Montrealers have been given the green light to drink their tap water, after tests on water samples incubated over the past 24 hours showed the water is safe to drink.

"The tests were done. They have come back clean," Mayor Michael Applebaum said.

Thus ends the largest boil-water advisory Montreal has ever experienced, more than 36 hours after it was first issued to a region covering 1.3 million residents.

The mayor advised residents to run their taps for a few minutes if the water is still discoloured, then turn the taps back off and on again. If the water runs clear, they can proceed to drink it.

'Rigorous analysis' to come, mayor says

Applebaum returned to City Hall for a hastily organized news conference at 10 p.m. on Thursday, interrupting a period of mourning for his brother who died after a long illness earlier this week.

"I decided it was important to be here tonight in order to reassure Montrealers," Applebaum said. "I fully realize the situation caused many inconveniences for many Montrealers, in addition to businesses, institutions and community groups."

"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."

Applebaum said there will be a "rigorous analysis" of the events that led to the sedimentation in the water, to determine if there were technical breakdowns or negligence involved.

"It is very clear we will look at how did we communicate — how long did it take, how fast did people get the information — in order that we can improve," he added.

As for compensation for restaurants and other businesses financially affected by the boil-water advisory, Applebaum said all claims would be referred to the city's legal department.

Warning system took too long, opposition charges

The first preventive boil-water advisory was issued shortly after 7:40 a.m. yesterday, when the city began receiving calls about discoloured water in Montreal's Sud-Ouest borough.

The problem was traced to sediment in water at the nearby Atwater water filtration plant.

The initial advisory was issued only for the Sud-Ouest borough.

The city's water services director, Chantal Morissette, said a decision was made an hour and 20 minutes later, at 9 a.m. Wednesday, to expand that advisory to cover a region affecting 1.3 million Montrealers. However, it took another hour — and in some boroughs, even longer — for that news to reach the media and to spread across the city.

Opposition municipal party leaders said the whole process simply took too long.

"We have emergency measures in place in Montreal," said Projet Montréal Leader Richard Bergeron. "Something simply didn't work."

Vision Montréal leader Louise Harel said the city needs a more effective strategy to ensure all residents are contacted quickly in cases like this.

"We certainly have questions for the civil security because some citizens received information and others not," Harel said.

Harel also criticized the city's executive committee, saying members should have made an effort to address the public formally on Wednesday.

'We did not hide'

The city official responsible for public security, Christian Dubois, defended the executive committee. He said all the members were held up in a meeting yesterday from 8 a.m. to 4 p.m.

"We have [a water specialist] that we asked to come forward and explain the situation. All the executive board was in a meeting," he said.

"We did not hide," Dubois said. "For the elected officials to come out — I guess in retrospect, yes, we could have come out and said, 'Everything is under control.' We did take the decision to leave it to the administrative part of the city. That's the choice we made. But we were aware of it."

Fire department division chief Gordon Routley said the city did everything it could to inform the public.

"The public security organization … immediately set about notifying people," Routley said.

He said the city took all necessary steps, using news outlets, social media and a reverse 911 system to call telephone numbers in the affected areas.

Quebec Premier Pauline Marois had nothing but praise for the city on Thursday.

"I must congratulate the City of Montreal," Marois said.

She emphasized the boil-water advisory was only a preventive measure, and the risk to the public was low.

City criticized for lack of communication

Many residents, however, were critical of how information was disseminated.

Even after the boil-water advisory had been issued in the Sud-Ouest borough, some borough residents who called the city’s information line said they were told that running their taps for a couple of minutes would fix the problem.

Other residents said they noticed issues with their tap water before the city sent out any warnings.

CBC Montreal’s morning radio talk show, Daybreak, received a text message from one listener about discoloured tap water at about 8 a.m. on Wednesday.

One employee at the Masson Hotdog and Deli restaurant in the city’s Vieux Rosemont district says he found out about the advisory from a customer.

"There wasn’t enough communication," employee Nick Milonakos said.

Fire chief creates 'whoops moment' buzz

City officials said the water problems stem from extensive renovations underway at the Atwater treatment plant.

In order for crews to work on the plant, the water in the reservoir was lowered, but on Tuesday evening, the level went lower than it normally should.

When the water was lowered, dust that had settled at the bottom of the reservoir was stirred up and mixed into the system.

On Wednesday, Routley gained some buzz when he referred to that point as the "whoops moment." On Thursday, he acknowledged to Daybreak that it probably was "not a very elegant quote."

Ronald Gehr, a McGill University engineering professor who has done consultation work at the Atwater plant, says the dust could actually be sand that has eroded from filters used to process the water.

"It's likely over the years the sand, itself, abrades and the grains rub against each other and very fine particles come off," he said.

Dominic Frigon, also a McGill engineering professor, said the contamination is probably not serious.

He said the presence of sediment would have set off an automatic warning system, which requires officials to follow up with water tests.