Combined storm overflows

Combined Storm Overflows (CSOs) and how water companies deal with wastewater have recently been receiving significant coverage in the media. As a result, customers understandably have a lot of questions and concerns about them. We hope that many of these can be answered here.

Today, CSOs play an essential role in keeping all of our homes and communities safe from the risk of flooding during heavy rain (which happens more often due to climate change). But, we agree, that in an ideal situation, these wouldn’t be necessary and if we were building the infrastructure from scratch today, we would build them very differently. Unfortunately, this isn’t possible.

At Welsh Water we have over 36,000 kilometers of sewers that take your dirty water away before its cleaned at our local treatment works and returned safely to rivers and seas. Most of this network was built over 100 years ago during the Victorian times. So, whilst we have a programme of activity to replace, repair and update this network, it would cost between £9 billion and £14 billion to remove CSOs completely from our network. This would take decades and would make water and sewerage bills unaffordable.

But there are lots of things we are doing right now to monitor, repair and to report on CSOs, our infrastructure and the potential impact they have on the environment. And we work closely with our environmental regulators and other organisations to investigate, to make any improvements we can and to further understand how we can reduce our dependence on CSOs without putting our customers at risk.

It’s a long-term process, one which we have to innovate and adapt and invest in for many years to come. Between now and 2025, we're investing almost £100 million on improving these CSOs to reflect our customers' changing expectations and priorities as part of a wider investment package of £836 million to protect the environment. As a not-for-profit company, we are committed to ensuring that our operations do not adversely affect our land, rivers, and seas for now and for future generations to come.

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

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 £10.5 million in improving the monitoring of our CSOs since 2015, and now have spill monitors on 99% 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 then help us decide and prioritise what, if anything, we need to do about the overflows and by when.

2021 EDM Data

Below are the links to the data from our monitors for 2021 split according to the geographic areas of our operating area.