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Wastewater Treatment Wetlands: Applications and Treatment Efficiency 1
William F. DeBusk2Introduction
Constructed and natural wetlands are often used as low-tech treatment systems for domestic wastewater effluent, from single-residence septic tank effluent wetlands to large municipal wastewater treatment facilities. Similarly, wetlands may be used effectively for treatment of animal and aquaculture wastes. The use of wetland retention basins for treatment of stormwater runoff has become relatively commonplace. The composition of stormwater varies greatly, depending on the surrounding land use. For example, urban runoff may contain soil particles, dissolved nutrients, heavy metals, oil and grease. Residential and agricultural runoff may also contain organic matter and pesticides. A variety of industrial wastes, including pulp and paper, food processing, slaughtering and rendering, chemical manufacturing, petroleum refining and landfill leachates, are amenable to wetland treatment. Pretreatment, such as primary sedimentation or aeration stabilization, is often required for industrial effluents.Although a broad spectrum of designs has been used for wetland treatment systems, all can be classified as either surface-flow (SF) or subsurface-flow (SSF) systems ( Figure 1 ). The SF design typically incorporates a shallow layer of surface water, flowing over mineral (sandy) or organic (peat) soils. Vegetation often consists of marsh plants, such as cattails and reeds, but may also include floating and submerged aquatic vegetation, as well as wetland shrubs and trees. Natural wetlands have also been effectively utilized as SF treatment wetlands. In a SSF wetland, the basin is filled with gravel or some other coarse substrate, and the water level is maintained below-ground. Water flows horizontally, or sometimes vertically, through the gravel and the root mat of the wetland vegetation. Each type of treatment wetland has characteristic advantages and limitations for treatment of various wastes. The references listed at the end of this document are recommended for more in-depth information on treatment wetland design and applications.
Figure 1. Conceptual diagram of surface-flow and subsurface-flow wetlands. Treatment Wetland Performance
Treatment performance criteria for contaminant removal in wetlands may be based on the contaminant concentration in the wetland outflow or on the total or percent mass removal of the contaminant. It is important that the selected criteria accurately reflect the actual performance of the wetland relative to the objectives and intended uses of the wetland treatment system. As a case in point, the efficiency of nutrient removal decreases significantly as inflow concentration approaches the natural background concentration of the nutrient in the wetland, while the outflow concentration may be well within the desired range. Conversely, nutrient removal efficiency, in terms of percent mass removal, may increase substantially as the loading rateis increased to moderate levels, yet the outflow concentration may exceed the desired level. Thus, the actual performance of treatment wetlands is dependent on a multitude of factors, including inflow concentration, mass loading rates, wetland design and climate. Summary data for contaminant concentration reduction and mass removal, based on the North American Wetland Treatment System Database of approximately 200 wetland sites (Knight et al., 1994), are presented in Table 1 . Outflow concentrations in Table 1 may be regarded as a rough approximation of the background, or minimum attainable, concentration for a typical treatment wetland. Data in Table 1 represent combined performance data for surface- flow and subsurface-flow wetlands, derived from wetland treatment systems that collectively represent a wide range of contaminant types and loading and design characteristics. Therefore, it should be recognized that these summary figures provide only a general estimate of specific contaminant removal efficiency in treatment wetlands.
A brief overview of treatment wetland performance for specific types of contaminants is presented below.
Suspended Solids Removal
Wetlands are capable of achieving a high efficiency of suspended solids removal from the water column. Suspended matter in the water may contain a number of types of contaminants, such as nutrients, heavy metals and organic compounds. These contaminants may themselves be in particulate form, or they may be physically or chemically bound to the particulate matter. Thus, in cases where the bulk of the contaminant load is associated with particulate matter, physical settling of suspended solids can result in efficient removal of the contaminants from the water or wastewater stream.The concentration of suspended solids in the water column is measured gravimetrically, by filtering a sample of water, drying the residue and weighing. Total suspended solids (TSS) concentration is expressed as milligrams per liter (mg L-1 ). Turbidity, which is primarily caused by suspended particulate matter, is sometimes used as a surrogate measurement for TSS.
Most wetland treatment systems are overdesigned with respect to TSS removal, having been designed for removal of other contaminants. Therefore, background concentrations of TSS are often attained a short distance downstream from the wetland inflow.
Organic Carbon (BOD) Removal
Organic matter contains approximately 45 to 50% carbon (C), which is utilized by a wide array of microorganisms as a source of energy. A large number of these microorganisms consume oxygen (O2 ) to break down organic C to carbon dioxide (CO2 ), a process that provides energy for growth. Therefore, the release of excessive amounts of organic C to surface waters can result in a significant depletion of O2, and subsequent mortality of fish and other O2 -dependent aquatic or marine organisms.Wetlands contain vast numbers of organic C-utilizing microorganisms adapted to the aerobic (O2-rich) surface waters and anaerobic (O2-depleted) soils. Thus, wetlands are capable of highly effective removal of organic compounds from a variety of wastewaters. Organic C in wetlands is broken down to CO2 and methane (CH4 ), both of which are lost to the atmosphere. Wetlands also store and recycle copious amounts of organic C, contained in plants and animals, dead plant material (litter), microorganisms and peat. Therefore, wetlands tend to be natural exporters of organic C as a result of decomposition of organic matter into fine particulate matter and dissolved compounds.
The more readily degradable organic C compounds typically found in municipal wastewater can be rapidly removed in wetlands. Biological removal of a variety of recalcitrant (not readily decomposed) organic C compounds, including lignin-based compounds and petroleum products, can also be achieved in wetlands, although removal rates may be substantially lower. A commonly-used parameter for biologically available C is biochemical oxygen demand (BOD), which is actually a measure of the rate of O2 consumption by microorganisms utilizing the available organic C in the water or soil. The normal procedure for determining BOD in water samples measures the amount of O2 depletion occurring over a 5-day period (BOD5).
Nitrogen Removal
Nitrogen (N) is a major component of municipal wastewater, stormwater runoff from urban and agricultural lands, and wastewater from various types of industrial processes. Environmental and health problems associated with excessive amounts of certain forms of N in the environment have been well-documented. For example, high concentrations of nitrate in drinking water supplies can cause methemoglobinemia, or "blue baby" syndrome, in infants. Un-ionized ammonia (NH3 ), found in certain types of wastewater effluent, is potentially toxic to many aquatic and marine organisms. In addition, eutrophication of surface waters frequently is linked with elevated N concentrations, especially in coastal and estuarine environments.Nitrogen exists in many forms in the environment, and transformations among different forms may occur rapidly and frequently. Municipal and industrial wastewater may contain significant amounts of both organic and inorganic forms of N. Inorganic N, which includes nitrate, nitrite and ammonium, may also be present at high concentrations in agricultural and urban runoff. In the environment, nitrate and nitrite are usually found in well-aerated waters, with nitrate being the predominant form, while ammonium is the more persistent form of inorganic N in anaerobic wetland soils.
Wetlands generally are well-suited for N removal, even though the natural background level of total N in wetland outflows is typically greater than 1 mg L-1. As with organic C in wetlands, it is common for organic N compounds to be exported as a consequence of naturally-occurring organic matter decomposition within the wetland.
Substantial removal of N may take place through settling of N-containing particulate matter in the wetland inflow. In addition, since N is an essential plant nutrient, it can removed through plant uptake of ammonium or nitrate, and stored in organic form in wetland vegetation. A large portion of this N may later be released and recycled, as plants die and decompose. Ammonium may be chemically bound in the soil on a short-term basis, while organic N from dead plant material can accumulate in the soil as peat, a long-term storage mechanism.
Nitrate removal efficiency typically is extremely high in wetlands. The biological process of denitrification, i.e., conversion of nitrate to nitrogen gas, provides a means for complete removal of inorganic N from wetlands, as opposed to storage within the vegetation or soil. Denitrification usually accounts for the bulk of the inorganic N removal in wetlands.
Phosphorus Removal
Phosphorus (P), like N, is a major plant nutrient, hence, addition of P to the environment often contributes to eutrophication of lakes and coastal waters. In many cases, wetlands do not provide the high level of long-term removal for P that they provide for N. This is due, in part, to the lack of a metabolic pathway for P removal, as compared to denitrification for N removal. Nevertheless, most wetlands can provide significant P removal from water and wastewater through a combination of physical, chemical and biological processes.Orthophosphate is the predominant inorganic form of P in surface waters. This form of P readily accumulates in wetland vegetation and soils, as a result of biological uptake and chemical bonding. Formation of iron and aluminum phosphate minerals (low-pH wetlands) and calcium phosphate minerals (high-pH wetlands) is the major pathway for P removal in some wetlands. Organic forms of P are generally not biologically or chemically reactive in wetlands. Particulate organic P may be removed by settling from the water column. Both dissolved and particulate organic P may be biologically broken down to inorganic P (mineralization), and subsequently removed through biological and chemical processes.
Trace Metals Removal
A number of metals are required in small amounts for plant or animal growth. Some of these micronutrients, such as copper, selenium and zinc, are toxic at higher concentrations, and may be found in certain types of wastewater. Other metals, such as cadmium, mercury and lead, found in industrial or other types of wastewater have no known biological benefit, and are toxic even at relatively low concentrations. Furthermore, certain metals have a tendency to become concentrated at higher levels of the food chain. This biomagnification effect can lead to serious health hazards to higher organisms, including humans.Removal of metals in wetlands may occur through a number of processes, including plant uptake, soil adsorption (binding to soil particles) and precipitation (formation of solid compounds). Plant uptake rates and tolerance of metals varies considerably among plant species. Some terrestrial plant species are known to be capable of storing high concentrations of metals in roots and other tissues. Metals may also tend to accumulate on the root surfaces of plants, rather than being absorbed into the plant.
Wetland soils are potentially effective traps, or sinks, for metals, due to the relative immobility of most metals in wetland soils. A number of metals, including cadmium, copper, nickel, lead and zinc, form nearly insoluble compounds with sulfides under anaerobic conditions in wetland soils. In addition, some metals, such as chromium, copper, lead and zinc, form strong chemical complexes (chemisorption) with organic matter in the soil or water. Metals (for example, chromium and copper) may also be chemically-bound to clays and oxides of manganese, aluminum and iron. Nickel also binds with organic matter, iron and manganese, but may become re-mobilized under certain conditions in wetlands.
Data on wetland performance for metals removal are relatively sparse. Based on a limited data set for treatment wetlands, metals removal efficiency is potentially very high, but also highly variable among sites. For example, reported mass removal efficiencies were 75-99% for cadmium, 40-96% for copper, 0-86% for lead, 49-88% for nickel, and 33-96% for zinc.
Removal of Toxic Organic Compounds
In addition to the easily-degradable organic C compounds collectively referred to as BOD, there are a multitude of degradation-resistant and/or toxic natural and man-made organic compounds that may be present in wastewater effluent or runoff. Quantitative data on removal of organics, such as pesticides and petrochemicals, are limited. However, measurable removal of a wide variety of organic compounds has been documented for wetland treatment systems.Both mineral and organic soils may adsorb organic compounds via chemisorption (strong interaction) or physical adsorption (weak interaction). Microbes are capable of degrading most classes of organic pollutants, but the rate of degradation varies considerably, depending on chemical and structural properties of the organic compound, and the chemical and physical environment in the soil. For example, highly halogenated hydrocarbons such as polychlorinated biphenyls (PCBs) are extremely resistant to decomposition, due to their low solubility in water and the lack of a structural site for enzyme attachment for degradation. Other possible mechanisms for removal of organics in wetlands are volatilization and photochemical (sunlight) degradation.
Studies have documented successful wetland treatment of PCBs, lindane, pentachlorophenol and atrazine. In most cases, actual removal processes -- for example, sediment retention or microbial degradation -- were not determined. However, there is substantial evidence that pentachlorophenol breaks down readily under the alternating aerobic and anaerobic conditions found in wetland soils.
Total Dissolved Solids
Total dissolved solids (TDS) measurements are often used to express the degree of contamination or amount of impurities in water and wastewater. One of the reasons for this is the relative ease of measurement -- a water sample is evaporated, and the residual solids are weighed. A wide variety of inorganic ions and organic compounds, many of which may not be considered contaminants, contribute to the sum total of dissolved solids. A number of these are biologically utilized or chemically reactive in wetlands. However, TDS often includes relatively high concentrations of "conservative", or relatively unreactive, dissolved compounds, which are not removed in wetlands. For example, wetlands have very little effect on the concentration of sodium and chloride ions, or more generally, on salinity levels. Therefore, reduction of TDS concentrations in wetlands is often insignificant despite high removal rates of target contaminants.Conclusion
Constructed and natural wetlands have been used extensively to treat several types of wastewater and runoff. High levels of removal can be achieved for a number of contaminants, including suspended solids, nutrients, metals and organic compounds, in treatment wetlands. However, there are inherent limitations to the effectiveness of wetlands for wastewater treatment. In some cases, it may not be possible to achieve the desired outflow concentration, due to high natural background levels of the contaminant of interest. Also, there is a relatively high degree of time-dependent variability in treatment efficiency associated with wetlands, especially when compared with conventional treatment technologies. Nevertheless, wetland treatment is often the best choice for treatment or pre-treatment of water and wastewater because of the low maintenance costs and simplicity of operation.References
Cooper, P.F., and B.C. Findlater. 1990. Constructed wetlands in water pollution control. Pergamon Press, Oxford.Hammer, D.A. (ed.). 1990. Constructed wetlands for wastewater treatment: municipal, industrial, and agricultural. CRC Press, Boca Raton, FL.
Kadlec, R.H., and R.L. Knight. 1996. Treatment wetlands. Lewis Publishers, Boca Raton, FL.
Knight, R.L., R.W. Ruble, R.H. Kadlec, and S.C. Reed. 1994. Wetlands for wastewater treatment performance database. In G.A. Moshiri (ed.), Constructed wetlands for water quality improvement. Lewis Publishers, Boca Raton, FL.
Moshiri, G.A. (ed.). 1994. Constructed wetlands for water quality improvement. Lewis Publishers, Boca Raton, FL.
Reddy, K.R., and W.H. Smith (eds.). 1987. Aquatic plants for water treatment and resource recovery. Magnolia Publishing, Orlando, FL.
Tables
Table 1. Summary of contaminant removal efficiency in treatment wetlands, based on the North American Wetland Treatment System Database (Knight et al., 1994). Average values of combined performance data for surface- and subsurface-flow wetlands are presented. (Table adapted from Kadlec and Knight [1996])
Contaminant Outflow concentration Concentration reduction* Mass removal Removal efficiency*** mg L-1 % kg/ha/day** % Total suspended solids (TSS) 13.0 72 11.9 71 Biochemical oxygen demand (BOD5)
8.1 73 7.5 68 Nitrate (as N) 2.1 62 0.54 55 Total ammonia (as N) 2.4 52 0.38 26 Total nitrogen 4.5 53 1.5 51 Orthophosphate (as P) 1.1 37 0.12 41 Total phosphorus 1.7 56 0.22 31 *Average percent reduction in concentration between wetland inflow and outflow. **Kilograms per hectare per day (kg/ha/day X 0.892 = lb/acre/day).
***Percent of total contaminant load removed by the wetland.
Footnotes
1. This document is SL156, a fact sheet of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Published May 1999. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.2. William F. DeBusk, former assistant professor and extension specialist, Soil and Water Science Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611-0510.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.
U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
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