Montreal residents near the Jean-R.-Marcotte wastewater treatment plant experienced increased foul odors last summer as the city experimented with reducing its reliance on a costly odor-control chemical. The city is attempting to balance budget concerns with complaints from locals about “rotten egg smells” emanating from the largely open-air facility, testing different combinations of ferric chloride and a cheaper choice, alum. While $1.8 million in savings were realized last year, finding the right chemical mix – and dosage – to minimize the nuisance odor remains a challenge.
Montreal is testing lower-cost chemicals for wastewater treatment after soaring prices for traditional compounds, but the move is resulting in increased odor complaints from residents near the Jean-R.-Marcotte treatment plant. The situation underscores the financial pressures facing municipalities as they grapple with rising operational costs and stringent environmental regulations.
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The city received 33 complaints last year regarding odors emanating from the site, where the metropolis’s sludge – human waste – is processed, compared to just five in 2024, according to municipal data. The increase in complaints coincides with a shift in treatment chemicals, raising questions about the trade-offs between cost savings and environmental impact.
Some residents believe the unpleasant smells are linked to ongoing construction at the Jean-R.-Marcotte plant, where the world’s largest ozonation facility is currently being built. However, city officials have stated the construction is not the source of the odor issue.
“If I had known I would have this really disgusting smell, I wouldn’t have bought [my home],” said Charly Chartouny, a resident of Trefflé-Berthiaume Street, located near the Jean-R.-Marcotte plant in Rivière-des-Prairies.
The price of certain chemicals used in wastewater treatment has risen sharply in recent years. The City of Montreal projects it will spend $30 million on these products next year, a 50% increase compared to five years ago. This cost escalation is impacting municipal budgets and forcing cities to explore alternative solutions.
“The Market Has Exploded”
“The market has really exploded,” said Chantal Morissette, director of the Water Department, adding that Montreal is still securing competitive pricing compared to neighboring cities. The rising costs are a concern for municipalities across the region.
According to Benoit Barbeau, a professor in the Department of Civil, Geological and Mining Engineering at Polytechnique Montréal, the price increases are driven by higher energy costs for production and the rising cost of raw materials like iron and aluminum. “In general, prices have increased significantly in the resource industry,” he explained.
Increased transportation costs and growing demand from municipalities, due to increasingly strict standards, are also contributing factors, according to a representative from a city supplier who was not authorized to speak to the media.
Lower Cost, Reduced Effectiveness
The city is attempting to reduce its use of ferric chloride, an iron derivative added to wastewater to clump fine particles into larger flakes for easier removal. The move is part of a broader effort to manage costs and optimize treatment processes.
For the past two years, municipal authorities have been trying to use more alum, another coagulant derived from aluminum, which is roughly half the price. However, alum is also less effective at controlling odors.
“Anything that produces the rotten egg smells reacts a lot with iron. So in the summer, when sludge that has sat for a very long time in the sewer network arrives […], when you add ferric chloride, it helps control odors in the plant,” Barbeau explained.
In 2025, the City of Montreal spent $14.3 million on alum and $1.08 million on ferric chloride.
Finding the Right Balance
However, officials have yet to determine the optimal dosage, leading to persistent odors. “We’re filing complaints, but it seems like nothing is happening,” said Stéphane Horvat, another resident of Trefflé-Berthiaume Street.
The city is attempting to balance economic efficiency with odor reduction at the plant, according to Stéphane Bellemare, director of wastewater treatment. “We’re juggling these two problems – to be economically viable and reduce odors at the station,” he said.
Different mixtures of ferric chloride and alum have been tested at the Jean-R.-Marcotte plant, resulting in savings of $1.8 million last year, Bellemare noted.
The unusually dry weather last summer also played a role.
PHOTO HUGO-SÉBASTIEN AUBERT, LA PRESSE
The Jean-R.-Marcotte wastewater treatment plant’s infrastructure is largely open-air.
“These conditions influenced the composition of the water arriving at the station, promoting the formation of hydrogen sulfide (H₂S), a very odorous gas, even at low concentrations. In a context where the infrastructure is largely open-air, the combination of these factors led to increased dispersion of odors towards neighboring areas,” the City of Montreal’s communications department wrote in an email.
Bellemare remains optimistic. “We expect that next summer, there will be many fewer complaints than there were this year,” he said.
