Sewage spills into rivers and the sea often go undetected because water companies are supplying faulty data to a government watchdog, analysis for The Times suggests.

Rhys Blakely www.thetimes.co.uk 

The findings will fuel concerns that a £56 billion plan to reduce discharges will be undermined if water companies are allowed to continue monitoring their own performances.

The issue will be in the spotlight in a High Court hearing this week at which Ofwat, the water industry regulator, will be accused of ignoring its duty to protect the environment.

The latest analysis looked at spills of sewage into rivers and the sea from “storm overflow” emergency outlets that are supposed to be used only in severe conditions.

By looking at detailed data collected in 36 sewage treatment works — information that the Environment Agency (EA) can access but does not routinely check — 1,516 days were identified between 2018 and 2021 when it appeared highly likely that sewage was released in conditions that did not meet permits, making them illegal.

The EA is supposed to be made aware of them but many were not reported. For example, the analysis included the sewage treatment works at Henley-on-Thames, a noted rowing area. Thames Water, which operates the sewage works, told the EA there were no spills in 2020. The analysis suggests the plant released sewage in breach of its permit on seven days that year.

The analysis also looked at the Woodstock sewage treatment works which discharges into the River Glyme which flows through lakes in the grounds of Blenheim Palace.

Thames Water told the EA in 2020 there were no spills but added that this was a mistake and the report would be corrected. The new analysis suggests the sewage plant, in a site of special scientific interest, spilled for about 363 hours that year.

The private utility company has not updated its 2020 report, which was submitted 18 months ago. It has not disputed the new analysis. The report also includes the Ambleside sewage plant in Cumbria, which discharges into the River Rothay and into Windermere, England’s largest lake. Pollution concerns were raised this summer because the lake suffered high levels of potentially toxic blue-green algae.

Ambleside appears not to have submitted any data on spills to the EA for last year. Data supplied by United Utilities for the analysis suggests that it spilled for more than 900 hours. United said it did not agree with the analysis, claiming it was “largely based on assumption rather than evidence”.

Overall, 16 of the 36 sewage works examined appear to have provided incorrect data to the EA.

Professor Peter Hammond, of Windrush Against Sewage Pollution (Wasp), carried out the analysis. His exposure of illegal spills helped to force the government to begin an investigation that led to a proposal last month to tackle the problem.

The spills involve “storm overflows” which act as relief valves for the sewage system. They are supposed to discharge excess storm water, combined with untreated sewage, directly into water bodies during periods of heavy rainfall.

Hammond has shown that many overflows are spilling waste into rivers when there is little or no rain and before treatment works are working at full capacity.

The government plan calls for a mandatory £56 billion investment programme by the water companies.

There are about 13,350 storm overflows in England, discharging to rivers. A report commissioned by the government looked at 9,240 of them and concluded that they were used more than 340,000 times in 2020.

The government plan calls for a 14 per cent reduction in these spills by 2030 and a 50 per cent reduction by 2040. The new analysis suggests, however, that the EA will not be able to monitor compliance if it continues to rely on water companies monitoring themselves.

Ofwat said that it was reviewing Hammond’s analysis. The regulator said: “What we are witnessing, with sewage being released into the environment, isn’t acceptable. Water companies do not take enough responsibility for their impact on the environment.”

A spokeswoman for the Rivers Trust said: “This report emphasises the inherent failures with the current system of self-reporting sewage spills. It’s clear the data has not been adequately audited and in many cases is invalid.

“We need a much better process of quality assurance from the water companies themselves, plus a stronger audit from the EA.”

Five of the ten water companies that were covered did not take issue with Hammond’s analysis of the performance of the treatment included in the study. However, United Utilities Severn Trent, South West, Anglian Water and Yorkshire said that they did not agree with his findings. An Anglian representative said: “We don’t recognise the data being put forward in this report as being correct.”

Hammond’s report also highlights the difficulty he has faced in getting some water companies to share data. “There are 40 STWs named [in the new report] but only 36 analysed because of data being withheld,” he said.

He said Severn Trent and United Utilities — judged by Ofwat to have met an “industry leading” standard for environmental performance — have refused to supply detailed data for sewage spills in 2020 and last year from 4,500 overflows whereas the other eight water companies complied fully with the request.

A spokeswoman for United said: “While there is an ongoing regulatory investigation into the performance of wastewater treatment works, it is right and proper that the relevant data is shared only with the investigating authorities.”