Water treatment turns to nature

Hardly a day goes by without a reminder that water management is a sensitive subject. A particularly contentious issue is river and sea pollution from wastewater treatment facilities. The National Engineering Policy Centre’s recent report, Testing the waters: priorities for mitigating health risks from wastewater pollution, investigated the role of engineering in tackling wastewater pollution to reduce exposure to human faecal pathogens. As the report says: “Our wastewater system is, at its core, an asset for the protection of public health, and it has been remarkably successful, including interrupting the transmission of major epidemics and protecting the environment by treating wastewater before it is returned to our rivers and seas.” The challenge now is to adapt how we treat wastewater to meet growing demands and changing aspirations.

An increasingly popular tool that water engineers are employing comes under the general umbrella of treatment wetlands. In essence, these engineered wetlands enlist nature to clean up after us. At its simplest, a wetland is a stretch of shallow water and plants where natural processes capture pollutants and keep them out of rivers and drinking water supplies. Engineers have created numerous artificial wetlands in the UK over the past 20 years or more, which have been cleaning up rivers and road water runoff, for example. In 2000, the Constructed Wetland Association was established to spread the word about the concept. And in recent years water companies have investigated engineered wetlands for processing wastewater after we have flushed it into the network of ‘sewage farms’ around the country.

The Environment Agency (EA) sees engineered wetlands as an emerging approach for wastewater treatment. The agency’s view is that “understanding the effectiveness of wetlands for nutrient removal is a rapidly evolving field of research where there are still evidence gaps which need to be understood”. The EA currently permits water companies to implement treatment wetlands for storm and nutrient removal on a trial or pilot basis. 

Pollution on the agenda

The growing interest in engineered wetlands for wastewater treatment is a response to new standards that the industry must meet to deal with pollutants from sewage processing plants. Phosphorus in particular has floated to the top of the wastewater treatment agenda. Phosphates pass into wastewater in human waste and detergents. Some estimates suggest that nearly 98% of the phosphorus consumed by humans in urban areas ends up in sewage sludge. Approaches used in conventional wastewater treatment, such as chemical dosing, remove much of this pollutant. In 2010 the government passed legislation that aimed to reduce the level of phosphates in detergents sold in the UK, but unacceptable amounts of phosphorus still get into waterways. This flush of nutrients into rivers and lakes encourages eutrophication – a rapid growth of microorganisms that drains the water of dissolved oxygen, killing fish and other wildlife. In effect phosphorus fertilises the growth of algae, leading to the ‘algal blooms’ that turn rivers green, a process bolstered by increasing water temperatures.

In a move to reduce how much phosphorus enters rivers from water treatment plants, in 2022 the Department for Environment, Food and Rural Affairs (DEFRA) released new standards specifying that by the end of 2024 water companies must reduce phosphorus levels in wastewater to 0.5 mg/l (milligrams per litre). Since 2020, the EA has had the ability to impose limits down to 0.25 mg/l on water company discharges. To reach this lower limit, water companies are investing in new ways of treating wastewater. 

One chemical dosing approach to remove phosphates involves treating water by adding iron. This reacts with phosphorus, forming a solid precipitate that can then be filtered out. However, chemical dosing has several drawbacks. One is the unpredictable cost of the dosing chemicals. There are also questions about the availability of these chemicals. According to Mott MacDonald, which manages the development of water treatment facilities, “the global production of ferric sulphate and ferric chloride is not sufficient to provide for demands and there is significant concern from all water companies surrounding the supply of these chemicals”. Dosing also creates additional solid waste that must be taken away by road, adding to local traffic. And it gets progressively harder for chemical processing to remove phosphorus as concentrations decline. Finally, extra chemical processing uses more energy, another unpredictable cost.

A new approach to water treatment 

The drawbacks of traditional treatment and other factors have prompted moves to use ‘nature-based solutions’ to meet social challenges. In its ‘strategic priorities’ for Ofwat, DEFRA has specified: “We want Ofwat to enable and encourage the increasing use of these nature-based solutions.” Water companies see wetlands as an increasingly attractive way of achieving this. In wetlands, plants and microbial activity take over from chemical processing to remove phosphorus. Plant roots take up some of the phosphates while bacteria that are naturally present in the wetland absorb phosphates in the form of adenosine triphosphate (ATP), a molecule that all living cells use for energy.

The most obvious sign of a wetland is its plant life. But Dr Gabriela Dotro, Principal Research Fellow in Ecological Engineering at Cranfield University’s Centre for Water, Environment and Development, says that while plants do filter out solids and provide shade to prevent algal blooms, they play a minor role in the direct uptake of pollutants. Plants are there mostly so that microorganisms can attach to their roots. “Most of the pollutant processing is performed by microorganisms and soils, sediments and substrates.” She has described constructed wetlands as “engineered systems designed to optimise processes found in natural wetlands, producing treated water and a multitude of additional co-benefits including biodiversity, amenity and aesthetic value”.

Dotro leads an industry-funded project at Cranfield gathering evidence to improve the design tools available for wetlands, to achieve predictable effluent concentrations of phosphorus. The first stage of the project involved analysing data from around the world on the use of surface flow wetlands for phosphorus removal. For example, Ireland has pioneered the use of so-called integrated constructed wetlands (ICW) for water treatment plants. “What the Irish have done,” says Dotro, “is create a flowsheet [flowchart] with a primary settling tank that is followed by a number of surface flow wetland cells or units. The wetlands are designed with the principles of integration into the surrounding landscape and using local materials wherever possible. They look like natural wetlands and provide habitat for a range of wildlife.” 

Since 2004, Ireland has seen a steady increase in the use of ICWs. These constructed wetlands were originally developed for farms and then expanded to sewage applications. Irish constructed wetlands have also featured in cleaning water from urban runoff and industrial wastewater. For example, the Irish consultancy VESI Environmental has created about 200 ICWs across Ireland and Europe. It was behind the development of a wetland system to treat wastewater from a potato processing plant at Palmerstown, Oldtown, in North  Dublin. VESI has even adapted the concept for homes that are not connected to a municipal wastewater network. Adding constructed wetlands to traditional septic tanks can even create a ‘water feature’ while cleaning up domestic wastewater. ICWs are now finding their way into the UK’s water treatment systems. Yorkshire Water installed an ICW at its Clifton Wastewater Treatment Works, near Doncaster in South Yorkshire, which came into operation in 2022. 

Wetlands for water treatment come in various guises. For example, in West Sussex, Southern Water is building a wetland close to the wastewater treatment plant in Bosham near Chichester to cope with heavy rainfalls. As Joff Edevane, Pathfinder Delivery Lead – Wetlands & Harbours with Southern Water, says: “We’re also looking at wetlands to treat storm overflows.” In winter months, rainwater can overload Southern Water’s local network of pipes. After periods of heavy rain, groundwater seeps into local sewers, overwhelming the wastewater treatment plant’s capacity. When this happens, a mixture of rainwater and sewage can end up in local waterways. As a part of the government’s storm overflow reduction plan, the Bosham wetland is designed to keep rainwater out of the treatment plant. “These overflows can continue for days or months after it rains,” says Edevane. Using a wetland to keep rainwater out of the sewage can stop this from happening.

Wetlands can also process discharges from smaller wastewater treatment facilities even when there is less risk of added rainwater. The UK is dotted with hundreds of small processing plants that handle wastewater from a few hundred houses. In one experimental project, Southern Water recently started work on its first ICW to ‘polish’ the outflow into a local river from wastewater treatment. The new project is near the small West Sussex village of Staplefield. Southern Water already operates a wastewater treatment plant just outside the village of about 200 residents. The idea is that water will flow into a new ICW created in a field alongside the existing works. The main contractor for the project, Mott MacDonald, describes the Staplefield ICW as “one of the first projects of its kind in England”. It says that the wetland will reduce total phosphorus discharge concentrations from 2 mg/l to 0.5 mg/l.

Cleaner water, naturally 

There are two main variants of wastewater treatment wetlands built alongside smaller treatment plants. Where there is limited space, the answer can be to install an aerated wetland. This entails growing reeds in a concrete box with an aeration grid beneath them. “They’re quite an intensive process,” says Edevane. Aeration grids have a smaller footprint in terms of land use, but they use more energy. “We don’t want to use those,” says Edevane, “But if that’s the land you’ve got available, you cut your cloth accordingly.”

Where there is enough land, as in Staplefield, the alternative is to create a variant of the ICW in what the water industry calls surface flow wetlands. These wetlands are outwardly low tech. You could say, you’re just digging a hole to act as a water store, which will then do some treatment, says Edevane. But there is more to it than that, he adds. “We look at the local geology, the local plant community and say, ‘If there was a wetland here naturally, what kind of plants would it have?’ Then we create that.” As Dotro explains: “You need to provide planting media, plants, design it for the right residence time for the pollutant you are targeting, and so on.” The idea, says Edevane, is that “if you walk past them with your dog, you wouldn’t know that they were anything to do with sewage treatment, apart from the fact there’s a sewage treatment works next to it.” 

When Southern Water assessed the site for its Staplefield wetland, it found that it sat on clay. This makes it possible to create a natural liner simply by moving clay around to create the wetland. The clay’s impermeability prevents treated water from leaking into groundwater. For its first free-surface ICW, Southern Water called on VESI for design advice. It came up with a series of four ponds, or ‘cells’. Over time, treated sewage flows through the sequence of cells. Operators manage the wetland by controlling the water flow through the cells. In this case, there is no need to put water through a chemical processing plant or aeration grids as gravity does the work. The details depend on the layout of the wetland. If there is a series of cells, weirs will automatically control the water flow between cells. These weirs can also be lifted or lowered manually for maintenance or when operators want to keep some water in the cells to avoid them drying out for too long, in a hot summer for example.

After the treated water has travelled through the ponds, it is pumped back into the effluent chamber in the existing waterworks before discharge through its outfall into the local river.

Enhancing the natural environment 

The rise of wetlands is also linked to new government regulations that require new construction projects to increase biodiversity. Mott MacDonald points out that: “Wetlands support about 10% of all wildlife species in the UK, including birds and plants, so creating a wetland habitat will enhance biodiversity.” In the case of the Staplefield wetland, it will be on farmland that was prone to becoming waterlogged, thanks partly to the clay that makes it easier to create a wetland. According to Mott MacDonald, replacing farm crops with a wetland is a “significant improvement in biodiversity”.

It isn’t just water pollution and biodiversity that water industry has to consider, there is also increasing awareness of its carbon footprint. Edevane points out that digging a wetlands means no need for concrete. “If we don’t have to pour concrete, it’s a lot less carbon.” In its Whole Life Carbon assessment of the Staplefield wetland, Mott MacDonald compared the wetland with two conventional alternatives. This showed that “carbon emissions are significantly lower for the ICW than for the other two conventional solutions”.

The wetland approach to wastewater treatment isn’t just good for biodiversity, a wetland can remove pollutants that are beyond the reach of chemical dosing with iron to remove phosphorus. Let nature have its way and a properly engineered wetland can reduce nitrates, for example. “You might get rid of some metals,” says Edevane. “You might get rid of some ammonia. It’s a whole range of things that it does. So for just one piece of technology, you’re getting five or six hits.” This is one role for the pioneering wetland at the Ingoldisthorpe Water Recycling Centre, which Edevane installed for Anglian Water with Norfolk Rivers Trust and Rivers Ecology.

One Irish project investigated the ability of ICWs to remove antimicrobial resistant organisms from farm wastewater. Agricultural use of antibiotics is implicated in the spread of antimicrobial resistant to humans. In Denmark, another group has carried out lab experiments to show that wetlands might be able to remove micro- and nanoplastics from water. 

In another project, which has received funding from the Royal Academy of Engineering, Cranfield University has helped two Jordanian pharmaceutical factories to build pilot-scale wetland systems for their wastewater treatment. The final effluent from the systems complied with Jordanian industrial wastewater reuse standards. The team is now seeking to implement the technology to help protect water resources in this water scarce country.

These may be early days for constructed wetlands for wastewater treatment in the UK, but if the concept catches on, we may get used to walking our dogs around the local water treatment facility. For that to happen, the trials now underway must prove their value. The EA says that wetland trials will operate for at least three years: “We continue to assess results from these trials and use this information to inform and improve our regulatory approach to wetlands. The use of these wetland trials allows the Environment Agency to obtain data on the efficacy of wetlands for nutrient removal and to understand performance, using a safe and controlled approach.”