Spanish infrastructure group Ferrovial has been awarded a contract worth about £80M to upgrade Slough Sewage Treatment Works (STW) on behalf of Thames Water.

The work will be carried out by an integrated joint venture between Ferrovial Construction and sister company Cadagua. Design work started in November 2025, with construction pencilled in to begin around 15 months later – i.e. early 2027.

According to the companies, the scheme is intended to enable the plant to meet updated Environment Agency wastewater quality standards. Planned improvements include expanding treatment capacity and tightening controls on pollutants such as ammonia and phosphorus. The upgrade will also target better management of high-strength return flows under storm conditions and will include a new power supply and standby systems intended to increase the works’ resilience and reduce environmental impacts on local watercourses.

The contract builds on a long-running relationship between Ferrovial Construction and Thames Water, notably the firm’s role in delivering the Central Section of the Thames Tideway Tunnel (alongside Laing O’Rourke in the Flo JV). Ferrovial highlights experience on other large UK projects such as the Silvertown Tunnel and the Northern Line Extension, while Cadagua brings more than five decades of experience in water and wastewater projects internationally.

Thames Water and regulators have been under pressure in recent years to reduce sewage discharges and improve river water quality across England. Upgrades to treatment works are one element of a wider response that also includes reducing storm overflows, improving network capacity and investing in monitoring and enforcement.

The companies say improved treatment will lead to “cleaner and healthier ecosystems” in nearby watercourses, but independent monitoring and regulatory oversight will determine whether the works meet those outcomes in practice.

The Slough STW contract follows continued public and regulatory focus on water industry investment amid growing political debate about how to finance and accelerate improvements to England’s rivers and wastewater infrastructure.

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