By Esme Stallard, Becky Dale, Jonah Fisher and Sophie Woodcock
BBC Climate and BBC Verify
Three major water companies illegally discharged sewage hundreds of times last year on days when it was not raining, a BBC investigation suggests.
The practice, known as "dry spilling", is banned because it can lead to higher concentrations of sewage in waterways.
Thames, Wessex and Southern Water appear to have collectively released sewage in dry spills for 3,500 hours in 2022 - in breach of their permits.
Water UK, the industry body, said the spills "should be investigated".
The BBC requested the same data from the other water companies in England, which said they could not respond due to being under an Environment Agency (EA) criminal investigation.
Releasing sewage into rivers and seas is allowed in the UK to prevent pipe systems becoming overwhelmed - but it has to have been raining.
Without rainwater the sewage is likely to be less diluted - leading to build-ups of algae which produce toxins "that can be fatal to pets and pose a health risk to swimmers", says Dr Linda May, a water ecologist at the UK Centre for Ecology and Hydrology.
Discharging in dry conditions is therefore illegal under environmental law.
Collectively throughout 2022, Thames, Southern and Wessex illegally started releasing sewage on dry days 388 times - research by the BBC's climate and data teams suggests - including during last summer when these regions were in drought.
There even appears to have been spills by all three companies on 19 July 2022, the hottest day on record, when temperatures topped 40C in some places and many people tried to cool off in rivers.
Environment Secretary Therese Coffey told BBC News: 'It does seem extraordinary on the hottest day of the year that there may be releases. The EA is the regulator; they are the people who do the detailed investigation of why that has happened."
All nine English water companies were sent environmental information requests for data on when their spills started and stopped. Only Thames, Southern and Wessex provided details - which the BBC then cross-referenced with Met Office rainfall data to identify dry spills.
Fewer spills are likely to have been recorded in 2022 by Thames and disclosed to the BBC. That's because the company only had 62% of its overflow points monitored - compared to Wessex with 91% and Southern with 98%.
The remaining six water companies in England said they couldn't provide information because they were already being investigated for potential illegal spilling by industry regulator Ofwat and the EA. If the companies shared data with the BBC, they said, then analysis could be carried out which could sway public opinion.
Thames, Southern and Wessex serve more than 22 million people.
Across the Wessex Water region - from the Dorset coast to the Bristol area - BBC analysis identified 68 sites where sewage may have been discharged illegally last year. The spills that started on dry days appear to have lasted for more than 1,500 hours.
In one case, the BBC's analysis suggested that sewage was discharged into the River Chew in north Somerset from a nearby wastewater treatment works for nearly 50 hours during dry periods.
Georgie Duckworth swims and rafts regularly in the river with her two young boys, like other local residents, and describes the spills as "outrageous".
Georgie Duckworth runs an outdoor activity company and regularly uses the River Chew with her two children
"We are all aware not to go swimming, not to get your heads under in the water when it is raining, but the thought that it is happening in dry weather too, it's alarming," she told the BBC.
Wessex Water said the spills into River Chew were caused by groundwater coming up into pipes and forcing it to spill. It said this dilutes the sewage and "the storm overflow is not identified as one of the factors affecting the ecological condition of the river".
However, the EA - England's environmental protection body - told the BBC that any dry spills due to groundwater are a breach of permit and illegal.
Wessex Water also contested some of the other spills highlighted by the BBC, citing doubts over the accuracy of its own data.
Budget cuts and reporting - 'firm link'
Nicholas Ostrowski, an environmental barrister and water industry expert, says there are three reasons why water companies may be spilling during dry weather - maintenance issues; "hydraulic incapacity" in the system, where there is not enough space for water to go through the pipes; and the company "deliberately sending effluent out in dry weather".
Any illegal spills should be investigated by the EA. Enforcement action can be taken, ranging from a warning to an unlimited fine.
The government has revealed that the EA recorded 115 cases of illegal operation in 2022 for the three water companies - less than a third of what the BBC analysis found.
One of the agency's officers - who works in environmental regulation - told the BBC anonymously there was a "firm link" between the EA's failure to identify and investigate dry spills, and budget cuts and staff losses.
The EA's environmental protection budget, funded by the government's Department for Environment, Food and Rural Affairs (Defra), was halved between 2010-20.
The officer also told us the agency was increasingly relying on water companies to report their own dry spill incidents because of these cuts.
The EA's chief executive, John Leyland, told the BBC: "The funding for the Environment Agency is a matter of public record... and we've seen a steady decline in some of our funds and so we've had to change.
"We've been focusing on digital monitoring, but earlier this year we announced a programme of increased investments in real people on your riverbank."
On the BBC's findings of dry spills, the EA was unable to comment because of its ongoing criminal investigation into water companies. But Mr Leyland said: "We are committed to increasing our regulatory presence to hold the water companies to account."
Water Minister Rebecca Pow told the BBC she considers the amount of sewage discharged into the English waters "utterly unacceptable", and said the protection budget had been increased by 12% since last year.
Environment Secretary Therese Coffey also defended the government's record to the BBC: "I think it is a really difficult issue, but actually we are the party that's cleaning it up, we have got the monitoring going that is how we are able to uncover the scale of the issue we are tackling.
"We're also…making sure we are going to get more investment into the water companies and infrastructure, and that's where we have a credible plan."
'River of effluent'
One apparent dry-spilling case by Southern Water picked up by BBC analysis also came to the Environment Agency's attention after a complaint from a member of the public.
In March 2022, Robert Bailey from the Clean Harbours Partnership noticed his local chalk stream, the River Lavant near Chichester, had become "discoloured for many miles and was starting to fill with a white plume".
His concerns correlate with two dry spills the BBC has now identified at the site.
"Because of the sewage being discharged, it is a river of effluent," he told us.
The BBC has seen the investigation report from the EA's subsequent visits in April and May - which confirms that Southern Water was discharging sewage when there was no rain, in breach of its permit.
The EA issued Southern Water with a warning - but said further action could be taken if there were more spills.
There were 91 more hours of sewage discharges starting during dry weather at the site during the rest of 2022 - BBC analysis suggests.
In total, the research indicates that Southern Water illegally released sewage at 25 sites across its area last year - from Hampshire to Kent - for a total of nearly 800 hours.
John Penicud, head of wastewater operations at Southern Water, told the BBC that "so-called 'dry spills' are a complex issue" and said discharges in dry weather can be caused by groundwater entering pipes.
He added that: "Lavant is in a catchment that is particularly prone to groundwater infiltration." He also said the company planned to invest more than £1.6m improving more than 4km (2.5 miles) of sewers in the area.
Spills in 'green haven'
Another frequent spiller appears to be Thames Water's Longbridge Road overflow site in east London, which releases sewage directly into Mayes Brook.
The river is located in the UK's first "climate" park in Dagenham - constructed at a cost of £3.8m to provide a green haven for the local community and wildlife.
But last year, the overflow spilled for nearly 200 hours - leaving excrement-soaked wet wipes on the riverbanks.
BBC research estimates that about a quarter of those hours were from dry spills.
"You've got this contrast of an improved park costing millions of pounds, getting polluted every four days on average by sewage and that's a scandal," says Theo Thomas, chief executive at water charity London Waterkeepers, who has been lobbying Thames Water and the local authority to resolve the issue.
A Thames Water spokesperson said they had been in contact with Mr Thomas and apologised for the storm overflows which occurred.
"We've been assessing how we can improve our Longbridge site and the surrounding network and will continue to work with the local communities on our investment plans," they added.
The BBC analysis suggests that Thames Water - with customers from the Cotswolds to the Thames Estuary - dry-spilled for 1,253 hours in 2022, at 49 overflow sites.
On these spills Thames Water said: "The Environment Agency's methodology for calculating dry day spills is still being determined and we will continue to work with our regulators as they define this. We regard all discharges of untreated sewage as unacceptable, and we have planned investment in our sewage treatment works."
Counting the dry spills - methodology
The figures were the result of a nine-month investigation by the BBC Data and Climate and Science teams.
Water and sewerage companies are responsible for outlets known as combined sewer overflows (CSOs), which release sewage from treatment works or the sewage network into the UK's waterways.
The majority of CSOs record when they discharge.
Every year the sewerage companies inform the Environment Agency (EA) in England how frequently and for how long each outlet discharges.
But the EA only publishes the annual summary of total spill counts and hours.
So the BBC sent Environmental Information Regulation requests to England's nine water companies to obtain the start and stop times of each discharge recorded at CSOs in 2022.
The start and stop times for the three companies that provided data were adapted into the standard 12/24-hour counting blocks used by the EA to determine individual "spills".
These were then cross-referenced with the highest-quality 1km-gridded rainfall data - available from the Met Office - to identify spills occurring in periods of dry weather.
This rainfall data is presented in gridded squares that cover the land area of the UK. Each grid cell is 1km by 1km square.
The rainfall values are calculated from a network of automatic rainfall gauges and observation stations.
The EA defines a dry day as one where there was less than 0.25mm of rain on that day and the day before.
The BBC took a conservative approach of four consecutive days without rain to allow for catchment drain-down time - when rainfall moves through the hydrological system.
The methodology was independently reviewed by three academic experts working in this field.
Additional reporting by Libby Rogers, Rob England and Nassos Stylianou. Graphics by Jana Tauschinski and Kate Gaynor
Methodology support from Dr Gemma Coxon, University of Bristol; Dr Nick Voulvoulis, Imperial College London; Dr Barnaby Dobson, Imperial College London