Constructed wetlands (CWs - also called “reed beds”) are natural wastewater treatment processes. They are shallow basins filled with filter material (substrate), usually sand or gravel, and planted with locally relevant, or ornamental, wetland vegetation tolerant of saturated conditions (most often reed, reed canary grass, and cattail, iris, reed sweet grass). Wastewater is introduced into the basin and flows over the surface or through the substrate, and is discharged out of the basin through a structure which controls the depth of the wastewater in the wetland (Rozkošný et al., 2014; UN-HABITAT, 2008).
CWs represent the biological treatment stage (secondary and/or tertiary) of WWTPs, based on slow filtration of pretreated wastewater. They may also be used for tertiary treatment of effluent from mechanical-biological treatment plants. Types of CWs treating raw wastewater (without sedimentation pretreatment) also exist (French systems), however they operate in a different mode (Rozkošný et al., 2014).
The basic principle of CW treatment is the flow of wastewater through the filtration system. Pollutants are removed by several complex physical, chemical and biological processes. Vegetation plays a vital role in the treatment, as it provides surfaces and a suitable environment for microbial growth and filtration. The filter environment must fulfil the pre-defined requirements in terms of hydraulic conductivity and load of wastewater by pollution, flow rate, frost penetration, or the possibility to bind phosphorus and heavy metals. Filter material must be permeable enough to avoid clogging and subsequent surface flow. (Rozkošný et al., 2004; UN-HABITAT, 2008).
Pollutant Removal Mechanisms in CWs
(Source: UN-HABITAT, 2008)
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