How the sewer network works

In total we take 3.1 billion litres of wastewater away from homes and businesses every day, including drainage from roads, highways and public spaces.

This water flows through approximately 93,000km of pipes and goes to our treatment works, of which we have more than 1,000.

Combined sewer systems in England transport rainwater runoff and wastewater, including sewage, in the same (combined) pipe to a sewage treatment works.

The combined sewer network is meant to accommodate some storm-water runoff, but not large amounts. When more storm and ground water enters the sewer system than the pipes were designed for, storm overflows act as relief valves to avoid homes and properties being flooded. 

We’re working hard to prevent river pollution

As part of our Get River Positive approach, we will ensure that our storm overflows and sewage treatment works do not harm rivers, based on the Environment Agency RNAGS measure.

The sector has made good progress in improving river health over the last 30 years. The water industry accounts for 27% of the remaining quality issues in England’s rivers and other sectors, including agriculture, housing and transport, accounting for 73% of the reasons for rivers failing to achieve good ecological status.

Not withstanding the progress made, we are responding to our stakeholders’ heightened focus on this important issue.

Our Get River Positive commitments and five River Pledges set out how we will make a positive impact on the environment and the communities we serve and help our region’s rivers be the healthiest they can be.

Climate change, population growth and an increase in hard landscaping – such as paved driveways - are contributing to more intense rainfall, more runoff and more sewage, increasing the pressure on the sewer system.

The Environment Agency (EA) sets strict permits to limit the impact of storm overflows on the environment. If permits are breached, then action will be taken against those responsible.

Severn Trent monitors its storm overflows to identify how often and for how long they are used, this is called Event Duration Monitoring (EDM). The data is used to track their performance and allowing us to respond to any issues to make sure they are working correctly.

The types of storm overflow

There are two main types of storm overflows:

What we’re doing to reduce the impact of storm overflows on rivers in our region

Combined Sewage Overflows are vital in protecting people from flooding and are widely used across the UK. Whilst CSOs are only responsible for 3% of reasons for rivers in the Severn Trent region not achieving good ecological status, we fully understand the concern around them and the impact they can have on rivers.  

Frankly, if we were designing a sewer network today, we wouldn’t create a combined system, we’d separate the sewer and surface water systems.

Our aim is to dramatically reduce the use of overflows, but with over 90,000km of sewer pipes, we can’t replace the system overnight. That said, there’s still lots we are doing.  

Increasing capacity

We’re also increasing sewer capacity and storage to slow the flow of water during wet weather, so overflows are less likely to be triggered.

Our commitment is to reduce the use of storm overflows to an average of 20 per year by 2025.​

We are continuing to invest £100m a year on our sewage network, improving our sewer systems, reducing sewer flooding and pollutions and installing more monitoring capability.

Monitoring

In December 2022, our teams successfully completed the installation of 100% of monitors on our c.2,400 combined sewer overflows (CSOs). These monitors record data every 2 or 15 minutes, providing us with over 300 million data records over the course of a year about how our CSOs are performing, ensuring that they are fulfilling the permit conditions as defined by the EA.  

100% of our sites have monitors installed as of December 2022.