How expanding Montreal's wastewater treatment plant will help the environment

The City of Montreal is looking to significantly reduce its own greenhouse gas emissions while vastly improving the St. Lawrence River's water quality by overhauling its wastewater treatment plant.

Retrofitting world's 3rd largest treatment plant with ozonation and improved incinerators will take 10 years

Industrial building's interior
Montreal's wastewater treatment plant, which opened in 1984, is the third largest in the world. It treats nearly half of Quebec's wastewater before releasing it into the St. Lawrence River. (Kwabena Oduro/CBC)

The city of Montreal is looking to significantly reduce its own greenhouse gas emissions while vastly improving the St. Lawrence River's water quality by overhauling its wastewater treatment plant.

As part of Montreal's 2023 budget (presented in November) the city earmarked $682 million over 10 years to replace the incinerators at the Jean-R.-Marcotte plant in Rivière-des-Prairies–Pointe-aux-Trembles. 

The incinerators there are responsible for more than 30 per cent of greenhouse gas emissions from municipal facilities, according to Mayor Valérie Plante.

There will also be $461 million spent over 10 years to improve the wastewater treatment process.

The city says the plant, which opened in 1984, is the third largest wastewater treatment facility in the world and treats nearly half of Quebec's wastewater before releasing it into the St. Lawrence River at a rate of between 2.5 and 7.5 million cubic metres — roughly the volume of the Olympic Stadium — per day.

But before that water is released, solids are removed. And where do those solids go? There are four huge incinerators on site which burn nearly 700 tonnes of waste per day. That's what produces the greenhouse gas emissions.

The city is looking to convert the incinerators into biodigesters, dropping emissions by as much as 92 per cent and creating energy, according to Maja Vodanovi, who is in charge of waterworks on the city's executive committee.

WATCH | Tour Montreal's wastewater treatment plant: 

Inside Montreal's wastewater treatment plant

5 months ago
Duration 2:51
CBC Montreal takes a tour of Montreal's wastewater plant on the east end of the island. It's the third largest facility of its kind in the world, processes 45 per cent of Quebec's wastewater and is responsible for 30 per cent of emissions from municipal buildings in Montreal.

At the same time, the city has to push forward with improved wastewater treatment to meet new government regulations, she said.

The plant will be turned into the largest ozonation plant in the world, said Vodanovi. 

"We have to retrofit it into an old building. So, it's quite a technological feat," she said.

"The ozonation facility we are implementing in there is huge. It has the size of eight locomotives that we are putting into the basins of the wastewater."

This, in turn, will have an impact on wildlife from the city all the way to Lake Saint-Pierre (150 kilometres away) and beyond, she said, as the water released into the river will be so much cleaner.

Reducing harm to St. Lawrence River

In 2021, a study found that high concentrations of E. coli bacteria colonies are entering the St. Lawrence River in the Montreal area and following the current at least as far as Lake Saint-Pierre and Trois-Rivières.

Ozonation is a procedure that consists of injecting ozone at the end of the treatment process.

Ozonation is "a very advantageous solution from an environmental perspective," the city says on its website. Work will be completed in several steps in order for ozonation to be implemented in 2025.

Once completed, nearly all viruses, bacteria and pharmaceutical products will be eliminated from the wastewater, the city says.

Former mayor Gérald Tremblay first announced the city would disinfect its wastewater using ozone gas in 2008, but there have been delays in the years since and cost estimates have continued to rise by hundreds of millions.

Wastewater doesn't just include what goes down the drain, but also all the runoff from city streets and all the melted snow.

That means the wastewater plant has to not only remove the trash and debris that people put down the drain, but also all the grit, grime, gravel and litter from the streets as well.

Sewer facility
Debris and trash must be removed from the wastewater and burned. (Kwabena Oduro/CBC)

During the expansion project, the treatment capacity of the plant will be reduced for two six-month periods between November 2022 and April 2023 and between November 2023 and April 2024, the city says.

That means things should run smoothly if the weather remains clear, but if there's heavy rain or snowmelt, the extra wastewater will be directed to the areas of the river that are the least likely to affect human and aquatic health, the city says.

Good 1st step toward improving plant

Sarah Dorner, a professor at Polytechnique Montréal, specializes in water quality and pollution. She attended a recent, city-hosted information session on the expansion plan and, she said, this is something that needs to be done.

The ozonation is important because there is a need for disinfection and Quebec doesn't allow chlorine to be used, she said. Ozonation is effective, she explained, and it will improve the river's water quality enough to allow for more uses downstream.

Water quality will improve not just for people, but for wildlife as well, she added.

"There absolutely is a need to improve the plant," said Dorner.

Many experts feel there is even more to be done in the coming decade to ensure wastewater is properly managed in the region, but this is a good first step, she said.

While the city moves forward with the expansion, the population can help reduce the plant's impact on the environment simply by thinking twice before flushing something down the drain, according to Simon Hamel, an assistant superintendent at the plant.

"It doesn't disappear magically," Hamel said. "Every time they use the garbage instead of the toilet, that's a good way to help the environment."


Isaac Olson


Isaac Olson is a journalist with CBC Montreal. He worked largely as a newspaper reporter and photographer for 15 years before joining CBC in the spring of 2018.

with files from Kwabena Oduro

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