From Lake to Tap

An article on the water treatment process

Services: Technical Writing

Ever wonder what happens to the water before it reaches your tap? Having access to clean potable water can be concern as it has a direct impact on our health. But rest assured, the City of Toronto and the Town of Markham employ strict measures to maintain good water quality. Here, we will explain the process by which the water is collected, treated, and distributed to your homes, offices, and institutions.

The Source

Lake Ontario is the sole source of drinking water for Metropolitan Toronto and the southern parts of York Region, which include Vaughn, Richmond Hill and Markham. Lake water is referred to as surface water (whereas water taken from underground is referred to as groundwater). The raw surface water collected from the lake contains various substances, many of which need to be treated and removed for the water to become potable. These substances include the following:

  1. Microbiological Substances: Viruses and bacteria from sewage treatment plants.
  2. Inorganic Substances: Salts and metals that come from runoffs, industrial and domestic discharge, and farming.
  3. Organic Substances: Naturally-occurring synthetics that come from industrial processes, oil production, urban storm water run-offs, and septic systems.

The Treatment and Distribution Process

The treatment and distribution process involves two systems: the metropolitan system and the regional system. The raw lake water is collected and treated first by the metropolitan system. It is then transported to the regional system, which acts as an extension to the metropolitan system, where the treated water is again tested, and then stored and distributed.

The Metropolitan System

1. Intake
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.

2. Screening
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.

3. Pre-chlorination
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.

5. Sedimentation
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.

6. Filtration
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:

  1. Carbon/Anthracite: This is the top layer of the filter that works to remove taste, colour, and odour producing chemicals.
  2. 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
    of sand.
  3. 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.

7. Disinfection
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.

8. Storage
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.

9. Stabilization
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.

10. Testing
The treated water is tested by Toronto Water on a regular basis for lead, bacteria, and other harmful substances.

11. Pumping
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.

12. Reserve
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.

13. Transmission
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.

Regional System (Town of Markham)

14. Testing
On a weekly basis, York Region and Markham Waterworks collect samples from the regional system to undergo testing for harmful substances.

15. Pumping
The Town of Markham owns two pumping stations that work to carry the water through the network of water mains and to your homes, offices, and institutions. These pumping stations function identically to the ones in the metropolitan system by maintaining even levels of water pressure throughout the town.

16. Reserve
The Town of Markham owns three reservoirs that work to compensate for water pressure during times of emergencies and high level of demand. These reservoirs work identically to the ones in the metropolitan system.

17. Supply
Finally, the water that has undergone the long process of treatment, testing, pumping, and reserve is ready to be supplied through the distribution network of water mains to your homes, offices, and institutions. The distribution network is maintained by Markham Waterworks.

18. Backflow prevention program
Normally, the treated water flows from the public water supply system to the private water supply systems. There are cases, however, when this normal flow direction is reversed—this is called backflow. When backflow occurs, undesired chemicals and pollutants may get into the public water supply system, causing contamination. The Backflow Prevention Program prevents this from happening by cutting off the cross connections between the drinking water system and private water systems.