Combined sewer overflows (CSOs)

During storms, when large amounts of rain get into the sewers, flows may arrive far more quickly than they can be collected and processed.

In these circumstances, we would provide storage facilities (normally large tanks) to store stormwater until the bad weather subsides.

Once that happens, the stored stormwater is returned to the works for full treatment.


Full storage tanks

However, if storm conditions continue once the storage tanks are full, then the stormwater arriving at the works is first filtered through screens and then diverted for immediate release through pipes, known as outfalls.

This type of ‘release valve’ in a sewerage system is vital. Without it the system would back up flooding homes, gardens, streets, highways and open spaces with diluted but untreated sewage from manholes, drains and toilets.

The release of stormwater during storms is a course of action regulated and policed by the Environment Agency through the issue of Discharge Consents

Practicalities dictate that a sewerage network cannot operate as a ‘closed’ system – it must have in place a stormwater release mechanism to safely get rid of excess flows during storm events.


High groundwater levels

On occasions, there are some locations where the groundwater level becomes higher than the sewers and drains.

If there are any defects in the pipes such as cracks or faulty joints, the groundwater may enter it. This overloading of the system may cause some CSOs and storm tanks to spill for longer periods than normal.

We are working in conjunction with the Environment Agency, uses sophisticated models to ‘build-in’ appropriate capacity into the whole sewerage system.

However, the UK climate continues to remain unpredictable and much of the sewer network was built many years ago.

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