Robert Geach, 54, was working on the sand filtration unit at the Falmouth Waste Water Treatment Works on 30 December 2013, Truro Crown Court was told, when he slipped and fell into the tank.
South West Water dispatched a colleague to the site four and a half hours later, in response to its lone worker alert system. He found Geach floating face-down in the water.
HSE inspector Georgina Speake said: “This tragic case could have been prevented if the company had reduced the size of the hatch used to access the sand filters, and properly considered hazards of the operations, including how close Mr Geach was to the water.
“Mr Geach was exposed to the risk of drowning which could have been easily controlled if the task had been properly planned and simple measures adopted earlier, which South West Water failed to do so adequately.”
South West Water, which provides services to Cornwall, Devon and some parts of Dorset and Somerset, pleaded guilty to breaching s 2(1) of the Health and Safety at Work Act. It must also pay £41,608 costs.