CHAPTER 5 WETLAND SYSTEMS

Transcription

1 CHAPTER WETLAND SYSTEMS Learning Objectives In this chapter we will examine the use of natural and constructed wetlands for use in wastewater treatment. After completing this chapter, the student will be able to: Describe the fundamental processes behind wetland systems; Explain the differences between natural and constructed wetlands; Outline in detail the physical components of both natural and constructed wetlands; Describe the operation and maintenance of these systems; and Identify the safety issues related to wetland systems. Introduction Natural wastewater treatment processes include soil-based and aquatic systems. In these systems, natural environmental components such as vegetation, soil, microorganisms (land and water-based), and, to a limited extent, simple animals like rotifers, provide treatment that proceeds at natural rates. In contrast, mechanical systems, such as the activated sludge process, which also depends on the activity of microorganisms, require high-energy inputs for mixing and oxygen supply to sustain a high rate of treatment. Natural systems typically also require fewer operational personnel, consume less energy, and produce less sludge than mechanical systems. Where sufficient land of suitable character is available, natural systems are often the most cost-effective option for construction and operation. They are typically better suited for small to moderate-sized communities, and commercial and industrial operations in rural areas because of the need for sufficient land area. Natural systems include soil-based systems, such as leach fields and land treatment systems (Figure.1), which depend on the subsurface soil environment, and aquatic systems that include stabilization ponds (as discussed in Chapter ) for municipal wastewater, aquatic systems with floating plants and wetlands with emergent plants. This chapter covers wetland systems. Wetlands are ecosystems in which the water surface is at or near the ground surface long enough each year to maintain saturated soil conditions and related vegetation. Wetland systems can be naturally occurring or constructed. Natural wetlands are part of contiguous surface waters; so wastewater applied to natural wetlands should not exceed the capacity of the area to absorb and process the water. The primary natural wetland types that can improve water quality are swamps, which are dominated by trees; bogs, which are characterized by mosses and peat; and marshes, which contain grasses and emergent macrophytes (plants associated with bodies of water). Most of the wetlands used for wastewater treatment are marshes, although there are examples of the other types. Wetland Systems -1

2 Figure.1 Land Treatment Natural Systems (U.S. EPA, ) Constructed wetlands are designed for similar treatment methods, but often offer better control. There are two types of constructed wetlands: the free water surface (FWS), in which water in the system is exposed to the atmosphere and surface reaeration is the major oxygen source, and the subsurface flow (SF) type (Figures. and.), in which the water level is maintained at or below the surface of the permeable media used in the bed. In both cases, bulk water in the system is essentially devoid of oxygen. - Operations Training/Wastewater Treatment

3 Figure. Elements of a Three Zone Free Water Surface (FWS) Wetland (USEPA, 000) Figure. Elements of a Subsurface Flow (SF) Wetland (USEPA, 000) Vegetation is an important component in both types of systems, but more for its physical presence than for direct uptake of contaminants. The submerged vegetation and litter in the FWS system provides substrate for the attached microbial growth that is primarily responsible for the biological treatment in the system. In other words, it provides a home for the bacteria and organisms needed for treatment. Substrate in the SF case is simply the surface of the media (typically gravel) itself and the roots of the plants. Natural and constructed wetland systems, both FWS and SF types, are being used more frequently to treat wastewater and improve water quality on a worldwide basis. This chapter describes the use of wetland systems to treat domestic, municipal, and industrial wastewater. Wetlands are also used for other purposes including the treatment of mine drainage, urban stormwater, combined sewer overflows, agricultural runoff, livestock and poultry wastes, landfill leachates, and groundwater recharge. Wetland Systems -

4 Process Description Most constructed FWS wetlands consist of one or more vegetated shallow basins or channels, with a barrier to prevent seepage, soil to support the emergent macrophyte vegetation, and with appropriate inlet and outlet structures. The water depth in this type of constructed wetland ranges from 0.0 to 0. m (0. to. ft). The design flows for operational FWS treatment wetlands range from less than m /d (00 gpd) to more than 000 m /d (0 mgd). In FWS wetlands, the submerged leaves and stems of the living plants, standing dead plants, and benthic litter layer provide a physical substrate for the development of the periphytic attached-growth microorganisms that provide much of the biological treatment. In SF wetlands, the substrate is composed of the surface of the submerged media as well as the surfaces of the roots and rhizomes of the emergent plants growing in the system. The treatment response in SF wetlands typically occurs at a greater rate than in FWS wetlands because of the increased availability of substrate in the gravel media. Figure. illustrates the relationship for removing - day biochemical oxygen demand (BOD ). Figure. BOD in a FWS Wetland System Subsurface flow (SF) wetlands consist of shallow basins or channels, with a seepage barrier and with inlet and outlet structures. The bed is filled with a porous media such as gravel, sand, or soil, and vegetation is planted in that media. Their depths range from 0. to 0. m (1 to ft) with design flows ranging from 1 m /d (0 000 gpd) all the way to m /d (. mgd). The water surface in SF wetlands is maintained beneath the upper surface of that media, hence the name subsurface flow. The water near the bottom of the wetland is in an anoxic anaerobic state, and a shallow zone near the water surface is often aerobic, with atmospheric re-aeration being the source of that oxygen. Because of the controlled substrate system and more stable environment for the microorganisms in a SF wetland, the treatment rate is typically greater than that of a FWS system. In addition to the greater treatment rate, SF wetlands offer several other advantages. Because the water surface is below the top of the gravel, mosquitoes are not a problem. The greatest advantage is the minimal risk of public exposure to or contact with the wastewater because the water surface is not directly or easily accessible. The major disadvantage is the cost of the gravel media. The unit costs for other system components (e.g., excavation, liner, inlets, and outlets) are approximately the same for either SF or FWS wetlands, but the gravel in the SF system adds significantly to project costs. Because of these costs, the SF concept is usually best suited only for smaller applications where public exposure is an issue such as individual homes, groups of homes, parks, schools, and other commercial and public facilities. FWS is more economical for larger municipal and industrial systems and other potential wetland applications. The FWS concept also offers greater potential for incorporating habitat values to a project. The treatment process occurring in FWS and SF wetlands is a complex, interrelated sequence of biological, chemical, and physical responses. Because of the shallow water depth and slow flow velocities, particulate matter settles rapidly or is trapped in the submerged matrix of plants and gravel. Algae are also trapped and cannot regenerate because of the shading effect in the densely vegetated portions of the wetland. These deposited materials undergo anaerobic decomposition in the bottom layers and release dissolved and gaseous substances to the water. 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 at spots on the living plant surfaces including the exposed roots and food nodes on those stems; therefore aerobic reactions are possible within the system. The removal of particulate BOD in wetlands occurs rapidly through settling and - Operations Training/Wastewater Treatment

5 entrapment in the submerged plants and litter in FWS wetlands or in the media in SF systems. The trapped material and soluble BOD are oxidized or reduced primarily by periphytic microorganisms. BOD is also produced by the internal decomposition of plant litter and other naturally occurring organic materials. Thus, some untreatable BOD is always present in the final wetland effluent, leaving a residual background BOD that ranges from to mg/l. Table.1 presents influent versus effluent BOD values and Figure. gives BOD removal percentages versus calculated hydraulic retention time (HRT) or the amount of time the liquid stays in the system. Table.1 Influent vs. Effluent BOD for Wetland Systems (adapted from USEPA, 000 and WPCF, ) Constituent Influent 1 Effluent FWS Effluent SF BOD 0 to 00 mg/l to 0 mg/l to mg/l 1 Adapted from Metcalf and Eddy () assuming typical removal by primary sedimentationsoluble BOD = % to % of total Wastewater applied continuously to head of channel containing emergent aquatic vegetation, typical surface hydraulic loading 0.0 m /m d (0. gal/sq ft/day), HRT to days Wastewater applied continuously to head of SF bed supporting emergent vegetation, typical surface hydraulic loading 0.0 m /m d ( gal/sq ft/day), HRT 1 to days Figure. BOD vs. HRT for Wetland Systems Table. presents input versus output of total suspended solids (TSS). Most of the systems produced an effluent less than 0 mg/l and the majority of the systems an effluent less than 0 mg/l. As with BOD, these systems produced residual background concentrations of TSS, from to mg/l, which are a byproduct of decomposing vegetation and other natural organics in the wetlands. Table. Influent vs. Effluent TSS for Wetland Systems (adapted from USEPA, 000 and WPCF, ) Constituent Influent 1 Effluent FWS Effluent SF TSS to 0 mg/l mg/l to 1 mg/l 1 Adapted from Metcalf and Eddy () assuming typical removal by primary sedimentationsoluble BOD = % to % of total Wastewater applied continuously to head of channel containing emergent aquatic vegetation, typical surface hydraulic loading 0.0 m /m d (0. gal/sq ft/day), HRT to days Wastewater applied continuously to head of SF bed supporting emergent vegetation, typical surface hydraulic loading 0.0 m /m d ( gal/sq ft/day), HRT 1 to days The type of nitrogen compounds entering a wetland system depends on the type of pretreatment used. Complete-mix aerated systems with sufficient HRT may produce significant nitrate concentrations. Most of the wetland influent nitrogen will be a combination of organic nitrogen and ammonia ammonium nitrogen which is typically characterized as total Kjeldahl nitrogen (TKN). The dissolved ammonia ammonium balance in the water is a function of ph and temperature (for convenience, the combination is typically referred to as ammonia nitrogen). The organic nitrogen is associated with wastewater solids and algae entering the system. These materials are rapidly separated from the flowing water and then made soluble and converted to ammonia. Several removal possibilities are available for ammonia; it can be removed by plant Wetland Systems -

6 uptake, adsorbed by the soil, or converted to nitrate in aerobic environments. Some surface ammonia is volatile and can be lost directly to the atmosphere. The plant uptake for nitrogen removal is temporary because much of that nitrogen reenters the wetland environment as organic nitrogen when the green parts of the plants die off each year. Soil adsorption can be significant but is typically limited in duration because the amount of storage is limited. It may take a newly constructed wetland one or more growing seasons to reach equilibrium with respect to plant growth and soil adsorption responses. During this early period, ammonia removals can be unusually high. The system attains equilibrium when the vegetation reaches the maximum expected density, and at that point ammonia removals stabilize at their long-term level. The only significant long-term mechanism available for ammonia removal is biological nitrification. Nitrification is an aerobic process, but the environment in the wetland is not aerobic, except near the water surface and on submerged plant and root surfaces. In theory, at least. g of oxygen are required to oxidize 1 g of ammonia. The available oxygen sources in FWS and SF wetlands do not provide the conditions necessary for rapid nitrification. Ammonia removal (nitrification) may require a few weeks to reach low levels without supplemental oxygen sources. Figure. gives ammonia removal data versus HRT. Several systems show a negative percent removal, or effluent ammonia higher than influent ammonia, which is caused by algae present in facultative lagoon effluents entering the systems. This is to be expected. Figure. Ammonia Removal Percentages vs. HRT Once full nitrification does occur, nitrate removal in wetlands can be rapid and effective because the anoxic conditions and carbon sources necessary to support the treatment reactions are naturally present. In a typical wetland system, most of the wastewater BOD has been removed before nitrification begins to dominate in the wetland, but the plant litter and other naturally present organic detritus provides the necessary carbon source. The other important nutrient to consider is phosphorus. As previously described for ammonia, a wetland system may display effective phosphorus removal during the first year or two because of soil adsorption and plant uptake by the vigorously expanding vegetation. However, when the system reaches equilibrium, the phosphorus removal is likely to be reduced. Plant uptake continues, but the decomposition of the litter releases much of that phosphorus back to the water. Direct settling can account for removal of any influent phosphorus associated with particulate matter. A reasonable expectation for phosphorus removal in a wetland system with an HRT of less than days is 0 to 0%. Removing phosphorus to low levels in typical emergent marsh-type systems requires either a large wetland area or some form of supplemental treatment. Fecal coliform removal is also an area where wetlands are not necessarily effective for treatment. This is of course also dependent on the amounts found in your influent. Natural sources of fecal coliforms in the wetland create a situation where a steady background concentration of fecal coliforms is likely to be present in the wetland effluent. This background level is estimated to be 00 to 00 cfu/0 ml. Many of the same mechanisms that remove TSS and other sediments also assist in removing bacteria and viruses. Many removal pathways for dissolved and particulate metals are the same as for phosphorus removal. The ultimate sink for metals seems to be with the organic bottom sediments. Fortunately, these wetlands continuously generate fresh organic material in the form of plant litter, but, in the long term or with high loadings, the metals may reach toxic levels for the wildlife inhabiting the wetland. The presence of bird and wildlife should be minimized in the first stages of a wetland treatment system, when most of the metal deposition occurs. Complex organic compounds, such as hydrocarbons, surfactants, pesticides, and phenols, from refineries, pulp and paper mills, pesticide and chemical manufacturers, food processing facilities, and landfill leachates can be treated in addition to the simple carbonaceous - Operations Training/Wastewater Treatment

7 organics that are characterized as BOD. Table. shows the percent removal for some of these trace organics and compounds. Table. Removal of Organics Using Wetland Systems Because of the earlier noted removal efficiencies, it is important to note that some form of treatment usually precedes a wetland treatment system. This can range from primary to tertiary levels. The level of pre-application treatment needed prior to the wastewater entering the wetland depends on the functional intent of the wetland component, on the level of public exposure expected, and on the need to protect habitat values. The minimal preliminary treatment for municipal wastewater is the equivalent of primary treatment, accomplished using septic tanks or primary tanks for small systems or pond units with deep zones for sludge accumulation for larger systems. Providing the equivalent of secondary treatment is considered prudent before allowing public access to the wetland components or developing specific habitats that encourage birds and other wildlife. This level of treatment can be accomplished at a first stage wetland unit where public access is restricted and habitat values are minimized. Tertiary treatment with nutrient removal may be necessary before discharging to natural wetlands where preservation of the existing habitat and ecosystem is desired. Physical Design and Construction The basic aspects of wetland design and construction are similar to those employed for shallow lagoons. These typically include earthen berms for lateral water containment and some type of seepage control for the bottom of the wetland cell. Unique features of a wetland system are vegetation and inlet and outlet structures that promote uniform flow across the wetland. Berms for wetland cells (Figure.) are typically built with to 1 (horizontal to vertical) interior side slopes, with a minimum of 0. m ( ft) of freeboard above the average water surface in an FWS wetland. The external berms for municipal wetlands should be at least m wide at the top to allow access by service vehicles. For a wetland system to meet its performance expectations, the water must flow uniformly across the entire surface area provided for treatment. Severe short-circuiting of flow can result from improper grading or nonuniform subsurface compaction of the berm. 0 1 Figure. Exterior Berm for Wetland System (USEPA, 000) Liners (Figure.) are also an important component for wetlands as in stabilization lagoons. Membrane liners have been used for FWS and SF wetland systems. Both 0 mil PVC and HDP (High Density Polyethylene) have been successful, as has been mil EPDM rubber (Ethylene Propylene) for smaller systems. If sharp edged crushed stone is used in an SF wetland, a thin layer of sand ( 0 mm) or a special fabric is placed on top of the liner to prevent punctures. Stones in subgrade soils can also puncture the liner and, in these cases, a fabric Wetland Systems -

8 under the liner is recommended. Clay liners can be constructed from locally available clay soils or commercially available products such as bentonite. Bentonite is typically mixed with in situ native soils, graded, and compacted. Typically, clay liners have to be 0. m more in depth to provide the necessary hydraulic barrier. Figure. Various Liners for Common Wetland Systems In the SF wetland, gravel provides the matrix for water flow and the media for planting emergent vegetation (Figure.). Depths in the main bed, which provides the treatment, range from 0. to 1 m (1 to ft); the typical depth is 0. m ( ft). Rock used in the treatment portion of the bed ranges from to 10 mm in diameter. Gravel that ranges in size from 1 to mm (0. to 1 in) is typically used. Rounded river gravel provides better service than sharp-edged crushed stone. Hard, durable stone (either river gravel or crushed) is preferred over crushed limestone. The material should be washed to eliminate soil and other fines that can contribute to clogging or short-circuiting. Figure. Gravel Bed for SF Wetland System In FWS wetlands, a layer of soil at least 10 mm ( in) thick is placed on the compacted bottom (or liner) to serve as the rooting media for the intended emergent vegetation. This soil can be topsoil reserved during the initial excavation of the site, or it can be imported. Any loamy soil with acceptable agronomic properties is suitable. Uniform influent distribution and effluent collection across the full width of each wetland cell in the system is absolutely essential in small to moderate-sized systems (both FWS and SF). Uniform distribution is typically accomplished using perforated manifold pipes for inlets and outlets. The size of the manifold, orifice diameter and spacing are functions of the intended flow rate. A single manifold pipe with a central inlet is not suitable for a wide wetland cell because it would be difficult to ensure uniform flow from all of the outlets. Multiple manifold pipes (in pairs) could be used for this purpose. Sequential sets of flow distribution structures could be used to uniformly divide the flow from the main influent line to the required number of manifold sets. Operational adjustment of submerged manifolds is not possible; so great care must be taken during construction to ensure that the manifold pipe remains level for the life of the system. At a minimum, sufficient compaction and careful grading in the inlet and outlet zones are required. It may be necessary to support the manifold on concrete footings in potentially unstable soils (e.g., clays). A clean-out area at each end of the submerged manifolds is recommended to allow for flushing. Also, in warm climates, the inlet manifold can be installed in an exposed position to allow access for maintenance and adjustment. Alternatives to simple drilled orifice holes allow greater control over flow distribution. Gated aluminum irrigation pipe has been used, but it is susceptible to clogging, depending on the influent water quality. Figure. shows us a basic inlet structure. - Operations Training/Wastewater Treatment

9 1 1 Figure. Wetland Inlet Structures and Piping (USEPA, 000) Submerged effluent manifolds must be connected to an outlet structure (Figure.) containing a device for controlling the water level in the wetland bed. This device can be an adjustable weir or gate, a set of stop logs, or a vertical telescoping pipe. Multiple weir or drop boxes are an alternative to using manifolds for inlet and outlet structures. Typically, these boxes are constructed of concrete, either cast-in-place or prefabricated. Several boxes must be used along the width of the cell to ensure uniform distribution. Spacing ranges from to 1 m (1 to 0 ft), center to center, depending on the width of the cell. The boxes have advantages as discharge structures because they contain weirs or gates that can be used to adjust water levels without requiring a separate structure for this purpose. However, they require an adjacent deep-water Wetland Systems -

10 zone to prevent vegetation encroachment Figure. Outlet Structures for Wetland Systems (USEPA, 000) Wetland plants can be established from seeds, root and rhizome material (tubers), seedlings (sprigs), and locally obtained clumps. Using seeds is a low-cost, high-risk endeavor. Sometimes, clumps of existing wetland species can be harvested from local drainage ditches or other acceptable sources. In these cases, most of the stem (to approximately 0. m) and leaves are stripped off, and the material is planted in clumps of a few shoots. Root and rhizome material can be obtained in the same manner. If sufficient lead time is available, an onsite nursery can be established so that seedlings or clumps are available on schedule for planting. The higher the plant density, the more rapid the development of a mature and completely functional wetland system. If planted on 1m ( ft) centers, a wetland system will typically take at least two full growing seasons to approach equilibrium and achieve expected performance objectives. Figure.1 demonstrates plant density for a typical system. - Operations Training/Wastewater Treatment

11 Figure.1 Plant Distribution for Wetland System Planting seedlings or clumps is the simplest because the green part goes up. Some experience with rhizomes is needed to identify the node that will be the future shoot. Soil moisture should be maintained after planting seeds or any of these other materials. The water level can be increased slowly as new shoots develop and grow. If the water level is higher than the tips of the green shoots; the plants will die. In SF wetlands, the wetland bed is typically flooded to the top surface of the gravel before planting, and that level is maintained until significant growth occurs. The water level is then lowered to its operational position within the bed. If the wetland is designed to receive septic tank or primary effluent, using clean water for this initial flooding purpose is suggested. Wastewater can be introduced to the system after a few weeks of plant growth, and the water level can be lowered within the bed. Because the gravel media does not provide the necessary nutrients, using an initial dose of commercial fertilizers should be considered if clean water is used during the initial planting and growth period. After the SF bed is planted, a layer of straw or hay mulch to 10 mm ( to in) thick should be placed on top of the bed. This protects new plants from the high summer surface temperatures that can occur on bare gravel surfaces. It may not be economical to plant large FWS wetlands on 1-m ( ft) centers if the total system area is over 00 hectares. In this case, the cost-effective amount of planting should be done in separate bands extending the full width of the wetland cells, with some plantings in each cell near the discharge end. These bands can serve as source material for the future spread of vegetation although it may require many years for such a wetland to reach complete plant coverage. The natural emergence of native plant species can also eventually vegetate large wetland areas if a suitable seed bank is available in local soils. Operation and Maintenance Under ideal conditions, startup of a constructed wetland system should not occur until at least weeks after planting the vegetation. This time period gives the new plants time to acclimate and grow. After growing is complete, startup procedures are simple and involve opening the inlet gates or valves and setting the desired wetland water level at the adjustable outlet or weirs. If the plants have not grown to a height that significantly exceeds the design water level, the water depth must be increased gradually as the plants grow taller. If startup is defined as the initial period before the system reaches optimum performance, then the startup period for a wetland system in cool temperate climates might require years in the best case and to years in the worst case. The system will not attain optimum performance levels until vegetation and litter are fully developed and reach equilibrium. The time required for that to occur is a function of planting density and season. During the startup period, the operator should inspect the site several times per week, observing plant growth and health, the integrity of berms and dikes, and the emergence of mosquitoes (for FWS systems). The operator should also adjust water levels as required. The developing experience during this initial period will indicate the inspection frequency required for the on-going operation once the system reaches equilibrium. During the first spring season after planting and startup, the plant coverage in all wetland cells should be carefully inspected. Any large unpopulated areas should be replanted to avoid the short-circuiting of flow. The water quality performance during the startup period will not represent long-term expectations. In some cases, the performance may be better than the long-term expectations, such as removal of phosphorus and nitrogen in FWS wetlands. The new wetland has freshly exposed soil surfaces (presumably with some clay content) and rapidly growing vegetation. Both conditions provide rapid, but short-term, removal pathways for phosphorus and ammonianitrogen. These systems may not reach equilibrium for these two parameters until the end of the Wetland Systems -

12 first or second full growing season, and then, the equilibrium effluent concentrations will likely be higher than those during startup. The opposite results can be expected for BOD and TSS. A new wetland system has minimal vegetation and minimal substrate for attached-growth organisms. Thus, removal of BOD may be marginal and TSS removal poor if algae develops in any exposed open water. The removal of these two parameters can be expected to improve as the plant canopy develops and increases in density. Operating a wetland system is simple. Much of the effort involves visually observing conditions and correcting any problems that develop. The major issues of concern are Water level maintenance; Uniformity of flow distribution and collection; Berm and dike integrity; Health and growth of system vegetation; Control of nuisance pests and insects; and Removal of undesirable vegetation. The key hydraulic requirement is maintenance of uniform flow conditions. The operator must routinely observe and adjust inlet and outlet structures as required, including flow distribution structures at the inlet end and water level controls at the effluent end. The SF wetland is designed to maintain a subsurface water level. Some temporary surface flow may be observed after a surcharge such as intense rainstorms. If persistent and large-scale surface flow is observed, the operator must lower the water level to account for this new flow. Surface flow on these systems negates their intent to eliminate the risk of public exposure, and it may result in the emergence of mosquitoes. Most municipal FWS wetland systems have at least two cells in parallel to allow better system control and temporary shut down of one side for maintenance if required. Seasonal water level adjustments for these systems may be suggested in the operation and maintenance (O&M) manual. If the O&M manual does not contain such guidance, it is suggested that the operator develop a plan, systematically try similar conditions, and observe results. Typically, a lag time of several weeks will pass before the effects of such a change are observed. The influent to the wetland should be sampled and tested periodically for the constituents of concern so that the operator can establish a record of performance for the wetland component. If problems then develop, these data will assist in determining the necessary adjustments. This data can also assist in developing a plan for optimized operation (e.g., seasonal water depths and flow distribution to different cells). Figure.1 provides hypothetical data to demonstrate the parameters to monitor during operation. Figure.1 Operational and Process Control Data for SF Wetland System A well-designed and properly operated wetland system will not require a routine harvest and removal of plant material and litter to achieve water quality goals or sustain expected hydraulic conditions. Harvesting or burning has been used on a few FWS systems to relieve the hydraulic resistance that develops in poorly designed systems with high aspect ratios and to control the habitat for mosquitoes. The plant litter in these wetlands decomposes over time but leaves a sediment residue that accumulates over time ( 1 mm/acre). When this accumulated sediment and accumulated refractory solids from the wastewater TSS begin to interfere with the design treatment volume or hydraulics in the wetland, then removal is necessary. The problem will be most acute near the head of the system because most of the influent TSS will be removed in the first 0% of the cell -1 Operations Training/Wastewater Treatment

13 length. Therefore, the access ramp to the cell should be located near the head end of the cell. It is estimated that removal might be needed every 0 to years, depending on wastewater characteristics, the types of plants used in the system, and the local climate. Maintenance requirements for constructed wetlands are simple and include the maintenance of berms and dikes (mowing and erosion control), maintenance of water tightness integrity (animal burrows and tree growth on berms), and control of nuisance pests and vectors such as mosquitoes. When the wetland is designed to operate at a shallow depth, the periodic removal of tree seedlings from the wetland bed may be required because if the trees are allowed to reach maturity, they will shade the emergent vegetation and not provide the necessary substrate for attached-growth organisms. Grass cover on berms should be routinely cut. Inlet and outlet structures and water level control devices must be periodically cleaned and adjusted. This includes removing debris and cleaning weir surfaces to remove bacterial growth and other clogging substances. Submerged inlet and outlet manifolds should be periodically flushed and cleaned with a high-pressure hose. Table. outlines routine maintenance requirements for various systems. Table. Maintenance Schedule for FWS and SF Wetland Systems Controlling mosquitoes is a critical issue. In warm climates, wetlands have been seeded with appropriate mosquito predators, such as fish and insects, or onsite houses for bats and mosquito-eating birds have been provided. Other control measures such as mosquito pupaecide (Altosid) and bacterial insecticides (Bacillus thuringiensis isrealiensis and Bacillus sphaericus [BTI and BS]) have also been successfully used. Fish cannot reach all parts of the wetland when accumulated litter becomes too dense. Thus, the harvesting and burning of vegetation have been used in some systems. In one system, plants were harvested from a narrow area parallel to each berm and from transverse bands perpendicular to the flow in the wetland bed to provide access for fish and for spraying when required. Although these efforts helped control mosquitoes, the open water parallel to the lateral berms provided an open channel for significant short-circuiting of flow. Safety The safety concerns for wetland systems are very similar to those of a stabilization lagoon. In the case of SF systems it is important to keep people away from the system due the hidden nature of the wastewater. Warning signs should state that the water is nonpotable and that there is no trespassing, swimming, or fishing allowed. Operators must also be aware that these natural systems are home to a variety of wildlife, not all are friendly. As noted earlier, be sure to control the mosquito population due to the ease with which they can spread illness to you and the surrounding population. If your system uses mechanical means for pretreatment, obey all the safety procedures for dealing with electrical and mechanical equipment. Make sure that the equipment is properly LOCKED OUT and TAGGED at the control panel before work is performed on the equipment. Another electrical safety concern is exposed wires, either near or on the berms that feed mechanical equipment. When mowing, use care to avoid cutting wires, and repair any damaged wire immediately. Although the depth of these systems are not great, you should still wear proper safety equipment such as lifejackets and waders when entering a FWS system. References U.S. Environmental Protection Agency () Process Design Manual: Land Treatment of Wetland Systems -1

14 Municipal Wastewater EPA-/1-1-01, Cent. Environ. Res. Inf., Cincinnati, Ohio Chapter Quiz Need Questions Written Here Chapter Quiz Answers Need Answers Written Here -1 Operations Training/Wastewater Treatment