There are two types of sewer systems in Toronto: the separated sewer system and the combined sewers system.
Sanitary sewers transport wastewater released from a drain, toilet, sink or appliance such as a clothes or dishwasher. This wastewater from residences and businesses flows to treatment plants where it is cleaned before being released into Lake Ontario.
Separated storm sewers capture rainwater or snowmelt. Much of this is captured through catch basins, the square grates on the side of the road. This water flows directly into nearby waterways including streams, rivers and Lake Ontario.
Some of the City’s older areas (approximately 23%), where the sewer system was built as long as a century ago, have combined sewers. Combined sewers have only one pipe that carries both sewage and stormwater. Most of the time, combined sewers carry all contents (rain, melted snow and sewage) to wastewater treatment plants for full treatment.
During periods of heavy rainfall, combined sewers may fill beyond capacity, causing a combined sewer overflow.