Storm Overflow Spills Cut by 97% at Carnon Downs Treatment Works

Major drop in discharges from Carnon Downs site thanks to new upgrades

South West Water has reported a 97% reduction in storm overflow spills at one of Cornwall’s worst-performing sites last year, with just one spill recorded so far in 2025.

Carnon Downs Wastewater Treatment Works, located between Truro and Falmouth, recorded 290 storm overflow activations in 2023. Following targeted upgrades by South West Water, that figure fell to just 8 in 2024 — despite it being the wettest hydrological year on record.

So far this year, only a single spill has been recorded at the site.

What’s caused the improvement?

The issue was traced back to groundwater — water found underground in soil, sand and rock — entering the sewers that feed into the works. During wet weather, this would overwhelm the system and trigger the storm overflow.

To tackle the problem, South West Water constructed a new storm tank at the site, adding up to half a million litres of extra storage. This helps hold the additional water and prevent it from spilling into the environment.

The upgrade also included:

• A new pumping system

• Monitoring equipment to manage overflows more effectively

• A screening unit to remove debris before water enters the tank

• A manual washdown system to keep the tank working at full capacity

Water stored in the tank is returned for treatment as soon as the site has the capacity to deal with it.

Local teams, local results

Andy Pettifer, Wastewater Operations Manager for West Cornwall, praised the work of the team and its impact.

“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.”

Bigger picture across the region

The work at Carnon Downs is part of a wider programme by South West Water, which is investing around £760 million over the next five years to reduce storm overflow spills.

The company is aiming to meet the government’s target of fewer than 10 spills per overflow per year by 2040 — a full decade ahead of schedule.