Storm overflow spills in the village of Altarnun in Cornwall are reducing following new engineering work by South West Water.
Altarnun Sewage Treatment Works on the edge of Bodmin Moor, was one of South West Water’s highest spilling storm overflows in 2024 with 206 spills recorded.
Engineers at the water company knew that during sustained downpours Altarnun’s treatment works could become overwhelmed, triggering the storm overflow designed to protect homes and streets from flooding.
In moorland areas like Altarnun, groundwater levels can fluctuate significantly, which can lead to more surface water and groundwater finding its way into local sewers.
The extra water can put additional pressure on treatment works, especially during wet weather, and this was the case in Altarnun.
To stop more water from finding its way into the sewers and to help the treatment works cope with demand, South West Water’s solution was two-fold.
Firstly, to keep water out of the sewers, teams completed CCTV surveys to identify the sections of the pipe that were worst affected by infiltration.
They then installed a protective lining to 40 metres of sewer in the area and carried out repairs across the local network, including upgrades to six manholes.
Secondly, teams installed a new storm storage tank at Altarnun Wastewater Treatment Works to increase capacity. This allows additional flows from heavy rain to be held at the treatment works when the site is at capacity, so it can be returned to the treatment process when flows reduce.
South West Water says that thanks to the targeted improvements, the number of spills fell by 20% in 2025.
Hazel Tranchant, Head of Tactical Asset Management at South West Water, said:
“Customers quite rightly want to see storm overflows operating less often.
“At Altarnun, we’ve focused on reducing the amount of extra water entering the sewer network and increasing storage for rainfall so the system can cope better.
“We know there is much more work to do to reduce spills across the region but it’s great to see how targeted improvements like this are making a difference and that we’re heading in the right direction.”
Altarnun formed part of South West Water’s £16 million Infiltration Reduction Programme, which targeted 175 high-risk sites after reviewing performance across more than 1,600 storm overflows.
With the UK government demanding a 50% reduction in storm overflow spills by 2029, the era of reactive management is over. Speaking in the House of Commons on 21 July 2025, then environment secretary Steve Reed said, “This Government will cut water companies’ sewage pollution in half by the end of the decade.”
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