The City of Montreal is going ahead with its controversial plan to dump eight billion litres of raw sewage into the St. Lawrence River.

Pierre Desrochers, the chairman of the city's executive committee, said at a news conference Friday morning that emptying a major sewer interceptor into the river is the only viable option that will allow necessary construction work to be completed.

This is the only option. - Pierre Desrochers , executive committee chairman

"After re-examining the situation, our administration has concluded that it's inevitable that we have to close the interceptor, even if it means diverting the wastewater to the river," said Desrochers, adding that the quality of drinking water will not be affected.

"We studied all other alternatives, and this is the only option."

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

Mayor Denis Coderre had temporarily put the brakes on the plan after a public backlash earlier this week, saying it needed a second look.

Project Montréal head Luc Ferrandez said that the city's administration is "indifferent" to the environment.

"Where's the mayor? If this was about baseball then he would be here," said Ferrandez.

Provisions to be taken by the city:

Any activity that involves direct contact with water will be prohibited from Oct.18-28 in Montreal's Southwest borough.

The same restriction applies to the King Edward Quay sector from Oct.18-Nov.15.

The city will ask businesses and residents along the St. Lawrence River from LaSalle to Rivière-des-Prairies to limit waste that goes in water during this period.

Snow dump to be moved

The construction work is tied to the demolition of the Bonaventure Expressway.

The city is moving a snow dump now located beneath the expressway.

Runoff from that snow dump empties into the interceptor, as does the industrial and residential effluent from many other sewer lines.

The sewage will be allowed to flow into the river beginning Oct. 18, and that will continue for about a week.

The province's Environment Ministry has approved the decision.

Up until the 1980s, it was common practice to clear Montreal's sewers this way, but it is no longer considered to be accepted practice. It's been six years since the city last dumped raw sewage into the river.

Pierre Desrochers says the city has studied all other alternatives and that this is the only option. (Stephen Rukavina/CBC)

Montreal will not be the first Canadian city to deliberately or inadvertently release untreated sewage into oceans, lakes and rivers. Victoria discharges millions of litres of raw sewage into the Strait of Juan de Fuca every day.

Since 2004, Winnipeg's outdated sewer system has led to 185 million litres of raw sewage being dumped into the city's waterways. In 2014, untreated sewage flowed freely into Halifax Harbour, years after Halifax's sewage treatment was finished.

City spokesman Philippe Sabourin said earlier this week that contamination is not a big concern because of the sheer size of the river and how quickly it flows. Water in the river flows at a rate of 6,000 to 7,000 cubic metres a second compared to the flow rate of the wastewater, expected to be just 13 cubic metres a second.

However, independent wastewater experts, biologists and other scientists have said the pollution will have an impact on aquatic life and on living organisms along the river's banks, and it could have a negative effect on communities downstream from the Island of Montreal.