Montreal Mayor Denis Coderre has blasted the federal Environment Ministry and given Environment Canada officials three days to meet in Montreal to discuss the city's plan to dump of eight billion litres of wastewater into the St. Lawrence River.

"We don't need to play politics on the back of Montrealers," Coderre said at a news conference in Montreal hours after the federal government demanded that the city suspend the sewage dump

"We are dealing with a government that has decided to use a situation that is essential for Montreal and to play politics."

On Tuesday, Federal Infrastructure Minister Denis Lebel, campaigning in Saguenay, Que., read a statement from his cabinet colleague, Environment Minister Leona Aglukkaq, in which she asks her department to "explore options to prevent this release" while gathering more information about the plan's environmental impact.

"It's a press release from an environment minister who I know very well, who as usual said nothing," Coderre said.

The mayor said the city's experts spent the last year working closely with the province's environment ministry, and repeatedly asked Environment Canada officials if they had any questions about the dossier.

"They have this file in their hands [since] September 2014 … We asked, 'Do you have more questions? Do you have more questions?' They had no questions," Coderre said.

"Montreal did its homework. Speak to the experts who will tell you very clearly that this was the only option ... If the federal government wants to try to teach us a lesson, I suggest they invest millions of dollars and tell us how they will invest in the green industry ... Until then, we have work to do."

Federal leaders weigh in

Montreal's controversial plan to dump eight billion litres of wastewater into the St. Lawrence River has become a hot political issue, playing into the federal election campaign today.

The city is set to release the raw sewage from 26 sewer mains along the Montreal shoreline for one week, beginning Oct. 18, pending Environment Canada approval.

At a campaign stop Tuesday at Montreal's Olympic Stadium to talk about lifting children out of poverty, Liberal Leader Justin Trudeau asked reporters to "imagine the Big O filled with kids." Trudeau was asked, in turn, by a reporter to picture four Olympic stadiums filled with sewage being poured into the river.

Montreal faces 'impossible choice'

"Obviously this is a question that is worrying people," Trudeau said, adding that a Liberal government would be a better partner with cities by investing $20 billion in green infrastructure programs, including water treatment.

That investment, he said, would help cities such as Montreal "not to have to make impossible choices like the one it's faced with right now."

Montreal has said it has no option but to dump the raw sewage for a week, in order to move a snow dump that empties into a major sewage interceptor beneath the Bonaventure Expressway, a raised highway that runs along the downtown shoreline, which is being torn down.

The city estimates it would cost $1 billion to build a temporary diversion or storage area for wastewater.

The plan must still be approved by Environment Canada, which said Monday it is still assessing it.

Federal ministers urge caution

On Monday, Lebel said his government was "very concerned" about the plan.

It's so preposterous, it's obvious an NDP government would not permit it. – NDP Leader Tom Mulcair

Campaigning in B.C., NDP Leader Tom Mulcair said an NDP government wouldn't allow it.

"The idea that in 2015, we would release billions of litres of raw wastewater into the St. Lawrence and its ecosystem: It's so preposterous, it's obvious an NDP government would not permit it," Mulcair said.

In Monday afternoon's news conference, Coderre shot back at the NDP leader: "My dear Tom, we did all our homework."

A 'fiasco,' the CAQ says

The provincial Environment Ministry has already granted its approval, prompting Quebec's second opposition party, the Coalition Avenir Québec, to call on Quebec Environment Minister David Heurtel to resign.

The floating spa Bota Bota in Old Montreal is near one of the 26 interceptors where the billions of litres in wastewater would be dumped, pending Environment Canada approval. (Bota Bota)

The CAQ said Heurtel's job is to protect the environment in Quebec, saying he has failed by not finding a way to prevent this "fiasco."

"He treated this issue the same way you'd treat a simple dump of 100 litres," the CAQ's environment critic, Mathieu Lemay, said in a statement.

Sewage to flow from 26 spots

Details of how the city will dump the sewage are now emerging.

The wastewater is to flow out of 26 interceptors, which correspond to areas used for sewer overflow during heavy rains, documents obtained by Radio-Canada show.

The interceptors run most of the length of the island's shoreline, from Pointe-aux-Trembles in the east to LaSalle in the west.

Tourists who find themselves walking along the Old Port, where several cruise ships dock, may encounter a bad smell while the wastewater runs into the river.

Two interceptors are found along the river in Old Montreal, including one near the floating spa Bota Bota.

The largest release of wastewater is expected to be near Monseigneur-Richard High School in Verdun, an area already known to receive a high volume of waste during heavy rain. In all, three interceptors are located directly next to Nuns' Island.

Timeline of wastewater controversy

Mid-September: The City of Montreal publishes the plans for the construction project on its site.

Sept. 28: Official Opposition Projet Montréal releases statement expressing concern for the weeklong dumping of eight billion litres of wastewater into the St. Lawrence.

Sept. 30: The city's administration says the plans to dump the wastewater have been put on hold as it looks into other possible options.

Oct. 2: Pierre Desrochers, chairman of the city's executive committee, says the plan will go ahead. "This is the only option," he said.

Oct. 3: A petition against the plan garners more than 25,000 signatures.

Oct. 5: New York Senator Patty Ritchie weighs in on Montreal's decision, sending a letter of protest to the International Joint Commission — the group tasked with protecting the St. Lawrence River and Great Lakes.

Oct. 5: Mayor Denis Coderre holds press conference saying project will go ahead, pending approval from Environment Canada.