Under certain conditions, combined sewers and sanitary sewers can overflow into the environment. These overflows represent a small part of the overall wastewater treatment system and are partly due to old infrastructure that is in the process of being replaced.
Metro Vancouver monitors the discharge from combined sewers in 14 locations and sanitary sewers in six locations across the region. Monitoring assesses the quality and environmental impact of the overflows, which helps municipalities determine which sewers are a priority for upgrading.
Combined sewers carry both sewage and stormwater in a single pipe. These sewers were built in the early 1900s, when wastewater was not treated and they still exist in the older parts of the region. Today, the contents of combined sewers normally go to wastewater treatment plants.
Combined sewers work well during dry weather, but during heavy rain can fill up with more water than they can handle. To prevent combined sewers from backing up into homes and businesses, they were designed to overflow directly into the nearest body of water.
We no longer build combined sewers, but they still exist in the oldest parts of the region, primarily Burnaby, New Westminster and Vancouver. Municipalities and Metro Vancouver are working to replace combined sewers with separate sanitary and storm sewers. This is a labour intensive and costly operation.
Sanitary sewers collect anything that is flushed down a toilet or emptied down a drain and carry it directly to wastewater treatment plants. These sewers contain wastewater from residences and from the commercial, institutional and industrial sectors.
Unlike combined sewers, sanitary sewers are not designed to carry stormwater or to overflow as part of their normal operation. Emergency sanitary sewer overflows can happen because of power or equipment failures at sewage pumping stations or broken sewer pipes. Sanitary sewers can also overflow in wet weather if too much rainwater or groundwater enters the sewers during or after storms. Metro Vancouver municipalities have plans in place to respond to these overflows.
What you can do
The actions of residents, business and industry can make a big difference to how well the region’s wastewater treatment system works. Putting grease and disposable wipes down the drain or toilet can damage equipment and block sewer pipes, causing sewage to overflow into the environment. Solvents, unused medications and other chemicals can also damage the environment. Find out what you can do to keep the region’s system running well.