CONSTRUCTED WETLAND SYSTEMS

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1 Fact Sheet T4 Performance & Cost for Decentralized Unit Processes TREATMENT SERIES DECENTRALIZED WASTEWATER SYSTEMS What are Constructed Wetlands? Constructed wetlands are passive wastewater treatment components used to produce secondary (and in some cases, tertiary) effluent. At a minimum, incoming effluent must have undergone primary treatment (liquid-solid removal). There are two different types of constructed wetlands. Free-water surface (FWS) wetlands use vegetation grown on bottom sediments and flooded to a specific depth. Subsurface flow (SF) vegetated bed wetlands also use vegetation, but effluent flows beneath the surface of the vegetated bed instead of on top of it. Each configuration has its advantages. Free-water Surface (FWS) Wetlands FWS system typically consists of channels or basins, with a natural or synthetic liner to prevent seepage. In a FWS constructed wetland, the emergent vegetation is flooded to a depth that ranges from 6 to 24 in. (100 to 450 mm). Plants in FWS constructed wetlands serve a number of purposes. Stems, submerged leaves, and litter provide a place for beneficial bacteria to grow. Leaves above the water surface shade the water and reduce the potential for algal growth. A complete FWS wetland system includes primary treatment via septic tanks or Imhoff tanks, screening with a rotary disk filter, or stabilization lagoons. Organic loading is generally between 50 and 100 pounds per acre per day but must be corrected for elevation since the rate of treatment slows with increasing elevation. The plants typically used in FWS wetlands include bulrush, cattail, common arrowhead, common reed, rushes, sedges, yellow flag, arrow arum and pickerel weed. Plants are chosen from locally grown hardy variations of these plant families.

2 2 Constituent removal in FWS wetlands occurs as a result of a variety of processes. Biochemical oxygen demand or BOD (a measure of the organic matter) is removed by microbial activity and the emergent plants help to trap and settle particulate matter suspended in the wastewater. Nitrogen is removed by providing optimum conditions for microbes that convert ammonia nitrogen to nitrate nitrogen (nitrification) and then convert nitrate nitrogen to nitrogen gas (denitrification). A FWS wetland may be specifically designed for nitrogen control if appropriately sized. Such systems require an operator with specific training and expertise. Phosphorous will be removed through adsorption, chemical precipitation, and plant uptake. Plant uptake of phosphorus is rapid, but the phosphorus is released back into the water as soon as the plant dies. Phosphorus can also be released during other times of the year, usually in response to changed conditions within the system. Pathogenic bacteria and viruses are removed in FWS constructed wetlands by adsorption, sedimentation, predation, and die-off from exposure to sunlight (UV) and unfavorable temperatures. Subsurface-flow (SF) wetlands SF vegetated bed systems consists of gravel or other coarse media and emergent vegetation. Compared to FWS wetlands, SF systems require less land area and (when operating properly) have fewer odor and mosquito problems. Disadvantages of the SF systems are increased cost due to gravel media and the potential for clogging of the media and occasional odor problems. A complete SF wetland system includes primary treatment via a septic tank or other primary treatment component. The wetlands are used to provide secondary treatment, but a more conservative application is for polishing effluent after secondary treatment by a facultative lagoon, a FWS wetland or some other treatment processes described in this series. An SF system is normally a lined earthen pond about 2 feet deep filled with rock media. The rock-filled cells typically have vegetation in a top layer of finer rock (pea gravel). Unless very lightly loaded, these systems tend to be anoxic or anaerobic (have low amounts of oxygen) through most of their depth. Using multiple parallel cells allow the operator to vary loading on the individual cells to create appropriate treatment environments. The plants add little oxygen, but do provide microclimates which assist in treatment. Common plants used in SF wetlands are reeds and cattails. Their purpose is

3 3 to provide oxygen and add to the surface area for biological growth in the root zone. The above ground portion of the plant provides little benefit except for nutrient uptake and plant growth. BOD removal in these systems occurs primarily under anaerobic conditions, but filtration of suspended matter also plays a part. The rate of removal is related to detention time and temperature. The limited free water surface limits oxygen transfer, so it has been suggested that these should be designed using lower BOD loading rates than that used for facultative ponds to encourage aerobic decomposition. Loading rates from 50 to 70 pounds of BOD per day have been used. Nitrogen removal is accomplished by nitrification/denitrification processes. Nitrification in SF constructed wetlands is limited, because the subsurface flow regime is nearly anaerobic, except for the top few inches and possibly aerobic microsites near the plant roots. Nitrification requires a supply of oxygen, either from the plant roots, surface aeration, effluent recirculation, or batch loading to induce oxygen flow down into the media between applications, or some type of supplemental diffused aeration. Phosphorous removal and pathogen reduction occur as a result of the same processes as in FWS wetlands. How can Constructed Wetlands be used? Constructed wetlands are not generally recommended for systems that treat large wastewater volumes because of the large land area required. Cluster-development or small community scale systems are most appropriate. FWS wetlands are used for achieving secondary treatment, polishing of secondary effluent, and providing wildlife habitat. Using parallel cells allows the operator to vary the flows and balance the loading on the individual cells to create appropriate treatment environments. The plants add little oxygen, but do provide microclimates which may assist in treatment. The plants also provide an aesthetically pleasing treatment unit. SF wetlands are used to reduce suspended matter after septic tank treatment at individual homes and clustered developments. The resulting effluent is then dispersed into the soil using appropriate methods. Parallel cells offer the flexibility to vary flow and loading.

4 4 Compatibility with Community Vision Constructed wetlands have an attractive natural appearance and may provide habitat for wildlife. They are an attractive landscape feature if they do not experience anaerobic conditions. FWS and SF wetlands are passive treatment systems with a large footprint. This has a number of implications both positive and negative. If there is space for the wetlands system it can be made into a very attractive green space. The green grass around the wetlands is attractive as is the vegetation growing in the wetlands. It is important that no deep-rooted vegetation be allowed on the banks or in the pond itself, but it can still result in a natural appearance. However, institutional and physical control of public access is required via fencing and signage in most settings. FWS wetlands have the potential to produce odors and attract vectors. Systems that receive a heavy BOD load may exhibit odor episodes associated with periodic loading or low pressure weather fronts. These usually last from a few hours to a day. The lighter the organic loading the less likely the system is to produce odor. These systems are expandable if space is available. Selection of any wastewater dispersal option must be considered within the context of a community s broad, long-range plans for land use. Changes in development patterns, population density, livability, and delivery of services will occur as a result of the choices made and these must all be taken into account. Constructed wetland systems can be made into attractive green space. Land Area Requirements The land requirement for FWS wetland systems is high. The total site area will include the surface area of the FWS wetlands, the dike area, the buffer zone (if required) around the wetlands, and the area of the roads associated with the site. As the size increases the buffer zone and the infrastructure area increases. A medium sized FWS system may have a 25 foot buffer zone and no road, a larger system will have a foot buffer strip around the site with a 2-lane access roads with lanes 8 or 10 feet wide. A SF system takes less space than a comparable FWS system. The total site area is primarily the surface area of the wetlands since there are no dikes or buffer zones due to less risk of human exposure.

5 5 Construction and Installation of Constructed Wetlands Systems Major issues in installation of both FWS and SF wetlands include providing sufficient flow from the inlet of the plant to the treatment cells to allow flow balancing between the cells. Site preparation for the wetland itself includes grubbing (root removal) and leveling the site. The basin is excavated and dikes are created with a 3:1 to 4:1 run to rise ratio. The excavation may be lined with clay or a 40 mil high-density polyethylene (HDPE) liner. Rock riprap may be installed to protect the liner. Weirs are installed to split the flow if necessary. The media used for SF wetlands is doublewashed hard rock in the size range of ¾ inch to 1 ¾ inch. A 4-6 inch deep pea rock cap is sometimes placed on top of the media for planting vegetation. Wastewater distribution in SF systems is provided by installation of either a perforated pipe in course rock or a chamber installed at the head of the wetland cell. If a pipe system is used, it is critical that the pipe be laid level to insure uniform distribution, and that risers be provided at both ends of the pipe for cleaning access. Wastewater collection components are installed at the end of the both types of wetlands. Collection is similar to distribution; with the addition of a water level control device. The effluent from the wetlands is either Installation of HDPE liner (Above) and establishment of disinfected for surface discharge, stored for plants (Below) irrigation purposes or dispersed into the soil. It is common for larger systems to require one standard upstream monitoring well and two standard downstream monitoring wells to insure liner integrity is maintained. Personnel who install constructed wetlands systems must have appropriate construction expertise in this type of technology. Certification of construction contractors may be required in certain jurisdictions.

6 6 Operation and Maintenance of Constructed Wetlands Systems Flows must be balanced, and levels in the wetlands adjusted occasionally. In some climates Regular service is important for all systems to the vegetation must be regularly harvested. ensure best long term performance to protect Typical failures in FWS wetlands are caused by public health and the environment. This also protects the investment. Frequency of operation and maintenance is dependent upon excess organic loading which turns the wetland anaerobic causing odors and potentially killing the wastewater volume, relative risk to public emergent vegetation. Excess solids will create health and the environment as well as the complexity of any pretreatment components used problems for emergent vegetation if allowed to prior to dispersal. settle in the FSW wetlands. Service providers who perform O&M for constructed wetlands must have appropriate training and expertise. Licensing and certification may be required depending upon the jurisdiction. Costs for Constructed Wetlands Systems There is a wide variation on cost for these systems owing to a lack of uniformity in design. The total capital cost of a FWS constructed wetland will include earthwork, pipe installation, liner, seeding and tank installation for overflow. SF systems will have the same families of costs plus those for washed rock media. These may both be lined systems and the largest variables are the cost of the liner. If a native clay liner is used, the cost may be very reasonable. If a synthetic HDPE liner is used instead, the cost may be much higher. A licensed installation contractor will likely be required. If properly designed with adequate land area available, this is a natural passive system that requires no power to run. Power would be required for blowers or recirculation pumps if supplemental aeration is added. Permitting and operation and maintenance costs must also be considered.

7 7 References 1. Onsite Sewage Treatment Program, University of Minnesota Manual for Septic System Professionals in Minnesota. St. Paul, MN. 2. U. S. EPA Subsurface Flow Constructed Wetlands for Wastewater Treatment. EPA-R , Office of Water, Washington, D.C. 3. U. S. EPA Constructed Treatment Wetlands. 843-F , Office of Water, Washington, D.C. 4. U. S. EPA Constructed Wetlands for Wastewater Treatment and Wildlife Habitat. 832-R , Office of Water, Washington, D.C. 5. Wallace, S.D Constructed Wetlands: Design Approaches - PowerPoint Presentation. in (M.A. Gross and N.E. Deal, eds.) University Curriculum Development for Decentralized Wastewater Management. National Decentralized Water Resources Capacity Development Project. University of Arkansas, Fayetteville, AR. This Fact Sheet was prepared by members of the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT), including: John R. Buchanan, PhD, PE University of Tennessee Nancy E. Deal, MS, REHS NC State University David L. Lindbo, PhD, CPSS NC State University Adrian T. Hanson, PhD, PE New Mexico State University David Gustafson, PE University of Minnesota Randall J. Miles, PhD University of Missouri These materials were reviewed by the WERF Project Subcommittee including: Tom Groves NE Interstate Water Pollution Control Commission Mike Hines Southeast Environmental Engineering Jim Kreissl Environmental Consultant Jack Miniclier Charles City County Barbara Rich Consultant Eberhard Roeder Florida Department of Health Larry Stephens Stephens Consulting Services Jeff Moeller WERF Senior Program Director 4/10 Water Environment Research Foundation 635 Slaters Lane, Suite G-110 Alexandria, VA