993). Experience with gravel bed treatment wetlands fortreating domestic wastewater has shown that on the longer term, removal of nutrientscan be optimized up to about 50% for nitrogen and 40% for phosphorus in systems withlow loading rates (800 population equivalent per hectare) (Brix, 1994b).
The relationship between mass loading and removal rates of total nitrogenarrived using data from 200 working treatment wetland systems in North America(USEPA 1993) is illustrated in Figure 15. It can be observed that nitrogen removalefficiencies decline at mass loading rates above 20 kg/ha/day. Total nitrogenremovals up to 79% are reported at higher nitrogen loading rates of up to 44 kg/ha/day from some systems in the US (Watson et al., 1990).
The average total nitrogen removal efficiencies of horizontal and vertical flowtreatment wetlands in Austria have been reported as 51% and 36%, respectively.Similarly, the performance efficiencies with regard to phosphorus removal are70% and 60%, respectively (Haberl et al., 1998).
The nitrogen removal performance of treatment wetlands in Germany is re-ported as an average performance of 45 systems is 54%. The phosphorus removalefficiency has been reported as 21%, which is the average of 74 operating systemsfor which long-term data are available (Borner et al., 1998). Experience with SSFwetlands in Denmark has shown that systems loaded with 800 population equiva-lent (PE) per hectare had a long-term efficiencies of 35% for nitrogen and 25% forphosphorus (Schierup et al., 1990)
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The nutrient removal performance of wetland systems in tropical climaticconditions presents a vastly different picture. A small wetland system designed totreat effluent from septic tank from an office complex in Santa Carina, Brazil,indicated high nitrogen and phosphorus removal efficiencies in the order of 78%and 72%, respectively (Philippi et al., 1999). The nitrogen and phosphorus removalefficiencies of a full-scale plant treating domestic wastewater in Bhubaneshwar,India, have been found to be over 70% and 43%, respectively (Juwarkar et al.,1995). A pilot scale study in the Central India also indicated higher removalefficiencies for both nitrogen and phosphorus in the range of 58 to 65% (Billoreet al. 1999). A full-scale gravel bed horizontal flow system built in Kathmandu,Nepal, reports nitrogen removal in the range of 80 to 99% and phosphorus removalfrom as low as 5% to 69% (Shreshtha et al., 2000).
Constructed wetlands have been also tried to treat nutrient rich effluent fromdairies. The results from a pilot-scale demonstration plant installed to treat thesecondary treated effluent from a dairy in Maitland, New South Wales, Australia,showed high variations in the nutrient removal efficiencies. The range of nitrogenremoval varied from 3 to 45% with an average of 26%. The phosphorus removalindicated negative efficiencies, and ranged from –8% to 56.7%, with an averageof 28% (Geary and Moore, 1999).
C. Pathogen Removal
Pathogens, the disease causing microorganisms, can be viruses, bacteria, pro-tozoa or helminths. Pathogens are derived mainly from human and animal feces.
FIGURE 15. Total nitrogen mass loading and removal rates in treatment wetlands (Source:Adapted from Knight et al. 1993).
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Some pathogens also originate from soils, sediments, and decaying organic matter.Removal of pathogens is an important objective for wastewater treatment systems.Conventional biological treatment systems such as activated sludge process, trick-ling filter, are not efficient in pathogen removal (Table 6). Therefore, it is custom-ary to add disinfection processes such as chlorination, ozonation and ultravioletradiation in sewage treatment plants. In domestic wastewater treatment plants withconventional biological units, sometimes polishing ponds that hold treated effluentfor 1 or 2 weeks are recommended in place of disinfection processes.
Alternately, waste stabilizing ponds have been found to be extremely efficientin removal of pathogens and are widely recommended process for this purpose,particularly in the tropical countries (refer to Table 6). CTWs also are consideredhighly efficient in removing pathogens in wastewater. Despite the presence ofwater, wetlands are hostile environment for pathogens. Various processes that maybe involved in the removal of pathogens in wetlands are (Kadlec and Knight,1996):
• Natural die-off;• Sedimentation• Filtration;• Ultraviolet radiation;• Unfavorable water chemistry;• Temperature effect; and• Predation by other organisms.
Significant pathogen removal occurs during the sedimentation process, espe-cially of those attached to particulate matters. The filtering effects of plants andbiofilms remove pathogens by direct contact. Bactericidal excretions by plants alsoplay a role in removal of pathogens in wetlands (Gesberg et al., 1990; Mitchell,1996b), but this is unlikely to be a significant removal process, as it may otherwiseprevent biofilm development on the substrate surface. Vegetated wetlands are moreeffective in pathogen removal than those without plant growth, because the plantsprovide habitat for a variety of microorganisms some of which (such as zooplanktons)are pathogen predators (Kadlec and Knight, 1996). Predation may be highly efficientin removing pathogens, especially in and around the root zones where zooplanktonsare likely to shelter and feed. Ultraviolet rays from sunlight also destroys pathogens,though this may not be effective in wetlands due to shadow effects of full-grownmacrophytes that may protect pathogens from effective exposure.
1. Pathogen Removal Performance
A majority of the treatment wetlands have been installed and operated mainlyfor removal of physico-chemical parameters such as organic and nutrient pollut-
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ants. Most of the research studies also are concerned mainly with performance ofconstructed wetlands in these aspects. Comparatively fewer studies have beencarried out on the removal of pathogens by constructed wetlands. Pathogen re-moval efficiencies of CTWs are of particular importance for their adoption as alow-cost treatment system for domestic wastewater in tropical and subtropicaldeveloping countries.
Early attempts for studying the pathogen removal performance of CTWsinclude the study by Gesberg et al. (1990), which evaluated the efficiency ofremoval/inactivation of total coliforms in a pilot gravel bed CTW system. Thesystem received primary treated domestic wastewater at a hydraulic loading rate of5 cm.day–1. The study included both horizontal flow and vertical flow beds. Thestudy reported that the pathogen removal by constructed wetlands is superior toconventional treatment processes. While the conventional treatment systems donot achieve more than 1 log removal, treatment wetlands easily achieved a mini-mum of 2 log removal of coliforms. The results indicated that vegetated beds aremore effective than unvegetated beds. The summary of reported results are illus-trated in Figure 16.
The first-order reaction equation derived by Bavor et al. (1990) to study thefecal coliform removal rates by gravel filled trenches in Richmond, Australia,predicted the need for a hydraulic retention time of 2 days at 20°C, to achieve 1log removal, and 3 days for the same removal efficiency in planted beds. Thecontradiction in the findings compared to results of other studies were explainedthat in planted beds, short-circuiting takes place, with the flow preferentiallyrunning below the plants.
Pathogen removal performance of constructed wetlands showed seasonal varia-tions in a full-scale study conducted at Cheshire, UK (Rivera et al., 1995). This
TABLE 6Comparison of Pathogen Removal Efficiencies of Various TreatmentProcesses
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FIGURE 16. Coliform removal performance of gravel bed constructed wetlands.
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study noted that the presence of reeds appears to make little difference in gravelbed wetlands, whereas it made a significant difference in soil beds in the removalof coliform bacteria. It is possible that the role of the reeds in providing oxygen tothe root zone may be important in soil beds but of relatively little importance in theporous gravel beds. Further, in removal of parasites (both protozoa and helminths),beds planted with reeds performed better than unplanted beds. The gravel bedsplanted with reed were found to be most effective with respect to protozoa andhelminths with removal efficiencies of up to 100% for a range of parasites. Also,the results indicated that in all cases, generally gravel beds perform better than soilbeds in pathogen removal. The removal efficiencies reported by this study arepresented in Table 7.
A comparative experimental study of pathogen removal performance by wastestabilization ponds, high-rate algal pond and treatment wetlands with retentiontimes being 24 days, 5 days, and 3 days, respectively, was conducted in Spain(Garcia and Becares, 1997). The results of this study indicated that the wetlandsystem showed higher removal efficiency for most of the pathogen groups fol-lowed by stabilisation pond and high-rate algal pond. The removal efficiencies ofwetland system for total coliforms, faecal coliforms and fecal streptococci werereported to be 99.48, 98.8%, and 98.67%, respectively.
Another pilot study of subsurface flow wetland system used for tertiary treat-ment of domestic wastewater at Leek Wootton, UK, achieved significant removalof E. coli and total coliforms, in the order of 1.5 to 2.1 log units, with hydraulicretention time of about 30 h (Green et al., 1997).
A laboratory study of surface flow wetlands for pathogen removal perfor-mance in Thailand indicated fecal coliform removal percentages were of 84.26,98.09, 99.56, and 99.62 at a HRT of 1.5, 3, 5 and 6 days, respectively (Khatiwadaand Polprasert, 1999).
A subsurface flow wetland that has been in operation for 10 years in Lauwersoog,Netherlands, reported removal performance of more than 99.9% for viruses, E. coliand fecal streptococci (Verhoeven and Meuleman, 1999).
A surveillance study for the microbial quality of wastewater in Ismailia, Egypt,where a gravel bed CTW has been installed to treat primary-treated domesticwastewater, found that the system could remove 96.2% of the human intestinal
TABLE 7Removal Rates of Fecal Coliforms in Reed Beds
a
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nematode eggs (helminths) at hydraulic retention time less than 1 day (Stott et al.,1997).
D. Heavy Metals Removal
‘Heavy metals’ is a collective term given to all metals having molecular weightabove calcium in the periodic table of elements. The densities of these metals aregreater than 5 g/cm3. The main heavy metals of concern in water quality manage-ment are lead, copper, zinc, chromium, mercury, cadmium, and arsenic. Thesemetals may be highly toxic when present in higher concentrations in water.However, some metals such as copper and zinc are also essential micronutrients forplants and microorganisms. Heavy metals in wastewater derive from many sourcesincluding corrosion of metal pipes, and paints.
Many varieties of wetland plant species are tolerant of high concentrations ofheavy metals, perhaps because of the protective effect of the iron plaque, which candevelop around the roots (Hadjichristova, 1994). Hence, wetlands can be designedand built for heavy metals removals. In fact, many constructed wetlands specifi-cally built for heavy metal removal are in operation in Australia (Morey, 1996).Similarly, iron and manganese removal are prime considerations in acid minedrainage wetlands (Weider, 1989). The three main wetland processes, whichremove heavy metals are (Kadlec and Knight, 1996):
• Binding to soils, sediments and particulate matter• Precipitation as insoluble salts; and• Uptake by bacteria, algae, and plants
Major proportion of heavy metal removal is accounted to binding processeswithin wetlands (Kadlec and Keoleian, 1986). Because of their positive charge, theheavy metals are readily adsorbed, complexed and bound with suspended particles,which subsequently settle on the substrate. Precipitation of heavy metals as in-soluble salts such as carbonates, bicarbonates, sulfides, and hydroxides is anotherprocess that leads to their long-term removal. These salts formed by the reactionof heavy metals with other chemicals present in water column are insoluble, andhence precipitate to the bottom to become fixed within the wetland substrate.
During the initial period of establishment of treatment wetlands, the bindingprocesses are limited and the uptake by the biota is dominant. Algae and microor-ganisms take up heavy metals available in the dissolved form, whereas macro-phytes can take up also from the sediments. However, the uptake by plants,bacteria, and algae accounts for less than 1% of the total heavy metals removal inCTWs (Hiley, 1995).
There is concern about the risks of adopting wetlands for treatment of heavymetal from wastewaters, as bioaccumulated toxic metals may enter the food chain.
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Therefore, the long-term consequences of such risks needs to be adequatelycovered while using CTWs for this purpose. Accumulation of heavy metals inwetland substrates may also have long-term implications. The sedimented heavymetals may be released into the system if the substrates are disturbed or oxidized.To avoid such an eventuality, substrates have to be permanently maintained inanoxic conditions (DLWC, 1998a).
1. Heavy Metals Removal Performance
In a subsurface flow wetland planted with common reed for treatment oflandfill leachate, Surface et al (1993) measured an iron removal efficiency of78.6%, which is equivalent to 963 kg.ha–1.yr–1.
A full-scale surface flow landfill-leachate treatment wetland installed in Mo-bile Country, Alabama, USA, indicated high removal rates of lead and nickel(Sanford, 1999). The reported removal efficiencies of these metals are 94% and88% respectively. Another similar system with 3 days retention operated at Isanti-Chisago sanitary landfill site in Minnesota, USA, reported removal efficiencies ofiron, zinc, manganese, arsenic, and mercury as 97%, 93%, 91%, 89% and 75%respectively (Loer et al., 1999). Strecker et al. (1994) reported averages of about50% for zinc and 60% for lead from a CTW for stormwater treatment.
Chromium concentration reduction in wetlands varied from zero at low influ-ent concentrations to 87.5% or greater at an influent concentration of 160 µg/L(Kadlec and Knight, 1996). Mass chromium removal rates in constructed wetlandsreceiving domestic wastewater has been estimated as up to 7.2 kg.ha–1.yr–1.
IX. CONCLUDING REMARKS
Constructed wetlands can be designed and operated to remove a wide range ofpollutants from wastewater. Given the low operation, maintenance, and energyrequirements, constructed wetlands could well be the systems for achieving sus-tainable urban wastewater management. Unlike conventional biological treat-ments, which merely transfer problems of pollutants in space and time, constructedwetlands systems do not generate sludge — the concentrated form of pollutants,handling and disposal of which is a major problem. In fact, constructed wetlandsare designed and installed also as systems for treatment and disposal of sludgegenerated from conventional sewage treatment plants. The large quantum ofresearch and the results have clearly established the versatile nature of wetlandsystems and their capabilities to achieve high-performance efficiencies at lowercosts. Hence, they ideally suit developing countries, where it has been reported thatless than 10% of wastewater is treated mainly because of the nonaffordability offinancial and energy-intensive conventional treatment systems. Unlike conven-
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tional processes, constructed wetlands can be installed and operated with the samefinancial and performance efficiencies even for small and isolated communities,and also as on-site disposal systems for individual households, in both developedand developing countries. The flexibility in designing constructed wetlands forwide-ranging treatment capacities thus offer the advantage of achieving effectivewastewater management and environmental protection irrespective of geographicand socio-economic constraints.
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