Water Treatment Process

Here’s how we get from raw lake water to drinkable tap water:

First, raw water is taken from Lake Ontario using large pipes in one of the four treatment plants in Ontario; The R.C Harris, the F.J Horgan, Island or R.L Clark Water Treatment Plants. Once the water enters the plant, screens are used to remove large objects and debris, and pre-chlorination happens; a process where chlorine is added to kill any micro-organisms (“Drinking water treatment,” 2015).

Alum, a chemical compound (Helmenstine, 2014), is also added at this point and causes small contaminates such as silt to clump together (“Drinking water treatment,” 2015). The chemicals are then mechanically mixed with the water and form larger groups of particles called “floc”. In a large tank, the heavy floc sinks to the bottom and is later on removed by being passed through several filters made of layers of gravel, fine sand and carbon or anthracite, a coal like mineral (“Drinking water treatment,” 2015). If there were any previous chemical, physical or biological impurities in the water, the filters would catch those, and the carbon layer has it’s own job of eliminating any taste or odour-producing chemicals as well.

   After being purified, the water will flow into holding basins before being distributed to North York citizens/ Torontonians. A safe level of chlorine is added to kill any micro-organisms left and excess chlorine is removed using sulphur dioxide. The City of Toronto also adds fluoride to water to help combat cavities (“Drinking water treatment”, 2015).

Finally, the plant adds ammonia to the water to stabilize any chlorine in the water and protect the water during it’s long journey from the plant to people’s taps (“Water treatment program,” 2014).

Here’s a graphic representation for any visual learners:

Screen shot 2015-01-12 at 6.53.56 PM

(“How is lake water turned into drinking water?”, 2014)


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