Thames Water has been in the news a lot this year for sewage spills and flooding. But why are people so angry at the water company and what is causing the issues?

The UK’s largest water firm, which covers an area of 16 million people, came in for heavy criticism earlier this year after the number of times it spilled sewage into rivers doubled in 2023.

The Environment Agency (EA) and water minister said the increasing number of ‘storm overflow’ spills by Thames Water and other companies was ‘disappointing’ and ‘unacceptable’.

Despite the revelations and criticism, an ongoing spill at Thames Water’s Chesham sewage treatment works has continued to flow into the River Chess for over 1,500 hours nonstop since February.

Meanwhile, in Maidenhead, the annual Boulter's to Bray Swim on the River Thames has just been cancelled because of health fears over sewage discharged into the waterway.

The cancellation follows rowers in this year’s Oxford and Cambridge boat race in London being told not to enter the Thames after high levels of E. coli were discovered.

In other news, Thames Water sewage leaks have also caused a family to be hospitalised and human waste to flow through people’s gardens.

What are storm overflows?

Across most of the UK, rainwater and wastewater flow through the same pipes in a ‘combined’ sewer system.

This means that during periods of heavy rainfall, the sewers are filled by excess water, creating a risk the risk of sewage backing up into people’s homes, gardens and streets.

To prevent this, storm overflow drains are in place, acting as automatic ‘relief valves’ to stop the sewer system becoming overwhelmed.

The drains work by diverting rainwater and untreated sewage into the watercourse when there is a large volume of water in the sewer network.

That is how things should work. However, some utility firms, including Thames Water, have been accused of even allowing sewage to flow into waterways during periods of dry weather.

On top of this, EA monitoring data shows that sewage spills from storm overflows are happening more often.

Why is the number of storm overflow spills by Thames Water increasing?

“The problem is groundwater getting into the sewer system,” says Paul Jennings, chairman of the River Chess Association, speaking to the Local Democracy Reporting Service.

The environmentalist has extensively studied the 18km River Chess, a rare chalk stream rich in minerals and wildlife, which is under threat due to sewage discharges.

A ‘substantial volume’ of groundwater is currently infiltrating the River Chess area’s sewer system, flowing through to Thames Water’s Chesham sewage treatment works.

“It is an incredibly inflated volume of fluid that Thames Water has got to deal with,” says Jennings, “That is why they have to discharge.”

Why is groundwater infiltrating the sewer system more often?

Responding to issues in Chesham, Thames Water said: “The prolonged wet weather and high groundwater and surface levels this winter means more flows have been coming into the site than it can cope with.

“We’ve been working to prevent excess water getting into our sewers by relining pipes and sealing manholes.”

Jennings welcomes some of the progress Thames Water has made with repairs but says that the degradation of its network of pipes over many years remains one of the main drivers of groundwater getting into the sewers.

He said: “Because that groundwater gets above the sewer pipes, it flows into the pipes wherever there are cracks and fractures.

“Over the years, because of a lack of maintenance, the number of those cracks and fractures have increased. Now we are in a high groundwater year, the sewer just gets inundated by this huge volume.”

What is Thames Water doing to resolve the problems?

Thames Water said: “Chesham sewage treatment works has been upgraded so it can treat more flows coming into the site.”

Ongoing upgrades to improve the quality of effluent being discharged from the site into the Chess are set to be completed this year.

Thames Water is upgrading and increasing the size of its Chesham equipment, so the site can hold an additional 112 litres of wastewater a second before overflowing to storm tanks.

The water company says: “This increased capacity means that we can now treat larger volumes of wastewater to stop it entering our rivers.”

The £20m of upgrades at Chesham are part of a £1.6 billion investment programme to improve Thames Water’s sewage treatment works and sewer networks.

What are the effects on the environment?

Jennings says there are currently ‘high levels’ of ammonia and phosphates from untreated effluence entering the Chess.

He says: “That is impacting the river. You have got sewage fungus growing everywhere. Huge numbers of vertebrates that we would normally expect to find in this river are not present.

“Whereas you go upstream of the effluent outpour, and you have got a fantastic river, crystal clear, with a wide variety of invertebrates and fish.”