A nature-based approach to combined storm overflows

A traditional ‘grey’ solution would have meant increasing the capacity of storm water storage to 22,000 m3. At a financial cost of £56m and a whole life carbon cost of 3000 tonnes CO2e, this was unaffordable and unsustainable. Instead, we devised a nature-based solution known as an integrated constructed wetland (ICW) – a proven technology, but the first of its kind on a UK network CSO. This solution would not only remove harmful pollutants from the CSO and improve river water quality, but would also provide local amenity and biodiversity benefits.

Selecting the right nature-based solution

Having water catchment teams and treatment works teams working closely together helps us identify opportunities to apply nature-based solutions to a broader range of water industry problems. Drawing on technology being successfully advanced in other parts of Europe, the project team identified an opportunity during site exploration for a nature-based facility that could provide the same or better outcomes for river quality as a traditional solution.

We used a CIRIA B£ST multiple capitals approach to assess the benefits of each potential solution against its whole-life cost. The CIRIA digital spatial tool estimates the value of multiple potential benefits of the project, reflecting nuance such as the level of confidence users have in the data, and the relative value of benefits in the context of the project. A nature-based solution was a clear preferred scheme.

A combined aerated reedbed and an ICW overflow management facility was shown to provide a threefold increase in total benefits, achieving better water quality in the river, providing new amenity space for the local community, and creating wildlife-rich habitats for nature recovery. The solution uses a two-stage approach integrated within the local landscape to reduce harm to the river. An aerated reedbed provides intensive pollution reduction and reduces the risk of foul odour nuisance and an ICW provides effluent polishing – a final stage of filtering to remove suspended solid materials.

Future research and innovation

This is the beginning of a new approach to CSO management and, as the first project of its kind in the UK, Pont-y-felin is supporting multiple research programmes. These include:

• Welsh-Water-funded research with Cranfield University to optimise plant selection and density with the aim of enhancing biological processes, attracting native wildlife, and supporting further scale-up of wetlands.




• Evidence-gathering to improve the understanding of pollution concentration in the intermittent discharge from sewer overflows. A current lack of understanding makes it challenging to design for pollution management. 

Early assessments project the Pont-y-felin scheme to be carbon neutral over its lifespan. Long-term carbon sequestration in wetland systems is not well understood and is the subject of academic research. The scheme will provide a well-monitored test case to supplement research in this area.