Constructed Wetlands

Concept

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).

Small Constructed Wetland Treatment Plant (Source: Rozkošný et al., 2014)

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).

Types There are three main types of CWs (US EPA, 1995):
  • Surface flow wetlands – SF,
  • Subsurface flow wetlands – SSF, and
  • Hybrid systems (combine different types of CW).
Surface Flow Constructed Wetland (Source: US EPA 1995) Surface Flow Constructed Wetland (Source: US EPA 1995)
Subsurface Flow Constructed Wetland (Source: US EPA 1995) Subsurface Flow Constructed Wetland (Source: US EPA 1995)
SSF CWs are divided in two types according to flow direction:
  • SSF CWs of horizontal flow, and
  • SSF CWs of vertical flow.
(a)
(b)
Schematic cross-section of vertical (a) and horizontal (b) flow SSF CWs (Source: UN-HABITAT, 2008)
Function

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)

Pollutant Removal Mechanisms in CWs

(Source: UN-HABITAT, 2008)

WASTEWATR CONSTITUENTS

REMOVAL MECHANISMS

Suspended Solids

  • Sedimentation
  • Filtration

Soluble organics

  • Aerobic microbial degradation
  • Anaerobic microbial degradation

Phosphorous

  • Matrix sorption
  • Plant uptake

Nitrogen

  • Ammonification followed by microbial nitrification
  • Denitrification
  • Plant uptake
  • Matrix adsorption
  • Ammonia volatilization (mostly in SF CW)

Metals

  • Adsorption and cation exchange
  • Complexation
  • Precipitation
  • Plant uptake
  • Microbial Oxidation /reduction

Pathogens

  • Sedimentation
  • Filtration
  • Natural die – off
  • Predation
  • UV irradiation (SF system)
  • Excretion of antibiotics from roots of macrophytes
References & Sources for Further Reading
  • Cooper, P. F., Job, G. D., Green, M. B. & Shutes, R. B. E. (1996). Reed Beds and Constructed Wetlands for Wastewater Treatment. Marlow, UK: WRc Publications
  • Rozkošný, M., Kriška, M., Šálek, J., Bodík, I., & Istenič, D. (2014). Natural Technologies of Wastewater Treatment. Global Water Partnership Central and Eastern Europe
  • UN-HABITAT. (2008). Constructed Wetlands Manual. UN-HABITAT Water for Asian Cities Programme Nepal, Kathmandu
  • US EPA. (1995). A Handbook of Constructed Wetlands (vol.1). Washington, D.C: USDA-Natural Resources Conservation Service/US EPA-Region III/ Pennsylvania Department of Natural Resources

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