Wetlands, permanently waterlogged areas populated by hydrophytic plants such as reeds, comprise a variety of sub-surface micro-habitats of differing oxygenation and redox potential. Constructed wetland systems are increasingly being employed for treatment of wastewater, sewage sludges and industrial effluents as a cost-effective, low energy and robust alternative to traditional engineered biological treatment such as the activated sludge process.
The natural ability of soils to filter suspended solids mechanically and chemical reactions (precipitation) with sewage constituents are other forms of cleaning mechanisms, which interact. Constructed wetlands are classified according to their mode of operation as free water surface-flow, horizontal flow, vertical downflow or vertical upflow type. They have been used successfully in the treatment of domestic sewage, urban, highway and stormwater runoff, acid mine drainage, agricultural wastewater and industrial effluents (including landfill leachate). BOD and solids reduction occurs through microbial activity and removal of nitrogen and phosphorus through the processes of filtration, denitrification, plant uptake and sorption.