A retired scientist has determined that the vast majority of sewage spills from storm overflows across England, Scotland, and Wales may have been illegal. Peter Hammond, whose work inspired the television docudrama *Dirty Business*, analysed discharge data from 482 storm overflow sites operated by water companies during 2024 and 2025. His findings suggest that between 80% and 90% of these spills did not occur during periods of “heavy rainfall,” as defined by the Environment Agency—namely, 25mm to 30mm of rain within 24 hours—raising questions about their legality.

Storm overflows are designed as emergency safety valves to release diluted sewage into rivers or coastal waters only during exceptional wet weather, preventing sewage from backing up into homes. Hammond’s study, covering a population of approximately 39 million, compared the timing of spills with local rainfall levels and found an apparent widespread failure to comply with legal requirements. For example, 95.5% of spills across 64 Thames Water treatment facilities reportedly did not meet the heavy rainfall threshold. Similarly, an analysis of six United Utilities overflow points discharging into Windermere and its feeder rivers showed non-compliance rates ranging from 68% to 77%, even after adjusting rainfall criteria for the local climate.

Hammond highlighted that the Environment Agency used the same rainfall-based methodology to prosecute Southern Water in 2021, resulting in a £90 million fine for thousands of illegal sewage discharges. He argued that if this approach was deemed robust enough for that case, it should be consistently applied across the water industry. He also called for increased scrutiny of both water companies and regulators, including the Environment Agency and the Department for Environment, Food and Rural Affairs, which he said have allowed widespread unlawful discharges to continue.

Windrush Against Sewage Pollution, the campaign group with which Hammond is affiliated, urged faster reforms and greater accountability. Ash Smith, the group’s founder, stated that Windermere has long been subjected to excessive sewage pollution.

Responses from regulators and industry highlighted the complexity of the issue. An Environment Agency spokesperson acknowledged that storm overflows must only operate during rainfall or snowmelt to avoid breaching permits and that violations prompt enforcement action. However, they emphasized that pursuing criminal convictions requires consideration of site-specific factors and permit conditions alongside rainfall data. Water UK, the trade association representing the sector, emphasized that discharges should only happen under exceptional circumstances or if alternatives are prohibitively costly. The association expressed a commitment to ending sewage discharges as rapidly as possible.

Scottish Water declined to comment on the analysis, stating they had not been notified of any identified illegal spills by the campaign group.

Additional data from environmental charity Surfers Against Sewage indicated that water companies discharged sewage into bathing waters for a total of 116,000 hours since mid-May, with 70% of these releases exceeding acceptable limits set by the Environment Agency. The findings underscore ongoing concerns about water pollution and regulatory enforcement across the UK’s water industry.