Sewage in UK Rivers: The Data Behind the Crisis

The Scale of the Sewage Crisis

In 2023, water companies in England discharged raw sewage into rivers and seas over 464,000 times, for a total of 3.6 million hours. That's not a historical relic — it happened last year, and it's getting worse. Spill counts have roughly doubled since 2020, though some of this increase is attributed to better monitoring rather than more pollution.

Storm overflows are the mechanism. They're designed as emergency release valves — when heavy rain overwhelms the combined sewer system (which carries both rainwater and household sewage in the same pipe), the excess is discharged into rivers and coastal waters rather than backing up into homes. The problem is that water companies have chronically underinvested in sewer capacity, so 'emergency' overflows have become routine events.

The data is now public. Since 2021, water companies have been required to install Event Duration Monitors (EDMs) on their storm overflows and publish the data annually. This transparency has enabled public scrutiny, environmental campaigning, and data-driven analysis of which overflows are the worst offenders.

Understanding Storm Overflow Data

Each storm overflow has two key metrics: spill count (how many separate discharge events occurred in a year) and spill duration (the total hours of discharge). A single spill event might last minutes or days.

An overflow that spills 200 times for a total of 3,000 hours is far worse than one that spills 10 times for 50 hours. But the picture is nuanced — some overflows are in unpopulated areas discharging into large, fast-flowing rivers where dilution is significant. Others discharge into small streams near bathing waters or drinking water intakes.

The Environment Agency's Event Duration Monitoring data covers approximately 15,000 storm overflows across England. Not all are monitored (coverage has been increasing from about 80% to over 90% in recent years), and the monitors measure duration rather than volume, so the actual volume of sewage discharged is estimated.

Our Water Intelligence API provides this data at the postcode level — how many overflows are within 5km, their total spill counts and hours, and the nearest overflow with distance and discharge details. The Water Quality Score combines this with waterbody health data and bathing water classifications.

Water Company Performance

Performance varies dramatically between water companies. Some have invested heavily in infrastructure and reduced spill events. Others have allowed their networks to deteriorate while paying billions in dividends to shareholders.

Ofwat, the water industry regulator, publishes annual performance assessments. Companies are rated on multiple metrics including leakage, supply interruptions, pollution incidents, and internal sewer flooding. The worst performers face enforcement action and financial penalties.

The Environmental Performance Assessment (EPA) run by the Environment Agency rates companies from 1 star (serious concerns) to 4 stars (industry leading). In recent years, no company has consistently achieved 4 stars, and several have been rated at 1 or 2 stars — triggering additional regulatory scrutiny.

Our API includes water company identification and performance data in the water endpoint, so you can see not just the overflow data for a specific postcode but which company is responsible and how they perform overall.

How to Check Your Area

The free Sewage Checker tool is the fastest way to assess water quality for any UK postcode. Enter a postcode and you'll see: the Water Quality Score (0-100), the number of storm overflows within 5km, total spill counts and hours for those overflows, the nearest overflow with operator and receiving water details, and the nearest waterbody with its Water Framework Directive classification.

For a deeper analysis, the full Water Intelligence API adds bathing water site classifications (Excellent, Good, Sufficient, Poor), water company details including Ofwat ratings, detailed spill history for individual overflows, and a full score breakdown showing which factors are driving the water quality assessment.

If you're buying a property, the water quality data should be part of your due diligence alongside flood risk and ground stability. A property near a storm overflow that spills 300 times a year will have a different relationship with its local river than one in an area with minimal discharges.

For environmental campaigners and journalists, the API enables systematic analysis — identifying the worst overflows, tracking year-on-year trends, and holding specific water companies accountable with data rather than anecdote.

The Water Quality Score

The Water Quality Score is a proprietary rating from 0 to 100 that synthesises multiple water-related datasets into a single assessment of water quality for a given postcode. Higher means better water quality.

The score considers: storm overflow proximity and intensity (how many overflows are nearby and how frequently they discharge), waterbody health (the Water Framework Directive classification of the nearest river, lake, or coastal water), bathing water quality (if there are designated bathing waters nearby, what are their classifications?), and water company performance (how does the responsible company rate on environmental metrics?).

Each factor is weighted based on its relevance to the postcode — an inland postcode won't be penalised for poor bathing water scores because there are no bathing waters nearby, but a coastal postcode will.

Scores are rated as: Excellent (80-100) means minimal pollution concerns, Good (60-79) means generally acceptable but some factors to monitor, Fair (40-59) means notable concerns — investigate specific factors, and Poor (0-39) means significant water quality issues in the area.

Using Data to Drive Accountability

The availability of structured, postcode-level water quality data creates new possibilities for accountability. Environmental organisations can use the API to build interactive maps showing the worst-affected areas, track improvements (or deterioration) over time, and correlate sewage discharges with ecological impacts.

Local councils and water companies can use the data for planning decisions — understanding the cumulative impact of new developments on already-stressed sewage systems. A proposed housing estate upstream of a storm overflow that already spills 200 times per year should face harder questions about drainage infrastructure.

Citizens can use the free tool to check their local area and understand what's happening to their rivers. Public awareness has been a major driver of regulatory pressure on water companies, and accessible data tools accelerate this.

The structured API format makes it possible to build tools that would be impractical to create manually — automated monitoring, trend analysis, cross-company comparisons, and integration with ecological and public health datasets.