In the 19th century, the water off the city's shores was severely polluted by the dumping of waste from residences and businesses.

Before 1842, Toronto's water supply was manually pumped from Lake Ontario, streams and wells.

[4] Outside of the pre-amalgamation City of Toronto each of the former municipalities had its own treatment plants and pumping stations.

It is the oldest of the operational water treatment plants in Toronto, being opened on November 1, 1941 after construction started in 1932.

[8] The intakes are located over 2.6 kilometres (1.6 miles) from shore in 15 metres (49 ft) of water, running through two pipes under the bed of the lake.

He was involved in projects including the Prince Edward Viaduct, the Mount Pleasant bridge, and the expansion of the Toronto Civic Railways' streetcar network.

[11] Known for its Art Deco style, the building is dubbed “The Palace of Purification”[8] and was designated under the Ontario Heritage Act in 1998.

[8] The cold, treated water produced by the plant passes through a heat exchange system, which enables Enwave's Deep Lake Water Cooling project to cool buildings across the harbour in Downtown Toronto.

[14] The F. J. Horgan Water Treatment Plant is located in Scarborough in the West Hill neighbourhood on Lake Ontario.

[15] As of 2022, it remains the only plant in Toronto that uses ozone as the primary disinfectant to control pathogens, seasonal taste, and odour.

[8] The R. L. Clark Water Treatment Plant is located in Etobicoke in New Toronto on Lake Ontario.

[19] They are mainly located in areas that cannot accommodate underground reservoirs due to space restrictions.

There are 10 underground reservoirs across Toronto and in Markham:[20] There are in-plant temporary storage tanks storing water as well:[33] Toronto's wastewater treatment plants treat wastewater in a 5 step process:[34][35] During screening, large objects such as rocks and sticks that get washed into the sewer system are removed.

After disinfection, the effluent wastewater meets all Ministry standards and is returned to the natural environment and reused within the facility for internal process use.

[35][36] Solids removed during primary treatment and secondary treatment are pumped into digesters which allow a biological process that uses microorganisms, heat, mixing, and a 10-20 day holding time to break down complex organic matter into simpler forms.

[37] The plant is one of several legacy projects of Roland Caldwell Harris, the Public Works Commissioner from 1912 to 1945.

The plant was partly created after North Toronto residents threatened to de-amalgamate over inadequate sewage treatment in the late 1920s.