High bacteria counts have kept swimmers off Black Rock Beach in Point Pleasant Park for 17 months. (CBC)

A sewage treatment plant that broke down last year in heavy rain is once again fully operational.

Crews have now installed ultraviolet lights to disinfect the water, which was the last step before bringing the plant back online, Halifax Mayor Peter Kelly said Thursday.

With clean water now flowing into Halifax harbour, city officials said, city beaches including Black Rock and the Dingle will be safe for swimming in about three days. Once the beaches reopen, lifeguards will take water samples every week to ensure the harbour continues to be safe for swimming.

The beaches were opened in 2008 to much fanfare. The sewage treatment plant had opened months earlier and bacteria counts plummeted, allowing swimming for the first time in decades.

"We owe the public, of course, a great deal of gratitude for their patience and understanding and also their support," Kelly said, "but also their willingness to make sure that the job got done,"

The $55-million plant was disabled in January 2009 when a power failure caused catastrophic flooding just weeks after the city took ownership, resulting in raw sewage once again flowing directly into the harbour.

Kelly said repairs cost about $10 million.

"For the bulk of the cost, it's being picked up by insurance that was in play for the owners," he said, "so that has been recouped through there."

Kelly said the water in the harbour will be even cleaner than it was 17 months ago because two other new plants in Dartmouth and Herring Cove are also operational.