Constructed Wetlands for Wastewater
Conventional on-site wastewater systems (septic systems), even when functioning properly, do not adequately prevent pollutants from reaching groundwater and surface water (see EBN). A constructed wetland is an alternative that uses plants and microorganisms living in association with plant roots to remove contaminants and nutrients from wastewater, significantly reducing groundwater contamination. Although constructed wetlands can be used to treat both stormwater and wastewater, this primer will focus on the latter.
Conventional septic systems collect wastewater in a septic tank, where organic solids settle out and anaerobic bacteria begin the process of biological decomposition. From there the wastewater is piped to a drainfield and released slowly into the soil, where aerobic bacteria continue to break down the remaining organic matter and pathogens. Conventional septic systems do not remove nitrogen and phosphorus, which fertilize bodies of water, causing algae blooms that deplete oxygen and kill aquatic life (a process known as eutrophication).
Installing a constructed wetland between the septic tank and the drainfield pre-treats wastewater before it is released into the soil. A diffusion pipe delivers effluent from the tank into the wetland, which is sealed off from the soil by a liner (usually PVC). The wetland consists of a layer of gravel or crushed-stone aggregate, and is planted with typical emergent wetland species—cattails, bulrushes, reeds, and sedges. Solids in the wastewater are filtered by the aggregate and roots, while organic compounds and nutrients are broken down or removed by bacteria and plants.
Most constructed wetlands for wastewater treatment are subsurface-flow wetlands in which the wastewater flows through the aggregate and remains below the surface. This prevents odors and mosquito breeding and minimizes the risk of human contact. Aerobic and anaerobic bacteria in a constructed wetland, through a series of biochemical processes, convert nitrogen compounds into harmless atmospheric nitrogen, which escapes into the air. The treated effluent is then delivered (by gravity or pump) to the drainfield. Depending on local codes, pretreating wastewater in a constructed wetland may allow drainfields to be downsized, which can offset some of the expense of the wetland.
Constructed wetlands require some maintenance, including regular inspections of all components and care of vegetation to ensure adequate growth. Water levels must be monitored and controlled: if too low, the wastewater may not make sufficient contact with the plants’ root structure to perform properly; too high and it may reach the surface of the gravel media, creating smelly conditions and prime mosquito habitat. Cold weather can be another problem for constructed wetlands, slowing the flow of water and retarding biological processes. But a uniform layer of well-decomposed organic mulch can allow subsurface-flow wetlands to operate throughout the winter even in very cold conditions.