Environmentalists panned Montreal’s controversial decision to divert, for a week earlier this month, raw sewage into the St. Lawrence River to repair a key wastewater tunnel. But earlier-than-expected completion of work ended the discharge sooner, on Nov. 14. About 1.3 billion gallons of sewage entered the river, down from an earlier estimate of 2.1 billion. The repairs are part of the city’s half-billion-dollar upgrade and expansion of the 2.5-million-cu-meter-per-day Jean R. Marcotte wastewater treatment plant. When work ends, it will be the world’s largest plant to use ozone filtration. 

Montreal officials say the discharge was necessary to remove degraded braces on a main line into the plant, enable crews to move a snow chute that feeds into the line and allow for a nearby elevated highway to be lowered to grade.

The city had no capacity to temporarily hold that amount of sewage to make the repairs, says Richard Fontaine, plant director. “It is not the type of thing we like to do,” he adds. Backlash from protesters and downstream mayors delayed the discharge for one month. But Sarah Dorner, engineering professor at Montreal’s École Polytechnique, says the rise in microbial contaminants is well within the river’s capacity to absorb and dissolve them. Alternatives such as building a giant sewage holding pen would have been impractical and prohibitively expensive, she says.

During the four-day line shutdown, crews removed wooden and steel braces installed two decades ago. Some steel was corroding while wooden pieces had broken off—threatening to damage pumps and other equipment that could have led to rain storm overflows, Fontaine says.

Marcotte, which serves 2 million people in Montreal and in areas of Quebec, is the world's third largest treatment plant. France-based Degrémont, a unit of Suez Environnement; Montreal-based SNC-Lavalin and Réal Paul Architecture won design and construction contracts earlier this year related to the $187-million ozone disinfection upgrade. “That should bring to our plant another dimension of treatment,” says Fontaine.