Safe Drinking Water: Treatment, Standards and Choices

What comes out of the tap

In most Ontario towns, you can fill a glass at the kitchen tap and drink it without a second thought. Behind that glass is a treatment plant, a network of pipes, a lab that tests water every day, licensed operators, and laws. In this final lesson you will put your solution chemistry to work: you will see the chemistry of treatment, compare test results with Canadian standards, and analyse the economic, social and environmental issues around drinking water.

How a municipal plant treats water

1. Intake and screening: water from a lake, river or wells passes through screens that remove fish, weeds and debris.
2. Coagulation and flocculation: a coagulant such as alum (aluminium sulfate) is added. Tiny clay and organic particles in raw water carry negative charges and repel each other, so they never settle. The positive Al³⁺ ions neutralize those charges, and in the slightly basic water they also form a sticky aluminium hydroxide precipitate, Al(OH)₃(s). Slow mixing grows these into fluffy clumps called flocs, which trap particles and many microbes.
3. Sedimentation: the flocs settle to the bottom as sludge.
4. Filtration: water passes through layers of sand, gravel and often activated carbon, which also removes some taste and odour compounds.
5. Disinfection: chlorine (or chloramine), ultraviolet light or ozone kills or inactivates bacteria, viruses and parasites. Chlorine is added so that a small chlorine residual stays in the water all the way to your tap.
6. Other adjustments: the pH may be raised slightly so the water does not dissolve lead or copper from old pipes, and some municipalities add fluoride to help prevent tooth decay.

Rural homes with private wells are responsible for their own testing; many public health units offer free bacteria testing kits for well water.

Standards: how clean is clean enough?

Health Canada publishes the Guidelines for Canadian Drinking Water Quality, which set maximum acceptable concentrations (MACs) based on health research. Ontario's Safe Drinking Water Act, 2002 and its regulations make standards like these legal requirements for municipal systems, require regular testing in licensed labs, and require operators to be certified. Many municipalities post an annual drinking-water report online.

There are also aesthetic objectives for things that affect taste, smell or colour but are not a health risk at normal levels, such as chloride (about 250 mg/L or less) and iron.

Walkerton, 2000: why the rules exist

In May 2000, the drinking water of Walkerton, Ontario became contaminated with E. coli O157:H7 and other bacteria after heavy rain washed manure from a nearby farm into a shallow town well. Chlorination and testing at the well were not done properly. Seven people died and more than 2300 became ill. A public inquiry led by Justice Dennis O'Connor recommended major changes, which led to the Safe Drinking Water Act, 2002, mandatory operator training and certification, licensed testing labs, and the Clean Water Act, 2006, which protects the sources of drinking water (lakes, rivers and aquifers) from contamination. This is the "source to tap" approach: protect the source, treat the water, and test it.

Drinking water in First Nations communities

Many First Nations communities in Canada have lived under long-term drinking water advisories, some for many years. Causes include water systems that were too small or poorly funded, difficulties keeping certified operators, and remote locations. Drinking water on reserves is a shared federal responsibility with First Nations. Since November 2015, more than 150 long-term advisories have been lifted (158 by September 2026, when 38 were still in effect in 36 communities) through new plants, upgrades and operator training, much of it led by the communities themselves, but some advisories remain in effect, including in Ontario. Safe water is also tied to culture: many First Nations, Métis and Inuit people describe water as a living relative to be protected, and Indigenous water protectors lead work to safeguard lakes and rivers for future generations.

Bottled water: economic, social and environmental issues

• Cost: in most Ontario municipalities tap water costs well under one cent per litre, while bottled water usually costs 100 times as much or more.
• Environment: making, transporting and disposing of plastic bottles uses energy and creates waste; not all bottles are recycled. Groundwater taking permits for bottling plants have caused public debate in Ontario.
• Health and regulation: bottled water is regulated as a food under federal law, while municipal tap water is tested under provincial rules. Both are safe in most of Canada. Bottled water can be essential during an advisory or emergency.
• Global equity: in many low-income countries, unsafe water and poor sanitation still cause hundreds of thousands of deaths each year, mostly from diarrhoeal diseases among young children. Affordable solutions include protected wells, household chlorination and filtration.

Emerging issue: pharmaceuticals and microplastics

Trace amounts of medicines and personal-care products (often measured in nanograms per litre) pass through wastewater plants and appear in rivers. Treatment plants were not designed to remove them, and scientists are still studying the long-term effects. Studies have also found microplastic particles in both tap and bottled water. These are active areas of research for environmental chemists.

Careers in water quality (A2.1)

• Water treatment or wastewater operator: runs plants and tests water; in Ontario, operators must be certified (college programs in water quality or environmental technology are common).
• Environmental or analytical chemist: measures pollutants in labs (university degree in chemistry).
• Laboratory technician/technologist: runs daily tests (college diploma).
• Public health inspector: investigates outbreaks and advises well owners (degree plus certification).
• Civil or environmental engineer: designs treatment plants and pipe networks (engineering degree and licence).