The Metropolitan System
The raw water from Lake Ontario is collected by intake pipes that extend up to 5 kilometers offshore into the lake. Low-lift pumps operate to move the collected water into and through each of the four water treatment plants in Toronto: the R.C. Harris, R.L. Clark, F. J. Horgan, and Island Water.
As the lake water enters the treatment plant, it passes through multiple moving screens. The screens separate large objects and debris such as plants, sticks, and trash from the water to prevent the damaging of equipment later in the treatment process.
Chlorine, a chemical element used to disinfect water, is added to the incoming lake water to kill harmful microbiological substances.
4. Coagulation and flocculation
A chemical called Alum is added to the water after the pre-chlorination stage. Alum is a coagulant, a substance that causes small particles to bind together to form larger particles called floc. Flocculation is the process by which the chemicals are mechanically mixed to encourage the creation of floc.
After flocculation, the water enters a massive chamber called the sedimentation basin. Gravity acts upon the heavy floc, forcing them to collect at the bottom of the basin where it is removed from the water. The clear water left in the higher levels is drawn into the next stage of the treatment process.
The water is drawn into the filtration layers to remove particles that had not settled at the bottom of the sedimentation basin. There are three layers that work to trap impurities:
- Carbon/Anthracite: This is the top layer of the filter that works to remove taste, colour, and odour producing chemicals.
- Fine Sand: This is the middle layer that works to filter out the majority of objects such as floc, algae, microbes, bacteria, and silt by trapping them in the spaces formed between the grains
- Graded Gravel: This is the bottom layer that acts as a bed for the sand to rest on, preventing it from falling out of the filter.
Once the water is filtered, safe levels of chlorine are added to kill any remaining microorganisms. Fluoride is also added to prevent tooth decay, particularly in young children.
Prior to distribution, the purified water is held in holding basins. Upon leaving the holding basins, the water is treated with sulphur dioxide to remove excess levels of chlorine.
The water undergoes one final treatment by having ammonia added to it. The ammonia and chlorine combine to form chloramines, which disinfect and stabilize the water, keeping it safe through the transmission process.
The treated water is tested by Toronto Water on a regular basis for lead, bacteria, and other harmful substances.
Once the water has been treated and tested, it is pumped through a series of pumping stations owned by the City of Toronto. These stations use high-lift pumps to raise the water pressure to elevate the water to higher areas such as the Town of Markham, which is located 150 to 220 meters above Toronto-based treatment plants. The pumping stations are situated approximately 30 meters apart from each other to ensure the water pressure is consistent throughout the various districts and regions.
The treated and tested water is also pumped into ground level and elevated reservoirs. During peak periods of water use and emergencies such as fires and power outages, these reservoirs serve to maintain adequate water supply to meet demands that can’t be met by the pumping stations alone.
The final stage in the metropolitan system is the transmission of large volumes of water from the pumping stations and reservoirs to the regional water system. To do this, a large network of steel pipes called water mains is used to transport the water. The Toronto water mains network connects with a separate network of water mains owned by and operated under York Region and the Town of Markham.