Lifting the lid on water quality with new data on CSO discharge

Storm outfalls or Combined Sewage Outfalls (CSOs) are a key source of pollution in the aquatic environment. Over the last year, there has been a massive increase in the number of monitoring systems. Now, with a new understanding of the number of discharge events taking place, the UK is now set to legislate for improvements.

Understanding the sewage outfall challenge

Stormwater discharge events releasing untreated sewage to the aquatic environment were designed as occasional events to prevent excess water backing up into homes and streets.

However, the latest data shows such events are becoming far more frequent than first anticipated in the UK.

Monitoring of stormwater discharge events is fundamental to understanding what is happening. As a result, Event Duration Monitoring (EDM) data is key to improving the environment.

As a result, the Environment Agency is pushing a programme to install monitors on every outflow in England and Wales by the end of 2023. Just over 12,000 of the 15,000 storm overflows in England were being monitored by the end of 2020. This is a significant achievement given that there were just over 800 monitored storm outfalls in 2016.

“A total of £1.1 billion is being invested by water companies to improve storm overflows over the next five years.”

Furthermore, although water companies are legally obliged to monitor and report EDM data from storm overflows, until recently, that data was not widely publicly available. Now it is reported annually, and the latest figures reveal the scale of the problem in the UK. Last year there were close to 15,000 separate overflow incidents across the country with an average duration of eight hours.

Achieving 100 per cent monitoring of the 14,630 overflows in England by 2023 could see £1.1 billion invested by water companies to improve storm overflows over the next five years, according to industry trade body Water UK.

The statement added that “Storm overflows account for only around 4 per cent of all the reasons for rivers and waterways not achieving good ecological status, so it’s essential to deal with all the other sources of harm.”

Taking action on water quality

Nonetheless, although the huge increase in monitoring is partly responsible for the increase in discharge figures, the numbers also reveal that the average number of spills per storm overflow was 33 over the year.

With figures showing a discharge on average every 10 days or so and some 400,000 discharge events, this cannot be considered an occasional event. It dwarfs the roughly 3,000 Combined Sewer Overflow (CSO) discharge notifications issued between 2019 and 2020 through the Safer Seas Service (SSS) from campaign group Surfers Against Sewage (SAS).

“The 2020 data indicates that, appallingly, almost one in five overflows across England are discharging more than 60 times per year, a number which is supposed to trigger an EA investigation,” noted Michelle Walker, deputy technical director of The Rivers Trust.

“This is a staggering statistic, and The Rivers Trust is now calling for greater transparency and publication of real-time monitoring of overflows which are discharging more than 20 times per year so the appropriate measures can be taken to improve the situation.”

Taking action on water quality

Given the growing evidence that storm outflow events are damaging UK waterways, earlier this year the Storm Overflows Taskforce – made up of Defra, the Environment Agency, Ofwat, Consumer Council for Water, Blueprint for Water and Water UK – agreed on a long-term goal to end the damaging pollution caused by storm overflows.

Through the Taskforce, water companies have committed to increasing the number of overflows they will improve by 2025, with a further 800 overflows to be investigated.

Even so, recording such a high frequency of storm outflow discharge events over the last year makes newly planned legislation to tackle water quality all the more significant. At the end of March UK Environment Minister Rebecca Pow announced that legislation designed to reduce the harm from storm overflows will be introduced.

"Increased data discovery and monitoring of storm outflows are already paying dividends in making the compelling case for action on water quality."

The Bill, which follows on from a private members bill from Philip Dunne MP, will create key duties for water companies, government and regulators.

For example, the government is to publish a plan to reduce sewage discharges from storm overflows by September 2022 and will have a duty to report progress on its implementation. Furthermore, under the Bill's terms, water companies will be required to publish annual data on storm overflow operation.

The government expects to open a consultation on potential options to eliminate storm overflow harm later this year.

In any event, increased data discovery and monitoring of storm outflows is the first step to managing and mitigating their impact. It is already paying dividends in making a compelling case for action on water quality.