Green infrastructure is becoming more widely used to manage stormwater runoff, providing a natural means of limiting or eliminating combined sewer overflows and improving overall water quality. Constructed wetlands and systems that mimic wetlands (for example bioswales, bioretention areas, and rain gardens) provide a cost-effective and environmentally conscious option that can result in an efficient and aesthetically pleasing wetland. Additionally, constructed wetlands can provide valuable habitat for wildlife, including many amphibians, songbirds, and small mammals. Constructed wetlands are classified according to their designed water flow; three common wetland designs are horizontal subsurface flow (HF), vertical flow (VF), or free surface wetlands (FSW).
Regardless of the type of treatment wetland used, each utilizes a process known as the root zone method (RZM) for removing bacteria, and excess nutrients, that negatively affect water quality. The RZM can be summarized as follows: Influent (incoming) water passes horizontally or vertically through the soil and percolates the wetland bed. The roots of wetland plants provide a pathway for the water to flow, and as the wastewater and solids move through the system they are treated by microbes that are contained near the plants’ roots. The leaves of the plants absorb oxygen and transport it to the roots through their stems, which are hollow, and act as a bio-pump. In the soil, or filter layer below the roots, anaerobic digestion treats the influent wastewater as well. The type of substrate and plants included in the wetland design will vary depending on what the wetland has been designed to control. Depending on the specific use of a treatment wetland, it may be necessary to pre-treat wastewater and remove large solids to prevent clogging of the substrate, which will reduce the effectiveness of the system.
In horizontal subsurface flow wetlands, water passes through emergent plants, and the RZM removes bacteria and excess nutrients at very high rates in a well functioning system. After construction is complete, HF wetlands do not require significant maintenance, and many can function several years without maintenance. Vertical flow wetlands utilize the RZM with a planted filter bed to treat wastewater as it flows through the system. Typically the top layer is planted gravel above a layer of sand. The deepest portion of the system is another layer of gravel that contains drainage pipes to collect and transport the filtered water as it percolates through the system. VF wetlands are designed to be most effective when wastewater is applied in discrete intervals at a rate of 4-12 doses per day, and the wastewater is allowed to slowly percolate through the unsaturated layers of soil and sand. Intermittent dosing is necessary to allow adequate oxygen transfer, which is necessary for aerobic degradation by the resident microbes.
Free surface wetlands are the most natural-looking treatment wetland option. In these systems, water flows above ground and plants are rooted in the soil layer at the base of the wetland. Typical design includes a basin lined with an impermeable layer, such as clay. The substrate consists of rocks, gravel, and soil. The basin is usually planted with native plants, and floating wetland islands can be used to supplement the plant coverage and increase system efficiency. FSW wetlands are usually flooded with wastewater to a depth of 3-18 inches above ground level. As the water slowly flows through the wetland and percolates into the soil, excess nutrients are taken up by the plants and potentially harmful bacteria may be trapped and degraded by microbial communities in biofilms on and near the plant roots.
Each type of treatment wetland has advantages and drawbacks that need to be carefully considered before developing blueprints and planning construction. MAD Scientist & Associates has a proven track record with wetland design and construction, so if you are considering a constructed wetland for treatment or mitigation, we can help from the initial planning stages through construction and monitoring to make sure your wetland is a success.
Diagrams for Horizontal Flow and Vertical Flow Wetlands credited to:
Morel, A.; Diener, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of different treatment systems for households or neighbourhoods. Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC). [Accessed: 19.02.2013].