The sewage treatment plant in Mont St-Hilaire, about 50 kilometres east of Montreal has been shutdown after being damaged in an early morning fire.
But it also means that wastewater from Mont-Saint-Hilaire, Beloeil, Otterburn Park and McMasterville is now being dumped untreated into the Richelieu River.
The cause of the fire is not known and no one was hurt.
Officials from Environment Quebec are assessing the situation.