A typical municipal water purification process involves several steps, from physical removal of impurities to chemical treatment. Below we highlight the main steps:

Coagulation, Flocculation and Sedimentation
Before the raw water is treated, it is passed through coarse filters to remove large debris like sticks and leaves. Sand and grit settle out of the water during this stage as well. During coagulation, a chemical such as alum is added to the raw water. When the water is stirred, the alum forms sticky globs that attach to small particles made up of bacteria, silt and other impurities.
The water is then sent to a settling tank where the globs – or floc – sink to the bottom. This is a much longer phase of purification called flocculation.
Next, the water is pumped very slowly across a large basin where much of the remaining floc and solid material accumulates at the bottom in a process called sedimentation or clarification.

Filtration
Filtration involves passing the water through layers of sand, coal and other granular material to remove microorganisms – including viruses, bacteria and protozoans such as Cryptosporidium – and any remaining floc and silt. This stage of purification mimics the natural filtration of water as it moves through the ground.
After the water is filtered, it is treated with chemical disinfectants to kill any organisms that might have made it through the filtration process. The most effective disinfectant is chlorine.

Chlorination
Chlorination is used in all of Canada's 3000 water treatment plants and, in many municipalities, it is the only chemical disinfection used. But it is not without its problems.
When the chlorine combines with organic material, such as dead leaves, it produces potentially dangerous trihalomethanes (THMs). While treatment plants in larger cities can filter out THMs to keep them at a safe level, those in small towns often don't, and THM warnings have been issued in many towns across the country, particularly in Newfoundland and Nova Scotia.

Ozone oxidation
Ozone oxidation is another effective disinfectant process, but unlike chlorine, ozone does not stay in the water after it leaves the treatment plant, so it offers no protection from bacteria that might be in the water pipes.

Ultraviolet Light
Water can also be treated with ultraviolet light to kill microorganisms, but it has the same limitation as oxidation: it is ineffective outside of the treatment plant. Nevertheless, UV treatment is being considered for water treatment in places such as North Bay, Ontario, Victoria, B.C. and Erickson, B.C., where residents oppose water chlorination.