Wastewater Technology Fact Sheet Wetlands: Subsurface Flow
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United States Office of Water EPA 832-F-00-023 Environmental Protection Washington, D.C. September 2000 Agency Wastewater Technology Fact Sheet Wetlands: Subsurface Flow DESCRIPTION Optional Inlet Vegetation Manifold Warm Climates Wetland systems are typically described in terms of the position of the water surface and/or the type of vegetation grown. Most natural wetlands are free water surface systems where the water surface is exposed to the atmosphere; these include bogs Outlet Zone Inlet Zone 2” to 3” 2” to 3” Gravel (primary vegetation mosses), swamps (primary Gravel Water Treatment Zone Surface Outlet 1/ ” to 11/ ” Gravel vegetation trees), and marshes (primary vegetation Inlet Manifold 2 2 Manifold Cold Climates grasses and emergent macrophytes). A subsurface Membrane Line or flow (SF) wetland is specifically designed for the Impermeable Soils treatment or polishing of some type of wastewater and are typically constructed as a bed or channel Source: Adapted from drawing by S.C. Reed, 2000. containing appropriate media. An example of a SF wetland is shown in Figure 1. Coarse rock, gravel, FIGURE 1 SUBSURFACE FLOW sand and other soils have all been used, but a gravel WETLAND medium is most common in the U.S. and Europe. The medium is typically planted with the same types systems. The biological reactions are believed due of emergent vegetation present in marshes, and the to the activity of microorganisms attached to the water surface is designed to remain below the top available submerged substrate surfaces. In the case surface of the medium. The main advantages of this of FWS wetlands these substrates are the subsurface water level are prevention of mosquitoes submerged portion of the living plants, the plant and odors, and elimination of the risk of public litter, and the benthic soil layer. In SF wetlands the contact with the partially treated wastewater. In available submerged substrate includes the plant contrast, the water surface in natural marshes and roots growing in the media, and the surfaces of the free water surface (FWS) constructed wetlands is media themselves. Since the media surface area in exposed to the atmosphere with the attendant risk of a SF wetland can far exceed the available substrate mosquitoes and public access. in a FWS wetland, the microbial reaction rates in a SF wetland can be higher than a FWS wetland for The water quality improvements in natural wetlands most contaminants. As a result, a SF wetland can had been observed by scientists and engineers for be smaller than the FWS type for the same flow rate many years and this led to the development of and most effluent water quality goals. constructed wetlands as an attempt to replicate the water quality and the habitat benefits of the natural The design goals for SF constructed wetlands are wetland in a constructed ecosystem. Physical, typically an exclusive commitment to treatment chemical, and biochemical reactions all contribute to functions because wildlife habitat and public water quality improvement in these wetland recreational opportunities are more limited than FWS wetlands. The size of these systems ranges from small on-site units designed to treat septic tank centimeters or ³ 0.25 in.) to large crushed rock effluents to a 1.5x107 liters per day (4 MGD) system (³15.2 centimeters or ³6 in.); A combination of in Louisiana treating municipal wastewater. There sizes from 1.3 centimeters to 3.8 centimeters (0.5 to are approximately 100 systems in the U.S. treating 1.5 inches) are most typically used. This gravel municipal wastewater, with the majority of these medium should be clean, hard, durable stone capable treating less than 3.8x103 m3/day (1 MGD). Most of of retaining it’s shape and the permeability of the the municipal systems are preceded by facultative or wetland bed over the long term. aerated treatment ponds. There are approximately 1,000 small scale on-site type systems in the U.S. The most commonly used emergent vegetation in treating waste waters from individual homes, SF wetlands include cattail (Typha spp.), bulrush schools, apartment complexes, commercial (Scirpus spp.), and reeds (Phragmites spp.). In establishments, parks, and other recreational Europe, Phragmites are the preferred plants for facilities. The flow from these smaller systems these systems. Phragmites have several advantages ranges from a few hundred gallons per day to since it is a fast growing hardy plant and is not a 151,400 liters per day (40,000 gallons per day), food source for animals or birds. However, in some with septic tanks being the dominant preliminary parts of the U.S. the use of Phragmites is not treatment provided. SF wetlands are not now permitted because it is an aggressive plant and there typically selected for larger flow municipal systems. are concerns that it might infest natural wetlands. In The higher cost of the rock or gravel media makes these cases cattails or bulrush can be used. In areas a large SF wetland uneconomical compared to a where muskrat or nutria are found, experience has FWS wetland in spite of the smaller SF wetland area shown that these animals, using the plants for food required. Cost comparisons have shown that at flow and nesting material, can completely destroy a stand rates above 227,100 liters per day (60,000 gallons of cattails or bulrush planted in a constructed per day) it will usually be cheaper to construct a wetland. Many of the smaller on-site systems FWS wetland system. However, there are serving individual homes use water tolerant exceptions where public access, mosquito, or decorative plants. The vegetation on a SF wetland wildlife issues justify selection of a SF wetland. One bed is not a major factor in nutrient removal by the recent example is a SF wetland designed to treat the system and does not require harvesting. In cold runoff from the Edmonton Airport in Alberta, climates, the accumulating plant litter on top of the Canada. The snow melt runoff is contaminated with gravel bed provides useful thermal insulation during glycol de-icing fluid and a SF wetland treating the winter months. The submerged plant roots do 1,264,190 liters per day (334,000 gallons per day) provide substrate for microbial processes and since was selected to minimize habitat values and bird most emergent macrophytes can transmit oxygen problems adjacent to the airport runways. from the leaves to their roots there are aerobic microsites on the rhizome and root surfaces. The SF wetlands typically include one or more shallow remainder of the submerged environment in the SF basins or channels with a barrier to prevent seepage wetland tends to be devoid of oxygen. This general to sensitive groundwaters. The type of barrier will lack of available oxygen limits the biological depend on local conditions. In some cases removal of ammonia nitrogen (NH3/NH4 - N) via compaction of the local soils will serve adequately, nitrification in these SF wetlands, but the system is in other cases clay has been imported or plastic still very effective for removal of BOD, TSS, membrane (PVC or HDPE) liners used. metals, and some priority pollutant organics since Appropriate inlet and outlet structures are employed their treatment can occur under either aerobic or to insure uniform distribution and collection of the anoxic conditions. Nitrate removal via biological applied wastewater. A perforated manifold pipe is denitrification can also be very effective since the most commonly used in the smaller systems. The necessary anoxic conditions are always present and depth of the media in these SF wetlands has ranged sufficient carbon sources are usually available. from 0.3 to 0.9 meters (1 to 3 feet) with 0.6 meters (2 feet) being most common. The size of the media The limited availability of oxygen in these SF in use in the U.S. ranges from fine gravel (³0.6 systems reduces the capability for ammonia removal via biological nitrification. As a result, a long The SF wetland does not provide the same level of detention time in a very large wetland area is habitat value as the FWS wetland because the water required to produce low levels of effluent nitrogen in the system is not exposed and accessible to birds with typical municipal wastewater influents unless and animals. However, wildlife will still be present, some system modification is adopted. These primarily in the form of nesting animals, birds, and modifications have included installation of aeration reptiles. If provision of more significant habitat tubing at the bottom of the bed for mechanical values is a project goal it can be accomplished with aeration, the use of an integrated gravel trickling deep ponds interspersed between the SF wetland filter for nitrification of the wastewater ammonia, cells. The first pond in such a system would be and vertical flow wetland beds. These vertical flow located after the point where water quality is beds usually contain gravel or coarse sand and are approaching at least the secondary level loaded intermittently at the top surface. The intermittent application and vertical drainage APPLICABILITY restores aerobic conditions in the bed permitting aerobic reactions to proceed rapidly. Cyclic filling SF wetland systems are best suited for small to and draining of a horizontal flow system has been moderate sized applications (£ 227,100 liters/day or successfully demonstrated at the 130,000 gallons £60,000 gallons per day) and at larger systems per day SF wetland system in Minoa, NY. The where the risk of public contact, mosquitoes, or reaction rates for BOD5 and ammonia removal potential odors are major concerns. Their use for during these cyclic operations were double the rates on-site systems provides a high quality effluent for observed during normal continuously saturated in-ground disposal, and in some States a significant flow.