Treatment wetlands are treatment systems that replicate the physical, biological and chemical treatment processes occurring in natural wetlands. They are designed to enhance biological, physical and chemical treatment processes found in natural wetlands to remove fine sediments, nutrients and other pollutants (e.g. pesticides, heavy metals). Used extensively to treat primary and secondary municipal sewage, landfill leachate, urban stormwater run-off and industrial wastewaters, treatment wetlands are equally suited to improving agricultural water quality (stormwater run-off and/or irrigation tailwater).
Treatment wetlands work best when water flow is regulated into and through the wetland, to ensure sufficient retention time and to maximise water contact with macrophytes and wetland soils while minimising the risk of vegetation disturbance or resuspension of pollutants. Particulate matter, nutrients, pesticides and any other pollutants in agricultural run-off are removed through:
enhanced sedimentation of particles
adsorption (attachment) of particles to soil and organic matter and subsequent storage in the wetland substrate
nitrification/denitrification and volatilisation converting nutrients and pesticides to gaseous forms
Treatment wetlands are often one of the last elements in a treatment train, to remove fine sediments, nutrients and some pesticides. Minimising the coarse sediment loading in treatment wetlands, through the use of sediment basins and vegetated buffers and swales is essential to prevent smothering of vegetation and wetland damage. It is critical that farm best management practices are implemented in the paddock as excess pollutants can damage or impact the efficacy of treatment wetlands.
Treatment wetlands are generally around 0.3-1m deep with over 80% of the area vegetated with macrophytes. A treatment wetland will typically include (Figure 1):
a sediment basin, to remove coarse and medium-sized sediments (>125μm)
an inlet zone connecting the sediment basin to the macrophyte zone, designed to allow only the design flows into the macrophyte zone
macrophyte zone, with dense vegetation (i.e. reeds and sedges) creating the environment necessary for water treatment
outlet zone, to regulate outflow and the water level in the treatment wetland
high-flow bypass, to direct excess water, above the design flows, away from the macrophyte zone.
The macrophyte zone is designed to facilitate enhanced sedimentation of fine particles (125µm) and allow the biological and chemical removal of nutrients and some pesticides. The role of wetland vegetation is to:
Slow water flow to facilitate sedimentation and adsorption of fine sediments, and any associated nutrients and/or pesticides.
Provide a surface for epiphytic biofilms, a thin but robust layer of mucilage adhering to a solid surface (such as stems or roots) and containing a community of bacteria and other microorganisms which are responsible for most of the microbial processing in a wetland. Reeds and sedges are recommended for the macrophyte zone as they have many small stems/leaves in contact with the water, maximising the surface area for biofilm growth.
Directly take up dissolved nutrients.
Trap and contain pollutants accumulated in the sediment to reduce the widespread resuspension of trapped sediments.
Provide a carbon source to support denitrification which is the dominant nitrogen removal process in wetlands.
Promote even water flow through the wetland (see design requirements)
Decrease erosion by reducing wave action and flow velocities (speeds) while binding soil particles with their root systems to minimise resuspension.
Provide shade and reduce light availability for algal photosynthesis and aquatic weed growth.
The regular wetting and drying of the macrophyte zone sediments progressively leads to improved fixation of pollutants in the sediments and reduces the likelihood of reversal and loss of pollutants. Wetland vegetation also inhibits the release of nutrients from the sediments by pumping oxygen into the soils.
Water treatment (fine sediments, nutrients, pesticides)
Habitat (needs to be carefully planned and may not be encouraged)
Water reuse (irrigation)
The information provided in the following pages is specific to enhancing the water treatment capacity of treatment wetlands. Changes to the design to provide habitat or amenity may reduce the treatment effectiveness of the treatment wetland. For information on other constructing or restoring wetlands for other purposes refer to wetland restoration.
In addition to the standard disclaimer located at the bottom of the page, please note the content presented is based on published knowledge of treatment systems. Many of the treatment systems described have not been trialled in different regions or land uses in Queensland. The information will be updated as new trials are conducted and monitored. If you have any additional information on treatment systems or suggestions for additional technologies please contact us using the feedback link at the bottom of this page.
^ Bachand, PAM & Horne, AJ 2000, 'Denitrification in constructed free-water surface wetlands: II. Effects of vegetation and temperature', Ecological Engineering, vol. 14, p. 17-32.