Storm overflows run by South West Water spilled 2.8 times as often on average as those run by Anglian Water in 2025
Average number of monitored spill events per storm overflow with spill data, by water and sewerage company, England, 2020 to 2025. Spills per overflow.
Latest figures
| Series | Period | Value |
|---|---|---|
| All companies | 2025 | 20.5 |
| South West Water | 2025 | 34.0 |
| United Utilities | 2025 | 26.8 |
| Yorkshire Water | 2025 | 23.6 |
| Dŵr Cymru Welsh Water | 2025 | 23.3 |
| Wessex Water | 2025 | 18.3 |
| Northumbrian Water | 2025 | 17.9 |
| Southern Water | 2025 | 16.0 |
| Thames Water | 2025 | 15.9 |
| Severn Trent Water | 2025 | 15.0 |
| Anglian Water | 2025 | 12.0 |
Notes
- The average is each company's spill events divided by the number of its storm overflows that returned spill data that year. Companies monitored very different numbers of overflows in 2025, from 122 (Dŵr Cymru Welsh Water) to 2,431 (Severn Trent Water).
- The number of overflows monitored grew over these years, as companies fitted monitors to more of them, so a company's average can change because different overflows are included.
- All companies uses the England totals from the long-term trends table. In 2020 the companies' figures add up to 204 fewer spills than that total.
- Spills are measured by event duration monitors fitted to each overflow, and counted by the 12/24 hour method: the first 12 hours of a discharge count as one spill, then each 24-hour block after that with any discharge counts as one more, until a 24-hour block with none.
- Figures are for England only, and include Dŵr Cymru Welsh Water's overflows in England. Storm overflows are designed to release diluted sewage into rivers and the sea when heavy rain would otherwise overwhelm the sewers, so the number of spills depends partly on how much rain fell that year.
Sources
More on climate
- Fine particles (PM2.5) averaged 8.0 µg/m³ at urban background sites in the UK in 2025, and nitrogen dioxide 20.4 µg/m³ at roadside sites in 2025
- UK greenhouse gas emissions were 367 million tonnes in 2025, 54% lower than in 1990
- Low-carbon sources generated 64.3% of UK electricity in 2025, renewables 52.1% and fossil fuels 32.3%
- England's recycling rate for waste from households was 43.8% in 2024
- Monitored storm overflows in England spilled sewage 20.5 times each on average in 2025: 292,119 spills from 14,239 overflows