UK River Pollution: 2024 Storm Overflow Data exposes sewage discharge impact
27th March 2025
The Environment Agency has published its Event Duration Monitoring (EDM) data for 2024 showing the frequency and duration of spills from storm overflows in England. We break down what this means for the health of our fresh water, and what actions we believe need taking.
Waste water treatment
Urban wastewater, commonly referred to as sewage, is a mixture of domestic wastewater, industrial wastewater and rainwater run-off from roads.
Every day in the UK about 347,000 kilometres of sewers collect over 11 billion litres of wastewater.
This is treated at 9,000 sewage treatment works before the effluent is discharged to inland waters, estuaries and the sea.
The purpose of wastewater treatment is to remove organic substances to protect the environment.
During heavy rainfall the capacity of sewers can be exceeded, which means the possible inundation of sewage works and the potential to back up and flood peoples’ homes, roads and open spaces.
Combined sewer overflows were developed as overflow valves to reduce the risk of sewage backing up during heavy rainfall.
The Environment Agency’s EDM data
Storm overflow spills are measured by event duration monitors (EDMs). All storm overflows across the water network have been fitted with EDMs since 2023.
The dataset these monitors generate can be accessed via the Government website, and are summarised as long-term trends, as in the table below.
| EDM Annual Return: long-term trends | 2020 | 2021 | 2022 | 2023 | 2024 |
| Total number of storm overflows listed | – | 14,470 | 14,580 | 14,530 | 14,285 |
| Total no. active storm overflows with event duration monitors | – | 12,707 | 13,323 | 14,241 | 14,254 |
| Total no. storm overflows with spill data | 12,092 | 12,393 | 13,080 | 14,031 | 14,182 |
| Total number of monitored spill events | 403,375 | 372,533 | 301,091 | 464,056 | 450,398 |
| Total duration (hrs) of monitored spill events | 3,101,150 | 2,667,452 | 1,754,921 | 3,606,170 | 3,614,428 |
| Average number of spill events per storm overflow with spill data | 32.6 | 29.4 | 23 | 33.1 | 31.8 |
| Average duration (hrs) of each spill event per storm overflow | 8.1 | 7.4 | 5.8 | 7.8 | 8 |
Storm overflow spills – what do these data tell us?
Key observations
The data from 2023 to 2024 shows that the high frequency and duration of storm overflow discharges continue to be a significant issue.
High Spill Numbers Continue:
- The total number of monitored spill events remained very high, decreasing only slightly from 464,056 in 2023 to 450,398 in 2024. This indicates that the problem of frequent storm overflow discharges persists.
- The total duration of monitored spill events also remained very high, increasing slightly from 3,606,170 hours to 3,614,428 hour.
Average Spill Rate Remains High:
- The average number of spill events per storm overflow remained very high, decreasing slightly from 33 in 2023 to 31.8 in 2024.
- The average duration of each spill event remained the same at 8 hours.
The total number of storm overflows with spill data has increased from 12,092 in 2020, through to 14,031 in 2023.
The Environment Agency highlights that this makes comparisons across this time period difficult, however, in 2024 the number of storm overflows with spill data was 14,182, which makes comparisons between 2023 and 2024 data more reliable.
The total number of spill events in 2024 was 450,398 compared to 464,956 in 2023 (a 2.9% decrease) and the total duration of monitored spill events increased from 3.60 million hours to 3.61 million hours (an increase of 0.2%).
The average number of spill events per storm overflow was 31.8 compared to 33.1 in 2023, and the average duration of each spill event was 8 hours compared to 7.8.
While 39% of storm overflows spilled less than 10 times in 2024, this is still less than the 40.5% in 2023, and although 12.5% of storm overflows did not spill at all in 2024, this is less than the 13.9% in 2023.
UK river pollution in 2024
Many will be disappointed, but not shocked by these findings. 2024 was full of bad news for our rivers:
- In February, the Rivers Trust’s 2024 State of Our Rivers Report found that no river in England or Northern Ireland is in good overall condition.
- In the summer there were warnings over alarmingly high levels of E. coli in the Thames ahead of the Henley Regatta.
- Earthwatch Europe’s first Great UK WaterBlitz revealed high-levels of nutrient pollution in June and September.
- Also in September, the Government’s Water (Special Measures) Bill was heavily criticised by campaigners.
- Analyses in November revealed that more than three-quarters of English river sites sampled by the Environment Agency between 2023 and 2024 were found to be contaminated with one or more harmful neonicotinoid pesticides.
- In December Ofwat’s final determinations for the 2024 price review sparked outrage by shifting some of the cost of the £24 billion environmental improvements investment on to customers. Water bills in England and Wales will increase by an average of £31 per year between 2025 and 2030.
There is no doubt that our freshwater ecosystems are in a terrible state, due to a complex range of pollution sources including sewage discharge, agriculture and urban run-off.
Our rivers have been historically stressed by farming and are being pushed to the brink by outdated and inadequate sewage treatment works.
Citizen science water quality monitoring
The alarming state of UK river pollution calls for a unified national strategy. But change begins locally and the passion of individuals for their local rivers is a driving force for that change.
Citizen science provides a tangible way for individuals to make a difference.
By actively collecting water quality data, individuals can contribute to a comprehensive national picture, enabling informed action and driving effective solutions.
The citizen science revolution has already begun – as part of Earthwatch Europe’s two Great UK WaterBlitzes.
The WaterBlitzes are four-day campaigns in which volunteers assessed nitrate and phosphate nutrient pollution in their local rivers, lakes and ponds.
In June, 2,600 participants assessed the quality of 1,300 freshwater bodies; and in September this rose to 4,500 participants investigating 2,300 sites, despite it being one of the wettest weekends of the year.
Across eight days, 7,000 people took 3,600 measurement and created two incredibly detailed snapshots of our nation’s freshwater health.
Two robust, reliable datasets which can be used to help uncover non-compliance, improve water company performance, and identify pollution sources.
When communities have first-hand knowledge of pollution or degradation in their waterways, they are better equipped to hold polluters and policymakers to account.
This bottom-up pressure is an essential counterbalance to top-down regulation and helps foster accountability from all stakeholders across the water industry.
Citizen science isn’t just about gathering data; it’s about igniting a cultural shift. It breaks down the barriers of traditional science, empowering everyday people to contribute to research that directly impacts their lives.
By participating, individuals not only learn about environmental challenges but also gain the confidence to become powerful advocates for change in the face of UK river pollution.
Fixing UK river pollution
There are solutions to the complex problems of pollution in the UK’s rivers.
- The law needs to be enforced, with more regular and rigorous environmental inspections of water treatment operations and stiffer penalties, through properly funded, resourced and empowered regulatory bodies.
- A radical shift to make pollution unprofitable to incentivise water companies to dramatically reduce their negative effect on freshwater systems needs to be paired with effective leveraging of the Environmental Land Management schemes to encourage nature-friendly farming practises which eliminate the need for harmful pesticides and fungicides.
- Investments should prioritise ecologically sensitive and biodiversity rich areas like chalk streams; and nature-based solutions – sustainable drainage systems and wetlands – that can cost-effectively enhance freshwater ecosystems while generating multiple co-benefits should be implemented.
Although much of the responsibility for the health of our rivers lies with large organisations – governments and water companies – there are actions that we as individuals can take.
Being mindful of what we put down our drains and toilets, reducing water usage, and using our voice to demand that those in power prioritise the environment. Individual actions which add up to collective impact.
At Earthwatch, we hope that this latest Event Duration Monitoring data encourages more people to take action for their freshwater systems.
We hope that authorities will begin to recognise the power of citizen science and integrate citizen generated data into their freshwater monitoring frameworks.
We hope that our collective knowledge will spark collaborative action to create truly meaningful environmental change and bring an end to UK river pollution.
Help fight pollution by supporting the Great UK WaterBlitz
It costs £6 for each water testing kit used in each Great UK WaterBlitz and we rely on voluntary donations to support our work. Every donation means that we can test more rivers in more locations across the UK. Please donate to make the Great UK WaterBlitz as big as possible – the more data, the louder the call for change.
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