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A council spokesman said the diver was one of a team of four external contractors ''brought into town specifically for the job''.
The job entailed being lowered into the sewage in a cage to carry out urgent repairs to the plant's ''aeration diffuser grid'' in one of the two wastewater tanks.
After completing the repairs, the diver went under again, in the second pond, just to check it was in good shape.
The work was planned by waste services company Veolia and a dive operator with ''extensive experience'' of diving in such environments.