The number of raw sewage spills into rivers by Thames Water more than doubled last year, it has been revealed.

The water company discharged sewage 16,990 times in 2023, data published by the Environment Agency yesterday shows.

This marks a 109 per cent increase in discharges by Thames Water compared to 2022, according to the new storm overflow data.

A Thames Water spokesperson told the Local Democracy Reporting Service (LDRS): “We regard any untreated discharges as unacceptable, and we’re committed to stopping them from being necessary, with the assistance of our regulators.

“Storm discharges are closely linked to rainfall and groundwater conditions and our region experienced above average rainfall for most of 2023, which saw an increase in the frequency and duration of storm discharges from our sites compared to 2022.”

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The new data showed that spills by Thames Water lasted for an average of 12 hours, longer than any other water company in England.

However, some discharges last far longer, such as the ongoing incident at Thames Water’s Chesham Sewage Treatment Works.

Sewage there has been pumped non-stop into the River Chess since February 29, with the discharge approaching 700 hours.

Chair of the River Chess Association Paul Jennings claimed that groundwater ingress was compromising the local sewage treatment network capacity in Chesham and other areas.

He told the LDRS: “High groundwater is the issue. A problem we have known about for 10 years, just a lack of investment and in appropriate work. It is the same across all of Bucks and Herts not just Chesham.”

The new data, which shows that Thames Water spills lasted for a cumulative 8,184 days in 2023, also highlights some of the worst sites for sewage spills.

Among them are the Gerrards Cross Sewage Treatment Works, where 120 spills into the River Misbourne lasted for 2,305 hours last year.

Thames Water said that upgrades of the site, due to be completed in 2026, will allow it to treat greater volumes of incoming sewage, reducing the need for untreated discharges in wet weather.

The water company said it was also upgrading other sites across Buckinghamshire and beyond to deal with sewage discharges.

A spokesperson said: “We’re currently increasing sewage treatment capacity at 250 of our sites across London and the Thames Valley, including Little Marlow, Chesham and Aylesbury sewage treatment works.

“These upgrades will improve our ability to treat the high volumes of incoming sewage and reduce the need for overflows during wet weather.”

“Taking action to improve the health of our rivers is a key focus for us and we are leading the way with our transparent approach to data.”

Thames Water is the only water company to provide live alerts for all untreated discharges on its ‘near real-time’ map.

The water company maintains over 68,000 miles of sewers across its network and manages 354 sewage treatment works, 5,235 pumping stations and 1.48 million manholes across the region.

Yesterday, the Environment Agency’s director of Water Helen Wakeham said: “Whilst it is disappointing that water companies have reported an increase in sewage spills in 2023, it is sadly not surprising.

“We are pleased to see record investment from the water sector, but we know it will take time for this to be reflected in spill data.”