The changing role of wetlands

Published 16 April 2024

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The first phosphorus wetland has struggled to meet tight nutrient limits but has shown promise for nitrogen and inland bathing applications. Lucinda Dann reports.

The first integrated constructed wetland built in the UK for phosphorus removal has struggled to meet tight nutrient restrictions and has been effectively ruled out for future rollout by the new Levelling-up and Regeneration Bill which was passed last October.

The Cromhall Integrated Wetlands were built by Wessex Water to protect and enhance the water quality and ecology of the Tortworth Brook in South Gloucestershire, which was failing to meet water quality targets, in 2020.

Monitoring of the wetland revealed that if the nature-based solution had a phosphorus (P) removal target it would have met this on an average annual basis. However, it would have breached it in autumn 2021, resulting in a penalty to Wessex.

This, combined with the acreage that would be required for the solution to meet the new limits brought in by the regeneration bill at wastewater recycling centres (WRCs), means the company has scaled back its plans for widespread adoption to just two new P wetlands during PR24.

However, the wetland has proved to be very effective at removing both nitrogen and faecal bacteria, meaning the company is now exploring other potential uses across its catchment areas.

The first of its kind

Wessex Water first looked at nature-based solutions for P treatment at its Cromhall WRC after water quality modelling showed that a 2.0mgP/l total P permit would be necessary at the site to meet water quality targets. This would require significant investment in tertiary treatment such as ferric dosing.

Rather than opt for the traditional energy and chemical-intensive solution, Wessex decided to trail an integrated constructed wetland as a more sustainable approach to reduce nutrient discharges – the first time a wetland had been used for this purpose in the UK.

Wessex had been in discussions with local landowner the Tortworth Estate and the Environment Agency (EA) since 2010, but without a driver for the project in the Water Industry National Environment Programme (WINEP) for PR14 the project had to wait until PR19 to go ahead.

Operational from April 2020, the wetland is 0.8ha in area and is arranged in a modular design with a total of 12 cells combining open water, surface flow march and wet grassland habitat types.

“Our experimental site at Cromhall was very much the first wetland in this country for P removal. We worked with the Wildfowl and Wetlands Trust consultancy to design it, but I think it is probably fair to say that we weren’t sure what the design should be given the limited data sets available.

“There were no templates for this, no one had done this before, so we used our combined experience and knowledge to design a wetland that we thought would probably hit a 2.0mgP/l concentration,” says Ruth Barden, director of environmental solutions at Wessex Water.

Intensive monitoring at the site until November 2021 revealed the wetland was achieving a 27.5% reduction in total P and meeting its proposed 2.0mgP/l permit limit, but the company is not entirely satisfied with the results.

“It broadly does achieve a 2mg/l discharge concentration on an annual basis, but not quite. Its providing about 20% P reduction, but we are not convinced that is P removal. We think its just being held up in the wetland temporarily and then might get re-released at another stage,” says Barden.

“One of the findings from Cromhall design is that wetlands are probably not all we had hoped they would be in terms of P.”

Wessex has identified some design issues with the wetland, such as the cells being too deep and some preferential flow, which is limiting the retention time within the wetland needed to enable maximum P uptake by the plants.

“If we reconfigured it and did some more work, which we are inclined to do, we think we could get a greater level of P removal but even so I don’t think its quite going to hit the level of the really stringent targets which are now needed.”

The Levelling-up and Regeneration Bill which was passed at the end of October has effectively ruled out P wetlands as being an option across the nearly half of Wessex’s operational area due to nutrient neutrality.

This legislation requires all sewage treatment works in areas above 2,000 population equivalent to achieve P removal down to the technically achievable limit, which Barden says is not possible using a P wetland due to the amount of land that would be required. And without the P driver, it will be difficult to make the case for the wetlands, she adds.

“The business case doesn’t really stack up as well as it could have done. They take up a huge amount of land and if the wider benefits – such as nitrogen, emerging contaminant, pathogen removal or biodiversity gain – that they are delivering are not truly valued then it’s difficult to make that case.”

The biodiversity natural capital gain of the wetlands was measured as being 111% for increased habitat, and 42% in increased hedgerow biodiversity units compared to the arable land before construction.

“At the moment the focus is very much on hitting a P target from an asset, it’s not about the wider benefits that asset might generate, or even the disbenefits that it might deliver in terms of increased carbon, be that embedded or operational carbon from a grey asset that can hit that P standard. That’s just not really taken into account – it is by companies but not by the regulator.”

The company has submitted new proposals to the department for environment, food and rural affairs for tackling nutrients, which sees its business plan proposals for P wetlands cut down to just two new schemes.

“In our proposal we are looking at delivering two wetlands for P removal whereas previously we were thinking of tens of wetlands. They are at very small sites, with between 100-150 population equivalent and where the modelled permits would only need to be 3.0mgP/l, and that’s it, which is disappointing, but that’s where we are,” says Barden.

Nitrogen and bacterial removal

While the wetlands have not proved as effective as had been hoped for P removal, they have produced promising results in other areas. Monitoring shows there has been a more than 60% reduction in nitrogen, with the rate being removed on a more consistent basis throughout the year than P, and less than 5% of microplastics are making it through the wetland.

Barden says these promising results in other areas means the company is looking to change the direction of its efforts.

“It might be that they don’t necessarily play such a part in terms of catchment nutrient balancing or part of end of pipe treatment for P but they might do for nitrogen reduction and for inland bathing.

“Certainly, in the Poole harbour area where the Levelling up and Regeneration Act requires P and nitrogen reduction it may well be they lend themselves to provide an element of nitrogen reduction at smaller sewage treatment works.”

As the first four cells of the wetland provides 60% of the nitrogen removal, wetlands for this purpose could be considerably smaller than the 0.8ha footprint of the Cromhall wetlands.

“I think there is more work that we would like to do to really understand exactly what that looks like, what is the optimum design for a nitrogen removal wetland and then what is the application for that.”

Barden says the other big selling point of the wetlands is its ability to treat sewage effluent to the point that human faecal bacteria cannot be detected.

“One of our PhD students couldn’t find human bacterial markers in the effluent, which is amazing because its all human bacteria that is coming into it,” she says.

The company is therefore considering opportunities to place wetlands at small sewage treatment works where there is the potential for river recreation. “This might be a really good solution because you can provide a wetland that we can have confidence delivers bacterial removal but also maybe even enhances the amenity there by having a habitat.

“I think the purpose of the wetland may be changing and we are still better understanding that application and then looking at the business case for them in wider situations instead of phosphorus removal.”