Combined sewers convey a mixture of rainwater, groundwater and sanitary sewage. They are a legacy from the early 20th century when wastewater was not treated and conveying sewage and drainage in one sewer pipe was considered cost effective. With the introduction of wastewater treatment around 1960, legacy combined sewer systems were retrofitted to convey all dry weather flows (sewage and groundwater) and significant amounts of stormwater to wastewater treatment plants for treatment. Interceptor sewers and treatment plants cannot handle all stormwater runoff coming from combined sewers during prolonged and heavy rain. This results in combined sewer overflows to Burrard Inlet, English Bay and the Fraser River from combined sewer outfalls in Burnaby, New Westminster and Vancouver.
Metro Vancouver and its member municipalities continue to work to eliminate combined sewer overflows – as sanitary sewage is isolated from combined sewers, combined sewer overflows will eventually transition to being stormwater outfalls.