What Is A Constructed Wetland?

Constructed wetlands are a manufactured design of watered substrates, riparian vegetation and associated wildlife that imitates a natural wetland.

Plants in wetlands are very good at filtering soil and chemical pollutants from runoff. Wetlands protect groundwater as they take up excess nitrogen and phosphorous. A wetland is like a sponge that soaks up excess water and slowly releases it into lakes, streams or underground aquifers. When water slowly moves through the area, sediments settle out and the slow movement allows for longer contact between organic and inorganic matter. All of the different masses in the wetland, and gas and water exchanges create a community of microorganisms that can change or break down the substances. When the vegetation dies off, it sinks to the bottom, allowing for more material for the microbes to exist on, creating a self-sufficient system. Wetlands also provide habitat for numerous species of birds and animals, with the possibility for recreational uses also, such as bird watching.

A constructed wetland is a human designed system of watered substrates with riparian vegetation, birds and animals that simulates the processes of a natural wetland. A constructed wetland can be used as a filtration device for pollutants from industrial plants and factories, as well as for storm water runoff, thereby lessening the contaminants entering a treatment facility or rivers or streams. The utilization of a more natural process, such as a constructed wetland, will reduce the need for a more invasive mechanical filtration system. Other benefits of constructed wetlands can be in providing habitat for numerous birds, mammals, reptiles, insects and amphibians. An existing wetland can be enhanced or restored by the construction of an artificial wetland. Constructed wetlands can be also be used for aesthetic purposes or for recreation such as fishing and bird watching. Additionally, such a wetland area can also be constructed on a natural groundwater recharge site to enhance the infiltration of water into an aquifer. Another advantage is that different geographical areas can be used, such as alpine or low lying land, increasing the possibility localities and reducing restrictions.

Studies have shown that a constructed wetland can comply with strict water quality standards with removal of up to 95% of fecal coliforms, a bacteria found in human and animal waste. There are also monetary benefits since the design and construction is relatively low cost, and the maintenance of a system requires little effort. The size and design varies depending upon the desired use, but most designs are similar, with an intake area where the polluted or contaminated water enters the wetland. This could also be a type of septic tank used as a holding area where the wastewater is pumped into the wetland. Then there could be several filtration areas containing various species of plants that will perform the decontamination process, and a substrate of soil and gravel. Different plants perform different functions, some are more effective than others at removing nitrogen and phosphorus than others, so the type of plants used will depend upon the desired results.

One of the most common designs in North America for treating sewage is the Free Water Surface wetland. This design consists of dense stands of emergent wetland vegetation that slowly filter the contaminants, which are then excreted onto the surface of the plants as a biofilm that becomes a harmless substance. A Vegetated Submerged Bed wetland has a larger surface area for the biofilms, filtration and adsorptions which makes the land area needed smaller. There is no surface flow, which also helps with keeping insects and odor to a minimum.

Constructed wetlands are not necessarily restricted to being used as water treatment facilities, but can also be developed as mitigation areas to alleviate the loss of wetlands during construction projects along roadways, commercial or residential development. Mitigation is usually carried out on an acre for acre basis, with monitoring of the vegetation and wildlife required following completion of the wetland. When constructed for a mitigation site, the area can become a multi-use site, with recreation and education possibilities. Some constructed wetland mitigation areas are used for bird watching and fishing with interpretive trails and tours.

There can be several problems associated with constructed wetlands. Temperature is important because some plants will stop taking up nutrients and can die when it gets too cold, so the amount of effective wastewater treatment may not be the same for the entire year. Routine maintenance of the wetland must be done on a regular basis to control unwanted aquatic plants and insects like mosquitoes. This means that there will be some cost and manual labor considerations to take into account. Weeds need to be chemically treated or pulled out before they can become a major problem. If the wetland was not designed well, sediment build-up can be a problem and require dredging to increase the efficiency of the wetland.

Even though a constructed wetland will not provide the filtration necessary for large scale projects, it can still be used as a more natural and less intrusive method for purification of smaller scale treatment projects with overall excellent results. As mitigation projects, constructed wetlands have proved to be an acceptable method of preventing the complete loss or degradation of necessary wetland habitat.

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