The Halifax Water Commission has ordered builders of the city's three new sewage treatment plants to replace faulty ductwork, CBC News has learned.

Water Commission spokesman James Campbell said "prematurely corroding" ductwork was discovered about a month ago in the Dartmouth Wastewater Treatment Plant. 

Crews checked the ducts after air pressure monitoring devices inside the plant indicated there was a problem.

"Essentially, what we've determined is that there is premature corrosion in the ductwork," Campbell told CBC News.

The problem is not confined to the Dartmouth treatment plant.

When the commission checked, it found faulty ductwork in its sister facilities in downtown Halifax and in Herring Cove. Campbell said faulty ductwork is also being removed in those facilities.

Halifax took ownership of the three plants in late 2009 and early 2010. They are integral pieces in the municipality's $332-million Harbour Solutions Project to treat sewage and storm water before it flows into Halifax Harbour.

The Water Commission said all three plants are still functioning normally and successfully treating sewage. 

Campbell could not say how much faulty duct piping needs to be replaced or what it will cost.

"We don't know at this time," he said. "It's being investigated. As we find chunks that need to be replaced, it's being identified to the contractor, and they are replacing it. It will take as long as it takes to replace."

Campbell said the contractors — Degrémont of France and Dexter Construction Company Ltd. of Bedford, N.S. — will pay for the repairs.

"We've identified a deficiency, and we're taking action to make sure the contractor pays for it on their nickel," he said. "It is not anything that will be borne by ratepayers. Halifax Water is holding their feet to the fire, and it will be fixed by the contractor."

Neither Degrémont nor Dexter responded to the CBC's requests for comment.

Not the first problem

The setback surfaces two months after Halifax Mayor Peter Kelly took a swim in Halifax Harbour to celebrate the return to operation of the Halifax plant.

The $54-million facility had been operating on a trial basis for only a month when it was shut down in January 2009 because of a malfunction that caused a catastrophic flood. Repairs and a redesign took nearly a year and a half.

On Wednesday, Mayor Peter Kelly said the repairs are covered by a three-year warranty.

"Thank God for the warranties that we have in place," Peter Kelly told CBC News. 

"With a project this large — a $332-million project — from time to time, you will pick up points of concern. Our staff did pick up on that."

While the Halifax Water Commission said it learned of the problems only recently, Kelly said unspecified problems with ductwork were known about for months and brought to the attention of the contractors before the city took ownership of the plants.