What are combined sewer overflows?

Ask someone to explain what a combined sewer overflow (CSO) is and they will probably look at you very confused.

However, these CSOs play a major role in preventing flooding in our streets and homes, but also, by doing this, they can affect bathing water quality.

Here’s how they work:

  • When the sewer system is operating normally, sewage leaves peoples’ homes and businesses, sometimes mixed with rainwater, and is sent to a nearby treatment works
  • If an area is hit by really heavy rain, like the kind we have seen in recent summers, the sewers sometimes become completely full of water and the sewage starts backing up
  • Imagine filling a bottle with water from the tap. Eventually the water will completely fill the bottle and start flowing out of the top
  • If there was no CSO in place, this sewage could enter people’s homes and streets, as the wastewater would force its way out of the network of pipes to the surface, often via manhole covers
  • With a CSO 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 sea
  • Under strict conditions, water companies 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 be times when storms completely fill it

It is important to remember that the Environment Agency estimates these CSOs only lead to around 30 per cent of sea pollution in the North West. We are committed to improving our beaches and working with our partners on tackling the other causes, such as rain running off highways and farm land, private drainage being incorrectly connected and people not cleaning up after their dogs.

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