A treatment technology for wastewater that aims to replicate the naturally occurring processes in wetlands. Constructed shallow ponds are lined and filled with some sort of filter material (substrate), usually sand, gravel, rock or soil, and planted with vegetation tolerant of saturated conditions (e.g. reeds). As wastewater flows through the ponds, the filter material filters out particles and microorganisms degrade the organics. See also free-water surface constructed wetland, horizontal subsurface flow constructed wetland, Vertical flow constructed wetland.