FEATURE
The Emergence of Treatment Wetlands As constructed wetlands for wastewater treatment grow in popularity, regulators and researchers strive to better understand this treatment option. S T E P H E N COLE udging by the growing number of wetlands built for wastewater treatment around the world, this "natural" technology seems to have firmly established roots. After almost 30 years of use in wastewater treatment, constructed "treatment wedands" now number over 500 in Europe and 600 in North America. Marsh-type "surfaceflow"systems are most common in North America, but "subsurface flow" wetlands, where wastewater flows beneath the surface of a gravel-rock bed, predominate in Europe. The inexpensive, lowmaintenance technology is reportedly in high demand in Central America, Eastern Europe, and Asia. New applications, from nitrate-contaminated groundwater to effluent from high-intensity livestock operations, are also increasing. But in the United States, treatment-wetland technology has not yet gained national regulatory acceptance. Projects are approved on a case-by-case basis. Some states and EPA regions are eager to endorse them, but others are wary of this nontraditional method of treating wastewater. In part, this reluctance exists because the technology is not yet completely understood. Knowledge of how the wetland works is not far enough advanced to provide engineers with detailed predictive models. And, being natural systems, their performance is variable, subject to the vagaries of changing seasons and vegetative cycles. These treatment wetlands also pose a potential threat to wildlife attracted to this new habitat—an ecosystem exposed to toxic compounds. New efforts are under way, however, to place the technology onto firmer scientific and regulatory ground. Long-term demonstration and monitoring field studies are currently probing the inner work-
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ings of wetlands and their water quality capabilities to provide better data on how to design more effective systems. Researchers are documenting the fate of toxic compounds in wetlands and the extent to which wildlife may be exposed to them. A recent study of U.S. policy and regulatory issues surrounding treatment wetlands has recommended that the federal government actively promote the technology and clear the regulatory roadblocks to enable wider use. Proponents argue that the net environmental benefits of constructed wetlands, such as restoring habitat and increasing wetland inventory, should be considered. A federal interagency work group is grappling with that recommendation, trying to balance the benefits and shortcomings of the increasingly popular technology. "The field has really exploded in the last five years," said Robert Knight of CH2M Hill in Gainesville, Fla., who has conducted research and designed treatment wetlands nationwide since the 1970s. International interest is growing, and systems are now in operation in southeast Asia, India, China, and South America. Since 1988 the International Association on Water Quality has sponsored a biannual conference dedicated to "the use of macrophytes in water pollution control." The sixth conference in the series will be held this September in Brazil (i). "Ten years ago this was considered an 'alternative innovative technology,'" said Robert Gearheart of California's Humboldt State University and a developer of the Areata (Calif.) Marsh and Wildlife Sanctuary, which has been treating millions of gallons of municipal wastewater a day since 1986. "It's past that horizon now. The technology has gained enough theoretical background to be regularly considered as a 0013-936X/98/0932-218A$15.00/0 © 1998 American Chemical Society
wastewater treatment option." Gearheart recently helped design a wetlands system at the Apache Powder Superfund site in southern Arizona. This summer the wetlands will begin treating nitrate-contaminated groundwater. The biological nitrification processes of the wetland should, over a seven-day retention time, reduce nitrate-nitrogen levels in the groundwater from 150 mg/L to less than 10 mg/L. The use of treatment wedands is well established in Europe, where the technology originated with laboratory work in Germany 30 years ago. Subsurface-flow systems are the norm because they provide more intensive treatment in a smaller space than marsh-type wetlands, an important design constraint in countries where open space is limited. "The European thrust has been for smallscale systems primarily for domestic wastewater treatment," says plant ecologist Hans Brix of the University of Aarhus in Risskov, Denmark, who has been working on wedand systems since the early 1980s. Denmark alone has 150 systems, most in small villages handling domestic wastewater. The use of "reed beds," the common European term for treatment wedands, is widespread in northern Europe, says Brix. With funding from the Danish Ministry of the Environment, Brix has been working with Polish engineers over the past five years to transfer the technology. Poland now has nearly 100 systems, he says. Constructed wedand systems in North America have been designed predominantly for large-scale treatment of municipal wastewater, ranging from 100,000 to 15 million gallons per day. A 1993 survey of more than 300 wetland cells in North America, treating primarily municipal wastewater (2), documented average performance levels for the mix of system types. Biochemical oxygen demand (BOD) was reduced 73% to 8 mg/L, total suspended solids were cut 72% (13 mg/L), total nitrogen was reduced 53% (4.5 mg/L), and total phosphorus was cut 56% (1.7 mg/L). Beyond municipal wastewater Since the 1980s, constructed wetiands have also been built to treat other types of waste waters, including acid mine drainage, industrial wastewater, agricultural and storm water runoff, and effluent from livestock operations. The petroleum industry is using constructed wetlands to treat a variety of waste waters from refineries and fuel storage tanks. "Every major company is implementing wedand technology in one way or another," said Robert Kadlec, professor emeritus at the University of Michigan and a leading consultant in the field. Amoco has been treating pretreated process wastewater at its oil refinery in Mandan, N.D., since 1975, and Chevron has op-
An extensive research program is under way at the Tres Rios Demonstration Wetlands project in Phoenix, Ariz. The municipal wastewater treatment project includes 12 wetland cells constructed in former sludge beds (above) to test different design parameters and a 4.5acre wetland built in the floodway of the Salt River (left). (Photos courtesy CH2M Hill, Gainesville, Fla.)
erated treatment wetlands at its refinery in Richmond, Calif., since 1988. The food-processing industry is a relative newcomer to treatment wetiands, reports Kadlec. Fullscale systems are in place at the American Crystal Sugar plant in Hillsboro, N.D., and at a potatoprocessing plant in Connell, Wash. The pulp and paper industry has conducted extensive pilot-scale projects but has not yet built many full-scale projects, said Knight. Since 1990, more than 65 pilot- or full-scale wetlands have been built in North America to treat livestock wastewater, according to a recent survey (3). These typically small systems (average size 1.5 acres) treat high-strength wastewater from swine, cattie, and dairy operations, predominandy in Kentucky (21 systems) and Ontario (9 systems). The Department of Agriculture's Natural Resources Conservation Service supports many of the U.S. projects. Stormwater runoff has recently become a focus of constructed wetlands work at the University of Florida's Center for Wetlands in Gainesville, according to Associate Director Mark Brown. The center was involved in some of the early work on this technology and participated in the design of the 1200-acre Orlando Wetlands Park in 1987 to polish highly treated municipal effluent. "As EPA has increased pressure MAY 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 1 9 A
Wastewater from the Chevron oil refinery in Richmond, Calif., on the edge of San Francisco Bay, has been treated by constructed wetlands (foreground) since 1988. Recent research by University of California-Berkeley scientists has quantified the extent to which the wetlands accumulate and volatilize selenium from the wastewater. (Photo courtesy Peter J. Duda, Chevron Products Co.)
on communities to deal with nonpoint source pollution, communities are trying to figure out how to retrofit their systems," said Brown. "We've constructed several wetlands in Florida, and we're also looking at the stormwater problem on a watershedwide level. We've been studying watershed dynamics to see how a network of wetland systems could be designed into the watershed." The Tennessee Valley Authority (TVA) has built 19 treatment-wetland systems since its program began in 1985 to treat acid drainage at coal mining and processing facilities as well as at coal-fired power plants in Alabama and Tennessee, according to Greg Brodie, program manager for environmental control technologies in TVA's technology advancement group. TVA has also operated a $10-million constructed wetland research facility in Muscle Shoals, Ala., since 1992. While many of the early TVA projects were marshlike surface flow systems, the most recent projects are "passive treatment systems" that link several different types of cells—vertical limestone drains as well as vegetated cells—to sequentially treat particularly "nasty" wastewater with low pH and high metal content (4). The Electric Power Research Institute (EPRI, Palo Alto, Calif.) has spent $2.5 million since 1994 on constructed wetland pilot projects, monitoring programs, and basic research. The treatment of runoff from the piles of coal at coal-fired power plants is one target of this effort. According to John GoodrichMahoney, research manager of EPRI's water quality program, development and verification of wetland treatment systems will allow utilities to meet possible future regulatory requirements. A pilot project now under construction at a coalfired plant in Birmingham, Ala., will treat wastewater with a pH of 2 and high levels of aluminum. The experimental system uses a series of "successive alkalinity-producing systems," a rich organic layer over 2 2 0 A • MAY 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
an anoxic limestone drain, to reduce the acidity in the wastewater before it flows into the wetiand cells. Results from this and other EPRI studies will be used to create a wedands system design manual and software to estimate the cost-effectiveness of different treatment applications for the electric industry, according to Goodrich-Mahoney. Continued growth in the use of treatment wetlands is expected as the result of new regulatory initiatives on nutrient management, including the Clean Water Act's "total maximum daily load" (TMDL) program. Gearheart notes that small- to medium-sized communities trying to meet new TMDLs in sensitive watersheds for phosphorus or ammonia "need something that's cost-effective," and wedands are a good option. In Denmark, new regulations on nutrient removal will encourage more wetlands, down to the individual household level, Brix believes. Looking inside the "black box" The rapid spread and diversification of treatmentwetland technology are, however, running ahead of the mechanistic understanding of how they work, which is needed to develop detailed design criteria. These complex natural systems are still somewhat of a "black box," according to many in the field. "We've got a huge, functioning mess called wetlands out there with all sorts of interesting things going on inside it," said Kadlec, who recently coauthored with Robert Knight a massive treatmentwetlands reference work (5). "But we don't have enough information about what goes on inside the system. We have a solid foundation of empirical understanding, but to advance our knowledge, we need to understand die internal processes that lead to the observed performance." "Basically, all we know now is that they work," said Gearheart. "We know that if you load it at certain rates, 90% of the time you'll be fine. But if you want
to be able to say, for example, what happens if I double that loading rate, we're not there yet. We can't model it." EPA is currently revising a design manual for small-community municipal treatment wetlands that it issued in 1988 (6). That manual was "premature," said Jim Kreissl, an environmental engineer in EPA's National Risk Management Research Laboratory in Cincinnati, Ohio, who worked on the manual and is currendy drafting the updated version. To shed some light inside the black box, Robert Bastian of EPA's Office of Wastewater Management initiated a "technology assessment" in 1995 of surface water wedands for municipal wastewater treatment to review the state of the art and to identify what areas needed more research. The assessment, which has involved many of the leaders in the field over the past two years, has sparked debate over some fundamental questions about how wedands work, according to participants. It has identified several research needs, including more monitoring data from full-scale systems receiving medium to high organic loads, the role of dissolved oxygen concentrations in removal of BOD and total nitrogen, and the effects of different plant communities on performance. The role of plants in transporting oxygen into the root zone to promote nitrification has been demonstrated in the laboratory but not convincingly in die field, according to Brix. "That is highly touted," said Gearheart, "but there is very litde data to say one way or the other whether that is an important factor or whether the plants are more or less passive." It is more likely, according to some researchers, that the ratio of open water to vegetated areas is more important in creating aerobic conditions in a wetland. "You can do some serious nitrogen removal with alternating open and vegetated areas," said EPA's Kreissl. Another volatile issue, according to Knight, is how important the volume of water in a wetland is to treatment performance. "Is it the bottom of the wetlands or the volume of water that's most important? The data coming in now are on the side of the wedand bottom. It apparently doesn't matter how deep the water is as long as the soil is wet. That is a real surprise to civil engineers who for years have designed treatment systems based on their volume and hydraulic residence time." The EPA technology assessment is scheduled to be completed this summer, Bastian said. The revised EPA design manual, which will cover small subsurface and surface-flow systems for secondary municipal wastewater treatment, will be published by the end of the year, said Kreissl. Flood of new research results Numerous research efforts, both broad-based and focused, are currendy generating a great deal of new information on treatment-wetland function. One of the largest and most recent research efforts began in 1995 in Phoenix, Ariz., at the Tres Rios Demonstration Wetlands, adjacent to the city's 91st Avenue wastewater treatment plant. Originally planned as part of a U.S. Bureau of Reclamation study for a proposed 800-acre wetland and to help meet stringent
Subsurface-flow wetlands (above), where water flows through a rock-and-gravel bed, are used at the Constructed Ecosystems Research Facility in Tucson to study the role of plants in wetland treatment. Freefloating aquatic plants, such as duckweed, are also studied at the facility in an aquatic system (left). (Photos by Stephen Cole)
permit requirements at the treatment plant, Tres Rios's 16 separate wetland basins are being used to evaluate many different design and performance parameters. Data are collected at different points within the systems and at different depths. The extensive monitoring program includes gathering conventional water quality data; measurements of metals, biotoxicity, and organics; bird surveys; and macroinvertebrate sampling. "Expanding the species pallet" of plants used in treatment wedands is one focus of research by University of Arizona scientists at Pima County's Constructed Ecosystems Research Facility in Tucson. "Most constructed wedands for treatment have been built around herbaceous species so far," said project director Martin Karpiscak of the university's Office of Arid Lands Studies. "We're experimenting witii a greater variety of plants to see how water quality changes when you start using multispecies systems." In operation since 1989, the facility contains six 200 ft. x 30 ft. rectangular cells, most of which are filled with rock and coarse gravel for subsurface flow. The plants and trees grow hydroponically in municipal effluent or potable water, allowing researchers to concentrate on the impact of plants, rather than soil, on water quality. Karpiscak has found that pathogen removal is higher in a multispecies system than in a single species system (7). "One of the things that may be important in pathogen removal is having multiple types of wedand components, say a duckweed system followed by a subsurface wetland." Looking deeper into the wetland, to the microbes in the soil and around the root systems of wetland plants, plant biologist Norman Terry and colleagues at the University of California at Berkeley are M A Y 1 , 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 2 1 A
wetlands database to include available data on wildlife use; toxic substances in water, biota, and sediments; and vegetation. Bastian hopes to make the entire database accessible over die Internet before the end of this year. Research is also being directed toward several critical issues. Norman Terry's research group is working to find out exactly where toxic trace elements from wastewater end up in a treatment wetland. The group is completing laboratory studies documenting trace element uptake potential of various wedand plants and identifying where the elements go in A multistage "passive treatment system," which includes wetland cells and an anthe plants. The group is also oxic limestone drain, treats coal ash leachate at the Kingston electric plant in Roane monitoring several active treatCounty, Tenn. High-metal-content leachate flows from the limestone drain into a deep ment wetlands to track trace oxidation pond (foreground) and then into an aerobic marsh. The plant's intake channel is in the background. (Photo by Gregory A. Brodie, TVA, Chattanooga, Tenn.) elements in the ecosystem. The four-year project, begun in 1996, is measuring 26 trace elements in sediment, water, studying the role that bacteria play in trace eleroots, stems, leaves, plant litter, and air. ment removal. Researchers in Terry's laboratory have found that bacteria in the root zone of bulrush inTerry has also studied the extent to which wetcrease the plants' ability to accumulate and volatil- land plants volatilize trace elements, removing them ize selenium twofold. They are now working to iden- from the wedand and effluent completely and retify which bacteria are most responsible, and will soon leasing them harmlessly into the air. A new study of move to mesocosm studies to see whether seeding selenium removal in the constructed wetlands at the soil with those bacteria increases trace element Chevron's Richmond oil refinery on San Francisco Bay removal. found that 10-30% of the selenium was volatilized Some researchers are experimenting with an in- (8). To address similar habitat-related issues, Phoenovative wetland design—a vertical flow system— that they hope will solve the oxygen depletion prob- nix's Tres Rios demonstration project recently exlem and boost nitrification. Effluent flows over a panded its research program, according to research porous surface and percolates through a vegetated field manager Roland Wass. Influent and effluent wasand filter, which is periodically allowed to dry to re- ter is now being analyzed for potential bioaccumuintroduce oxygen to the system. Hans Brix and other lation and mutagenic activity from organic comresearchers in Germany, Austria, and the United King- pounds. Toxicity tests will look for physiological dom are working with different variations on these impacts on biota living in the system. Work also systems to establish design parameters, such as load- continues on the control of an unplanned threat to ing rates and length of rest periods, for maximum human health—mosquitoes. Fish have been introduced to the wetlands to consume the mosquito nitrification. larvae, but the density of the particular bulrush variety used at Tres Rios prevents the fish from Potential "attractive nuisances" Aside from the research issues surrounding the de- reaching certain parts of the wetland. Sections of sign and performance of the treatment-wetlands die wedands are being reconfigured and replanted "black box," another scientific issue looms large for to raise the water level and give the fish greater the future of the technology: Do treatment wet- access. lands pose a threat to wildlife? Many wetland projects are designed with habi- Regulatory uncertainty and barriers tat creation as one of their primary objectives. "All Treatment wetlands do not appeal to all wasteof our projects are habitat-driven," said Jim Sarto- water engineers, however. They lack the traditional ris of the U.S. Bureau of Reclamation Biological Re- "handles" of engineered pollution control systems; sources Division in Denver, who is involved in sev- they are not easy to control and may be hard to preeral municipal wastewater projects in southwestern dict. Regulators in the United States have similar states. EPA's Bastian also promotes the approach. "You problems with treatment wedands. They do not fit can justify the land use for a constructed wedand more easily into existing regulatory categories. Surfaceeasily if you also use it for habitat restoration." flow treatment wedands can be a point source disAs part of its technology assessment effort, EPA charge and a protected environment at the same has expanded the 1994 North American treatment- time. 2 2 2 A • MAY 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
There is no national guidance on the use of treatment wetlands and no uniform acceptance of them by states, according to researchers and consultants. In this atmosphere of regulatory uncertainty, questions abound. EPRI's John Goodrich-Mahoney expressed concerns that under a strict reading of the Clean Water Act, certain treatment wetlands could be considered "waters of the United States," and thus discharges into them could be tightly regulated. Similar concerns among the planners of the Phoenix Tres Rios wetlands project have recently raised the issue of regulatory uncertainty to the federal level. An interagency work group on constructed wetlands for wastewater treatment has been meeting since last summer to work out a federal position on the technology. An initial position statement is expected this year. The Tres Rios planners, realizing that they had several regulatory questions regarding their proposed wetland, approached EPA for assistance. With the help of EPA's Robert Bastian, the Phoenix office of the U.S. Bureau of Reclamation received an Environmental Technology Initiative (ETI) grant in 1995 to study the policy and permitting barriers to siting surfaceflow wetlands for municipal wastewater treatment, especially ones that would restore or create valuable wildlife habitat. The ETI Treatment Wetland Policy and Permitting Team of representatives from federal, state, and local agencies issued a report in January 1997 that recommended "changes in regulations and/or policy that would facilitate, where appropriate, implementation of beneficial treatment wetland projects." It also advocated that the "net environmental benefits" of habitat creation, reduced use of energy and treatment chemicals, and recreational value—not just the water quality impact of a treatment wetland project—should be considered in approving them. The report cataloged numerous regulatory and policy issues. Should disinfection of effluent be done at the inlet rather than the outlet of a wetland? When should they be lined to protect groundwater? Should treatment wetlands be allowed to mitigate for permitted wedand losses? Under what conditions should constructed treatment wetlands be considered "waters of the United States"? The report also noted that more research is needed concerning the "fate and effect of potential wastewater toxins and ecological risks in treatment wetlands." The federal interagency work group, including representatives from EPA wetlands and wastewater offices, the U.S. Army Corps of Engineers, the National Oceanic and Atmospheric Administration, the Bureau of Reclamation, and the U.S. Fish and Wildlife Service, was created to take up these issues. "This group is trying to take what we learned on the ETI Tres Rios wetiands project and see what kind of guidance we might issue on a national basis," said EPA's Bastian. The work group's challenge is to clarify existing regulations, not propose new ones, said Phil Oshida, chief of the Wetlands Strategies and State programs Branch of EPA's Wetiands Division and a work group participant. "We're trying to find areas of com-
mon agreement on siting, monitoring, and permit compliance from a national perspective." The question of where treatment wetlands should be sited has been a particularly difficult issue, said Oshida, but the group has reached consensus on the need to handle wetland systems differently depending on whether their primary purpose is water treatment or habitat restoration. The work group is currentiy drafting a discussion document to lay out the federal view of treatment wetlands and initiate a broader discussion with other interested parties. Oshida does not foresee federal agencies proposing any policy or regulatory changes before those discussions are held and a consensus document is There is currently completed by early 1999. There is still some disagree- no national m e n t within the work group about the habitat value of treat- guidance on the m e n t wetlands, according to Bastian, and there are concerns use of treatment about the negative impact they could h a v e o n t h e e n v i r o n - wetlands and no ment. But others agree that they uniform provide a potentially valuable resource. "We're looking for opporacceptance of tunities to restore wetlands, their function, and habitat," said John them by states. Meagher, director of EPA's wetlands division, "and wastewater effluent may be an untapped source of water to do that in dry areas." EPA is not currently developing the type of specific guidance documents and formal agency actions that were recommended in the ETI study to promote the use of treatment wetlands. Nevertheless, wetlands experts are encouraged that the issues are now being discussed at the national level. "Although it may still be some years before EPA actually does anything, the agency is on the right path now," said Kadlec.
References (1) 6th International Conference on Wetland Systems for Water Pollution Control. Aguas de Sao Pedro, Sao Paulo, Brazil, Sept. 27-Oct. 2, 1998. http://tender.igce.unesp.br/ib/ cea/wet.html (accessed March 1998). (2) North American Wetlands for Water Quality Treatment Database; U.S. Environmental Protection Agency, Office of Research & Development, National Risk Management Research Laboratory: Cincinnati, OH, 1994. (3) CH2M Hill and Payne Engineering. Constructed Wetlands for Livestock Wastewater Management, EPA Gulf of Mexico Program, Nutrient Enrichment Committee: Stennis Space Center, MS, January 1997. (4) Brodie, G.; Stage, Aerobic Constructed Wetlands to Treat Acid Drainage. In Constructed Wetlands for Water Quality Improvement, G. A. Moshiri, Ed.; CRC Press: Boca Raton, FL, 1993. (5) Kadlec, R. H.; Knight, R. L. Treatment Wetlands; CRC Press: Boca Raton, FL, 1996. (6) Design Manual: Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment, EPA/625/188/022; U.S. Environmental Protection Agency, Office of Research and Development: Cincinnati, OH, September 1988. (7) Karpiscak, M. et al. Water Sci. Technol. 1996, 33(10/11), 231-36. (8) Hansen, D. et al. Environ. Sci. Technol, 1998, 32, 591597. Stephen Cole is managing editor o/ES&T. MAY 1, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 2 2 3 A
