Sewage map

Our Sewage Map: Q&A

Emily Cooper


Last week, we released the 2020 version of our Sewage Map, helping people to make more informed decisions about how and where they use rivers recreationally.

Although the data set has been released into the public domain, there is a long way to go in terms of accessibility. Unless you're as nerdy about rivers as our Data team are, it can be pretty tricky to interpret. That’s why we’re answering some of your questions in this blog.

Our Data team created this map using data from the Consented Discharges to Controlled Waters database, managed by the Environment Agency (England) and Natural Resources Wales (Wales).

The data on our Sewage Map is called Event Duration Monitoring (EDM) data. EDM data is collected using water level sensors. These sensors detect the emergency releases at the points in the sewer network which are designed to allow untreated sewage to overflow directly into the environment during storm events. The sensors send a signal when untreated sewage and storm water starts to overflow and again when it stops and these start and finish times are used to calculate the duration of the overflow event.

No – for three reasons:

  1. The data is only available for ~80% of known overflows
  2. There are still a number of unknown sewer storm overflows
  3. Even monitored points may have inaccuracies in the recordings.

On point number one, we should have a fix by the end of 2023 as water companies have pledged to monitor 100% of known overflows. However, point two will be a bit tricker to fix. We want to also enlist citizen scientists to help us identify and map the currently unknown storm overflows, helping to build an even more comprehensive picture of river health. Sign up to our newsletter to keep updated on our citizen science programme, or contact your local Rivers Trust.

We know there are some errors in the database we were provided with, which means that sometimes a discharge may appear in the wrong place on the map. In addition, the Consented Discharges to Controlled Waters database only includes discharges with a permit. This means that discharges operating without permits are not included on the map.

This is could be due to a sewer misconnection (when the foul sewer drain is accidentally connected to a rainwater drain) or in some cases there is historic infrastructure that was never recorded, which is a legacy issue for water companies who are now trying to map the underground sewer network in more detail.

We don’t know. We all want to see improvements in the way storm overflows are handled—but it’s not possible to make a comparison year-on-year. This year, there has been a 50% increase in the number of storm overflows monitored. This in itself is a good thing, because it helps us to understand where the problems are, but when more outflows are observed, the number and duration of spills increases. This doesn't necessarily mean that sewage is spilling more frequently, but it makes it impossible to draw any conclusions about whether or not the situation is improving.

No. River pollution is complex, and there are a whole host of sources that contribute to polluted rivers. However, tackling sewage pollution is a great first step towards cleaner, healthier rivers.

Well, in an ideal world, we want wild, healthy, natural rivers, valued by all - but until then, we have a few key asks.

  • Improved accessibility

    While it’s great that this data is available in now available in the public domain, it’s far from accessible and understandable. Even with our team of Data & Evidence experts, interpreting the data proved difficult—and this shouldn’t be the case. When you’re talking about the health of the rivers that we all love and use, it’s important that the information is understandable to everyone.
  • Real-time monitoring

    We are now calling for real-time monitoring of storm overflows which are discharging more than 20 times per year so the appropriate measures can be taken to improve the situation. This will also help people to decide whether or not to use the river for recreation. After all, who wants to swim, paddle, catch or play in a smelly river?!
  • A resilient sewage system fit for the 21st Century

    Water companies have committed to investigating and improving 800 overflows before 2025 at a cost of £1.1 billion. This sounds great on paper, but in reality, it will only deal with 5% of the +14,000 SOs that are declared by water companies. We want to see an accelerated investment in fixing overflows and the upstream problems that cause them to spill more frequently than they should. In addition, investment in Nature Based Solutions will help to boost the resilience of our rivers while providing a whole host of other benefits for biodiversity, flood management and more.

Your support!

There are three really easy ways to support our movement.

  • Donate - your donations give us the freedom to keep fighting for causes we believe in. Ending sewage pollution is one of them!
  • Sign up to our newsletter - stay up to date on the latest developments in our fight to end sewage pollution
  • Learn more about sewage pollution - equip yourself with the knowledge you need to support our call for cleaner rivers.
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