Do You Encounter Constructed Wetlands When Providing Septic Service?

Running effluent through constructed wetlands can be an effective treatment option as well as an attractive landscape feature if installed and maintained properly.

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I have been involved in a project monitoring and evaluating constructed wetlands for treatment of tile drainage water from fields where manure has been applied. It is funny how sometimes there are parallel activities or questions that pop up. I have been asked whether constructed wetlands are a viable treatment option? The short answer is yes. Of course, as always, whether it is the type of system to use at a specific site is dependent on the residence or facility characteristics and soil and landscape conditions. Since I received the question, it’s a good time to look at the purpose and design of wetland systems to treat septic tank effluent.

         First, I’d like to highlight that these systems are constructed; it is not allowable to run septic tank effluent into an existing wetland! The wetland systems I have been involved with were in shoreland areas where one purpose of including wetlands as a part of the treatment train was to protect the lakes from phosphorus additions which can contribute to detrimental algal blooms. They are meant to handle flows up to 10,000 gpd so they can serve individual residences, clusters of residences or small resorts.


         There are two types of constructed wetland systems: surface (also known as free water flow) and subsurface flow. In both cases wastewater enters one end of the lined excavation (usually 30 mil polyvinyl chloride PVC) and exits at the other end. The goal in terms of treatment is to have a biochemical oxygen demand less than 30 mg/L; total suspended solids less than 20 mg/L and less than 10,000 fecal coliform bacteria/100 milliliters. This highly treated effluent can be discharged to a soil treatment area such as a mound or in some locations directly to surface water.

         Surface flow wetlands can be of two types but in both cases, there is ponded wastewater at the surface. One type has vegetation rooted in sand or pea gravel and the other has floating vegetation at the surface without sand. Subsurface flow wetlands, as the name implies, are wetlands where flow is maintained beneath the surface by a drainage control structure in a gravel substrate.

         Subsurface flow wetlands are preferred in cold climates like mine for sewage treatment to minimize potential human contact, mitigate odor problems, reduce mosquito breeding and provide some protection from freezing. The systems require more area and are more expensive to construct, but they function better in winter.


         Treatment is accomplished by physical filtration within the gravel and plant roots. Organic material is broken down through biodegradation within the wetland. Nutrients are taken up and stored by the plants. Part of regular maintenance involves periodic removal of the vegetation. Some designers include an aeration component to the system to help break down the organic matter and improve nitrogen removal by nitrifying the ammonium to nitrate, which can then be reduced in the largely anaerobic environment in the media.

         Constructed wetlands have four parts: the liner, distribution media, plants and underdrain system. The liner should be PVC and not clay-lined because clay can crack and leak. The distribution media would be the typical drainfield rock 3/4 to 2 1/2 inches in diameter, clean and durable. Distribution can be by pressure or gravity. The media in the filter is 3/8 to 3/4 inch in diameter with a depth of 18 to 24 inches. Plants rooted in the media are most often cattails but can be a combination of bulrushes, reeds or sedges. The underdrain system at the end of the wetland is a 4-inch slotted pipe covered with drainfield rock. It moves the effluent out of the wetland through a drainage control structure to keep the effluent below the gravel level.

         The size of the wetland depends on the level of desired treatment, the strength of the wastewater and the amount of daily flow. In cold climates, they should be sized to provide 10 to 14 days detention time to ensure high-quality effluent. Length should be two to three times the width to ensure the wastewater does not flow through the system too fast. In addition, the volume of the rock needs to be included in calculating the dimensions. Using a 30% rock porosity value is appropriate. This increases the volume needed for treatment.


         Freezing problems can be avoided by allowing initial freezing at the surface; then dropping the level of water in the gravel media by using the control structure or adding insulating material to the surface. This is especially needed in winters when snow cover is less than normal. Treatment performance will be reduced during winter months, due to less plant uptake and reliance solely on movement of effluent through the gravel media.

         As with any other type of onsite sewage system, the wetland should be constructed on the contour with any surface drainage directed away from the wetland. Surface water inflow can overload the system. Use of good landscaping around the wetland cell can reduce problems due to surface runoff.        

         Final treatment and dispersal can be accomplished through an unlined seepage cell, standard soil treatment trenches, mounds or at-grades, or drip distribution systems. As always, how viable an alternative these systems are depends on homeowner preferences, cost and having the proper site conditions. Some homeowners desire to use what they consider to be a more “green” technology and are willing to incur additional costs. At one site I worked on, the homeowner incorporated different types of wetland plants in their system, which added thousands of dollars to the cost, but they had the desire to view these plants on their landscape. 


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