The potential of nature-based solutions is increasingly being recognised. Katharine Cross reviews practical experience of integrating these approaches into wastewater treatment.
Nature-based solutions (NbS), as defined by the International Union for the Conservation of Nature, are “actions to protect, sustainably manage and restore natural or modified ecosystems that address societal challenges effectively and adaptively, simultaneously providing human wellbeing and biodiversity benefits” (WCC-2016-Res-069-EN, Defining Nature-based Solutions). More specifically, NbS use plants, soil, bacteria, and other natural elements and processes to remove pollutants from wastewater; this reduces chemical use and saves energy. These approaches can treat wastewater in a sustainable, low-cost, low-impact manner, and can be designed to have a long performance life.
Use of NbS as part of wastewater management is not a new concept. For centuries, communities across the world have directly discharged wastewater to surface water, promoting the development of natural wetlands, due to biosolids and nutrient accumulation followed by the emergence of vegetation. These natural wetlands would reduce pollutant loads to receiving waters. As populations have increased and landscapes have changed, technologies for pollution control have become more finessed to treat higher loads of wastewater. This led to the increase in conventional wastewater treatment plants, which tend to be carried out as an ‘end of the pipe’ solution, using a combination of physical, chemical and biological processes and operations to remove solids, organic matter and, when needed, nutrients from wastewater to protect against contamination of downstream water bodies.
NbS can be integrated into these systems, although the treatment potential of NbS depends on a variety of factors. These include the type or combination of NbS used, the quantity and quality of water to be treated, and local conditions (climate, precipitation patterns, etc.). NbS such as treatment wetlands, ponds and lagoons, and soil infiltration process have been actively used as reliable and economically efficient wastewater treatment technology for decades. Innovative approaches for applying NbS to treat wastewater are growing, such as living walls, green roofs and willow systems.
There is a well-established evidence base in science, and practice, that demonstrates the effectiveness and efficiency of NbS – see, for example, ‘Treatment Wetlands’, ‘Waste Stabilisation Ponds’, ‘Wetland Technology’. What is important to emphasise when considering such systems is not only the ability to deliver the primary functions of treating wastewater, but also possible co-benefits that can generate greater overall societal advantages. These benefits include improvement of water quality, as well as benefits beyond this for people and nature, such as: increasing biodiversity; providing social co-benefits, such as recreational areas and wellbeing through green spaces; improving urban microclimates; flood and storm peak mitigation; biomass production; and enabling water reuse.
NbS technologies and case studies
Easily accessible technical information and case studies for using NbS as part of wastewater treatment can provide a foundation for wastewater utility managers and operators, local governments and municipalities, and regulators to consider cost-effective NbS options. Some examples of well-established and more recently developed NbS approaches with cases are highlighted below, and further information can be found in the book Nature-Based Solutions for Wastewater Treatment (see below). A key message across the examples is that the planning and design of wastewater systems needs to take into account the numerous co-benefits of NbS beyond treatment.
Slow-rate soil infiltration is used as a way to treat primary or secondary wastewater through controlled application to a vegetated land surface, providing treatment as well as irrigation to agricultural fields, pastures or forest lands. Wastewater infiltrates into the soil, and may percolate to the native groundwater or to underdrains or wells for water recovery and reuse of the effluent. In the case of Lubbock, Texas, USA, slow-rate soil infiltration started in the 1930s. As the city grew, the Lubbock Land Application System was expanded to avoid groundwater contamination. The application of municipal wastewater to agricultural lands in Lubbock has been demonstrated as a cost-effective treatment method, resulting in increased water conservation by reducing the demand on freshwater resources from surface water and groundwater use. This improves climate resilience in the face of increasing water shortages because of climate change, alongside growing demand.
Another case study highlighting NbS benefits beyond pollution control is of a vertical-flow treatment wetlands in Shenzhen, China, which was designed as a polishing step to meet environmental standards. Treated wastewater was intermittently loaded on the surface of the filter to percolate vertically through it. As wastewater is applied, air re-enters the pores and aerates the filter to enhance aerobic degradation processes. The effluent from the treatment wetland serves as an additional source of water into the Pingshan River and improves the water quality. This low-cost option has the added benefit of providing a green recreational area for the residents of Shenzhen. The wetlands provide habitat for plants and animals along the Pingshan River, and increase the biodiversity in the area, and are expected to help regulate floods, control stormwater, and provide regulation of carbon sequestration.
The application of newer NbS technologies, such as green walls and green roofs in dense urban areas, are used to treat greywater, which can then be reused for other purposes, such as irrigation and toilet flushing. There are many co-benefits, including mitigating run-off, heat mitigation, building insulation, and improving aesthetics in urban environments. A demonstration of an application combining a green roof and treatment wetland was in Tilburg, in the Netherlands, which had the aim of reusing wastewater for toilet flushing. The construction provided a green space for treating domestic wastewater locally and promoting water reuse, and transformed the roof into an ecosystem to support biodiversity. The system was also found to balance the temperature of the building, which reduced the costs of air conditioning, and helped to reduce the heat island effect in its surroundings. Furthermore, the green roof system contributed to slowing rainfall run-off, which can reduce urban flooding and pressures on the drainage system.
An example of application of a green wall (or living wall) was in Marina di Ragusa, Italy. The system collected greywater from bathroom showers. This greywater was treated through the living wall, which is a system of modules with plants that are set up to allow the filtration and biological treatment of the water. The water was then reused for irrigation and toilet flushing. The treated greywater was successfully reused throughout the tourist summer season of 2018, highlighting the system’s capacity for proper treatment efficiency for reuse purposes. The low surface area requirements also make this option economically viable for water reuse and efficiency measures, while creating a hotspot of biodiversity in the urban environment. Evapotranspiration by plants within the living walls supports the reduction of the urban heat island effect, which is particularly relevant for a beach resort in the summer season, and the system enhanced local aesthetics, promoting a green and sustainable image of the resort.
Another, newer NbS approach relevant for peri-urban or rural settings is the use of willow systems, which are treatment wetlands dominated by willows. They are used for onsite wastewater treatment and reuse by production of woody biomass. They are designed to treat all inflow water through evapotranspiration and there is little to no outflow from the system, meaning that this can be a zero-discharge system. There is very little impact on the surrounding environment, with a fully circular operation, with uptake of nutrients and binding of carbon in the willows. The system can produce a significant amount of biomass that can be used for energy purposes, as well as soil amendment. In addition, there are the benefits of providing habitat for flora and fauna, and flood mitigation.
An example of successful application is on Zaeland Island, Denmark, where a zero-discharge willow system has been in operation since 2017, treating wastewater for a community of 190-250 person equivalents. There is little impact on the surrounding environment, as there is no discharge, and it is a fully circular operation, with uptake of nutrients and binding of carbon in the willows. An important part of the maintenance of a willow facility is the harvesting of the willows. The biomass is harvested in a three- or four-year rotation and grows again from the leftover root stem.
Selecting a NbS for wastewater treatment
It can be challenging to determine which NbS to use for wastewater treatment. It is often difficult for wastewater utility managers to know how best to combine traditional infrastructure, such as a wastewater treatment plant, with natural solutions, such as wetlands. To help navigate options, a web-based tool is under development, and includes different NbS technologies, as well as case studies from across the world, to help communities learn more and identify which NbS might be suitable for treating wastewater. This includes information on the co-benefits, inputs needed in terms of manpower and skills, and level of biohazard. It builds on an extensive evidence base from the scientific literature and case studies. For details, see: https://multisource.eu/.
Such tools can raise awareness and deepen understanding of the viable options and requirements for using NbS. However, wastewater operators should use further technical guidance and expertise to select the best NbS or combination of NbS, which can then be designed for their context. Application of NbS is context specific and needs to be designed and implemented to meet local conditions and needs, while also carefully considering any trade-offs.
Integrating NbS into policy and action
NbS are being increasingly integrated into policy and practice as a solution for wastewater and water management and a way to achieve climate action. For example, within the European Commission, NbS have formed part of the Horizon 2020 programme, a financial instrument with the goal of ensuring Europe produces world-class science, removing barriers to innovation and making it easier for the public and private sectors to work together in delivering innovation.
Horizon 2020 has promoted the alignment of biodiversity and ecosystem services with goals of innovation for growth and job creation. Another example is the World Bank, which has integrated NbS into more than 100 projects across 60 countries, and has developed its knowledge base to support the uptake of NbS to manage disasters and water resources.
Looking ahead, according to a recent UN working paper (Smart, Sustainable and Resilient cities: the Power of Nature-based Solutions), three quarters of 2050’s infrastructure doesn’t yet exist, which provides an opportunity to explore how to include nature in its design. COVID-19 recovery plans offer an opportunity to generate economic growth in a way that protects and restores the natural environment and addresses climate change, as well as promoting the energy transition and supporting livelihoods. Coupled with the need for infrastructure, it is an opportune time to build back better in a way that promotes, protects and integrates nature. In addition, NbS can support mitigation and adaptation objectives of the Paris Agreement on Climate, which acknowledges the interlinkages between resilience of communities, livelihoods and ecosystems. NbS are also fundamental to meeting the Sustainable Development Goals, especially SDG6 (Water), SDG13 (Climate Change), SDG14 (Oceans), SDG2 (Zero hunger) and SDG15 (Ecosystems).
This growing commitment and interest across the policy landscape means there is more support for implementation, which can build on a solid evidence base from ongoing experience across the wastewater and water sectors. Learning and knowledge exchange is an important component that can help managers – from wastewater utilities to municipal governments to regulators – better build a business case, including the multiple benefits of integrating NbS as assets into water and wastewater treatment processes. IWA has been a platform supporting such exchange and is continuing do so through a recently launched working group. •
Available from IWA Publishing:
Nature-Based Solutions for Wastewater Treatment doi.org/10.2166/9781789062267
Treatment Wetlands iwaponline.com/ebooks/book/330/Treatment-Wetlands
Waste Stabilisation Ponds iwaponline.com/ebooks/book/83/Waste-Stabilisation-Ponds
Wetland Technology www.iwapublishing.com/books/9781789060164/wetland-technology-practical-information-design-and-application-treatment
Smart, Sustainable and Resilient cities: the Power of Nature-based Solutions wedocs.unep.org/handle/20.500.11822/36586