Treatment wetlands

Treatment wetlands — Key Considerations

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• Key Considerations
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What makes an effective treatment wetland?

• Macrophyte zone with dense vegetation covering at least 50%, but ideally around 80% of the wetland area[5][2].
• A small amount of open water (i.e. around 20% of wetland area)[5] to minimise the risk of water stagnation and ensure sufficient dissolved oxygen for the nitrification/denitrification process.
• Sediment basin to remove coarse to medium sediments (e.g. >125 µm), prior to the water entering the treatment wetland[5].
• High-flow bypass for excess flows to bypass the macrophyte zone to avoid erosion or damage to the macrophyte zone or resuspension of pollutants.
• Even bathymetry in the macrophyte zone to ensure uniform flows through the macrophyte zone, avoid short-circuiting and provide the necessary conditions for macrophyte growth[4].
• Correct depth of the macrophyte zone suited to the climate and the species of wetland plants to be established. Macrophytes are sensitive to water depth and will die if water is too deep for too long. Therefore the depth should not be more than half the plant height for more than 20% of the time[3].
• Elongated shape with a length to width ratio at least 3:1 to maximise hydraulic efficiency (i.e. the even distribution of water energy flow within the wetland to promote effective treatment processes)[2][5][4]. Baffles or levees can be used within wetlands to slow flows and increase retention time.
• Retention time of water within the wetland determined by the target pollutant reduction and the site hydrology. A detention time of at least 48 hours for 90% of the time is generally recommended for urban stormwater treatment[5]. Shorter detention times (e.g. 24 hours) may be suitable in tropical environments in order to treat larger volumes of run-off, particularly if pollutant concentrations are relatively low. The quality of water treatment won't be as high, but the overall pollutant load reduction may be greater if more water is treated. Increasing the size of the wetland; decreasing the amount of water entering the wetland (hydraulic load); and installing baffles or bunds within the wetland to increase the time taken for water to move through the wetland, are options to increase detention time.

Treatment processes

Suitability and Limitations

Treatment wetlands are suitable for and have been tested in a range of agricultural situations:

• Sugarcane (irrigated or rain fed)
• intensive horticulture (including bananas)
• irrigated cotton and grain
• production nurseries
• dairy (though solids separators are the preferred method of treating and reusing dairy effluent)
• aquaculture wastewater, including modified designs containing marine plants for treating discharge from land-based marine aquaculture[1].

They are suited to a variety of locations/climatic conditions including wet tropics and drier, irrigated production areas. However, elements of the design (e.g. depth) will differ depending on the local climate.

References

1. ^ Buhmann, A & Papenbrock, J (August 2013), 'Biofiltering of aquaculture effluents by halophytic plants: Basic principles, current uses and future perspectives', Environmental and Experimental Botany. [online], vol. 92, pp. 122-133. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0098847212001566 [Accessed 27 April 2022].
2. ^ a b Kavehei, E, Hasan, S, Wegscheidl, C, Griffiths, M, Smart, JCR, Bueno, C, Owen, L, Akrami, K, Shepherd, M, Lowe, S & Adame, MF (22 November 2021), 'Cost-Effectiveness of Treatment Wetlands for Nitrogen Removal in Tropical and Subtropical Australia', Water. [online], vol. 13, no. 22, p. 3309. Available at: https://www.mdpi.com/2073-4441/13/22/3309 [Accessed 21 December 2021].
3. ^ Melbourne Water (2017), Wetland Design Manual, Melbourne Water, Melbourne.
4. ^ a b Persson, J, Somes, NLG & Wong, THF (1999), 'Hydraulics efficiency of constructed wetlands and ponds', Water Science and Technology, vol. 40, no. 3, pp. 291-300.
5. ^ a b c d e Water by Design (2017), Wetland Technical Design Guideline. [online], Water by Design, Brisbane. Available at: https://waterbydesign.com.au/.

Last updated: 27 April 2022