Natural wetlands (e.g., swamps, bogs, marshes, fens, sloughs, etc.) are long recognized as providing many benefits, including food and habitat for wildlife, water quality improvement, flood protection, shoreline erosion control, and opportunities for recreation and aesthetic appreciation. Many of these same benefits have been realized by projects across the country, which involve the use of wetlands in wastewater treatment.
Wetlands are constructed as either surface flow (see Fig. 6.10) or subsurface flow systems (see Fig. 6.11). Both types of wetlands treatment systems typically are constructed in basins or channels with a natural or constructed subsurface barrier to limit seepage (USEPA, 1993). Surface flow systems require more land, but generally are easier to design, construct, and maintain. They consist of shallow basins with emergent and submergent wetland plants that tolerate saturated soil and aerobic conditions. Water flows in one end of the basin, moves slowly through, and is released at the other end. These systems provide habitat and public access. Subsurface flow systems are designed to create subsurface flow through a permeable medium, keeping the water being treated below the surface, thereby helping to avoid the development of odors and other nuisance problems. Such systems have also been referred to as root-zone systems, rock-reed-filters, and vegetated submerged bed systems. The media used (typically soil, sand, gravel or crushed rock) greatly affect the hydraulics of the system. These systems demonstrate higher rates of contaminant removal than surface flow wetlands. Both systems utilize the roots of plants to provide substrate for the growth of attached bacteria that utilize the nutrients present in the effluents and for the transfer of oxygen. Bacteria do the bulk of the work in these systems, although there is some nitrogen uptake by the plants. The surface flow system most closely approximates a
natural wetland. Typically, these systems are long, narrow basins, with depths of less than 2 feet, which are planted with aquatic vegetation such as bulrush (Scirpus spp.) or cattails (Typha spp.) (Fig. 6.12). The shallow subsurface flow systems use a gravel or sand medium, approximately 18 inches deep, which provides a rooting medium for the aquatic plants and through which the wastewater flows.
Organic and inorganic matters are removed from wastewater through several mechanisms. Processes of sedimentation, microbial degradation, precipitation, and plant uptake remove most contaminants. Organic compounds can be broken down for consumption by microorganisms in a wetland system. This biodegradation removes the organic compounds from water as they provide energy for the organisms. Organics can also be degraded when taken up by plants. They can also sorb to surfaces in the wetland, usually to plant debris. Organic compounds containing nitrogen sorb to the surface in the wetland, and organic nitrogen is converted to ammonia. Ammonia can volatilize, be exchanged with other cations in the sediment, or be nitrified if oxygen is present. Nitrate is the form of N taken up by plants, so emergent plants use it during the growing season. Excess nitrate in an anaerobic system is reduced to N2 and N2O gases as a result of denitrification, the main mechanism of nitrate removal.
The design considerations for constructed wetlands systems are varied and site-dependent. Food and agricultural wastewater treatment systems are most concerned with the reduction of suspended solids, organic matter, pathogens, phosphates, ammonia, and organic nitrogen. Some system designs anticipate exactly what kinds of contaminants the wetlands will receive, and at what levels; others face variable and unpredictable wastewater streams.
Food and agricultural wastewater destined for wetlands treatment often travels through a treatment train, although in some cases wastewater is released directly into a wetland system. The initial step is usually passage through a traditional wastewater treatment plant, where excess ammonia is removed, followed by a sedimentation chamber where any remaining suspended material is removed. Depending on the levels of fecal coliforms and the requirements for effluent contaminant levels, the water may be disinfected with chlorine before release into the constructed wetland system. If the water is to be discharged into a waterway, the minimum contaminant criteria may be different than for a system in which the wetlands are the final destination for the water.
Engineered wetlands for other kinds of wastewater may also consist of a series of treatment steps that have been built according to the expected flow and loading rates. In general, the heavier the load a system receives, the larger the wetlands system will need to be to effectively remove contaminants. The heavier load could be a large volume of water discharged into the system, or volumes with higher concentrations of contaminants.
A series of lined settling and aeration ponds, or lagoons, may be the initial step in treatment, followed by release into the actual wetland. The wetland designs can vary from more traditional systems, with populations of native plants, to aerobic systems that function without aquatic plants and treat waste primarily with added bacteria. An aerobic system may use aquatic plants in a final polishing step.
The principal design parameters for constructed wetland for wastewater treatment include hydraulic detention time, basin depth, basin geometry, BOD5 loading rate, and hydraulic loading rate (Metcalf and Eddy, Inc., 1991). Table 6.7 provides the typical ranges of design parameters for constructed wetland.
Simplified mathematical expressions for wetland systems are available if we view the systems as aerobic attached-growth biological reactors with plug-flow kinetics for BOD5 and nitrogen removal. The basic equation for both systems of wetland treatment is the following (Equation 6.16) (Reed et al., 1995):
Ce = effluent pollutant concentration, mg/l C0 = influent pollutant concentration, mg/l
Table 6.7. Typical design parameters for constructed wetlands (source: USEPA, 1992).
Minimum surface area Maximum water depth
Bed depth Minimum hydraulic residence time Maximum hydraulic loading rate Minimum pretreatment
Range of organic loading as BOD
Surface Water Flow
23-115 ac/mgd Relatively shallow
Not applicable 7 days
Primary (secondary optional) 4.1-8.2 kg/acd
Subsurface Water Flow
2.3-46 ac/mgd Water level below ground surface 12.30m 7 days
2.03-40.5 cm/d Primary
0.82-63.6 kg/acd kT = temperature-dependent first-order reaction rate constant,d 1 t = hydraulic residence time, d
The hydraulic residence time in the wetland can be computed as the following (Equation 6.17):
L = length of the wetland cell, m or ft W = width of the wetland cell, m or ft y = depth of water in the wetland cell, m or ft n = porosity, formed by vegetation, media, roots, and other solids depending on the system, expressed as a percent Q = the average flow through the wetland (Q = (Qin + Qout)/2), m3/d or ft3/d The surface area of the wetland is As = L W
One common problem associated with surface flow wetland systems is mosquitoes when certain pockets of the wetland hold still waters. This problem can be controlled by biological means. There are certain fish species (e.g., mosquito fish, Gambusia affinis) that feed upon mosquito's egg, larva, pupa; they can be introduced into the wetland system provided that the DO level in the wetland is above 1 mg/l in order to maintain fish populations.
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