Wetlands: constructed

Te Rere I Maniatutu – the Bay of Plenty wetland constructed by kiwifruit company Baygold. Photo by Les Anstis.

Constructed wetlands are hybrid systems that use an engineered approach to work with vegetation, soil, and organisms (Scholz, 2023). Constructed wetlands mimic the natural ability of wetlands to act as a sponge to manage stormwater in urban contexts and can also be used to filter and clean water. In peri-urban and urban locations constructed wetlands could help with flood prevention, wastewater treatment and contaminated water run-off (Agaton & Guila, 2023).

Various types of constructed wetlands exist, such as free water surface flow, horizontal flow, vertical flow, hybrid, aerated, anaerobic flow or evapotranspiration beds (Agaton & Guila, 2023; Reis et al., 2023). 

Household waste water can be separated roughly into two categories, highly polluted black water, e.g. water from toilet flushes, and grey water, e.g. water from showers or sinks. Grey water carries less nutrients, pathogens and organics and is therefore easier to treat. To treat black water with wetlands a pre-treatment step such as a pond or septic tank has to be taken. Therefore, it can be beneficial to keep these waste streams separate. The water is treated by moving through the wetland, so it is ideally built on a slight angle. However, the flow time and other specifics will depend on the available space and water influx. Vertical flow wetlands combined with ultraviolet radiation and chlorine disinfection results in the most stable water treatment. (Arden & Ma, 2018). Living Machines and Restorers are specific types of water treatment constructed wetlands. 

A wetland does require horizontal space. When horizontal space is not available, a vertical green wall filtration system could be considered as an alternative for water treatment. The specific choice of plants, and horizontal or vertical system will depend on the local context.

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Name of NbS

Wetlands: constructed

Type of NbS

 Hybrid living / engineered interventions

Location

Constructed wetlands could work in urban, periurban and rural places in all climatic zones, because wetlands occur naturally everywhere (Wetlands International South-Asia, n.d.)  

Constructed wetland. Copyright: Courtesy of Green Growth Group Mexico SA de CV.

Relationship to Indigenous knowledge

Wetlands are hugely valued by Indigenous peoples across Te Moananui Oceania. In Aotearoa New Zealand for example, Māori find spiritual and cultural significance in wetlands (Clarkson et al., 2013).  This is due to being an important source of food and resources (Clarkson et al., 2013). Māori are responsible for protecting the mauri (life force) of the environment and in exchange receive the fruits of the earth (Forster, 2012). Therefore, Māori have always employed extensive waste management systems (Pauling & Ataria, 2010). Human waste is considered tapu (forbidden, spiritually unsafe) and discharge of human waste directly into freshwater sources harms its mauri. Wastewater can only become noa (unrestricted, safe, good) through contact with Papatūānuku, The Earth Mother. (Simmonds et al., 2019). Therefore, Maori object to any wastewater directly entering bodies of water, even if it has been pre-treated (Bradley, 2012).

Climate change benefits
  • Reduced water quality
  • Loss of food production
  • Increased temperatures
  • freshwater flooding
  • Soil erosion
  • Reduced fresh-water availability

Wetlands have the highest carbon density in comparison to other terrestrial ecosystems giving them benefits that include contributing to both carbon cycles and reducing climate change (Salimi & Scholz, 2021). Wetlands absorb carbon dioxide and capture it within the sediment acting as a large carbon sink (Kayranli et al., 2010). Due to high productivity, high water tables, and low decomposition rates this allows wetlands to store carbon through the soil, and the captured sediment (Kayranli et al., 2010).

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Societal / socio-cultural benefits
  • Food security and quality
  • Waste management and hygiene
  • water security and quality

One of the main focuses of constructed wetlands is to easily remove degradable pollutants as well as toxic chemicals in the landscape, thus contributing to human health and resilience (Bhamidimarri, R. et al. 1991). Along with this, they are able to retain a high-water table when areas are prone to flooding.

Constructed wetlands create multiple social benefits including amenity areas, places for fish habitat, treatment of point source waste, and the filtering of pollutants (Everard et al., 2012). They are often part of a network of water-sensitive design systems that create a network of areas to facilitate social spaces in the urban environment (Metcalfe et al., 2017). 

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Ecological and biodiversity benefits
  • disturbance prevention
  • food production
  • habitat provision
  • medicinal resources
  • nutrient cycling

Constructed wetlands provide various ecosystem services that regulate, provide and support ecological and biodiversity benefits (Agaton & Guila, 2023). Planting can be used to create biodiversity in a landscape and therefore encourage wildlife species into the ecosystem. Wetlands are strongholds of biodiversity being rich in both flora and fauna (Clarkson et al. 2013).

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Diagram of vertical flow constructed wetlands, used for wastewater treatment. Image by: Eawag: Swiss Federal Institute of Aquatic Science and Technology Technical drawings: designport, Paolo Monaco, Zurich. (2014)  

Technical requirements

To determine the specifics of a constructed wetland, it is important to consider the local climate and the main objective, e.g. biodiversity, flood prevention, or waste water treatment. There are minimim sizes for constructed wetlands. This is due to the amount of space needed for water to achieve maximum filtration efficiency. The more land area it has the more effective the constructed wetland is often (Parde, 2021). Both Arden & Ma (2018) and Cross et al. (2021) give good overviews of technical requirements of wetlands for treatment of wastewater.

Issues and Barriers

There are a few disadvantages when it comes to constructed wetlands. The main one is, the vegetation used in constructed wetlands has higher biomass so regular harvesting is needed to ensure optimal removal efficiency. This increases the maintenance costs in the long term (Parde et al. 2021). The soil must have a low clay content; salinity of discharge can cause clay ground to collapse (Simmonds et al., 2019).

People often do not know how to take good care of their water treatment systems, leading to potential decrease in water quality. (Leonard et al., 2016). And wetlands are not always consistent in the level of treatment due to the many biological interactions (Arden & Ma, 2018). Therefore, it is important to carefully design and maintain wetlands used for wastewater treatment. An extra disinfection step, such as chlorination or UV-radiation could be applied to ensure clean water.

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Opportunities

Constructed wetlands have the advantage of being able to be used for various types of wastewater. This means there is an opportunity in Te Moananui Oceania to be able to use constructed wetlands to filter water runoff from agricultural landscapes which in some parts of the region is highly damaging to water ways (Parde et al. 2021). 

In rural and remote communities, constructed wetlands could be used to locally treat domestic wastewater.

Financial case

Typically low operation and maintenance costs make constructed wetlands affordable both in the short term and long term (Cross et al. 2021; Parde et al. 2021; Agaton & Guila, 2023). They end to not require expensive machinery and chemicals and provide, as described multiple potential co-benefits.

References
  • Agaton, C. B., Marie, P., & Guila, C. (2023). Ecosystem Services Valuation of Constructed Wetland as a Nature-Based Solution to Wastewater Treatment. Earth 2023, Vol. 4, Pages 78-92, 4(1), 78–92. https://doi.org/10.3390/EARTH4010006
  • Arden, S., & Ma, X. (2018). Constructed wetlands for greywater recycle and reuse: A review. Science of The Total Environment, 630, 587–599. https://doi.org/10.1016/J.SCITOTENV.2018.02.218
  • Bhamidimarri, R. et al. (1991). Constructed wetlands for wastewater treatment: The New Zealand experience. Water science and technology, London, Vol. 24, Iss. 5, pp 247 – 253. 
  • Bradley, J. (2012). Maori cultural considerations in developing and operating wastewater systems-Case history experiences. Wellington: MWH New Zealand Ltd..
  • Cross, K., Tondera, K., Rizzo, A., Andrews, L., Pucher, B., Istenič, D., Karres, N., & Mcdonald, R. (Eds.). (2021). Nature-Based Solutions for Wastewater Treatment: A series of factsheets and case studies. IWA Publishing. https://iwaponline.com/ebooks/book/834/Nature-Based-Solutions-for-Wastewater-TreatmentA
  • Everard, M., Harrington, R., & McInnes, R. J. (2012). Facilitating implementation of landscape-scale water management: The integrated constructed wetland concept. Ecosystem Services2, 27-37.
  • Forster, M.E. (2012). “Hei Whenua Papatipu: Kaitiakitanga and the politics of enhancing the Mauri of the wetlands”. (Master’s thesis)
  • Kayranli et al. (2010). Carbon storage and fluxes within freshwater wetlands: a critical review. Wetlands, 30, pp. 111 -124. https://doi.org/10.1007/s13157-009-0003-4
  • Leonard, M., Gilpin, B., Robson, B., & Wall, K. (2016). Field study of the composition of greywater and comparison of microbiological indicators of water quality in on-site systems. https://doi.org/10.1007/s10661-016-5442-9
  • Parde, D., Patwa, A., Shukla, A., Vijay, R., Killedar, D. J., & Kumar, R. (2021). A review of constructed wetlands on type, treatment and technology of wastewater. Environmental Technology & Innovation21, 101261.
  • Pauling, C., & Ataria, J. (2010). Tiaki Para.  A Study of Ngai Tahu Values and Issues Regarding Waste. Manaaki Whenua Press, Landcare Research.
  • Reis, M. C. G., Borges, A. C., Cunha, F. F. da, & Silva, R. R. da. (2023). Evapotranspiration beds as a zero-discharge nature-based solution for wastewater disposal: A review. Ecological Engineering, 189, 106896. https://doi.org/10.1016/J.ECOLENG.2023.106896
  • Salimi, S. & Scholz, M. (2021). Impact of climate change on wetland ecosystems: A critical review of experimental wetlands. Journal of Environmental Management, vol. 286. 
  • Scholz, M. (2023). Wetlands for water pollution control. Elsevier.
  • Simmonds, K., Austin, D., & Madison, M. (2019). Cultural Drivers Toward Land Based Disposal And Applications Enabling This. Water New Zealand. Available online: https://www.waternz.org.nz/Resources/Article?Action=View&Article_id=1756. Date accessed 9 May, 2024.
  • The Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitat (The Convention on Wetlands) (1971). Ramsar, Iran.

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