Across the UK, more than 20,000 combined storm overflows (CSOs) discharge a combination of rainwater and wastewater into rivers during periods of heavy rain. This can have a significant impact on river health. At Pont-y-felin Lane in Torfaen, Wales, a CSO discharges into the Afon Lwyd. As part of measures to boost water quality in the Afon Lwyd and Usk rivers, Dwr Cymru Welsh Water wanted to reduce the number of CSO spills during heavy rainfall.  

In 2018, Arup was appointed to lead the root cause investigation, data analysis, hydraulic and water quality modelling, optioneering optimisation and detailed design of a scheme to address the problem. 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.  

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. 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.