Constructed Wetlands
Primary Biological Filters
Before describing the proven capabilities of constructed wetlands it is essential to understand that constructed wetlands are only one part of a multi-part treatment system. When properly designed, built, and operated by experienced engineers, contractors, and wastewater treatment operators, constructed wetlands can be depended on for the removal of the following pollutants:
BOD (Biological Oxygen Demand), TSS (Total Suspended Solids), Nitrates, Metals- ionic and solid form, Petroleum hydrocarbons
They can be counted on to remove 40 – 80% of the total nitrogen in wastewater. Removal rates vary seasonally, being greater in the summer. Additionally, they will remove 99.0 to 99.9% of fecal coliforms, as well as other pathogens, including viruses.
Constructed wetlands are primarily biological filters that are very effective in removing BOD, TSS, and organic nitrogen; nitrates are almost totally removed. The rate of removal is a function of detention time. When comparing performance of wetlands, the comparison should be based on the performance of complete systems remembering that wetlands are only one part of a multi-part system. As an example, consider the system illustrated in Graph 1. Because of the reduced BOD and TSS, subsequent treatment processes such as the infiltration basin, or other soil based systems (overland flow, irrigation) work much more efficiently. The efficiency of other treatment process, such a nitrification filters, also increases.
Reviewers have been critical of nitrogen removal capabilities of constructed wetlands. To understand their proper role in the treatment process, it is important to understand that the form of nitrogen plays a significant role in soil treatment. Wetlands do not produce nitrates; because they are anoxic environments they do not do well in treating and/or converting ammonia. However, ammonia is easily captured by the clay particles in soil, whereas nitrates will move directly to ground water. Therefore as a design principal, nitrogen in the effluent from an on-site treatment system as ammonia is preferable to nitrate. Nitrifying technologies such as sand mounds or package treatment systems are more likely to create pollution problems because nitrates are not bound to soil particles. Wetlands do not produce nitrates, and therefore are very much less likely to contribute to groundwater pollution as the attached graph show.
Below Graph 1. Total Nitrogen Influent, Effluent, & Permit Limit Concentrations
The Two Types of Constructed Wetlands
There are two types of wetlands – free water surface wetlands (FWS) and subsurface flow (SF) wetlands. Figures 1 and 2 show their respective cross sections. Each type has its advantages and disadvantages and they must be properly evaluated in the context of the collection system, the possible methods of discharge of the treated effluent, and the permit requirements. Although the technology is apparently simple, understanding the proper role of each type of wetlands is a non-trivial process requiring experienced designers to properly evaluate the most appropriate system.
Below Figure 1. Free Water Surface Constructed Wetlands
Click on image to enlarge
Below Figure 2. Subsurface Constructed Wetlands
Click on image to enlarge
Wetlands, as one part of a multi-part treatment system allow the designer to meet State Environmental Department criteria for both total nitrogen and nitrates in the ground water while maintaining very simple operating conditions. Graph #2 summarizes water quality from an operating constructed wetlands with septic tanks as pretreatment and an infiltration basin for disposal of treated effluent, that is currently serving 145 homes. It demonstrates the capabilities of this technology.
A second example in Nebraska, which is designed for stream discharge is currently being monitored by both an independent laboratory and the University of Nebraska College of Engineering under the direction of Prof. M.F. Dahab. This system serves 120 homes plus the clubhouse. Except for the discharge pumps, this system does not use any energy. The operating bill, including testing, is $12,000 per year, or $100/year/ home. The following data set has been included as an example of a simple, yet effective, technology.
Clearly, wetlands can consistently meet design parameters established by regulatory agencies, and unlike mechanical systems, they are able to treat low flow volumes as well as those more nearly approaching the maximum. As an additional comparison, the energy for a small package treatment facility treating 25,000 gpd is approximately $300/month. The energy cost for the wetlands is $0.
Wetlands rely on self maintaining, self-regulating biological processes. In this respect they are similar to wastewater stabilization lagoons. However, unlike lagoons they are more efficient and therefore require less land. Their big advantage over other technologies that accomplish the same tasks is that they do not need energy.
Below Graph 2. BOD Influent, Effluent and Permit Limit Concentrations
Below Graph 3. TSS Influent, Effluent and Permit Limit Concentrations
Some Commonly Asked Questions:
- What happens when it rains? Rainwater shortens the treatment time, while diluting the wastewater. There is no effect on treatment. Surface water is kept out by a surrounding berm. No wastewater is allowed to overflow out of the wetlands.
- What happens in cold weather? Wetlands, like all wastewater treatment processes, are temperature dependent. The Engineer must design for the worst case, which is the low temperatures occurring in Winter. Snow cover actually helps. Our firm has designed operating municipal systems for the Wind River Range in Wyoming, and mountain communities in the Rockies. The Norwegians have operating systems above the Arctic Circle.
- What about odor? One of the virtues of subsurface wetlands is that wastewater flow is under the gravel surface. Noxious odors are trapped and in fact become food for the microorganisms attached to the gravel and plant root surfaces. A similar event takes place with surface flow wetlands on plant stems; often small floating plants such as duckweed and azolla contribute to odor removal. However, primary treatment must be aerobic in surface flow wetlands.
- Can they be included in the Landscape, for example at visitor centers, schools or next to golf course fairways? Campbell and Ogden wrote a book documenting numerous examples of treatment wetlands in the landscape. The EPA has a publication showing 17 examples of municipal treatment wetlands that serve both as parks and wildlife habitat. The PGA has a publication showing man made wetlands adjacent to fairways and greens and their effects on improving habitat.
- How much land is required and how much do they cost? Depending on water quality and winter temperatures, wetlands should average between ½ to 1 sq. ft. per gallon of water treated per day. Cost per acre is $22,000 to $50,000 for surface flow wetlands. Add an additional $50,000 for subsurface flow remembering that subsurface flow wetlands are more efficient and are therefore smaller for the same water quality.
For more detailed information please refer to the references or contact NSI.