Constructed wetlands are small artificial wastewater treatment systems consisting of one or more shallow treatment cells, with herbaceous vegetation that flourish in saturated or flooded cells. There are two basic types of Constructed wetlands, Free Water Surface Constructed wetlands (FWS) and Vegetated Submerged Bed Constructed wetlands (VSB).

Figure 6: Constructed wetlands

FWS wetlands have a combination of open water areas with some floating vegetation as well as emergent plants rooted in the soil bottom. They are usually more suitable to warmer climates, because biological decomposition rates are temperature dependent, decreasing with decreasing water temperature. In these systems wastewater is treated by the processes of sedimentation, filtration, digestion, oxidation, reduction, adsorption and precipitation.

VSB Constructed wetlands, also known as subsurface flow wetlands, Consist of gravel soil beds planted with wetland vegetation. These systems have many of the same features of the FWS but are distinguished by their subsurface hydraulic grade. Unlike the FWS wetland, the wastewater stays beneath the surface, flows in contact with the roots and rhizomes of the plants and is invisible or unavailable to animals. VSB Constructed wetlands are generally lower in cost and maintenance requirements than the FWS Constructed wetlands.