The water is first drawn from the streams surrounding the island — the St. Lawrence River, Lac St. Louis and the Rivière des Prairies — using numerous intakes that supply six drinking water production plants:

• Atwater
• Charles-J.-Des Baillets
• Dorval
• Lachine
• Pierrefonds
• Pointe-Claire

Once in the treatment plants, the water undergoes different stages:

• Clearance: Removal of the largest debris.
• Coagulation, flocculation and decantation: Agglomeration and deposit of smaller debris at the bottom of basins.
• Filtration and ozonation: Destruction of bacteria and viruses, and treatment of taste and odours.
• UV treatment: Inactivation of recalcitrant parasites.
• Chlorination: Final disinfection to maintain the purity of the water during its long journey through the approximately 5,300 kilometers of pipes (the distance between Montreal and Vancouver).

Drinking water is then transported and stored in 14 huge tanks while waiting to be used. Six of these reservoirs are built into Mount Royal, enabling gravity-fed distribution.

As the water leaves the reservoirs and travels underground, its pressure is calibrated. The water first passes through large-diameter pipes, which become smaller along the way. It’s almost like a road network, with major thoroughfares, boulevards, streets, alleys and thousands of intersections.

Water is now ready for use in our homes, industries, businesses and institutions, as well as through the 30,751 fire hydrants on the island.

Water discharged after use is collected by the sewer system. It then converges on a pipe with a larger diameter called a collector. In winter, some of the snow on our roads is added to the wastewater.

Water from the collector spills into huge, very deep pipes called interceptors. Even larger than a metro tunnel, these pipes are real underground rivers that surround the island and carry large quantities of wastewater to the wastewater treatment plant. Thirty-nine control structures make it possible to monitor these water supplies in real time, assisted by radar forecasts and a network of 51 rain gauges.

Every day, billions of litres of wastewater flow by gravity to the Jean-R.-Marcotte wastewater treatment plant. On average, the plant treats the equivalent indoor volume of 365 Olympic stadiums. This annual treated volume represents almost half of Québec’s total wastewater.

All of this water ends up in two titanic suction wells buried at a depth of 55 meters, where it is then purified by different successive treatments:

• Pumping: Suction of water to ground level. Coagulants are injected into the water to promote agglomeration of the suspended particles.
• Clearance: Retention of the largest debris.
• Desanding: Removal of sand, gravel and heavy particles.
• Decantation: Deposition of heavy matter at the bottom of basins so that 75% of suspended matter and 80% of phosphorus is removed from the water.

After a 58-hour cycle, the loop is closed and the water is ready to return to the river.