An engineered system designed to mimic natural wetland processes for the treatment of wastewater, stormwater, or agricultural runoff. Constructed wetlands use vegetation, soil, and microbial communities to remove pollutants.
A constructed wetland is an engineered treatment system that uses natural wetland processes—including sedimentation, filtration, adsorption, plant uptake, and microbial metabolism—to improve water quality. Two main design types exist: surface flow (free water surface) wetlands where water flows over a vegetated soil bed, and subsurface flow wetlands where water passes through a gravel or sand medium below the surface, either horizontally or vertically. Constructed wetlands effectively remove a range of pollutants including suspended solids, biochemical oxygen demand (BOD), pathogens, nitrogen (through nitrification-denitrification), phosphorus (through adsorption and plant uptake), and some metals and trace organics. They are used for secondary and tertiary wastewater treatment, combined sewer overflow management, stormwater treatment, acid mine drainage remediation, and agricultural runoff polishing. Operating costs are significantly lower than conventional mechanical treatment systems, making constructed wetlands particularly suitable for small communities, developing countries, and decentralized treatment applications. Common wetland plants include cattails (Typha), bulrush (Schoenoplectus), common reed (Phragmites), and various sedges (Carex). Design considerations include hydraulic loading rate, retention time, media composition, plant species selection, and seasonal performance variation in cold climates. Well-designed constructed wetlands can also provide ancillary benefits including wildlife habitat, carbon sequestration, and aesthetic green space.