Firefighters were alerted to a fire in Swanage yesterday afternoon.
Dorset and Wiltshire Fire and Rescue Service were called ar 1.12pm.
Nick Corben, watch manager at Swanage Fire Station, said: "The fire was located in a lower ground floor area of a plant room at Wessex Water Sewage Treatment works.
"There was no one on site so we had to force entry to the plant room.
"The area was very heavily smoked logged and visibility was very poor.
"BA teams had to make their way down to the lower floor and they extinguished a fire which was in a large electric motor that operated a vent fan.
"The fire also involved a rubber drive belt, a fibreglass safety cover and a flexible joint in the ducting.
"It also damaged and melted a wall and overhead lighting - eight breathing apparatus, two hose reel jets, one main line were used and four fire engines were in attendance."
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