During periods of heavy rainfall, pipes known as storm overflows aim to prevent excess water overflowing into the sewage system. But this is not always the case.
During intense rainfall, combined sewers (which carry both wastewater from homes and businesses and rainwater running off roads and buildings into drains) can exceed their capacity. To reduce the risk of sewage flooding homes, businesses and roads, storm overflow pipes can contain excess water.
Then when the time is right, these pipes can provide a controlled release point for excess water to flow out into rivers and the sea. These discharges are legally permitted under very specific conditions, mainly during very heavy rainfall when the sewer network would become overwhelmed. But public concern has grown over how frequently they are used and what effect they may have on the environment.
To address these concerns, since January 2025, water companies in England have been required to publish near real-time sewage discharge data via websites like Sewage Map or Waterwatch.
The aim was to improve transparency and allow the public, journalists, environmental organisations and researchers to better understand when sewage was entering rivers. For many people, particularly swimmers and other recreational water users, maps that show live discharges have become a way to decide if its safe to enter the water.
However, there is a problem: the data does not work in the way many people assume it does.
In December 2025, I began monitoring sewage discharges to investigate whether the information could help explain some of the microplastic pollution I was finding in the River Thames. I quickly realised that interpreting the data was far more complicated than expected. The issue was not only that the data changed over time, but that the information being provided did not answer the questions needed to understand environmental impact.
What sewage spill hours represent
England’s event duration monitoring (EDM) system records when a sewage overflow starts and stops operating. Essentially, it tells us how long a discharge appears to have lasted.
But duration alone tells us very little about the pollution released.
The data does not show how much sewage entered a river, lake or coastal water, what pollutants were present, whether the discharge was diluted by rainfall, or what effect it had on aquatic ecosystems. Nor do they measure bacterial contamination or indicate whether the water is safe for swimming. They also cannot tell us whether a spill caused harm to fish, plants or other wildlife.
A one-hour sewage discharge in a small river during dry weather could have a very different environmental impact from a one-hour discharge into a river that is high due to heavy rainfall. Both occurrences would appear identical in the EDM data, despite potentially having very different consequences.
Similarly, several shorter spills may release more pollution overall than one longer discharge. Duration is therefore not a direct measure of environmental harm.
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Live data is not always final data
Another challenge is that sewage discharge duration figures can change after they have been published.
By monitoring 91 sites along the tidal River Thames between December 2025 and January 2026, I observed that reported sewage spill durations (1,988 hours or almost 83 days) were later reduced by 82.9% (339 hours, 14 days) after validation.
During the monitoring period, Anglian Water’s publicly reported live discharge durations were reduced by 93% from more than 1,551 hours (64 days) to just 108 hours (4.5 days). At one site near Southend-on-Sea, a coastal city in Essex, an apparent spill lasting more than 850 hours (33 days) was later revised to just nine minutes.
These changes occur because live EDM systems depend on automated sensors that can later be reviewed and corrected by water company staff. Equipment faults, telemetry problems or precautionary alerts may result in data being amended once further checks are completed. So in some cases, overreporting of spill duration can occur, then be amended. While sometimes a spill will not be reported on the monitoring sites, yet still be actively spilling.
This does not mean the original data was deliberately misleading. Instead, it shows us that live figures should be viewed as provisional information rather than confirmed environmental records.
Transparency is not the same as understanding
Publishing live sewage data represents an important step towards environmental transparency. Without it, many sewage discharges would remain hidden from public scrutiny.
However, transparency alone does not tell us whether rivers are improving or declining.
To understand the true environmental consequences of sewage discharges, monitoring needs to go beyond spill duration. EDM data should be combined with measurements of discharge volume, river flow, microbial contamination, chemical pollutants and ecological indicators such as fish and invertebrate populations.
This broader approach would help answer the questions that matter most. How much pollution entered the river? What was in that pollution? Where did it travel? And what damage did that cause? Those are also the questions swimmers, recreational water users and researchers want answered to determine whether its safe for wildlife and for people entering the water.
The challenge is becoming increasingly important. Climate change is expected to increase the frequency of intense rainfall events and droughts, while population growth and ageing Victorian sewer infrastructure continue to place additional pressure on wastewater systems.
Live sewage monitoring has opened the door to greater public awareness, but the next step must be improving what is measured. Knowing when a spill happened is useful. Understanding what that spill means for water quality, ecosystem health and public safety is essential.
