Natural and Enhanced Remediation Systems - Chapter 6 pot

Suthersan, Suthan S. “Constructed Treatment Wetlands” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 6 Constructed Treatment Wetlands CONTENTS 6.1 Introduction 6.1.1 Beyond Municipal Wastewater 6.1.2 Looking Inside the “Black Box” 6.1.3 Potential “Attractive Nuisances” 6.1.4 Regulatory Uncertainty and Barriers 6.2 Types of Constructed Wetlands 6.2.1 Horizontal Flow Systems 6.2.2 Vertical Flow Systems 6.3 Microbial and Plant Communities of a Wetland 6.3.1 Bacteria and Fungi 6.3.2 Algae 6.3.3 Species of Vegetation for Treatment Wetland Systems 6.3.3.1 Free-Floating Macrophyte-Based Systems 6.3.3.2 Emergent Aquatic Macrophyte-Based Systems 6.3.3.3 Emergent Macrophyte-Based Systems with Horizontal Subsurface Flow 6.3.3.4 Emergent Macrophyte-Based Systems with Vertical Subsurface Flow 6.3.3.5 Submerged Macrophyte-Based Systems 6.3.3.6 Multistage Macrophyte-Based Treatment Systems 6.4 Treatment-Wetland Soils 6.4.1 Cation Exchange Capacity 6.4.2 Oxidation and Reduction Reactions 6.4.3 pH 6.4.4 Biological Influences on Hydric Soils 6.4.5 Microbial Soil Processes 6.4.6 Treatment Wetland Soils ©2001 CRC Press LLC 6.5 Contaminant Removal Mechanisms 6.5.1 Volatilization 6.5.2 Partitioning and Storage 6.5.3 Hydraulic Retention Time 6.6 Treatment Wetlands for Groundwater Remediation 6.6.1 Metals-Laden Water Treatment 6.6.1.1 A Case Study for Metals Removal 6.6.2 Removal of Toxic Organics 6.6.2.1 Biodegradation 6.6.3 Removal of Inorganics 6.6.4 Wetland Morphology, Hydrology, and Landscape Position References Creating or constructing a natural wetland sounds like an oxymoron, but this doesn’t mean that an “unnatural wetland” is by definition bad. It doesn’t mean we can’t mimic Mother Nature in giving natural birth to a desirable wetland. Constructed rice paddies have been responsible for feeding more people than any other enterprise on earth. 6.1 INTRODUCTION Natural wetlands are land areas that are wet during part or all of the year because of their location in the landscape. Historically, wetlands were called swamps, marshes, bogs, fens, or sloughs, depending on existing plant and water conditions and on geographic setting. Wetlands are frequently transitional between uplands (terrestrial systems) and continuously or deeply flooded (aquatic) systems. They are also found at topographic lows (depressions) or in areas with high slopes and low permeability soils (seepage slopes). In other cases, wetlands may be found at topo- graphic highs or between stream drainages when land is flat and poorly drained (blanket bogs). In all cases, the unifying principle is that wetlands are wet long enough to alter soil properties because of the chemical, physical, and biological changes that occur during flooding, and to exclude plant species that cannot grow in wet soils. 1 The structural components of natural wetland ecosystems are shown in Figure 6.1. These components are highly variable and depend on hydrology, underlying sediment types, water quality, and climate. Starting with the unaltered sediments or bedrock below the wetlands, these typical components are 1 • Underlying strata — unaltered organic, mineral, or lithic strata, typically saturated with or impervious to water and below the active rooting zone of the wetland vegetation • Hydric soils — the mineral-to-organic soil layer of the wetland, infrequently to continuously saturated with water and containing roots, rhizomes, tubers, funnels, burrows, and other active connections to the surface environment ©2001 CRC Press LLC • Detritus — the accumulation of live and dead organic material in a wetland, consisting of dead emergent plant material, dead algae, living and dead animals (primarily invertebrates), and microbes (fungi and bacteria) • Seasonally flooded zone — the portion of wetland seasonally flooded by standing water and providing habitat for aquatic organisms including fish and other verte- brate animals, submerged and floating plant species that depend on water for buoyancy and support, living algae, and populations of microbes • Emergent vegetation — vascular, rooted plant species containing structural com- ponents that emerge above the water surface, including both herbaceous and woody plant species Natural wetlands have been used as convenient wastewater discharge sites for as long as sewage has been collected (at least 100 years in some locations). Examples of old treatment wetland sites can be found in Massachusetts, Wisconsin, Florida, and Ontario. Judging 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 wetlands” now number over 1000 in Europe and in North America. 1 Marsh-type “surface flow” systems are most common in North America, but “subsurface flow” wetlands, where wastewater flows beneath the surface of a gravel-rock bed, pre- dominate in Europe. This inexpensive, low-maintenance technology is reportedly in high demand in Central America, Eastern Europe, and Asia. New applications, Figure 6.1 Structural components of natural wetland ecosystems (adapted from Kadlec et al., 1996). Unaltered Sediment Hydric Soils Detritus Rhizomes Emergent Vegetation Cypress Kness Subcanopy Tree Shrub Seasonal High Water Seasonally Flooded Zone Seasonal Low Water Canopy Tree Buttressed Stem ©2001 CRC Press LLC from nitrate-contaminated groundwater to effluent from high-intensity livestock operations, are also increasing. In the U.S., treatment-wetland technology has not yet gained universal 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 and contaminated groundwater. 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 pre- dictive models. Because wetlands are 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 within an ecosystem exposed to potentially toxic compounds. When utilized for benign, pretreated wastewaters, wetlands do not generally pose a threat to human or wildlife health. In these circumstances, there may be significant ancillary benefits in terms of habitat creation and beneficial human use. In those situations where a potentially hazardous condition exists, the extra expense of a gravel media is warranted. 1 Water and associated particulates, organisms, and sedi- ments are then located below ground, and thus out of reach of human and wildlife contact. Subsurface wetland waters are typically anoxic or anaerobic, which is optimal for some processes such as sulfide precipitation or denitrification, but unsat- isfactory for other processes, such as nitrification of ammonium nitrogen. New efforts are underway, however, to place the technology onto firmer scientific and regulatory ground. Long-term demonstration and monitoring field studies are currently probing the inner workings 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 this 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 this increasingly popular technology. 6.1.1Beyond Municipal Wastewater Constructed wetland systems in North America have been designed predomi- nantly for large-scale treatment of municipal wastewater, ranging from 100,000 to 15 million gallons per day. 1,2 The use of treatment wetlands is well established in Europe, where the technology originated with laboratory work in Germany 30 years ago. 3 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 small-scale systems primarily for domestic wastewater treatment; for example, Denmark alone has 150 systems, most in small villages handling domestic waste- water. The term “reed beds” is commonly used for treatment wetlands in Europe. ©2001 CRC Press LLC Since the 1980s, constructed wetlands have also been built to treat other types of wastewaters, including acid mine drainage, industrial wastewater, agricultural and storm water runoff, and effluent from livestock operations. 1,2 The petroleum industry is using constructed wetlands to treat a variety of wastewaters from refineries and fuel storage tanks. Food processing and pulp and paper industries are relative new- comers to treatment wetlands. Stormwater runoff also has recently become a focus of research in using constructed wetlands as a treatment method. While many of the early acid mine drainage treatment systems were marsh-like surface flow systems, the most recent projects are “passive treatment systems” that link several different types of cells — vertical limestone drain as well as vegetated cells — to sequentially treat particularly “nasty” wastewater with low pH and high metals content. 2 A wetland system for the treatment of runoff from coal piles at coal-fired power plants with a pH of 2 and high levels of metals uses a series of successive alkalinity-producing systems, a rich organic layer over an anoxic lime- stone drain, to reduce the acidity in the wastewater before it flows into wetland cells. Landfill leachates are a subset of polluted waters requiring substantial levels of treatment. Leachates vary considerably, depending upon the materials accepted at the landfill. They may contain large concentrations of volatile and toxic organics, both as individual compounds and as COD, chlorinated organics, metals, and nitrog- enous compounds. 2 Wetland treatment of landfill leachates has been successfully tested at several locations. Cold climate systems are functioning properly in Norway, as well as at several locations in Canada; reed beds are used to treat leachate in the United Kingdom, Slovenia, and Poland. 4 Based on current understanding of the effectiveness of wetland treatment of leachates, several U.S. projects are in planning and design phases. In addition, there are about a half-dozen other projects in various locations, such as Mississippi, Indiana, Pennsylvania, and West Virginia. Wetlands have been proposed for control of stormwater runoff from capped landfills. 1,2 Continued growth in the use of treatment wetlands is expected as a result of new regulatory initiatives on nutrient management, including the Clean Water Act’s total maximum daily load (TMDL) program. Small- to medium-sized communities trying to meet new TMDLs in sensitive watersheds for phosphorus or ammonia need something that is cost-effective, and wetlands are a good option. 6.1.2Looking Inside the “Black Box” The rapid spread and diversification of treatment-wetland technology are running ahead of the mechanistic understanding of how they work. These complex natural systems are still, somewhat, a “black box,” according to many in the field. For example, the role of plants in transporting oxygen into the root zone to promote nitrification has been demonstrated in the laboratory but not convincingly in the field, according to many researchers. There is very little data to say whether that is an important factor or whether the plants are more or less passive. It is likely, according to some researchers, that the ratio of open water to vegetated areas is more important in creating aerobic conditions in a wetland. ©2001 CRC Press LLC Another issue quite often debated is how important the volume of water in a wetland is to treatment performance. Is it the bottom of the wetland or the volume of water that is more important? The data coming in now are on the side of the wetland bottom: 1,2 it apparently does not matter how deep the water is as long as the soil is wet. That is a surprise to civil engineers, who, for years, have designed treatment systems based on their volume and hydraulic residence time. Numerous research efforts, both broad based and focused, are currently gener- ating a great deal of new information on treatment-wetland function. 1,2,5 The exten- sive research activities include gathering conventional water quality data; measure- ments of metals, biotoxicity, and organics; bird surveys; and macroinvertebrate sampling. Expanding the species pallet of plants used in treatment wetlands is another focus of research among researchers in this field. Most constructed wetlands for treatment have been built around herbaceous species so far, and many researchers are experimenting with a greater variety of plants to see how water quality changes when multispecies systems are used. Many have found that pathogen removal is higher in a multispecies system than in a single species system. One of the things that may be important in pathogen removal is having multiple types of wetland components, for example, a duckweed system followed by a subsurface wetland. 5 Looking deeper into the wetland, to the microbes in the soil and around the root systems of wetland plants, some researchers are studying the role that bacteria play in trace element removal. Researchers have found that bacteria in the root zone of bulrush increase the plants’ ability to accumulate and volatilize selenium twofold. They are now working to identify which bacteria are most responsible, and will soon move to mesocosm studies to see whether seeding the soil with those bacteria increases trace element removal. Some researchers are experimenting with an innovative wetland design — a vertical flow system — to solve the oxygen depletion problem and boost nitrifica- tion. 1,2 Effluent flows over a porous surface and percolates through a vegetated sand filter, which is periodically allowed to dry to reintroduce oxygen to the system. 6.1.3Potential “Attractive Nuisances” Aside from research issues surrounding the design and performance of the treatment wetlands black box, another scientific issue looms large for the future of the technology: do treatment wetlands pose a threat to wildlife? 1,5 This question is an important one, since many wetland projects are designed with habitat creation as one of their primary beneficial objectives. It is easier to justify the land use for a constructed wetland if it is also used for habitat restoration. Research is also being directed toward several critical issues. Some researchers are working to find out exactly where toxic trace elements from wastewater end up in a treatment wetland. They are completing laboratory studies documenting trace element uptake potential of various wetland plants and identifying where the ele- ments go in the plants: roots, stems, leaves, or plant litter. They are also monitoring several active treatment wetlands to track trace elements in the ecosystem: sediment, water, air, plant tissues, and animal tissues. ©2001 CRC Press LLC To address similar habitat-related issues, influent and effluent water have been analyzed for potential bioaccumulation and mutagenic activity from organic com- pounds. 5 Toxicity tests were designed to look for physiological impacts on biota living in the system. Work also continues on the control of an unplanned threat to human health: mosquitoes. Fish have been introduced to the wetlands to consume mosquito larvae, but the density of the particular bulrush variety used may prevent the fish from reaching certain parts of the wetland. Sections of the wetlands can be reconfigured and replanted to raise the water level and give the fish greater access. 6.1.4Regulatory Uncertainty and Barriers Treatment wetlands do not appeal to all wastewater engineers because they lack the traditional “handles” of engineered pollution control systems, are not easy to control, and may be hard to predict. Regulators in the U.S. have similar problems with treatment wetlands because they do not fit easily into existing regulatory categories. Surface-flow treatment wetlands can be a point source discharge and a protected environment at the same time. No national guidance on the use of treatment wetlands and no uniform acceptance of them by states exist, according to researchers and consultants. In this atmosphere of regulatory uncertainty, questions abound. Concerns have been expressed that under a strict reading of the Clean Water Act, certain treatment wetlands could be considered “waters of the U.S.,” and thus discharges into them could be tightly regulated. USEPA’s environmental technology initiative (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 regulation and/or policy that would facilitate, where appropriate, implementation of beneficial treatment wetland projects.” 6,7 It also advocated that “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 it. The report catalogued numerous regulatory and policy issues. Should disinfec- tion of effluent be done at the inlet rather than the outlet of a wetland? When should a wetland be lined to protect groundwater? Should treatment wetlands be allowed to mitigate for permitted wetland losses? Under what conditions should constructed treatment wetlands be considered “waters of the U.S.?” 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 USEPA 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. 6,7 The question of where treatment wetlands should be sited has been a particularly difficult regulatory issue, and consensus must be reached on the need to handle wetland systems differently depending on whether their primary purpose is water treatment or habitat restoration. There is still some disagreement about the habitat ©2001 CRC Press LLC value of treatment wetlands and concerns about the negative impact they could have on the environment. USEPA currently is not developing the type of specific guidance documents and formal agency actions recommended in the ETI study to promote the use of treatment wetlands. Nevertheless, wetlands experts are encouraged because the issues are now being discussed at the national level. 6.2 TYPES OF CONSTRUCTED WETLANDS 6.2.1 Horizontal Flow Systems The purposeful construction of treatment wetland ecosystems is a relatively new technology. Constructed wetlands for pollution control, wastewater treatment, and, recently, for contaminated groundwater treatment are divided into two basic types: free water surface (FWS) and subsurface flow (SSF) wetlands. Both types consist of a channel or a basin with some sort of barrier to prevent seepage and utilize emergent aquatic vegetation as part of the treatment system. The difference between FWS and SSF wetlands is that SSF uses some kind of media as a major component (Figures 6.2a and b). In an FWS treatment wetland, soil supports the roots of the emergent vegetation; water at a relatively shallow depth of 6 to 24 inches flows through the system with the water surface exposed to the atmosphere. Oxygen is provided by diffusion through the water surface. An SSF treatment wetland bed contains a suitable depth (1.5 – 3.0 feet) of permeable media, such as coarse sand or crushed stone, through which the water flows. The media also support the root structure of the emergent vegetation. The surface of the flowing water is beneath the surface of the top layer of the medium, determined by proper hydraulic design and appropriate flow control structures. In both systems the polluted water undergoes physical, biological, and chemical treat- ment processes as it flows through the wetlands. The rate at which organic contaminants move through wetlands can be deter- mined by several transport mechanisms. These mechanisms often act simultaneously on the organics and may include such processes as convection, diffusion, dispersion, and zero- or first-order production or decay. Figure 6.2a Free water surface (FWS) wetland. Inlet Outlet Weir ©2001 CRC Press LLC Currently, constructed wetlands for municipal wastewater treatment are designed based on the assumptions of plug-flow hydrodynamics and first-order biochemical oxygen demand (BOD) removal kinetics. The first assumption implies that dispersion in the system is negligible and all the fluid particles have a uniform detention time traveling through the system. The plug-flow model seems to give a reasonably accurate estimate of the performance of SSF-constructed wetlands. However, some designers have recognized the limitation of using the plug-flow model for constructed wetlands design. Three types of hydraulic inefficiencies may occur in treatment wetlands: one caused by internal islands and topographical features, a second caused by preferential flow channels on a large-distance scale, and a third caused by mixing effects, such as water delays in litter layers and transverse mixing. 6.2.2 Vertical Flow Systems Vertical flow constructed wetlands are vegetated systems in which the flow of water is vertical rather than horizontal as in FWS and SSF wetlands (Figure 6.3). Figure 6.2b Subsurface ßow (SSF) wetland. Figure 6.3 Vertical ßow constructed wetland. Inlet Effluent Porous Media [...]... emergent macrophyte-based wastewater treatment systems; 3) submerged macrophyte-based wastewater treatment systems; and 4) multistage systems consisting of a combination of the above-mentioned concepts and other kinds of low-technology systems (e.g., oxidation ponds and sanitary filtration systems) 6. 3.3.1 Free-Floating Macrophyte-Based Systems Free-floating macrophytes are highly diverse in form and habit,... negative electric potential between a standard platinum electrode and the concentration of oxygen in the soil This measure of electric potential is called reduction-oxidation or REDOX potential (ORP) and provides an estimate of soil oxidation or reduction potential (Figure 6. 9) Figure 6. 9 Typical depth proÞle for potential oxidation-reduction reactions taking place in a treatment wetland system ©2001... accumulations of organic carbon and reduced elements such as iron and sulfur typical of natural wetland soils Many of the changes that occur during wetland development and succession are the result of biological factors that occur in wetlands such as growth of bacteria and fungi, algae and macrophytic plants, micro- and macroinvertebrates, and larger animals While many of these natural biological processes... REDOX potentials 6. 4 .6 Treatment Wetland Soils The sediments that form in treatment wetlands are often different from those that form in natural wetlands, for a number of reasons First, the enhanced activity of various microbes, fungi, algae, and soft-bodied invertebrates leads to a greater proportion of fine detritus compared to leaf, root, and stem fragments There is significant formation of low-density... forms of rooted wetland and aquatic vascular plants (adapted from Kadlec and Knight, 19 96) more than 67 00 plant species on their list of obligate and facultative wetland plant species in the U.S Obligate wetland plant species are defined as those which are found exclusively in wetland habitats, while facultative species are those that may be found in upland or in wetland areas.1 Wetland macrophytes are... algae in wetlands (adapted from Kadlec and Knight, 19 96) 10 pH 10 pH Dissolved Oxygen (mg/L) 20 DO 0 0 12 MN 6 AM 12 NOON 6 PM 12 MN TIME Figure 6. 5 Typical diurnal plots of DO concentration and pH in a wetland dominated by Þlamentous algae ©2001 CRC Press LLC Cattail Duck Potato Emergent Herbaceous a Buttonbush Shrub Emergent Woody b Water Lilly c Floating Leaved Hydrilla d Figure 6. 6 Submerged Growth... of aerial and/ or floating leaves and well-developed submerged roots (e.g., water hyacinth, Eichhornia crassipes) to minute surface-floating plants with few or no roots (e.g., duckweeds, Lemna, Spirodella, Wolffia sp.) (Figure 6. 7a).2 Influent Effluent Figure 6. 7a Schematic description of a free-ßoating water hyacinth-(Eichhornia crassipes) based treatment wetland system Water Hyacinth-Based Systems: The... growth and regeneration of hydrophylic vegetation.8 Since most wetlands are constructed in former uplands, most constructed wetlands are initially dominated by mineral soils As constructed wetland treatment systems mature, the percent of organic matter in the soil generally increases, and in some systems, soils might eventually cross the arbitrary line between mineral and organic (Figures 6. 8a and b)... stems and leaves that fill the niche between sediment surface and the top of the water column Floating and submerged species may appear in treatment wetlands when water depths exceed the tolerance range for rooted, emergent species Aquatic macrophyte-based wetlands treatment systems may be classified according to the life form of the dominating macrophyte into 1) free-floating macrophyte-based treatment systems; ... to suspended solids and aerobically biodegradable organics, ammonia, and phosphorus 6. 3.3.5 Submerged Macrophyte-Based Systems Submerged aquatic macrophytes have their photosynthetic tissue entirely submerged (Figure 6. 7d) The morphology and ecology of the species vary from small, rosette-type, low-productivity species growing only in oligotrophic waters (e.g., Isoetes lacustris and Lobelia dortmanna) . Wetland Systems 6. 3.3.1 Free-Floating Macrophyte-Based Systems 6. 3.3.2 Emergent Aquatic Macrophyte-Based Systems 6. 3.3.3 Emergent Macrophyte-Based Systems with Horizontal Subsurface Flow 6. 3.3.4. Macrophyte-Based Systems with Vertical Subsurface Flow 6. 3.3.5 Submerged Macrophyte-Based Systems 6. 3.3 .6 Multistage Macrophyte-Based Treatment Systems 6. 4 Treatment-Wetland Soils 6. 4.1 Cation. Mechanisms 6. 5.1 Volatilization 6. 5.2 Partitioning and Storage 6. 5.3 Hydraulic Retention Time 6. 6 Treatment Wetlands for Groundwater Remediation 6. 6.1 Metals-Laden Water Treatment 6. 6.1.1 A Case