Natural Wastewater Treatment Systems - Chapter 6 ppsx

259 6 Free Water Surface Constructed Wetlands Wetlands are defined for this book as ecosystems where the water surface is at or near the ground surface for long enough each year to maintain saturated soil conditions and related vegetation. The major wetland types with potential for water quality improvement are swamps that are dominated by trees, bogs that are characterized by mosses and peat, and marshes that contain grasses and emergent macrophytes. The majority of wetlands used for wastewater treatment are in the marsh category, but a few examples of the other two types also exist. The capability of these ecosystems to improve water quality has been recognized for at least 30 years. The use of engineered wetland systems for wastewater treatment has emerged during this period at an accelerating pace. The engineering involved may range from installation of simple inlet and outlet structures in a natural wetland to the design and construction of a completely new wetland where one did not exist before. The design goals of these systems may range from an exclusive commitment for treatment functions to systems that provide advanced treatment or polishing combined with enhanced wildlife habitat and public rec- reational opportunities. The size of these systems ranges from small on-site units designed to treat the septic tank effluent from a single-family dwelling to 40,000- ac (16,200 ha) wetlands in South Florida for the treatment of phosphorus in agricultural stormwater drainage. These wetland systems are land intensive but offer a very effective biological treatment response in a passive manner so that mechanical equipment, energy, and skilled operator attention are minimized. Where suitable land is available at a reasonable cost, wetland systems can be a most cost-effective treatment alternative, while also providing enhanced habitat and recreational values. 6.1 PROCESS DESCRIPTION For engineering purposes, wetlands have been described in terms of the position of the water surface. The free water surface (FWS) wetland is characterized by a water surface exposed to the atmosphere. Natural marshes and swamps are FWS wetlands, and bogs can be if the water flows on top of the peat. Most constructed FWS wetlands typically consist of one or more vegetated shallow basins or channels with a barrier to prevent seepage, with soil to support the emergent macrophyte vegetation, and with appropriate inlet and outlet structures. The water depth in this type of constructed wetland might range from 0.2 to 2.6 DK804X_C006.fm Page 259 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC 260 Natural Wastewater Treatment Systems ft (0.05 to 0.8 m). The design flows for operational FWS treatment wetlands range from less than 1000 gpd (4 m 3 /d) to over 20 mgd (75,000 m 3 /d). The biological conditions in these wetlands are similar, in some respects, to those occurring in facultative treatment ponds. The water near the bottom of the wetland is in an anoxic/anaerobic state; a shallow zone near the water surface tends to be aerobic, and the source of that oxygen is atmospheric reaeration. Facultative lagoons, as described in Chapter 4, have an additional source of oxygen that is generated by the algae present in the system. In a densely vegetated wetland, this oxygen source is not available because the plant canopy shades the water surface and algae cannot persist. The most significant difference is the presence, in the wetlands, of physical substrate for the development of periphytic attached-growth microorganisms, which are responsible for much of the biolog- ical treatment occurring in the system. In FWS wetlands, these substrates are the submerged leaves and stems of the living plants, the standing dead plants, and the benthic litter layer. In subsurface flow (SSF) wetlands (see Chapter 7), the substrate is composed of the submerged media surfaces and the roots and rhi- zomes of the emergent plants growing in the system. Many of the treatment responses proceed at a higher rate in a wetland than in facultative lagoons because of the presence of the substrate and these periphytic organisms, and the response in SSF wetlands is typically at a higher rate than in FWS wetlands because of the increased availability of substrate in the gravel media. In addition to a higher rate of treatment than FWS wetlands, the SSF wetland concept offers several other advantages. Because the water surface is below the top of the gravel, mosquitoes are not a problem as the larvae cannot develop. In cold climates, the subsurface position of the water and the litter layer on top of the gravel offer greater thermal protection for the SSF wetland. The greatest advantage is the minimal risk of public exposure or contact with the wastewater because the water surface is not directly, or easily, accessible; however, the major disadvantage for the SSF concept is the cost of the gravel media. The unit costs for the other system components (e.g., excavation, liner, inlets, outlets) are about the same for either SSF or FWS wetlands, but the cost of gravel in the SSF system adds significantly to project costs. For design flow rates larger than about 50,000 gpd (190 m 3 /d), the smaller size of the SSF wetland does not usually compensate for the extra cost of the gravel. Because of these costs, the SSF concept is best suited for those smaller applications where public exposure is an issue, including individual homes, groups of homes, parks, schools, and other commercial and public facilities. It will be more economical to utilize the FWS concept for larger municipal and industrial systems and for other potential wetland applications. The FWS concept also offers a greater potential for incorporation of habitat values in a project. An example of a FWS wetland is shown in Figure 6.1. The treatment processes occurring in both FWS and SSF wetlands are a complex and interrelated sequence of biological, chemical, and physical responses. Because of the shallow water depth and the low flow velocities, particulate matter settles rapidly or is trapped in the submerged matrix of plants or gravel. Algae are also trapped and cannot regenerate because of the shading DK804X_C006.fm Page 260 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC Free Water Surface Constructed Wetlands 261 effect in the densely vegetated portions of the wetland. These deposited materials then undergo anaerobic decomposition in the benthic layers and release dissolved and gaseous substances to the water. All of the dissolved substances are available for sorption by the soils and the active microbial and plant populations throughout the wetland. Oxygen is available at the water surface and on microsites on the living plant surfaces and root and rhizome surfaces so aerobic reactions are also possible within the system. 6.2 WETLAND COMPONENTS The major system components that may influence the treatment process in con- structed wetlands include the plants, detritus, soils, bacteria, protozoa, and higher animals. Their functions and the system performance are, in turn, influenced by water depth, temperature, pH, redox potential, and dissolved oxygen concentra- tion. 6.2.1 T YPES OF P LANTS A wide variety of aquatic plants have been used in wetland systems designed for wastewater treatment. The larger trees (e.g., cypress, ash, willow) often preexist on natural bogs, strands, and “domes” used for wastewater treatment in Florida and elsewhere. No attempt has been made to use these species in a constructed wetland nor has their function as a treatment component in the system been defined. The emergent aquatic macrophytes are the most commonly found species in the marsh type of constructed wetlands used for wastewater treatment. The most frequently used are cattails ( Typha ), reeds ( Phragmites communis ), rushes ( Juncus spp.), bulrushes ( Scirpus ), and sedges ( Carex ). Bul- rush and cattails, or a combination of the two, are the dominant species on most FIGURE 6.1 Free water surface (FWS) wetlands at Arcata, California. DK804X_C006.fm Page 261 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC 262 Natural Wastewater Treatment Systems of the constructed wetlands in the United States. A few systems in the United States have Phragmites , but this species is the dominant type selected for con- structed wetlands in Europe. Systems that are specifically designed for habitat values in addition to treatment usually select a greater variety of plants with an emphasis on food and nesting values for birds and other aquatic life. Information on some typical plant species common in the United States and a discussion of advantages and disadvantages for their use in a constructed wetland are provided in the following text. Further details on the characteristics of these plants can be found in a number of references (Hammer, 1992; Lawson, 1985; Mitsch and Gosselink, 2000; Thornhurst, 1993). 6.2.2 E MERGENT S PECIES 6.2.2.1 Cattail Typical varieties are Typha angustifolia (narrow leaf cattail) and Typha latifolia (broad leaf cattail). Distribution is worldwide. Optimum pH is 4 to 10. Salinity tolerance for narrow leaf is 15 to 30 ppt; broad leaf, <1 ppt. Growth is rapid, via rhizomes; the plant spreads laterally to provide dense cover in less than a year with 2-ft (0.6-m) plant spacing. Root penetration is relatively shallow in gravel (approximately 1 ft or 0.3 m). Annual yield is 14 (dw) ton/ac (30 mt/ha). Tissue (dw basis) is 45% C, 14% N, 2% P; 30% solids. Seeds and roots are a food source for water birds, muskrat, nutria, and beaver; cattails also provide nesting cover for birds. Cattails can be permanently inundated at >1 ft (0.3 m) but can also tolerate drought. They are commonly used on many FWS and SSF wetlands in the United States. The relatively shallow root penetration is not desirable for SSF systems without adjusting the design depth of bed. 6.2.2.2 Bulrush Typical varieties are Scirpus acutus (hardstem bulrush), common tule, Scirpus cypernius (wool grass), Scirpus fluviatilis (river bulrush), Scirpus robustus (alkali bulrush), Scirpus validus (soft stem bulrush), and Scirpus lacustris (bulrush). Bulrush is known as Scirpus in the United States but is referred to as Schoeno- plectus in the rest of the world (Mitsch and Gosselink, 2000). Distribution is worldwide. Optimum pH is 4 to 9. Salinity tolerance for hardstem, wool grass, river, and soft stem bulrushes is 0 to 5 ppt; alkali and Olney’s, 25 ppt. Growth of alkali, wool grass, and river bulrush is moderate, with dense cover achieved in 1 yr with 1-ft (0.3-m) plant spacing; growth of all others is moderate to rapid, with dense cover achieved in 1 yr with 1- to 2-ft (0.3- to 0.60-m) plant spacing. Deep root penetration in gravel is approximately 2 ft (0.6m). Annual yield is approximately 9 (dw) ton/ac (20 mt/ha). Tissue (dw basis) is approximately 18% N, 2% P; 30% solids. Bulrush seeds and rhizomes are a food source for many water birds, muskrats, nutria, and fish; they also provide a nesting area for fish when inundated. Bulrushes can be permanently inundated — hardstem up to 3 ft (1 m), most others 0.5 to 1 ft (0.15 to 0.3 m); some can tolerate drought DK804X_C006.fm Page 262 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC Free Water Surface Constructed Wetlands 263 conditions. They are commonly used for many FWS and SSF constructed wet- lands in the United States. 6.2.2.3 Reeds Typical varieties are Phragmites australis (common reed) and wild reed. Distri- bution is worldwide. Optimum pH is 2 to 8. Salinity tolerance is <45 ppt. Growth is very rapid, via rhizomes; lateral spread is approximately 3 ft/yr (1 m/yr), providing very dense cover in 1 yr with plants spaced at 2 ft (0.6 m). Deep root penetration in gravel is approximately 1.5 ft (0.4 m). Annual yield is approxi- mately 18 (dw) ton/ac (40 mt/ha). Tissue (dw basis) is approximately 45% C, 20% N, 2% P; 40% solids. With regard to habitat values, reeds have low food value for most birds and animals and some value as nesting cover for birds and animals. They can be permanently inundated up to about 1 m (3 ft), and are also very drought resistant. They are considered by some to be an invasive pest species in natural wetlands in the United States. They have been very successfully used at constructed wastewater treatment wetlands in the United States. They are the dominant species used for this purpose in Europe. Because of its low food value, this species is not subject to the damage caused by muskrat and nutria which has occurred in constructed wetlands supporting other plant species. 6.2.2.4 Rushes Typical varieties are Juncus articulatus (jointed rush), Juncus balticus (Baltic rush), and Juncus effusus (soft rush). Distribution is worldwide. Optimum pH is 5 to 7.5. Salinity tolerance is 0 to <25, depending on type. Growth is very slow, via rhizomes; lateral spread is <0.3 ft/yr (0.1 m/yr), providing dense cover in 1 year with plants spaced at 0.5 ft (0.15 m). Annual yield is 45 (dw) ton/ac (50 mt/ha). Tissue (dw basis) is approximately 15% N, 2% P; 50% solids. Rushes provide food for many bird species, and their roots are food for muskrats. Some rushes can tolerate permanent inundation up to <1 ft (0.3 m), but they prefer dry- down periods. Other plants are better suited as the major species for wastewater wetlands; rushes are well suited as a peripheral planting for habitat enhancement. 6.2.2.5 Sedges Typical varieties are Carex aquatilis (water sedge), Carex lacustris (lake sedge), and Carex stricata (tussock sedge). Distribution is worldwide. Optimum pH is 5 to 7.5. Salinity tolerance is <0.5 ppt. Growth is moderate to slow, via rhizomes; lateral spread is <0.5 ft/yr (0.15 m/yr), providing dense cover in 1 year with plants spaced at 0.5 ft (0.15 m). Annual yield is <4 (dw) ton/yr (5 mt/ha). Tissue (on a dw basis) is approximately 1% N, 0.1% P; 50% solids. With regard to habitat values, sedges are a food source for numerous birds and moose. Some types can sustain permanent inundation; others require a dry-down period. Other plants are better suited as the major species for wastewater wetlands; sedges are well suited as a peripheral planting for habitat enhancement. DK804X_C006.fm Page 263 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC 264 Natural Wastewater Treatment Systems 6.2.3 S UBMERGED S PECIES Submerged plant species have been used in deepwater zones of FWS wetlands and are a component in a patented process that has been used to improve water quality in freshwater lakes, ponds, and golf course water hazards. Species that have been used for this purpose include Ceratophyllum demersum (coontail, or hornwart), Elodea (waterweed), Potamogeton pectinatus (sago pond weed), Pot- amogeton perfoliatus (redhead grass), Ruppia maritima (widgeongrass), Vallisne- ria americana (wild celery), and Myriophyllum spp. (watermilfoil). The distribu- tion of these species is worldwide. Optimum pH is 6 to 10. Salinity tolerance is <5 to 15 ppt for most varieties. Growth is rapid, via rhizomes; lateral spread is >1 ft/yr (0.3 m/yr), providing dense cover in 1 year with plants spaced at 2 ft (0.6 m). Annual yields vary — coontail, 8.9 (dw) ton/ac (10 mt/ha); Potamogeton , 2.7 (dw) ton/ac(3 mt/ha); and watermilfoil, 8 (dw) ton/ac (9 mt/ha). Tissue (dw basis) is approximately 2 to 5% N, 0.1 to 1% P; 5 to 10% solids. These species provide food for a wide variety of birds, fish, and animals; sago pond weed is especially valuable for ducks. These species can tolerate continuous inundation, with the depth of acceptable water being a function of water clarity and turbidity as these plants depend on penetration of sunlight through the water column. Some of these plants have been used to enhance the habitat values in FWS constructed wetlands. Coontail, Elodea , and other species have been used for nutrient control in fresh- water ponds and lakes; regular harvesting removes the plants and the nutrients. 6.2.4 F LOATING S PECIES Several floating plants have been used in wastewater treatment systems. These floating plants are not typically a design component in constructed wetlands. The species most likely to occur incidentally in FWS wetlands is Lemna (duckweed). The presence of duckweed on the water surface of a wetland can be both beneficial and detrimental. The benefit occurs because the growth of algae is suppressed; the detrimental effect is the reduction in transfer of atmospheric oxygen at the water surface because of the duckweed mat. The growth rate of this plant is very rapid, and the annual yield can be 18 (dw) ton/ac (20 mt/hat) or more. The tissue composition (dw basis) is approximately 6% N, 2% P; solids 5%. Salinity toler- ance is less than 0.5 ppt. These species serve as a food source for ducks and other water birds, muskrat, and beaver. The presence of duckweed on FWS wetlands cannot be prevented because the plant also tolerates partial shade. Open-water zones in FWS wetlands should be large enough so wind action can periodically break up and move any duckweed mat to permit desirable reaeration. The decom- position of the unplanned duckweed may also impose an unexpected seasonal nitrogen load on the system. 6.2.5 E VAPOTRANSPIRATION L OSSES The water losses due to evapotranspiration (ET) should be considered for wetland designs in arid climates and can be a factor during the warm summer months in DK804X_C006.fm Page 264 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC Free Water Surface Constructed Wetlands 265 all locations. In the western United States where appropriative laws govern the use of water, it may be necessary to replace the volume of water lost to protect the rights of downstream water users. Evaporative water losses in the summer months decrease the water volume in the system; therefore, the concentration of pollutants remaining in the system tends to increase even though treatment is very effective on a mass removal basis. For design purposes, the evapotranspira- tion rate can be taken as being equal to 80% of the pan evaporation rate for the area. This in effect is equal to the lake evaporation rate. In the past, some controversy existed regarding the effect of plants on the evaporation rate. It is the current consensus that the shading effect of emergent or floating plants reduces direct evaporation from the water but the plants still transpire. The net effect is roughly the same rate whether plants are present or not. The first edition of this book indicated relatively high ET rates for some emergent plant species (Reed et al., 1988). These data were obtained from relatively small culture tanks and containers and are not representative of full-scale wetland systems. 6.2.6 O XYGEN T RANSFER Because of the continuous inundation, the soils or the media in a SSF wetland are anaerobic, which is an environment not well suited to support most vegetative species; however, the emergent plant species described previously have all devel- oped the capability of absorbing oxygen and other necessary gasses from the atmosphere through their leaves and above-water stems, and they have large gas vessels, which conduct those gasses to the roots so the roots are sustained aero- bically in an otherwise anaerobic environment. It has been estimated that these plants can transfer between 5 and 45 g of oxygen per day per square meter of wetland surface area, depending on plant density and oxygen stress levels in the root zone (Boon, 1985; Lawson, 1985). However, current estimates are that the transfer is more typically 4 g of oxygen per square meter (Brix, 1994; Vymazal et al., 1998). Most of this oxygen is utilized at the plant roots, and availability is limited for support of external microbial activity; however, some of this oxygen is believed to reach the surfaces of the roots and rhizomes and create aerobic microsites at these points. These aerobic microsites can then support aerobic reactions such as nitrification if other conditions are appropriate. The plant seems to respond with more oxygen as the demand increases at the roots, but the transfer capability is limited. Heavy deposits of raw sludge at the head of some constructed wetlands have apparently overwhelmed the oxygen transfer capability and resulted in plant die-off. This oxygen source is of most benefit in the SSF constructed wetland, where the wastewater flows through the media and comes in direct contact with the roots and rhizomes of the plants. In the FWS wetland, the wastewater flows above the soil layer and the contained roots and does not come into direct contact with this potential oxygen source. The major oxygen source for the FWS wetland is believed to be atmospheric reaeration at the water surface. To maximize the benefit in the SSF case, it is important to encourage DK804X_C006.fm Page 265 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC 266 Natural Wastewater Treatment Systems root penetration to the full depth of the media so potential contact points exist throughout the profile. As described in Chapter 7, the removal of ammonia in a SSF wetland can be directly correlated with the depth of root penetration and the availability of oxygen (Reed, 1993). 6.2.7 P LANT D IVERSITY Natural wetlands typically contain a wide diversity of plant life. Attempts to replicate that diversity in constructed wetlands designed for wastewater treatment have in general not been successful. The relatively high nutrient content of most wastewaters tends to favor the growth of cattails, reeds, etc., and these tend to crowd out the other less competitive species over time. Many of these constructed wetlands in the United States and Europe have been planted as a monoculture or at most with two or three plant species, and these have all survived and provided excellent wastewater treatment. The FWS wetland concept has greater potential for beneficial habitat values because the water surface is exposed and accessible to birds and animals. Further enhancement is possible via incorporation of deep open-water zones and the use of selected plantings to provide attractive food sources (e.g., sago pond weed and similar plants). Nesting islands can also be constructed within these deep water zones for further enhancement. These deep- water zones can also provide treatment benefits as they increase the hydraulic retention time (HRT) in the system and serve to redistribute the flow, if properly constructed. The portions of the FWS wetland designed specifically for treatment can be planted with a single species. Cattails and bulrush are often used but are at risk from muskrat and nutria damage; Phragmites offers significant advantages in this regard. A number of FWS and SSF wetlands in the southern United States were initially planted with attractive flowering species (e.g., Canna lily, iris) for esthetic reasons. These plants have soft tissues which decompose very quickly when the emergent portion dies back in the fall and after even a mild frost. The rapid decomposition has resulted in a measurable increase in biological oxygen demand (BOD) and nitrogen leaving the wetland system. In some cases, the system managers utilized an annual harvest for removal of these plants prior to the seasonal dieback or frosts. In most cases, the problems have been completely avoided by replacing these plants with the more resistant reeds, rushes, or cattails, which do not require an annual harvest. Use of soft-tissue flowering species is not recommended for future systems, except possibly as a border. 6.2.8 P LANT F UNCTIONS The terrestrial plants used in land treatment systems described in Chapter 8 of this book provide the major pathway for removal of nutrients in those systems. In those cases, the system design loading is partially matched to the plant uptake capability of the plants and the treatment area is sized accordingly. Harvesting then removes the nutrients from the site. The emergent aquatic plants used in wetlands also take up nutrients and other wastewater constituents. Harvesting is DK804X_C006.fm Page 266 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC Free Water Surface Constructed Wetlands 267 not, however, routinely practiced in these wetland systems due to problems with access and the relatively high labor costs. Studies have shown that harvesting of the plant material from a constructed wetland provides a minor nitrogen removal pathway as compared to biological activity in the wetland. In two cases (Gearheart et al., 1983; Herskowitz, 1986), a single end-of-season harvest accounted for less than 10% of the nitrogen removed by the system. Harvesting on a more frequent schedule would certainly increase that percentage but would also increase the cost and complexity of system management. Biological activity becomes the dominant mechanism in constructed wetlands as compared to land treatment systems, partially due to the significantly longer HRT in the former systems. When water is applied to the soil surface in most land treatment systems, the residence time for water as it passes from the surface through the active root zone is measured in minutes or hours; in contrast, the residence time in most con- structed wetlands is usually measured in terms of at least several days. In some cases, these emergent aquatic plants are known to take up and transform organic compounds, so harvesting is not required for removal of these pollutants. In the case of nutrients, metals, and other conservative substances, harvesting and removal of the plants are necessary if plant uptake is the design pathway for permanent removal. Plant uptake and harvest are not usually a design consideration for constructed wetlands used for domestic, municipal, and most industrial wastewaters. Even though the system may be designed as a biological reactor and the potential for plant uptake is neglected, the presence of the plants in these wetland systems is still essential. Their root systems are the major source of oxygen in the SSF concept, and the physical presence of the leaves, stems, roots, rhizomes, and detritus regulates water flow and provides numerous contact opportunities between the flowing water and the biological community. These submerged plant parts provide the substrate for development and support of the attached microbial organisms that are responsible for much of the treatment. The stalks and leaves above the water surface in the FWS wetland provide a shading canopy that limits sunlight penetration and controls algae growth. The exposed plant parts die back each fall, but the presence of this material reduces the thermal effects of the wind and convective heat losses during the winter months. The litter layer on top of the SSF bed adds even more thermal protection to that type of system. 6.2.9 S OILS In natural wetlands, most of the nutrients required for plant growth are obtained from the soil by emergent aquatic plants. Cattails, reeds, and bulrushes will grow in a wide variety of soils and, as shown in the SSF wetland concept, in relatively fine gravels. The void spaces in the media serve as the flow channels in the SSF wetland. Treatment in these cases is provided by microbial organisms attached to the roots, rhizomes, and media surfaces. Because of the relatively light loading in most SSF wetlands, this microbial growth does not produce thick layers of attached material such as typically occur in a trickling filter, so clogging from DK804X_C006.fm Page 267 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC 268 Natural Wastewater Treatment Systems this source does not appear to be a problem. The major flow path in FWS wetlands is above the soil surface, and the most active microbial activity occurs on the surfaces of the detrital layer and the submerged plant parts. Soils with some clay content can be very effective for phosphorus removal. As described in Chapters 3 and 8, phosphorus removal in the soil matrix of a land treatment system can be a major pathway for almost complete phosphorus removal for many decades. In FWS wetlands, the only contact opportunities are at the soil surface; during the first year of system operation, phosphorus removal can be excellent due to this soil activity and plant development. These pathways tend to come to equilibrium after the first year or so, and phosphorus removal will drop off significantly. Soils have been tried in Europe for SSF wetlands, primarily for their phosphorus removal potential. This attempt has not been successful in most cases, as the limited hydraulic capacity of soils results in most of the applied flow moving across the top of the bed rather than through the subsurface voids so the anticipated contact opportunities are not realized. The gravels used in most SSF wetlands have a negligible capacity for phosphorus removal. Soils, again with some clay content, or granular media containing some clay minerals also have some ion exchange capacity. This ion exchange capability may contribute, at least temporarily, to removal of ammonium (NH 4 ) that exists in wastewater in ionic form. This capacity is rapidly exhausted in most SSF and FWS wetlands as the contact surfaces are continuously under water and contin- uously anaerobic. In vertical-flow SSF beds, described in Chapter 7, aerobic conditions are periodically restored, and the adsorbed ammonium is released via biological nitrification, which then releases the ion exchange sites for further ammonium adsorption. 6.2.10 O RGANISMS A wide variety of beneficial organisms, ranging from bacteria to protozoa to higher animals, can exist in wetland systems. The range of species present is similar to that found in the pond systems described in Chapter 4. In the case of emergent aquatic vegetation in wetlands, this microbial growth occurs on the submerged portions of the plants, on the litter, and directly on the media in the SSF wetland case. Wetlands and the overland flow (OF) concept described in Chapter 8 are similar in that they are both “attached-growth” biological systems and share many common attributes with the familiar trickling filters. All of these systems require a substrate for the development of the biological growth; their performance is dependent on the detention time in the system and on the contact opportunities provided and is regulated by the availability of oxygen and by the temperature. 6.3 PERFORMANCE EXPECTATIONS Wetland systems can effectively treat high levels of BOD, total suspended solids (TSS), and nitrogen, as well as significant levels of metals, trace organics, and pathogens. Phosphorus removal is minimal due to the limited contact opportunities DK804X_C006.fm Page 268 Friday, July 1, 2005 4:37 PM © 2006 by Taylor & Francis Group, LLC [...]... –0.2 59.0 55 .6 3.4 53.5 52.4 1.1 64 .9 61 .1 3.8 63 .3 60 .9 2.4 May 67 .5 59.9 7 .6 67.0 62 .5 4.5 June 72.1 71.8 0.3 72.8 68 .0 4.8 July 74.8 73 .6 1.2 73.7 69 .1 4 .6 August 78.4 72.7 5.7 73.1 66 .9 6. 2 Reduction (°F) September 76. 1 68 .5 7 .6 70.3 64 .5 5.8 October 64 .2 58 .6 5 .6 59.5 55.9 3 .6 November 60 .6 57.2 3.4 52.2 50 .6 1 .6 December 56. 3 50.2 6. 1 48.4 47.5 0.9 — — 5.5 — — 3.0 Average a b Five-year average... performance data for both FWS and SSF systems The principal removal © 20 06 by Taylor & Francis Group, LLC DK804X_C0 06. fm Page 2 76 Friday, July 1, 2005 4:37 PM 2 76 Natural Wastewater Treatment Systems TABLE 6. 6 Removal of Metals with Length in a Free Water Surface Constructed Wetland at Nucice (Prague) Metal 0m 5m 16 m 32 m 48 m 60 m 62 m Aluminum 451 1 26 65 47 46 . 73.7 69 .1 4 .6 August 78.4 72.7 5.7 73.1 66 .9 6. 2 September 76. 1 68 .5 7 .6 70.3 64 .5 5.8 October 64 .2 58 .6 5 .6 59.5 55.9 3 .6 November 60 .6 57.2 3.4 52.2 50 .6 1 .6 December 56. 3 50.2 6. 1 48.4 47.5. 1.1 February 62 .4 51.3 11.1 50.2 50.4 –0.2 March 59.0 55 .6 3.4 53.5 52.4 1.1 April 64 .9 61 .1 3.8 63 .3 60 .9 2.4 May 67 .5 59.9 7 .6 67.0 62 .5 4.5 June 72.1 71.8 0.3 72.8 68 .0 4.8 July 74.8 73 .6 1.2 73.7 69 .1. 1.0 55 Average 2. 26 4.98 2. 36 46 DK804X_C0 06. fm Page 273 Friday, July 1, 2005 4:37 PM © 20 06 by Taylor & Francis Group, LLC 274 Natural Wastewater Treatment Systems 6. 3 .6 TEMPERATURE REDUCTION Temperature