South West Water cuts storm overflow spills by 97% at one of Cornwall’s top spilling sites

Carnon Downs Wastewater Treatment Works, between Truro and Falmouth, had one of the region’s largest spilling overflows in 2023, activating 290 times.

Thanks to targeted improvements from the teams at South West Water these spills were reduced to just 8 in 2024, despite it being the wettest hydrological year on record, and just one spill in 2025 so far.

Groundwater, the water found underground in the cracks and spaces in soil, sand, and rock, was found to be entering the sewers which flow into the works and overwhelming it during periods of wet weather.

To hold these additional flows and prevent them from being released through the storm overflow, a new storm tank was built, providing up to half a million litres of extra storage.

The project also included a new pumping system and monitoring equipment to manage storm overflows effectively and ensure stored water is returned from the storm tank back to the treatment process as soon as site has capacity to treat it.

A screening unit also removes debris before water enters the tank, and a manual washdown system helps keep the tank clean and working at full capacity.

Andy Pettifer, South West Water’s Wastewater Operations Manager for West Cornwall, said:
“Our teams have worked hard to make these changes and it is great to see real improvements taking shape which reduce our storm overflow use and helps to protect the local environment.

“Our teams live and work in these areas, so we’re personally invested in getting it right. There’s more to do, but we’re committed to keeping up the momentum and making a lasting difference.”

South West Water is investing around £760 million over the next five years to reduce storm overflow spills across the region as it works to become the first water company to meet the government’s target of achieving less than 10 spills per overflow, per year, by 2040 – a decade ahead of target.