South West Water (SWW) has moved to explain why some apparent sewage spills in Cornwall and Devon may not be actual spills. The company, facing criticism from Surfers Against Sewage (SAS) after a series of alerts, says that storm overflow alerts can sometimes be triggered by something other than a discharge.
Background: Alerts during a dry summer
Earlier this month, data from the Surfer-led campaign group, based in St Agnes, showed several sewage discharges from combined sewage overflow (CSO) outlets in Cornwall. These systems allow combined household sewage and surface run-off to be released, and are only intended for use during heavy rainfall when the network is at capacity. Alert locations included Looe, Mylor Bridge, Fowey, Falmouth, and Truro city centre — a surprise during what SAS described as the hottest and driest summer Cornwall has seen for a while, when overflows wouldn't typically be expected.
At the time, South West Water said many of its sensors were so sensitive they could give false readings because leaves or litter could create blockages; high-tide waters were also cited as a cause of occasional false alerts.
SWW's explanation: Event Duration Monitors and potential glitches
Now, the company has come forward with a more detailed explanation, saying that its storm overflow alerts on its data app can sometimes appear even when no spill has taken place. Alerts on its WaterFit Live system are generated by event duration monitors (EDMs) — sensors installed at storm overflow locations that detect when a spill event begins and ends.
But, as Helen Dobby, SWW's head of environmental performance, put it: "People understandably assume that a storm overflow alert means a spill has taken place. In the vast majority of cases that's exactly what it means, but occasionally we investigate an alert and find it was triggered by something other than a discharge."
SWW says those alerts can occasionally be caused by debris , obstructions, or by sensors and telemetry systems not working correctly. They cited a recent Falmouth case as an example of an alert that was not a spill after investigation.
Data sharing and 48-hour delays
Once an investigation shows no spill occurred, SWW says it updates WaterFit Live and shares corrected information with organizations like SAS. However, the same data used by SAS's Safer Seas & Rivers Service remains published for 48 hours, which the company said could lead to variations between what the public sees and the corrected status.
SWW said it is continuing to invest in monitoring equipment, sensors, and technology to help identify and correct false alerts as quickly as possible.