"A previously unknown pipe discharging to the river was discovered as part of the dye test. This has come from a pipe which isn't operated by the district council and our building staff are working with the owners of the pipe to make sure this problem is fixed."
He said the dye tests had nothing to do with checking for issues with the city's wastewater network.
"Dye tests are carried out on a case-by-case basis as part of one-off investigations.
"The frequency of testing varies, but it averages about one a month.
"The dye has no odour so any odour would be associated with the water in that particular pipe," Mr Carter said.