Wetland-based treatment is continuing to evolve as a sustainable means of improving water quality. Gabriela Dotro, Carlos Arias and Günter Langergraber review progress, the focus of a recent IWA Scientific and Technical Report.
Water quality standards across the world are increasingly being focused on the promotion of healthier ecosystems and their associated services, including the protection of potable water sources, biodiversity, and ecological functions. This has applied to developed countries and to less industrialised nations, putting the challenge of developing wastewater treatment technologies that are suitable for both at the forefront of the development agenda.
Treatment wetlands have been used over the past three decades for water quality improvement of a variety of sources, including agricultural and urban runoff, industrial effluents, combined sewer overflows, and municipal wastewater. What makes wetlands particularly well suited for current and emerging challenges are their simplicity to run, their ability to cope with variable influent loads, and their potential for provision of additional benefits in the form of ecosystem services. The latter can include biodiversity, public education, buffering of flows and environmentally hazardous compounds, and provision of resources. Notably, wetlands can be – and have been – successfully implemented in developed and developing countries, so facilitating technology transfer across geographic, social and economic boundaries.
The development of wetland technology began in the late 1950s, with a new surge of applications and research since the start of the 21st century. Nowadays, treatment wetlands are a state-of-the-art technology applied worldwide for treating different types of wastewater, at scales ranging from single-household sewage systems up to several hundred hectares for industrial applications. The need for enhanced treated effluent qualities has led to a number of novel designs, from intensification strategies (see panels right on page 33) to entire flowsheets composed of wetland systems, thereby negating the need for sludge transport and associated infrastructure.
“Treatment wetlands can become part of a community rather than a technology that needs to be hidden”
Along with innovation came the globalisation of knowledge through modern technology and travel, which has facilitated the development of a more fundamental understanding of the science that enables wetland technology to cope with such a diverse array of applications.
New opportunities for wetland development and implementation are emerging, with the renewed focus on nature-based solutions for water and land management (see panel).
Nature-based water and wastewater management, blue-green spaces in futuristic urban visions, and international networking initiatives are highlighting the many opportunities for the use of treatment wetlands to deliver UN Sustainable Development Goals (SDGs) and a more balanced way of living with nature.
Conventional wastewater applications
The original application of treatment wetlands was for urban sewage. Treatment was initially believed to be primarily down to the plant species used. Professionals now know wetlands are complex ecosystems, in which a combination of physical, chemical and biological processes take place for the capture, storage and transformation of pollutants.
There are two broad categories of wetland systems, based on the location of the water table: free water surface (FWS) wetlands and subsurface flow (SSF) wetlands. The former are typically larger and mimic more closely natural wetlands, with high plant species diversity and areas of shallow and deep open-water zones. They are designed to enhance settling processes, with the biological transformation processes taking place at the water-sediment interface and in the area closer to the roots of the plants.
For SSF wetlands, filtration of particulates is achieved as the water flows vertically or horizontally through the porous media – typically sand or gravel – with the biochemical transformation of pollutants primarily performed by the biofilm growing on the media and plant root surfaces.
Both systems can be designed and operated to work under aerobic, anaerobic or a combination of conditions within the same treatment unit, offering opportunities for the removal of pollutants that require specific sequential steps for their biochemical transformation. The combination of more than one type of wetland system (for example, subsurface vertical flow followed by horizontal flow) for a given treatment flowsheet is referred to as a hybrid system.
In Europe, the technology has been primarily researched in terms of SSF modes, as these are more compact than FWS systems for a given mass flow. Systems are typically planted with a few species of emergent plants, and one of the species will become dominant after a few growing seasons. This type of wetland design, also referred to as a “reed bed”, can provide significant habitat for fauna, with recent research showing treatment wetlands have comparable biodiversity – in terms of stem-dwelling invertebrates, small mammals and moths – to similarly located natural wetland systems. When it comes to microbial communities, the systems are known to be highly diverse, both in the subsurface and in the accumulated sludge layer.
In parts of the world where land availability is less of an issue, FWS have predominated, with industrial applications at the forefront of implementation. The largest engineered treatment wetland in the world is the Nimr system in Oman, currently 360ha and due to be expanded by 120ha. As with many treatment wetland systems, the original intention was the delivery of water treatment, but opportunities are increasingly arising for these systems to contribute to the circular economy and additional provision of ecosystem services.
The need for new applications
The need to further integrate green spaces into urban agglomerations is well established worldwide. More recently, the benefits associated with blue-green infrastructure, such as treatment wetlands, are being quantified. This is evidenced in research, new scientific journals (for example, Blue-Green Systems, IWA Publishing), innovative town planning (such as flood risk management in Milton Keynes, UK), and public awareness generally. Treatment wetlands can include green roofs, green walls, and sustainable urban drainage systems, and can provide cooling effects in an urban environment. In addition, treatment wetlands can be designed and established to enable biodiversity and ecosystem connectivity in an otherwise fragmented landscape.
Productive wetland systems are a relatively new area of research. An example is the removal of phosphorus and metals, where the focus has shifted towards the recovery of those captured resources and their potential reuse. Reactive porous media is usually marketed as potentially reusable in agriculture once the media has exhausted its capacity to retain phosphorus. However, there are few studies currently published on the quality, rates of release, effectiveness, and longevity of the material when applied to different target crops. Metal-rich sediments can face limitations in terms of their disposal once treatment wetlands are dredged, typically after two decades of operation, for the removal of metals (for example, coal mining, industrial effluents, and chemically dosed sludge from sewage applications). New technologies, a better understanding of biogeochemical cycles, and policy drivers all support the case for assessing metal and phosphorus recovery applications, so enabling treatment wetlands to contribute to the circular economy and SDGs.
Another emerging area of work is the application of combined natural and engineered treatment flowsheets to enable water reuse (for example, the Horizon 2020-supported AquaNES project). Treatment wetlands have been coupled with energy-intensive technologies for tertiary polishing of effluents that can then be reused for non-potable applications. European projects have funded research into compact flowsheets that, coupled with renewable energy-powered disinfection, can provide effluents that are suitable for irrigation of crops and cleaning activities within the treatment plant (for example, the EU-supported SWINGS project).
As with all reuse applications, public perception and acceptance is key for implementation. Treatment wetlands have an advantage over grey technologies because they can be made aesthetically pleasing, and give multiple benefits (biodiversity, flow attenuation). With vision and social engagement, they can become an integral part of a community, acting as a “garden” feature rather than a technology that needs to be hidden from view. This also has the benefit of enabling the public to make a better link between individual actions (for example, what is poured down the sewer) and its impact on the environment.
Awareness of our individual environmental footprints is key to developing a more sustainable way of living, and treatment wetlands are ideally placed to help deliver this.
Nature-based solutions and their role in water and land management
Many recent initiatives have focused on developing nature-based solutions (NBS) for water and land management. In the UK, the emphasis has been on implementing a variety of configurations of wetlands for treating sewage, urban and rural runoff, industrial effluents, and natural flood management. At a workshop in November 2019, organised by Cranfield University, regulators, water utilities, international consulting agencies, SMEs that design and construct wetland systems, academics and environmental NGOs identified the skills gaps in the emerging market of NBS implementation. They prioritised topics, which were then clustered to form the base for new online development training courses on NBS. The first short course, developed by Cranfield University, will start in May 2020 (see www.cranfield.ac.uk/nbs). The group also plans to form a network on engineering NBS for water and land management, which will be linked to the IWA Task Group on NBS, the EU COST action on Circular Cities, and other initiatives.
Meeting phosphorus consents with reactive media
Conventional treatment wetlands are good at removing organics, solids and nitrogen, but very low phosphorus removal efficiencies are typically achieved after the sand or gravel media’s natural adsorption capacity is exhausted. A wealth of research has been conducted in the past 20 years to try to find an alternative reactive media that can sustainably achieve high removal rates of phosphorus from sewage. Sustainability in this context translates into media that: keeps the effluent pH at neutral levels; retains its structural integrity when placed in a full-scale system and is exposed to building machinery; and that is a by-product of or waste from another industry, so contributing to the circular economy, instead of resulting in increased mining of natural resources. There are multiple ongoing projects on the topic, with full-scale application of apatite, steel slag, and light expanded clay aggregates operating for a number of years throughout Europe. New research into engineered materials placed separately to the wetland system as stand-alone contactors is also under way.
New Scientific and Technical Report on Wetland Technology
The new IWA Scientific and Technical Report (STR) Wetland Technology: Practical information on the design and application of treatment wetlands (Langergraber et al, 2019) was launched at the IWA Water and Development Congress in Colombo, Sri Lanka, in December 2019. The STR was prepared by the IWA Task Group on “Mainstreaming the Use of Treatment Wetlands”, with contributions from more than 50 authors, and is available as an Open Access book. The STR has engineers focusing on wetland design as its main target group, and comprises practical, simple-to-use information on the design of treatment wetlands. It describes design considerations for 15 specific applications, and practical information for the design of 11 wetland types. Additionally, 10 case studies are presented. The content of the new STR builds upon the Open Access eBook Treatment wetlands (Dotro et al, 2017), which includes the fundamentals of wetland technology and is designed to be used in a biological wastewater treatment course at undergraduate level.
Dotro G, Langergraber G, Molle P, Nivala J, Puigagut J, Stein OR, Von Sperling M (2017): Treatment Wetlands, Biological Wastewater Treatment Series, Volume 7, IWA Publishing, London, UK, 172p. eISBN: 9781780408774; Open Access eBook from iwaponline.com/ebooks/book/330/Treatment-Wetlands
Langergraber G, Dotro G, Nivala J, Rizzo A, Stein OR (Eds, 2019): Wetland Technology – Practical information on the designing treatment wetlands. IWA Scientific and Technical Report No.27, IWA Publishing, London, UK, 190p. eISBN: 9781789060171. Open Access eBook from iwaponline.com/ebooks/book/780/Wetland-Technology-Practical-Information-on-the
Minimising treatment footprint by favouring the use of electro-active bacteria
A recent innovation in treatment wetland intensification is the use of electro-conductive media instead of sand or gravel. The use of material that can convey electricity enables electro-active bacteria to colonise the media, creating a naturally occurring biofilm that houses a diverse community of bacteria operating in a mutual association. The different types of bacteria can pass the electrons in a mechanism called “direct interspecies electron transfer”. In practice, this means the reach of any one microorganism is greatly extended, accessing electron sinks (for example, oxygen, nitrate, iron) that are centimetres away from it. This makes the electroactive wetlands much more efficient at the oxidation of organic matter and other compounds, such as antibiotics, emerging pollutants, and metals. While a conventional secondary treatment wetland can require 3-10m2 per population equivalent, this innovation enables sizing at <1m2 per population equivalent. The technology was developed through EU-funded projects and is now at commercial scale, being marketed in Spain under the trademark of METfilter. More information on this technology is available on the research group website www.bioelectrogenesis.es