The boil-water advisory, issued Wednesday when the city began receiving calls about discolored water in Montreal's Sud-Ouest distract, was the largest the city has ever experienced, covering a region with 1.3 million residents, Canadian Broadcasting Corp. News reported Friday.
After testing the problem was traced to sediment in water at the nearby Atwater water filtration plant, city officials said.
"The tests were done. They have come back clean," Mayor Michael Applebaum said.
There would be a "rigorous analysis" of the events that led to the sedimentation in the water, he said, to determine if there were technical breakdowns or negligence involved.
"In cases like this, we have a responsibility not to take any chances and to take every necessary measure to ensure the health and safety of Montrealers," he said.