Troubled waters

31st May 2024

Rivers and waterways across England and Wales are increasingly polluted by sewage spills. What is causing the crisis and what is being done to tackle it? Huw Morris reports

For an idea of the state of rivers across England and Wales, consider the roach. A hardy, small silver fish found in most rivers near where people live, the roach has fascinated scientists for 40 years. They have looked for endocrine disruption – the effect of natural or man-made chemicals that interfere with hormones. This can cause male fish to produce female eggs in their testes.

Treated sewage pumped into rivers carries a cocktail of chemicals related to oestrogen, the female sex hormone. Last year, researchers from Brunel University London and the University of Exeter took samples from 10 river sites. Male roaches had essentially become intersex in 60% of locations. They also displayed high concentrations of female proteins in 90% of samples.

That’s what happens with treated sewage. A bigger issue is the level of raw sewage pumped out into waterways by combined sewer overflows (CSOs). These act as an automatic safety valve that releases excess pressure on the network from flooding and heavy rain, preventing sewage from backing up into properties and stopping widespread mains pipe bursts across the country. However, they should only be used under strict permit conditions.

In 2010, just 7% of CSOs had monitors. Now they are all fitted with event duration monitors, which measure when and for how long a CSO is in operation. These reveal when discharges happen and help the government and regulators to hold water companies to account for illegal sewage spills and improve knowledge of overflow operation to identify where improvements can be made.

Pollution detection is getting better and what it reveals is highly disturbing. The Environment Agency is looking closely at more than 2,200 wastewater treatment works that discharge into English waters, admitting this is its largest-ever criminal investigation into potential widespread breaches of environmental permit conditions by all water companies. Water industry regulator Ofwat is also investigating the issue.

The latest data from the Environment Agency, which is informing the two investigations, reveals that 2023 was the worst year on record for storm water pollution (see ‘Storm water pollution in 2023’, p20).

Raw sewage was discharged into rivers and seas for more than 3.6 million hours, compared with 1.75 million hours in 2022, a rise of 105%. There were 1,271 spills a day on average across England, compared with 825 in 2022. The data for 2023 shows a 54% increase in the number of sewage spills compared with 2022. This increase is partly blamed on exceptional weather, with 2023 named by the Met Office as the sixth wettest year since records began in 1836.

In May 2023 – well before the latest statistics were unveiled – water companies in England apologised for not acting quickly enough. Dubbed by observers as the mea culpa, the industry pledged to put things right with plans to make the largest-ever investment in CSOs as part of a major programme to reduce spills into rivers and seas (see boxout below).

Water companies and ministers have consistently argued that sewage outflows are a result of antiquated Victorian wastewater networks. However, that was debunked last year by Peter Hammond, a data expert and former professor of computational biology at University College London.

His analysis of a report by Arup and Vivid Economics – ironically, sponsored by water giant United Utilities – reveals that less than 12% of the sewerage network in England and Wales was constructed in the 19th century. Most of the network was built before water privatisation in 1989, with around a fifth built during the 1960s and 1970s. Just 6.5% has been built this century, Hammond reveals.

“The disparity of investment before and since water industry privatisation must surely bear the brunt of blame for leaving us with sewers where only 6.5% were constructed in the 21st century,” he says. “In future, comments on the cases of untreated sewage discharges must specify which components of the sewerage system are being blamed and compare pre-privatisation to the period post-privatisation and to the water industry’s mea culpa of May 2023.”

Ofwat data shows that sewerage infrastructure investment has fallen in real terms from an annual average of £3bn in the 1990s to £2.7bn in the 2020s so far. This is despite a 16% increase in the population in the past two decades.

The industry had all debts written off at privatisation in 1989, with the government giving £1.5bn for improvements to the network. By March 2022, water companies had borrowed £60bn, according to company accounts, and have paid out around £72bn in dividends to shareholders since 1989.

Water companies regularly blame wet weather for sewage spills. But even this has been debunked by research from Imperial College London, which probed the reasons for spills given by water companies, covering 1,565 CSOs (see table, p19). A lack of hydraulic capacity was responsible for 79.4% of spills, while exceptional weather was reported as a reason for only 1.3% of the CSOs that spilled and blockages for under 1%. In short, the spills happen during dry weather. “By design, CSOs are meant to operate intermittently and only in response to heavy rainfall events,” the researchers concluded. “Today, the situation is quite different.

“Many CSOs operate frequently, even under normal or no rainfall, discharging diluted or raw untreated sewage into water systems.”

The water industry’s response to sewage spills

Water companies claim they have a comprehensive programme to end sewage spills. The £10.2bn plan will fund improvements through to 2030, representing a threefold increase in spending. All of England’s 14,187 overflows would meet or exceed current regulatory standards when completed. Approaching 9,000 improvements to the sewerage network have been identified, Almost two-thirds of spills at bathing areas would be stopped in the first five years of the project, and almost 50% of discharges near conservation areas. Iconic river basins, including the Solway Tweed, North West, Severn and Humber basins, would see spills cut by between 75% and 85%.

“This is the first plan in the world to set out such a detailed and expansive programme for upgrading overflows right across the country,” said Water UK chief executive David Henderson. “In just five years, it cuts annual spills into rivers and coasts by 150,000.”

The plan is currently being considered by Ofwat.

The dangers posed by sewage spills

A study by Imperial College London last year summed up the dangers to human and ecological health of sewage spills.

Exposure to pollutants can “bioaccumulate in wildlife, with trace concentrations building up over time and polluting the food chain, as well as potentially contaminating groundwater intended for potable use”.

Discharges can also deteriorate the ecological and chemical status of waterbodies, altering their physical characteristics, threatening drinking water supplies and leading to public health concerns. They also raise concentrations of pathogens downstream from CSOs, with contamination levels above acceptable risks for recreational use. People who come into contact with waterbodies polluted by CSOs face significant health risks, such as gastrointestinal illness, the study warned. Indeed, the River Thames is so bad that crews taking part in this year’s Oxford and Cambridge boat races were advised to cover all open wounds and not to throw the winning cox into the water.

The real reasons cited by water companies for sewage spills at combined sewer overflows

79.4% Lack of hydraulic capacity

8.2% Infiltration, including groundwater inundation

5% Asset configuration

1.3% Exceptional weather

1% blockages


Sewers built across 10 regions of England and Wales, 2001-2023


Sewers built post-2001 (km)

Sewer network by
July 2023 (km)

Post- 2001 (%)

Anglian Water




Dwr Cymru – Welsh Water




Northumbrian Water




Severn Trent Water




South West Water




Southern Water




Thames Water




United Utilities




Wessex Water




Yorkshire Water









Storm water pollution in 2023

Water company

Number of
sewage spills

Total duration
of spills, hours

Anglian Water



Dwr Cymru – Welsh Water



Northumbrian Water



Severn Trent Water



South West Water



Southern Water



Thames Water



United Utilities



Wessex Water



Yorkshire Water




Huw Morris is a freelance journalist


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