Most of the UK has a combined sewerage system, meaning that both rainwater and waste water – from toilets, bathrooms and kitchens – are carried in the same pipes.
Wastewater is usually sent to a sewage treatment works.
But capacity can sometimes be exceeded during heavy rainfall, especially if dry ground cannot quickly absorb large quantities of water.
This could lead to inundation of sewage works and potential flooding of homes, roads and open spaces.
The system is therefore designed to overflow occasionally, and discharge excess wastewater into the sea and rivers from Combined sewer overflows , external(CSOs)
However, according to Ofwat, some water companies have been “routinely releasing sewage” outside times of heavy downpours as a result of failing to manage their wastewater treatment plants in breach of their permits – known as dry spilling.
Earlier this year, the BBC found 6,000 potential dry spills by England’s water companies in 2022.
Water company permits also require them to treat a minimum amount of sewage before releasing it, even when it is raining.
BBC Panorama has found evidence that for the last three years United Utilities was discharging sewage into Lake Windemere before reaching this threshold.
