Sewage discharges are the release of raw, untreated sewage into waterbodies, such as rivers. These discharges often take place through the operation of storm overflow valves. These are designed to release water from the sewer network when the volume of water is too great for it, for example due to heavy rainfall.
Some use of storm overflows is permitted, to avoid the sewer network becoming overwhelmed and risking sewage backing up into homes and businesses. However, environmental campaigners have alleged that many water companies are using storm overflows far more regularly than they ought to.
For further background and statistics, see the Library briefing on Sewage discharges.
Explore data on sewage discharges
Data sources and definitions
This analysis of sewage discharges is based on annual event duration monitoring (EDM) data published by the Environment Agency. The data covers England only (Dŵr Cymru Welsh Water is included in the dashboard, but data is only shown for areas of England that it supplies).
Sewage discharges are the release of raw, untreated sewage into waterbodies, usually through storm overflow outlets. They are also referred to as sewage spills. The source data records the number of times each storm overflow released sewage in 2024, and the duration of discharges.
The number of spills is counted using ‘the 12/24 counting method’, described by the Environment Agency as follows:
- Start counting when the first discharge occurs
- Any discharge (or discharges) in the first 12-hour block are counted as one spill
- Any discharge (or discharges) in the next, and subsequent 24-hour blocks, are each counted as one additional spill per block
- Continue counting until there is a 24-hour block with no discharge.
For the next discharge after the 24-hour block with no discharge, you begin again with the 12-hour and 24-hour block spill counting sequence.
The duration of spills is calculated before the data are processed through this method, so is not affected by how individual spills are counted.
The data does not record how much sewage was released in a discharge event. It also does not allow us to identify ‘dry spills’, which are where sewage is discharged at times of little or no rainfall.
Constituency and river figures
Constituency figures have been calculated by matching the coordinates in the EDM data with constituency boundaries. Some overflows that spill into the sea are outside the official boundaries of any constituency. These have been allocated to the nearest constituency instead.
Figures for river catchments and waterbodies have been calculated by matching the coordinates in the EDM data with operational catchments and waterbody catchments, respectively, in the Environment Agency’s Catchment Data Explorer.
The catchment of a waterbody is the area of land from which water flows into the waterbody. Operational catchments are larger and typically include tributaries to the primary river. However, in some cases waterbodies within operational catchments will have been grouped because they face similar water quality issues, rather than because they physically connected.
For tidal sections of rivers the Catchment Data Explorer records the high-tide extent of the river rather than an area of land around them. Overflows have been allocated to these river sections if they are within 500 metres of the river and are not within the catchment of any other waterbody.
Not all storm overflows are located within the official catchment area of a waterbody. These are excluded from the map and tables on the rivers tab (but are included in the constituency and water company maps and tables).
Data updates
We aim to update this dashboard once per year, shortly after the Environment Agency publishes new annual event duration monitoring data.