Record sewage spills in 2024, says Environment Agency

What can monitoring professionals learn from yet another dire set of results for the water sector?

By Jed Thomas

In 2024, raw sewage was discharged into England’s rivers and seas for a record 3.61 million hours, according to new Environment Agency data published on March 27.

While the number of individual spill events was slightly lower than in 2023, the average duration per spill increased, driving up the overall hours.

But this year’s data marks more than just a grim milestone — it represents a transformational shift in monitoring capacity. For the first time, 100% of England’s Combined Sewer Overflows (CSOs) were equipped with real-time monitoring, offering an unprecedented, high-resolution view of sewage discharges across the country.

A milestone in environmental data collection

Prior to 2024, the lack of comprehensive monitoring meant the true scale of sewage pollution was largely unknown. That changed when every known CSO — over 15,000 sites — was fitted with Event Duration Monitors (EDMs).

EDMs are sensor-based instruments that detect and timestamp when a CSO begins and ends a spill. While they don’t measure volume or chemical content, they deliver critical temporal data: the start and end times of each discharge. This enables authorities to calculate both the number of spills and total duration with precision.

These monitors are typically installed in overflow chambers or outfall pipes and operate year-round, transmitting data to both the water company and regulators.

This complete dataset offers a level of granularity that was impossible in earlier years — and lays a foundation for accountability, enforcement, and infrastructure planning.

Building the dataset

Monitoring coverage has grown steadily over the past decade, driven by public pressure and regulatory mandates. In 2021, the UK government set a legal requirement for full CSO monitoring by the end of 2023. That target was met, just in time to generate the 2024 dataset — the first truly system-wide record of raw sewage discharges.

The result: a 3.614 million-hour record, revealing not a sudden worsening, but a longstanding problem finally seen in full.

Why monitoring matters

Accurate monitoring is not just a technical achievement — it’s a critical enabler of data-driven environmental policy. It allows stakeholders to:

• Benchmark performance across water companies.

• Pinpoint hotspots for chronic pollution.

• Track progress over time as infrastructure investments are made.

• Hold companies and regulators accountable using transparent, independently verifiable data.

Case in point: the River Ver in Hertfordshire — a globally rare chalk stream — was shown to have endured over 3,300 hours of sewage discharge in 2024. That figure would have been unknowable before comprehensive monitoring was implemented.

What is still missing from discharge data?

While 2024 marks a breakthrough year for sewage data transparency in England, it’s important to understand what the current monitoring system does — and doesn’t — capture.

Event Duration Monitors (EDMs), now installed at every Combined Sewer Overflow (CSO) site in England, tell us when each overflow starts and stops. From this, we can determine how many times a year each CSO spills and for how long — giving us, for the first time, a complete timeline of sewage discharge activity nationwide.

However, EDMs do not measure the volume or content of what’s being discharged. That means while we now know how long raw sewage is flowing into rivers and seas, we still don’t routinely track what’s in it, how much is coming out, or how dangerous it might be in any given instance.

To fully understand the environmental and public health risks posed by CSO spills, three key pieces of data are still largely missing from standard monitoring:

Volume of discharge:

We don’t know how many litres of sewage are released during each event.
A two-hour spill could release very different amounts depending on the size of the pipe, flow rate, and rainfall intensity.

Pollutant composition:

CSOs can carry a mix of harmful substances:

• Pathogens (e.g. E. coli, enteroviruses, antibiotic-resistant bacteria)

• Nutrients (nitrates, phosphates)

• Pharmaceutical residues

• Microplastics

• Heavy metals

• Endocrine disruptors

But there is no automated system at overflow points measuring the presence or concentration of these.

Real-time or near-real-time water quality data:

While some water quality sensors exist in rivers and coastal areas, they are limited in number and location.
Most water chemistry data comes from spot sampling, not continuous tracking — and often after pollution has already occurred.

Why this matters

Without this kind of granular pollutant data, the full impact of sewage discharges remains partially hidden, even with complete EDM coverage. For instance:

• A short spill from a highly contaminated treatment plant may cause more damage than a longer spill diluted by stormwater.

• High concentrations of bacteria or drug residues may pose a greater risk to swimmers and wildlife than nutrient pollution alone.

• Certain water bodies — like chalk streams, which are uniquely sensitive ecosystems — may be disproportionately harmed by pollutants that aren’t even being tracked.

This data gap makes it harder for regulators to prioritise interventions, enforce rules, or assess risk in real time. It also complicates efforts by researchers and activists to hold companies accountable for ecological damage or threats to public health.

Closing the monitoring gap

To truly understand the environmental and health impacts of sewage pollution — and to make meaningful improvements — future monitoring efforts need to expand beyond just when spills happen.

Emerging technologies could help, including:

• Automatic samplers that collect water during spills for lab analysis.

• Real-time sensors that can detect chemical or biological markers in discharge.

• Machine learning models trained on weather, land use, and historical water quality data to estimate pollutant loads when direct measurement isn’t feasible.

For now, the 2024 data gives us an essential foundation: a complete map of sewage discharge duration across England. But to fully see — and fix — the problem, we’ll need to match that timeline with equally robust data on volume, composition, and impacts.