Combined storm overflows


One of the most important things we do is protect public health by taking away wastewater from homes and communities to be treated before being returned safely to the environment.

We do this by operating a vast network of sewer pipes which if laid out in one line would stretch from Wales to Australia and back.

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 wastewater treatment works, where it is cleaned, treated and returned safely to the environment through watercourses or the sea.

During heavy rain storms, more water can find its way into our pipes than they are designed to cope with, so they have been designed to safely relieve the pressure on our systems. This is done through having release points - known as Combined Storm Overflows or CSOs - built into the system which release into watercourses.

Without these release points, the sewerage system would back up causing sewage flooding to buildings, streets, highways – or worse still cause toilets to overflow inside properties.

It is important to point out that CSOs are designed to operate during heavy rain so that if they do operate then any sewage present is heavily diluted with rain and surface water. 

The operation of our CSOs is highly regulated and is permitted and monitored by our environmental regulators Natural Resources Wales and Environment Agency. 

With climate change bringing more frequent extreme weather conditions it’s important to recognise this is having an impact on our CSOs.  During 2020, the UK experienced nine named storm events and was deemed the third wettest year on record since 1910.  February alone saw record rainfall as Storm Dennis hit causing flooding to thousands of homes and businesses.  As our sewerage system was not designed to deal with such intense storm conditions, it is inevitable that this will have increased the frequency that some CSOs have operated.

CSO performance and monitoring

As a company so closely linked to the environment, we take our environmental performance very seriously. Safeguarding our coastal waters and rivers from pollution is one of our top priorities.

We recognise that with environmental legislation tightening and customer expectations changing, there is increased scrutiny on how CSOs operate. That is why we are committed to being transparent with customers and stakeholders about how our CSOs are operating.

We have already invested £8.1 million in improving the monitoring of our CSOs since 2015, and now have spill monitors on 96.7% of all of our CSOs.  The monitors record the number and duration of spills and this data – known as EDM data - is published on our website as can be seen below.

We also provide real time spill information for key bathing waters to interested bodies, including Surfers Against Sewage.

If the monitors tell us that a CSO isn’t working as we’d expect, we’ll investigate to understand what is happening so that we can plan any improvements or investment that may need to be made.

To make sure this is acceptable to Natural Resources Wales and the Environment Agency, we share and discuss our EDM data with them on an annual basis. They can the help us decide and prioritise what, if anything, we need to do about the overflows and by when.

2020 EDM Data

Below are the links to the data from our monitors for 2020 split according to the geographic areas of our operating area.  As mentioned above, this year’s data should be viewed against the backdrop of the record rainfall our area received during the course of 2020.

Combined Sewer Overflow Booklet Thumbnail

Combined Storm Overflow Booklet

PDF, 411kB

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