Raw (untreated) water contains suspended solids, sediment, bacteria, and microorganisms that must be removed to produce drinking water. These are removed by full conventional treatment, described below:
- Screening - On its way from the river to the Dalecarlia and McMillan treatment plants, raw water passes through a series of screens designed to remove debris such as twigs and leaves.
- Pre-sedimentation – While the water moves slowly through Dalecarlia Reservoir, much of the sand and silt settles to the bottom.
- Coagulation - A coagulant, aluminum sulfate (alum), is added to the water as it flows to sedimentation basins. Coagulants aid in the removal of suspended particles by causing them to consolidate and settle. Alum contains positively charged atoms called ions which attract the negatively charged particles suspended in water causing them to gather into clumps of particles heavy enough to settle.
- Flocculation – The water is gently stirred with large paddles to distribute the coagulant; this causes particles to combine and grow large and heavy enough to settle. This process takes approximately 25 minutes.
- Sedimentation – The water flows into quiet sedimentation basins where the flocculated particles settle to the bottom. After about four hours, approximately 85 percent of the suspended material settles.
- Filtration – Water at the top of the basins flows to large gravity filters, where the water flows down through filter media consisting of layers of small pieces of hard coal (anthracite), sand, and gravel placed in the bottom of deep, concrete-walled boxes. Filtered water passes through to a collecting system underneath.
- Disinfection – Chlorine is added with precision equipment to kill pathogenic microscopic life such as bacteria or viruses. Ammonia is then added. The chlorine and ammonia combine to form chloramine compounds. The concentration of chloramines in the water is closely monitored from the time it is added at the treatment plants to points near the furthest reaches of the distribution systems. Disinfection is considered by many to be one of the most important scientific advances of the 20th century.
Fluoride, in the form of hydrofluorosilicic acid, is added to reduce tooth decay; this is especially beneficial for children.
Orthophosphate is added to control corrosion in pipes, service lines, and household plumbing throughout the distribution system. It works by building up a thin film of insoluble material in lead, copper, and iron pipes and fixtures. This thin film acts a barrier to prevent leaching of metals into the water. Calcium hydroxide (lime) is also added to adjust the pH of the water to ensure optimal performance of the orthophosphate.
Powdered activated carbon is occasionally used for taste and odor control.
After the water has completed its path through the treatment process, it is referred to as finished or potable water. Most people simply call it drinking water.
Another site which provides information on treatment processes and interesting links is the Environmental Protection Agency's How is Drinking Water Treated web site.