Combined sewer overflows

We don't usually like to think too much about what happens when we flush the loo but sewer overflows are an important part of the sewerage network which takes away the used water from our homes.

We call them combined sewer overflows (CSOs) and, while they probably don't crop up in your everyday conversation, they play a major part in preventing flooding in our streets and homes. However, if and when we need to use them, they can also affect bathing water quality in a particular area.

How these overflows work

  • When the sewer system is operating normally, sewage leaves our homes and businesses, sometimes mixed with rainwater, and is sent to one of our nearest treatment works
  • If an area is hit by really heavy rain, like the kind we have seen in more recent summers, the sewers sometimes become completely full of water and the sewage starts backing up
  • If there was no sewer overflow in place, this sewage could enter homes and streets, as the wastewater would force its way out of the network of pipes to the surface, often rising up through manhole covers
  • With a sewer overflow in place, the rain water, mixed with sewage, will rise inside the sewer and eventually enter a separate pipe which runs off the main sewer and flows into a river or the sea
  • Under strict conditions, and with the permission of the Environment Agency, water companies like United Utilities are allowed to spill wastewater into the sea because it is accepted there is a finite capacity inside sewer pipes
  • Even if a sewer is completely unobstructed and of the approved size, there could still be times when storm waters completely fill them

The Environment Agency estimates these sewer overflows lead to around 30 per cent of sea pollution in the North West, with water quality along our coastline also affected by lots of other causes. We are committed to improving our beaches and working with our partners on tackling these issues, such as rain running off highways and farm land, private drainage being incorrectly connected and people not cleaning up after their dogs.

You can read more about what we're doing to keep our waters clear of pollution, here.