Thames Water Utilities Limited has awarded a major works contract for the upgrade of Maple Lodge Sewage Treatment Works (STW), intended to deliver a higher quality of treated effluent and support compliance with new requirements.
The contract, with an estimated value between approximately £140 million and £280 million, has been awarded to Kier Infrastructure, confirmed on 17 February 2026, with the standstill period ending on 26 February 2026 and signature on 31 March 2026. The works are scheduled to run from 1 April 2026 until 29 March 2030.
According to the contract scope, the project includes extensive treatment and infrastructure upgrades across the works. These comprise new inlet screening and storm separation assets, a new storm tank and river outfall, upgrades to existing primary settlement tanks, and the conversion of three storm tanks into primary settlement tanks.
The scheme also includes new activated sludge treatment capacity, eight new final settlement tanks, additional sludge thickening, new tertiary solids removal plants, chemical dosing systems to support phosphorus removal, electrical and power network upgrades, SCADA enhancements, standby generation, and the demolition of redundant assets.
Thames Water has stated that the upgrade will ensure a higher quality of treated effluent, reducing phosphorus levels entering the river to below an average of 0.25 mg/l. Storm tank capacity will also be increased, reducing the need for untreated sewage discharges during storm conditions.
In parallel, the company is investigating the impact of groundwater on the local sewer network to inform longer-term planning. Thames Water expects the site to meet all government targets for storm overflows between 2035 and 2040.
