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.