Combined sewer overflow permits and how to check sewage spills
A combined sewer overflow releases diluted untreated sewage when rain overloads the sewer. How CSOs are permitted, monitored and mapped in England.
A combined sewer overflow (CSO) is an outlet on a combined sewer, one pipe for sewage and rainwater (Defra), that releases diluted untreated sewage to a river, the sea or groundwater when capacity is exceeded (s.141E). In England these are storm overflows: each needs an Environment Agency permit (regulation 12), each has an event duration monitor (EA, March 2026), and the revised trends file puts 2025 at 292,119 spills (annual returns). Wales, Scotland and Northern Ireland use other regulators.
What is a CSO in law?
"CSO" is the engineering term. Storm overflow is the legal one. Section 141E defines it as a structure or apparatus that discharges into inland waters, underground strata or the sea when the capacity of the downstream system, or of the storage tanks at a sewage treatment works, is exceeded. The Environment Act 2021 inserted that definition, in force since 9 January 2022.
Section 141E(2) excludes discharges from electrical power failure or mechanical breakdown at sewage disposal works, rising main failure, and a blockage downstream of the overflow. Those are emergency overflows. Section 4 of the Water (Special Measures) Act 2025 would add a definition and a publication duty in new sections 141F and 141G. It is not yet in force.
Defra describes combined systems as carrying "both rainfall runoff and foul flow". England has about 14,500 storm overflows. The 2025 return lists 14,272 with a monitor.
Which permit allows a storm overflow to spill?
A spill from a CSO is a water discharge activity under Schedule 21 of the Environmental Permitting (England and Wales) Regulations 2016. Regulation 12 says nobody may "cause or knowingly permit a water discharge activity" except "under and to the extent authorised by an environmental permit". The sewerage company that owns the sewer is treated as causing the discharge.
The permit does not allow a sewage spill at any time. Most permits set pass-forward flow conditions, and overflows that run in dry weather are classed as unsatisfactory (EA permit guidance). Permits are meant to limit use to rainfall and snowmelt (EA, March 2025).
A dry day means no rainfall above 0.25mm on that day and the preceding 24 hours (EA, August 2024). Each dry-day spill is a potential permit breach until someone investigates it. Since January 2025 the EA has classed every dry-day spill as a pollution incident (EA, March 2025).
How EDM data counts a sewage spill
An event duration monitor records when an overflow starts and stops. Spills use the 12/24 method in the EA permit guidance. Any discharge in the first 12-hour block counts as one spill. Each later 24-hour block with a discharge adds one more. Counting stops once a 24-hour block has no discharge. The next discharge starts the count again. So read spill counts alongside hours.
The EA annual returns are the national history. The 2025 column is the August 2026 revision of the long-term trends file. Averages are per overflow that returned spill data, and they are rounded.
| 2024 | 2025 (revised Aug 2026) | |
|---|---|---|
| Spill events | 450,398 | 292,119 |
| Hours spilling | 3,614,428 | 1,878,240 |
| Overflows with EDM | 14,254 | 14,272 |
| Average spills per overflow | 31.8 | 20.5 |
| Average hours per spill | 8.0 | 6.4 |
The March 2026 release reported 291,492 spills, 35% down on 2024, before that revision. The revision corrected Anglian Water and Northumbrian Water data. But the EA said the figures were heavily influenced by rainfall in a dry 2025. Fewer spills alone do not prove better performance.
How to check a sewage spill near you
Live spills on the company sewage map
Since 1 January 2025, section 141DA has required sewerage companies to publish a storm overflow's start and location within one hour of the start, and the end within one hour of the end. The near real-time sewage map is the company's page. Southern Water's sewage discharge map is one. Thames Water runs another. The EA return is history, not a live feed.
Section 141DB provides for continuous upstream and downstream water quality monitoring of storm overflows.
Spill history in the EA return
For past spills, use the EDM annual return: counts and hours for each of the 10 sewerage companies, 2020 to 2025, with trends back to 2016, plus an interactive spill map. Companies must also publish their own reports before 1 April the following year. That annual report duty was inserted by section 80 of the Environment Act 2021. It is not the EA return.
Permits behind the outfalls
To see the permit for an outfall, search the EA public register of water discharge consents by name, postcode or local authority. CSO entries show site type "Combined Sewer Overflow" and effluent type "Sewage - Storm Sewage", with the permit number, operator, grid reference and dates. Check the revocation date. Plenty of entries are old permits that have been replaced. The bulk file is Consented Discharges to Controlled Waters with Conditions.
If you see sewage in rivers or the sea, report it on GOV.UK or call the EA incident hotline on 0800 80 70 60.
Discharge reduction targets
The Storm Overflows Discharge Reduction Plan (2022, updated 2023) set the targets. The December 2025 progress report states them as follows.
- By 2035, storm overflows discharging into or near designated bathing waters must significantly reduce harmful pathogens.
- By 2035, 75% of storm overflows discharging into or near high priority sites must not cause local adverse ecological impacts. By 2045 that covers the rest of those sites. By 2050 it covers every storm overflow in England.
- By 2050, no storm overflow should have more than an average of 10 rainfall-driven spills a year, averaged over 10 years.
The same report says the share of overflows near high priority sites meeting a 10-spill proxy rose from 22.6% in 2022-23 to 33.7% in 2024-25. About £8bn (2022-23 prices) is going into more than 2,300 schemes in 2025-2030. The government has also pledged to halve storm overflow spills by the end of December 2029, against 2024.
On 21 July 2025 the government said it will replace Ofwat with one regulator, also taking water functions from the EA, Natural England and the Drinking Water Inspectorate. Permitting stays with the EA until that body exists.
You can ask RegAffairs AI about combined sewer overflows and get answers with links to the EA register entries behind them.