Putting Wetlands to Work
- By Lori D. Pfeil
- Mar 01, 2003
Wetlands are land areas that are wet during all or part of the year. Frequently, wetlands are transitional between uplands (terrestrial areas) and persistent or deeply (greater than two meters) flooded (e.g., streams, rivers, deep ponds and lakes) systems. Wetlands are among the most endangered ecosystems. Viewed as wastelands to be put to more productive use, wetlands have been drained for agriculture, urban expansion, industrial sites, home sites and dumps. Since settlement, the United States has lost over 53 percent of its wetlands.
Wetlands have been referred to by a host of terms including marsh, wet meadow, bog, swamp and bottomland forests. There are three key attributes of wetlands: (1) hydrology -- the degree of flooding or soil saturation; (2) wetlands vegetation -- the presence of water-loving plants called hydrophytes; and (3) hydric soils -- soils that are saturated, flooded or ponded long enough to develop anaerobic conditions in the upper part. All areas considered wetlands must have enough water at some time during the growing season to stress plants and animals not adapted for life in water or saturated soils. Although a number of definitions exist, most wetlands typically fall into one of the following four categories: (1) areas with both hydrophytes and hydric soils (e.g., marshes, swamps and bogs); (2) areas without hydrophytes, but with hydric soils (e.g., tidal flats), (3) areas without hydric soils but with hydrophytes (e.g., seaweed, covered rocky shores); and (4) periodically flooded areas without soil and without hydrophytes (e.g., gravel beaches or stream beds). Wetlands delineation is not easy and often is controversial. Knowledge and application of indicator vegetation, hydrologic conditions and soil properties is required.
Since settlement, the United States has lost over 53 percent of its wetlands.
Wetlands ecosystems have an intrinsic ability to modify or trap a wide spectrum of water-borne substances commonly considered pollutants or contaminants. Wetlands have been used as a sink for wastes and wastewater for a long time. Though the concept of deliberately using wetlands for water purification has only developed within the last 40 years, in reality human societies have indirectly used natural wetlands for waste management for thousands of years. Observations of the water purification phenomenon of natural wetlands systems have led to the stimulation of development of constructed wetlands for treatment from a variety of wastewater sources. Constructed wetlands, in contrast to natural wetlands, are human-made systems that are designed, built and operated to emulate wetlands or functions of natural wetlands for human desires or needs. Constructed wetlands have recently received considerable attention as low cost, efficient means to clean-up many types of wastewater. Natural or constructed wetlands have been used and/or designed to not only treat municipal wastes but also point and non-point wastes, such as acid mine drainage, agricultural wastes, landfill leachate, paper pulp, petro-chemicals, as well as industrial wastes. The use of wetlands for treatment of wastewater is an emerging technology in North America and worldwide.
How Do Wetlands Treat Wastes
Wetlands accomplish water improvement through a variety of physical, chemical and biological processes operating independently in some circumstances and interacting in others. Wetlands have a higher rate of biological activity than most ecosystems, they can transform many of the common pollutants that occur in conventional wastewater into harmless by-products or essential nutrients that can be used for additional biological productivity. These transformations are accomplished by virtue of the wetlands' land area, with its inherent natural environmental energies of sun, wind, soil, plants and animals.
The mechanisms in wetlands ecosystems that modify dissolved and particulate substances in wastewater include sedimentation, adsorption, filtration, precipitation, volatization, complexing, microbial modification and vegetation uptake. Sedimentation may be the most significant initial process, since suspended solids have a strong tendency to adsorb pathogenic microbial organisms, refractory organics, hydrocarbons, heavy metals and nutrients. Vegetation obstructing the flow and reducing the velocity enhances the sedimentation. Filtration, precipitation and complexing are interrelated and to a large extent, dependent upon hydraulic resistance from vegetation and soils that enhances sedimentation. The increased retention time, that wetlands provide, improve secondary treatment via oxidation and volatilization of many substances.
Wetlands ecosystems have an intrinsic ability to modify or trap a wide spectrum of water-borne substances commonly considered pollutants or contaminants.
Microbial populations in the water column, attached to vegetative and other substrates and within soils modify hydrocarbons, metals, pathogens and nutrient loads causing precipitation of some pollutants and recycling with subsequent settling out of others. Not only does a dense stand of wetlands vegetation provide a large surface area within the water column for microbial attachment, but the natural oxygen loss from wetlands plant root structures creates a substantial aerobic environment for microbial populations within the soil. Hydrophytic, or wet-growing plants, have specialized structures in their leaves, stems and roots somewhat analogous to a mass of breathing tubes (i.e., "snorkels") that conduct oxygen down into the roots. Because the outer covering on the root hairs is not a perfect seal, oxygen leaks out creating a thin film aerobic region -- the rhizosphere -- around every root hair. The larger region outside the rhizosphere remains anaerobic but the positioning of a large thin film aerobic region surrounded by an anaerobic region is crucial to the growth of microbial populations involved in the transformation of nitrogenous compounds and other substances.
Types of Wetlands Treatment Systems
Wetlands treatment systems (natural or constructed) use rooted, water-tolerant plant species and shallow, flooded or saturated soil conditions to provide various types of wastewater treatment. The three basic types of wetlands treatment systems include natural wetlands (NW), constructed surface-flow (SF) wetlands and constructed subsurface-flow (SSF) wetlands.
Although there are many types of naturally occurring wetlands, only those types with plant species that are adapted to continuous flooding are suitable for receiving continuous flows of wastewater. Also, because of their protected regulatory status, discharges to NW must receive a high level of pretreatment (a minimum of secondary wastewater treatment using biology treatment in which bacteria consumes the organic parts of the waste; this is in addition to primary treatment by screening, sedimentation and flotation). Constructed wetlands mimic the optional treatment conditions found in NW and provide the flexibility of being built at almost any location. In addition, constructed wetlands can be used for pretreatment of primary and secondary wastewaters from a variety of sources.
Surface-flow wetlands (natural and constructed) are densely vegetated by a variety of plant species and typically have water depths less than 0.4 meters. Open water areas may be incorporated into the design to provide for optimization of hydraulics and for wildlife habitat. Sub-surface-flow wetlands use a bed of soil or gravel as a substrate for growth of rooted plants. Pretreated wastewater flows by gravity, horizontally through a bed substrate where it contacts a mixture of faculative microbes living in association with the substrate and plant roots. Bed depth in SSF wetlands is typically less than 0.6 m, and the bottom of the bed is sloped to minimize the water flow overland. Typical plant species in SSF wetlands include reed (Phragmites), cattail (Typha) and bulrush (Scirpus).
Conditions Influencing Microbial Populations
The diverse microbial populations in wetlands include the diverse flora of bacteria, fungi and algae, which are important for nutrient cycling and pollutant transformations. The variety of microbial species in wetlands function in a wide range of physical and chemical conditions. Because of this variety of species and the niches they occupy, wetland ecosystems can operate consistently to treat water. Since many of these organisms are the same as those important in conventional treatment systems, their growth requirements and characteristics are known to many operators/engineers.
The mechanisms in wetlands ecosystems that modify dissolved and particulate substances in wastewater include sedimentation, adsorption, filtration, precipitation, volatization, complexing, microbial modification and vegetation uptake.
Overall ecosystem parameters, such as dissolved oxygen, water temperature and influent constituent concentrations, must be controlled through design and system operational control to keep the microbial community in harmony for optimal treatment. Wetlands vegetation substantially increases the amount of environment (aerobic and anaerobic) available for microbial populations both above and below ground. There is no doubt that macrophytic plants are essential for the high quality water treatment performance of most wetland treatment systems. The numerous studies measuring treatment with and without plants have concluded that almost invariably performance is higher when plants are present. This finding led some researchers to conclude that wetland plants were the dominant source of treatment because of their direct uptake and sequestering of pollutants. It is now known that direct uptake is the principal removal mechanism only for some pollutants and only in lightly loaded systems. During an initial successional period of rapid plant growth, direct pollutant immobilization in wetland plants may be important. For many other pollutants, plant uptake is generally of minor importance compared to microbial and physical transformations that occur in wetlands.
Macrophytic plants are essential in wetland treatment systems because they provide structure for the microbes that mediate most of the pollutant transformations. Any direct uptake of nutrients, metals and organic substances by plants provides some removal for a limited time but litter decomposition usually releases these compounds back into the water column after the growing season. Consequently, the most important role of plants is to simply grow up and die (creating detritus) and the water quality improvement function is principally dependent on the high conductivity of the litter/humus (detritus) layer and the large area for microbial attachment. Sedimentation and subsequent microbial modification -- metalization, oxidation, uptake, etc., are likely the most important processes for direct removal of pollutants from wastewater to soil substrate "sinks" inaccessible to future inflows and flushing actions.
In recent years, natural and constructed wetlands have been widely applied to address water quality degradation associated with municipal, industrial and mining sources of pollutants. The success of these installations, in both water quality and economic terms, has led to rapid expansion in wetland construction. Constructed wetlands may not be a panacea for all wastewater treatment problems, however, their lower cost and equipment demands should be a consideration when addressing point and non-point source pollution.
In summary, wetland systems use larger land areas and natural energy inputs to establish self-maintaining treatment systems providing environments for many more types of microorganisms because of the diversity of microenvironments in a wetland. The latter, along with a large treatment area, frequently provide more complete reduction and lower discharge concentrations of water borne contaminants. In the end, the wetlands have acted as a natural water filter that should be self-sustaining. For more detailed information on wetland microbial communities (bacteria, fungi, algae), as well as design and loading parameters, the reader is referred to Kadlec and Knight (1996) and The Water Environment Federation Manual of Practice FD-16, "Natural Systems for Wastewater Treatment," 2001. Information on the ecology of vascular plant species found in wetlands can be found in Mitsch and Gosselink (1993) and Kadlec and Knight (1996).
This article originally appeared in the 03/01/2003 issue of Environmental Protection.