Retention basins: Limiting wastewater discharge

Last updated August 8, 2022
Reading time: 2 min

The city is building retention basins, which are underground structures that reduce sewer backup and prevent wastewater from being discharged into our waterways.

Why are we building retention basins?

During rainstorms, if a large amount of rain falls all at once, it can cause the sewer system to overflow. When this situation arises, the risk of overflow towards homes or natural environments increases.

Retention basins help reduce the risk of wastewater being discharged into natural habitats while increasing the level of service of the sewer system. These pools temporarily hold wastewater. When the timing is deemed to be right or when water flow normalizes, water stored in the basin is redirected to the station for treatment.


  • The reduction of wastewater discharge and improvement of water quality.
  • Increased capacity of the sewer network to respond to the impacts of weather events related to climate change. 
  • Reduced risk of sewer backup.

​​​​​​​How are they built?

There are several types of retention basins. Most are underground structures made of reinforced concrete.

Building a retention basin is a major project that can take two to three years. If the basin is big and deep, the city may need to use explosives to excavate the ground. In such cases, the following measures are implemented:

  • Preventive inspection of buildings near the work site using photo or video surveys before and after work begins.
  • Installing measurement instruments inside and near the work site to limit noise and vibration caused by the dynamite. The data is monitored closely to ensure relevant standards are met.

​​​​​​​After excavation, the contractor begins building the different parts of the basin. Once it is complete, the basin is connected to the existing sewer system. It is then integrated into the management system for wastewater that is sent to the Rivières-des-Prairies treatment plant.

Did you know?

The Lavigne retention basin, which is currently under construction in Ahuntsic-Cartierville, can hold the equivalent of eight Olympic swimming pools. Its objective is to reduce the volume of wastewater discharged into the Rivière des Prairies and improve the Gouin collector’s level of service.