Hence, one of the first decisions made by the new Municipality of Metropolitan Toronto government in 1954, was to stop the construction of small wastewater treatment plants and replace them with larger ones that could handle the wastewater of several municipalities. Consequently, some of the smaller wastewater treatment plants were converted into wastewater pumping stations for the four large wastewater treatment plants which were in operation by 1960. In addition to the Ashbridges Bay Treatment Plant, three other wastewater treatment plants are in operation today. They are the North Toronto Treatment Plant, the Highland Creek Treatment Plant and the Humber Treatment Plant. These plants are owned and operated by Works and Emergency Services' Water and Wastewater Services division.
Wastewater is collected by an extensive underground sewer system.
It flows by gravity down the slope of Toronto, and occasionally pumped, to a wastewater treatment plant where it undergoes treatment processes that remove solids, chemicals and other undesirables before the water is released into the natural water supply. Toronto's four wastewater treatment plants operate under the guidelines set by the Ministry of Environment. Their removal effectiveness for suspended solids, biological oxygen demand and phosphorous ranges from 72 to 97 percent. In addition to operating four treatment plants, Water and Wastewater Services maintains 358 km of trunk collector sewer and eight pumping stations. They in turn retail the supply of water and sewage treatment services to their residents.