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. 

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 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 England 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

The rollout of our monitors has driven an acceleration of improvements
along our sewage network.

We are continuing to install monitors at all of our CSOs and sewage treatment works; these help us to assess performance and make the required improvements.

Currently 92% of our sites have monitors and 100% will be installed by the end of 2022.