Storm overflows
Storm overflows are locations where raw (untreated) sewage may discharge into rivers, streams and the sea. They operate when sewers are full or sewage treatment works are at capacity, to stop sewage backing up into homes.
Before 2019, there was no legal requirement to monitor the frequency of untreated sewage discharges via storm overflows.
In 2019 only 66% of storm overflows on the network had been installed with 'event duration monitors' (EDMs), which record storm overflows stopping and starting. In 2023, this figure was up to 97%.
In 2020, the extent of raw sewage discharges was revealed to the public for the first time: in the previous year, raw sewage discharged into rivers and seas in England more than 200,000 times, for more than 1.5 million hours.
The table below shows the national trend of sewage discharges over the past 5 years. Apart from 2022, which was an exceptionally dry year, it shows a general increase in the number and total hours of sewage discharges as more monitors are fitted on the network:
| Year | % of overflows with monitors installed | Number of sewage discharges | Total hours of sewage discharges |
|---|---|---|---|
| 2019* | 66% | 204,134 | 1,528,097 |
| 2020* | 80% | 403,043 | 3,101,234 |
| 2021 | 87% | 466,675 | 3,457,096 |
| 2022 | 91% | 384,568 | 2,348,439 |
| 2023 | 97% | 579,581 | 4,608,495 |
The Environment Act in 2021 passed with added amendments on sewage pollution after a public outcry and a groundswell of campaigning (see Data makes a difference). The law now requires sewerage companies to:
- Report within 1 hour the start and stop of a storm overflow. Other than Thames Water's storm overflows, this has not been implemented yet, but is expected later in 2024.
- Continuously monitor the water quality upstream and downstream of a storm overflow or outlet from a sewage treatment works. Roll out is expected in 2025 and to be completed by 2035.
- Secure a progressive reduction in the adverse impacts of discharges from the undertaker's storm overflows.
- The Secretary of State must prepare a report on reducing and eliminating storm overflows (see below).
The Storm Overflows Discharge Reduction plan was published in September 2023. It has the following headline targets:
- by 2035, water companies will have to improve all storm overflows discharging into or near every designated bathing water; and improve 75% of overflows discharging to high priority nature sites.
- by 2050, this will apply to all remaining storm overflows covered by our targets, regardless of location.
Thames Water have their own internal goals. They aim to reduce the number of sewage discharges per storm overflow to 24 per year by 2025, and then to 17 per year by 2030.