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Water treatment was originally established in order to provide safe drinking water. In the 19th century, the water off the city's shores was severely polluted by the dumping of waste from residences and businesses.
Early Toronto Water Supply
Before 1842, Toronto's water supply was manually pumped from Lake Ontario, streams and wells.
Water carters would take the water and distribute it to customers across the city.
Private Water Supply: Toronto/Furniss Water Works
From 1843 to 1873, water was privately provided by Furniss Works or Toronto Water Works, a subsidiary of Toronto Gas Light and Water Company, which was owned by Montreal businessman Albert Furniss.
Following Furniss's death in 1872, the City of Toronto bought out Furniss Works and transformed the water supply to public hands under the Toronto Water Works Commission.
Outside of the pre-amalgamation City of Toronto each of the former municipalities had their own treatment plants and pumping stations:
- North York
- Oriole 1923 - ruins of pumping station found near Duncan Mills Road and Don Mills Road
- Steeles 1930s
- Sheppard West 1940s
- Scarborough 1921
- New Toronto 1924
Prior to the 1950s, the municipalities were responsible for water treatment and water came from local water sources like wells and streams.
From Metro Toronto Public Works to Toronto Water
The current system was introduced in the mid-1950s, with the formation of Metro Toronto in 1954, and was managed by Metro Toronto. Following amalgamation in 1998, Toronto Water was created from the Toronto Works and Emergency Services and once part of Metro Toronto Works department. Since 1975, Toronto has supplied water to York Region (mostly to residents in the south end of York).
Drinking water operations
- Flocculation and Sedimentation
- Chloramination, prior to distribution
The City of Toronto uses four water treatment plants:
- R.C. Harris Treatment Plant 
- Queen Street East and Victoria Park Ave.
- Capacity of 950 megalitres per day
- Island Treatment Plant 
- Located on Centre Island
- Capacity of 410 ML/d
- F. J. Horgan Treatment Plant (formerly Easterly) 
- Copperfield Road
- Capacity of 487 ML/d
- R.L. Clark Treatment Plant (formerly Westerly) 
- 23rd Street in Etobicoke
- Capacity of 557 ML/d
- Opened in 1968 replacing the New Toronto Filtration Plant (opened 1915 at Lake Shore Boulevard and Seventh Street and which became Lakeshore Pumping Station 1968-2000)
The drinking water distribution system operated by Toronto Water comprises 6 pressure zones and approximately 520 km of watermains (greater than 150 mm in diameter) 
Pumping stations are located across the city to pump water from the filtration plants to residences. They are particularly critical since the city gains in elevation as it moves northwards away from Lake Ontario. Some pumping stations are located outside the city.
|Armour Heights Pumping Station||West - North York||Eastbourne Avenue at Wilson Avenue|
|Hanlan's Point Sewage Pumping Station||Toronto Island||West of Ned Hanlan's Memorial||Dry well reservoir with three (3) Flygt NP3203 centrifugal self-cleaning pumps|
|Ashbridge's Bay Pumping Station||South - Port Lands||Lake Shore Boulevard East west of Coxwell Avenue|
|Dufferin Pumping Station||West - North York||Finch Avenue West east of Dufferin Street||Pumping station located south of G. Lord Ross Reservoir|
|Ellesmere Pumping Station||East - Scarborough||Ellesmere Road near Scarborough Golf Club Road|
|Eglinton Pumping Station||East - Scarborough||Eglinton Avenue East near Pharmacy Avenue||Pumping station attached to Ashtonbee/Eglinton Reservoir|
|Gore Park Pumping Station||South - Downtown||Fleet Street and Strachan Avenue|
|John Street Pumping Station||South - Railway lands||Lake Shore Boulevard West and Rees Street||relocated and rebuilt due to construction of Skydome|
|Keele Pumping Station||West - North York||Keele Street and Steeles Avenue West||Attached to underground reservoir|
|Lawrence Pumping Station||West - North York||Lawrence Avenue West and Caledonia Road||Attached to underground reservoir|
|Parkdale Pumping Station||South - Toronto||The Queensway and Glendale Avenue|
|High Level/Poplar Plains Pumping Station||South - Toronto||Cottingham Street and Poplar Plains Road|
|Rosehill Pumping Station||South - Toronto||Mount Pleasant Road and Carstowe Road||Attached to Rosehill Reservoir|
|Scarborough Pumping Station||East - Scarborough||22 Fishleigh Drive|
|St. Albans Pumping Station||West - Etobicoke||St Albans Road and Bloor Street West|
|Island Pumping Station||South - Toronto||Lakeshore Avenue at Gilbratar Point||Attached to Island Water Treatment Plant|
|West Toronto Pumping Station||West - York||Old Weston Road and Junction Road; formerly New Toronto|
There are two pumping stations located outside of the city limits and part of the water service provided to York Region by Toronto:
|Milliken Pumping Station ||North - Markham||located on 14th Avenue west of Kennedy Road in Markham||undergoing expansion with new underground reservoir; serves current water tower|
|Bayview Pumping Station||North - Thornhill||8190 Bayview Avenue at Willowbrook Road in Markham||serves current underground reservoir|
Toronto water stores water in three formats:
- Floating Reservoir: newer Water tower or older Water tank with limited capacity
- Ground level Reservoir: underground water storage with grass covered top with large capacity
- Temporary storage: stored at a water treatment plant with limited capacity
Water in the city is stored once it is treated, ensuring uninterrupted water supply.
There are 5 water towers located in the city and 1 outside the city. They are mainly located in areas that cannot accommodate underground reservoirs due to space restrictions.
|Ashbridge's Bay||steel reinforced concrete tower||South - Toronto||Lake Shore Boulevard East|
|Leslie Tank||steel water tank||North Central - North York||Leslie Street and Lawrence Avenue East at Talwood Drive|
|Rouge Tower||steel reinforced concrete tower||North East - Scarborough||8200 Sheppard Avenue East near Morningside Road|
|Warden Tank||steel water tank||South East - Scarborough||Warden Avenue and Civic Road|
|Whitlam Tank||steel water tank||South West - Etobicoke||Whitlam Avenue and Twenty Second St|
|Milliken Tower||steel reinforced concrete tower||North - Markham||14th Avenue west of Kennedy Road|
There are 10 underground reservoirs across Toronto and in Markham:
|1||Ashtonbee/Eglinton Reservoir||underground reservoir lies below Ashtonbee Park; grass sportsfield||East - Scarborough (Golden Mile)||80 Ashtonbee Rd and 885 Pharmacy Avenue|
|2||Bayview Reservoir||underground reservoir lies below Bayview Reservoir Park (City of Markham); grass soccer pithces and baseball diamonds||North - Markham (Thornhill)||8190 Bayview Avenue south of Sycamore Drive|
|3||Dufferin Reservoir||underground reservoir lies on grass covered property; land on top being developed||North - Vaughan (Thornhill)||8851 Dufferin Street south of Autumn Hill Boulevard|
|4||Ellesmere Reservoir||underground reservoir next to Ellesmere Ravine park||North East - Scarborough (Morningside)||2904 Ellesmere Road near Neilson Road|
|5||Keele Reservoir||underground reservoir with grass soccer pitch on top||North West - North York (York University Heights)||4995 Keele Street south of Steeles Avenue West|
|6||Caledonia Park Reservoir||underground reservoir below Caledonia Park, a grass surface multi-use sports field||North West (Caledonia Park)||1141 Caledonia Road at Lawrence Avenue West|
|7||Milliken Reservoir and Tower||steel reinforced contrete tower (1970s) with underground reservoir (2010)||North - Markham (Hagerman's Corner)||4347 14th Avenue west of Kennedy Road|
|8||Richview Reservoir||underground reservoir below Richview Park and Carmen Bush Field||South West - Etobicoke (Richview)||59 Clement Road - Martin Grove Road and Eglinton Avenue West|
|9||Rosehill Reservoir||underground storage next to David Balfour Park; open reservoir 1874-1966, now covered with park||South - Toronto (Summerhill)||75 Rosehill Avenue|
|10||St. Clair Reservoir||underground reservoir at Sir Winston Churchill Park||South (1931) - Toronto (Casa Loma)||323 St. Clair Avenue West and Spadina Road|
There are in-plant temporary storage tanks storing water as well:
|Frank J Horgan Treatment Plant||temporary storage - 68 mega litres||South - Scarborough (East Point)||201 Copperfield Road; formerly Easterly Filtration Plant|
|Island Treatment Plant||temporary storage - 34 mega litres||South - Toronto (Toronto Islands)||Lakeshore Avenue|
|R.C. Harris Treatment Plant||temporary storage - 54 mega litres||South - Toronto (Neville Park)||Queen Street East at Neville Park Road|
|R.L. Clark Treatment Plant||temporary storage - 27 mega litres||Southwest - Toronto (Humber Bay); former site of New Toronto plant|
|Scarborough Pumping Station||temporary storage - 25 mega litres||Southeast||22 Fishleigh Drive|
In addition to water storage, the City has also constructed the following subterranean storm water detention tanks:
|Kenilworth Detention Tank||Storm water detention tank (2.25 mega litres)||East Toronto||South side of Kew Beach Avenue, between Kenilworth and Pantry Park. Control room located east of tank|
|Maclean's Detention Tank||Storm water detention tank (8 mega litres)||East Toronto||West side of Glen Manor Drive, south of Hubbard Boulevard.|
The city of Toronto has four facilities that process wastewater before it is returned to the lake.
- Ashbridges Bay Wastewater Treatment Plant 
- Built in 1910
- North Toronto Wastewater Treatment Plant 
- Opened in 1929
- Highland Creek Wastewater Treatment Plant 
- Opened in 1956
- Humber Wastewater Treatment Plant 
- Opened in 1960
As of April 2005, the departments and commissioners were replaced by divisions under the City Manager (and Deputy Managers):
Toronto Water is now under Toronto Water Division.
- Aulakh, Raveena (12 November 2010). "The digging begins on Avenue Rd". The Star. Toronto Star. Retrieved 23 March 2012.
- The Culture of Flushing: A Social and Legal History of Sewage, p 66
- The Star. Toronto https://www.thestar.com/news/gta/2010/11/12/the_digging_begins_on_avenue_rd.html. Missing or empty
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- "About – Island Water Treatment Plant". City of Toronto. 2011-06-21. Retrieved 2012-03-23.
- "About – F.J. Horgan Water Treatment Plant". City of Toronto. 2011-05-15. Retrieved 2012-03-23.
- "About – R.L. Clark Water Treatment Plant". City of Toronto. 1968-11-22. Retrieved 2012-03-23.
- "New Toronto Filtration Plant/Lakeshore Pumping Station Historical Plaque". Toronto's Historical Plaques at Torontohistory.org. June 2002. Retrieved 2012-03-23.
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- http://app.toronto.ca/tmmis/viewAgendaItemHistory.do?item=2009.PW25.6 Contract Award - Contract 09FS-42WS, Tender Call 59-2009 and Purchase Order Amendments for Milliken Pumping Station and Reservoir Expansion City of Toronto August 5, 2009
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- "St. Clair Reservoir photographs catalogue record". City of Toronto Archives catalogue. Retrieved 2 October 2011.
- St. Clair Reservoir (Map). Cartography by OpenStreetMap contributors. OpenStreetMap. Retrieved 2 October 2011.
- Strax, Kiril. "Underground reservoir at Sir Winston Churchill park". Kiril Strax's photostream. Flickr. Retrieved 2 October 2011.
- "Page Not Found". Archived from the original on 2014-10-29.
- "About – Ashbridges Bay Wastewater Treatment Plant". City of Toronto. 2011-05-15. Retrieved 2012-03-23.
- "About – North Toronto Wastewater Treatment Plant". City of Toronto. Retrieved 2012-03-23.
- "About – Highland Creek Wastewater Treatment Plant". City of Toronto. 2011-05-15. Retrieved 2012-03-23.
- "About – Humber Wastewater Treatment Plant". City of Toronto. 2011-05-15. Retrieved 2012-03-23.