an image of River Usk, Wales; an image of River Usk, Wales;

Pont-y-felin Lane, Torfaen

A nature-based approach to combined storm overflows

Most of the UK has a combined sewerage system, meaning that both rainwater and wastewater (from toilets, bathrooms and kitchens) are carried in the same pipes to a sewage treatment works. During heavy rainstorms, more water enters the pipes of these older combined systems than they are can cope with, so they have been designed to relieve the pressure through release points – known as combined storm overflows or CSOs. Across the country, more than 20,000 CSOs discharge 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.

Project Summary

1st project of its kind in the UK

80%cost saving over traditional solution

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.

A sketch of Pont-y-felin A sketch of Pont-y-felin
Pont-y-felin uses an integrated constructed wetland to remove harmful pollutants from the Combined Storm overflows and improve river water quality. ©Arup

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.

Pont-y-felin benefit assessment Pont-y-felin benefit assessment Pont-y-felin benefit assessment
The Ciria Best tool helped the team to evaluate the relative value of the project’s community, water quality and biodiversity benefits. ©Arup

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.


A place-based, community-led approach

After working collaboratively with Welsh Water Operations and engaging a specialist supply chain to confirm viability of the scheme, the team took a place and community-led approach to refining detailed designs, responding to public consultation. The final designs include educational areas, seating areas and an outdoor classroom to enable the community to connect with the environment. Footpaths and walkways will allow pedestrian access between different points of the site. As a result, the scheme has gained local support, generating a sense of ownership and excitement.

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.