A constructed wetland is an artificial wetland created for the purpose of treating anthropogenic discharge such as municipal or industrial wastewater, or stormwater runoff. It may also be created for land reclamation after mining, refineries, constructed wetlands are engineered systems that use natural functions of vegetation, soil, and organisms to treat different water streams. Depending on the type of wastewater that has to be treated the system has to be adjusted accordingly which means that pre- or post-treatments might be necessary. Constructed wetlands can be designed to emulate the features of natural wetlands, some constructed wetlands may also serve as a habitat for native and migratory wildlife, although that is usually not their main purpose. The two main types of constructed wetlands are subsurface flow and surface flow wetlands, the planted vegetation plays a role in contaminant removal but the filter bed, consisting usually of a combination of sand and gravel, has an equally important role to play. Many terms are used to denote constructed wetlands, such as reed beds, soil infiltration beds, constructed treatment wetlands, treatment wetlands, beside engineered wetlands, the terms of man-made or artificial wetlands are often found as well. A biofilter has some similarities with a wetland, but is usually without plants. However, the term of constructed wetlands can also be used to describe restored and recultivated land that was destroyed in the past through draining and converting into farmland, ponds for wastewater treatment or water purification are not considered as constructed wetlands. They are referred to as ponds or treatment ponds, respectively. A constructed wetland is a sequence of water bodies designed to filter and treat waterborne pollutants found in sewage. Constructed wetlands are used for treatment or for greywater treatment. They can be used after a tank for primary treatment. Some CW designs however do not use upfront primary treatment, vegetation in a wetland provides a substrate upon which microorganisms can grow as they break down organic materials. This community of microorganisms is known as the periphyton, the periphyton and natural chemical processes are responsible for approximately 90 percent of pollutant removal and waste breakdown. The plants remove about seven to ten percent of pollutants, different species of aquatic plants have different rates of heavy metal uptake, a consideration for plant selection in a constructed wetland used for water treatment. Constructed wetlands are of two types, subsurface flow and surface flow wetlands. Many regulatory agencies list treatment wetlands as one of their recommended best management practices for controlling urban runoff, in most cases, the bottom is lined with either a polymer geomembrane, concrete or clay in order to protect the water table and surrounding grounds. The substrate can be either gravel—generally limestone or pumice/volcanic rock, depending on local availability, subsurface flow wetlands can be further classified as horizontal flow and vertical flow constructed wetlands
Constructed wetland in an ecological settlement in Flintenbreite near Lübeck, Germany
Effluent from a constructed wetland for greywater treatment at an ecological housing estate in Hamburg-Allermoehe, Germany
Constructed wetland for domestic wastewater treatment in Bayawan City, the Philippines
Schematic of a free-water surface constructed wetland: It aims to replicate the naturally occurring processes, where particles settle, pathogens are destroyed, and organisms and plants utilize the nutrients.