Southwaite Wastewater Treatment Works: addressing stormwater overflow with an innovative nature-based solution

Project summary

The Southwaite Wastewater Treatment Works (WwTW) project represents a groundbreaking advancement in stormwater treatment, cleverly blending engineered and natural systems to treat water from the local population of about 1000 residents and the adjacent service station. Initiated during Asset Management Programme 7 (AMP7), this flagship project marks a shift from traditional, carbon-intensive stormwater management to a sustainable solution that bolsters local biodiversity.

The background

Situated near the rural village of Southwaite, Cumbria, and adjacent to the bustling Southwaite Services, the ageing infrastructure at Southwaite WwTW was burdened by rising demand and environmental pressures. The existing facilities, restricted by the village’s boundary and the need to discharge into the River Petteril, were insufficient to meet the stringent Water Industry National Environment Programme (WINEP) targets set for AMP7. Together, United Utilities and MWH Treatment achieved a complete overhaul to increase treatment capacity and decrease environmental impacts.

The challenges

The site presented several limitations, including a narrow land area bisected by the existing works and bordered by the West Coast Main Line, which required a compact and considerate design approach. Moreover, the project had to address the ambitious objective of treating stormwater flows, as the facility was tasked with operating under a ‘treat all flow’ condition, prohibiting any storm outfall. Additionally, there was the urgent need to upgrade the treatment capabilities to meet the WINEP targets for AMP7, aiming to enhance nutrient removal processes and achieve ‘good’ status for the downstream watercourse of the River Petteril, all while managing the effects of population growth and climate change.

The solutions

In response to these complex challenges, MWH Treatment devised a dual-stream treatment strategy. This innovative solution combined mechanical and biological treatment processes with a pioneering integrated constructed wetland (ICW) for managing excess stormwater. The design avoided the dependence on conventional storm tanks, which posed risks owing to high embodied carbon and septicity issues. Instead, a buried storm detention tank and a robust series of primary and secondary treatment tanks were incorporated, complemented with submerged aerated filters (SAF) and precision dosing systems to efficiently process variable inflows.

The ICW itself was designed to not only improve the treatment process but also foster biodiversity, supported by an intricate system of ponds lined with geotextile membranes and planted with a diverse array of species for maximum ecological benefit. RSK company Salix helped to choose and cultivate the plant species, which would establish a fully functioning ecosystem within two to three growing seasons. Once the plants mature, overall maintenance of the wetland will be minimal, creating a self-sustaining habitat. This hybrid treatment model facilitated effective management of storm events and positioned Southwaite WwTW as a leading example of sustainable stormwater treatment in practice.

The impact

The Southwaite WwTW now stands as a model for environmental innovation in wastewater treatment. The ICW enhances ecological diversity, with the wetland becoming a haven for varied flora and fauna, adding to the site’s natural beauty and offering educational opportunities for the local community. The project also highlights the significance of collaborative engagement with regulatory bodies, exemplified by the operating techniques agreement with the Environment Agency. This innovative operational framework enables monitoring and sharing of performance data, promoting a more informed and adaptive approach to wetland management. The success of this project acts as a catalyst for the broader adoption of nature-based solutions in the water industry, providing a replicable model for sustainable water management and habitat creation.