Treatment wetlands

Treatment wetlands

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• Overview
• Key Considerations
• Planning and design
• Construction and operation
• Links and references

Other name/s

Constructed wetlands, landscape wetlands, embellished wetlands, surface flow wetlands, free-water wetlands

Note this does not include sub-surface flow or vertical wetlands. These systems require a gravel or rock medium, which is unlikely to be cost-effective in treating the volumes of water required in many Queensland agricultural systems.

Description

Treatment wetlands are engineered systems that replicate and enhance the physical, biological and chemical treatment processes occurring in natural wetlands to remove fine sediments, nutrients and other pollutants (e.g. pesticides, heavy metals)[9][4]. They differ from restored or natural wetlands in that they are designed and managed primarily to improve water quality. Although treatment wetlands can be designed to provide other co-benefits in addition to water treatment, such as aesthetics and wildlife habitat. To create or rehabilitate wetlands for other purposes or ecosystem services, refer to aquatic ecosystem rehabilitation.

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 water quality in agriculture and aquaculture[4][3]. Treatment wetlands are generally designed to intercept surface flows (rainfall or irrigation run-off or wastewater) and can also be effective at treating shallow groundwater[7]. They can be formed through extensive earthworks to create a relatively uniform bathymetry or through relatively minor earthworks and hydrological modifications to retain water and support conditions conducive to macrophyte growth and water treatment (i.e. landscape wetlands) (Figure 1).

Particulate matter, nutrients, and toxicants 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 toxicants to gaseous forms
• uptake (absorption) by vegetation[4].

Treatment wetlands are often one of the last elements in a treatment train, to remove fine sediments, nutrients and some toxicants. It is essential to minimise the amount of coarse sediment entering treatment wetlands, through best management practices, sediment basins, vegetated drains and vegetated buffers and swales to prevent smothering of vegetation, damage and costly maintenance.

Case study: Treatment train approach on a banana farm

Treatment wetlands are generally around 0.3-1m deep with over 50% (ideally 80%) of the area vegetated with macrophytes (e.g. reeds and sedges)[5]. A treatment wetland will typically include (Figure 2):

• a sediment basin, to remove coarse and medium-sized sediments (>125μm)[9]
• 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 fine sediment and nutrient removal
• outlet zone, to regulate outflow and the water level
• high-flow bypass, to direct excess water, above the design flows, away from the macrophyte zone.

The macrophyte zone is the critical part of a treatment wetland. Without dense vegetation covering at least half of the wetland surface, wetlands will act more like a sediment basin (i.e. removing sediment and some particulate nutrients). Dense vegetation provides the environment for nitrification/denitrification, to remove nitrogen, specifically nitrate[7]. The macrophyte zone is designed to also facilitate enhanced sedimentation of fine particles (<125µm) and the biological and chemical removal of nutrients and some pesticides. Vegetation is important to:

• Slow water flow to facilitate sedimentation and adsorption of fine sediments, and any associated nutrients and/or pesticides. The drag caused by vegetation helps lower dissolved oxygen particularly near the base of the wetland, providing ideal conditions for denitrification.
• Provide carbon and oxygen in the soils, which promotes nitrification/denitrification, the dominant nitrogen removal process in wetlands[2]. The highest denitrification rates occur in carbon-rich wetland soils associated with vegetation[1].
• Provide a surface for epiphytic biofilms, containing a community of bacteria and other microorganisms which are responsible some nitrogen removal through denitrification[4][1]. 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 (absorb).
• Trap and contain pollutants accumulated in the sediment to reduce the widespread resuspension of trapped sediments.
• Promote even water flow through the wetland (see design section)
• Decrease erosion by reducing wave action and flow velocities (speeds) while binding soil particles with their root systems to minimise resuspension.
• Dense reeds and sedges could minimise the risk of aquatic weeds and algal blooms.
• 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[8].

Note that treatment performance is compromised if vegetation growth is excessive, for instance if the treatment wetland is overgrown with weeds. In this case, water stagnates with very low or no dissolved oxygen, limiting nitrification and in turn denitrification[7]. These conditions can lead to high dissolved organic nitrogen or ammonium export and even methane and nitrous oxide emissions[7][6]. Hence vegetation management is important, refer to the establishment and maintenance sections for more information.

Effectiveness as a treatment system

Well-located, designed and managed treatment wetlands in Queensland can reduce dissolved inorganic nitrogen (DIN) by approximately 80%[5]. Effective treatment wetlands have the following conditions that favour nitrification/denitrification. These are potential indicators of wetland nitrogen removal performance[5][7]:

• relatively high DIN concentrations in the water to be treated (greater than 0.2mg/L)
• vegetation covering at least 50% of the wetland area
• carbon-rich, low-oxygen wetland soils with high oxidation-reduction potential (ORP) between -100 or 100mV
• dissolved oxygen concentrations between 40-70% in outflow water
• neutral soil pH values between 6 and 8
• elongated shape with length to width ratio of at least 3:1
• slow flow, less than 6m³/s
• low suspended solids in the water.

With these conditions, treatment wetlands in agricultural areas can remove a kilogram of DIN for less than $50[5]. This cost-effectiveness calculation includes design, project management, construction, plant establishment and maintenance costs.

The actual cost-effectiveness of a treatment wetland will vary for each individual site. The cost-effectiveness needs to be considered relative to other treatment systems or management intervention options. Refer to cost considerations for more information.

Services and benefits

• Water treatment (fine sediments, nutrients, pesticides)
• Habitat (needs to be carefully planned and may not always be desirable)
• Aesthetics
• Flood management
• Water reuse (irrigation or recirculation in aquaculture systems)

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 aquatic rehabilitation.

References

1. ^ a b Adame, MF, Waltham, NJ, Iram, N, Farahani, BS, Salinas, C, Burford, M & Ronan, M (8 July 2021), 'Denitrification within the sediments and epiphyton of tropical macrophyte stands', Inland Waters. [online], pp. 1-10. Available at: https://www.tandfonline.com/doi/full/10.1080/20442041.2021.1902214 [Accessed 31 July 2021].
2. ^ Bachand, PAM & Horne, AJ (2000), 'Denitrification in constructed free-water surface wetlands: II. Effects of vegetation and temperature', Ecological Engineering, vol. 14, pp. 17-32.
3. ^ Department of Employment, EDI (2011), Wetland Management Handbook: Farm Management Systems (FMS) guidelines for managing wetlands in intensive agriculture.. [online], Queensland Wetlands Program, Brisbane. Available at: https://wetlandinfo.des.qld.gov.au/resources/static/pdf/resources/reports/fms/fms_025_handbook_web.pdf.
4. ^ a b c d Kadlec, RH, Knight, RL, Vymazal, J, Brix, H, Cooper, P & Haberl, R (2000), Constructed wetlands for pollution control: processes, performance, design and operation, IWA Publishing, London.
5. ^ a b c d 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].
6. ^ Kavehei, E, Iram, N, Rezaei Rashti, M, Jenkins, GA, Lemckert, C & Adame, MF (January 2021), 'Greenhouse gas emissions from stormwater bioretention basins', Ecological Engineering. [online], vol. 159, p. 106120. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0925857420304080 [Accessed 27 April 2022].
7. ^ a b c d e Kavehei, E, Roberts, ME, Cadier, C, Griffiths, M, Argent, S, Hamilton, DP, Lu, J, Bayley, M & Adame, MF (November 2021), 'Nitrogen processing by treatment wetlands in a tropical catchment dominated by agricultural landuse', Marine Pollution Bulletin. [online], vol. 172, p. 112800. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0025326X21008341 [Accessed 27 April 2022].
8. ^ Melbourne Water (2017), Wetland Design Manual, Melbourne Water, Melbourne.
9. ^ a b Water by Design (2017), Wetland Technical Design Guideline. [online], Water by Design, Brisbane. Available at: https://waterbydesign.com.au/.

Last updated: 24 May 2022