Dwr Cymru Welsh Water

Combined Sewers and Combined Sewer Overflows (CSOs)





One of the most important things we do is protect public health by taking away wastewater—which can include foul sewage and rainwater from roofs, gardens, and roads—from homes and communities to be treated and returned safely to the environment.

Most of our sewers are ‘combined sewers’. This is usually one single pipe that carries both wastewater from our homes and businesses and surface water from our gutters, drains and roads.


How Combined Sewers Work

When our sewer system is operating normally, combined sewers collect rain water that runs off gutters, drains and roads, as well as sewage. We call this wastewater, which then gets taken to our treatment works, where it is cleaned, treated and returned safely to watercourses and the sea.

This means that during heavy storms, more water is getting into these pipes than they can cope with so they have been designed to safely relieve the pressure on our systems. These release mechanisms are known as Combined Sewer Overflows, or CSOs.

Sometimes, during heavy rainfall, we have to protect our customers and their homes and businesses from sewer flooding so this CSO ‘release’ system is vital. Without it, sewerage systems would back up, flooding buildings, streets, highways and open spaces with diluted sewage and rainwater from manholes, drains and toilets.

Usually, this only happens when there’s been heavy rain, so any water that is released into rivers or the sea is unlikely to cause environmental damage.

We have permission for sewers to overflow via environmental permits from our environmental regulators, Natural Resources Wales and the Environment Agency.


Environmental performance and investment

As a company so closely linked to the environment, we are very aware of the impact our work has on our surroundings and take our environmental performance very seriously. Safeguarding our coastal waters and rivers from pollution is one of our top priorities and most important responsibilities.

Since 2001, we have invested over £1billion in sewage treatment and network improvements to improve bathing water quality which has resulted in Wales securing 47 Blue Flags for the 2016 summer bathing season (44 beaches and 3 marinas) – around a third of the UK’s Blue Flags – despite managing only around 15% of the British coastline.

As part of our Environmental Investigations Programme, we have placed monitors on many of our CSOs. These tell us the number of times a CSO has overflowed which confirms if our sewerage network is operating as it should – it does not measure how much water has been released, the quality of the water that has been released or environmental impact.


You can find out more about the CSOs with monitoring systems in your area here



Useful Information

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