Chemical Water Treatment Plants

Treating water to meet the standards of the Safe Drinking Water Act involves a number of steps, most of which involve chemicals. Whether the water source is groundwater (wells) or surface water (rivers, lakes, streams), the same general steps apply but there is a difference in the chemicals used. The specific chemical chosen for a step in the process will largely be determined by cost and the makeup of the particular water source. (Desalination of seawater for drinking purposes is a different process and will not be discussed here).

Coagulation

In most water-treatment plants, coagulation is the first step (although some begin with a primary disinfection). Coagulant chemicals, which cause particles suspended in the water to adhere to larger and heavier composite particles, are mixed into the incoming raw water. Chemicals used for coagulation include, but are not limited to: aluminum sulfate, ferric chloride, ferric sulfate, ferrous sulfate and cationic polymers.

Sedimentation

The raw water and coagulant mix then goes into one or more large tanks for the sedimentation stage. In these tanks, the compound particles sink to the bottom, sweeping up more particles as they settle. In multiple-stage sedimentation operations, the exit end of the tank is opposite the input end, and the clearest water at the top will flow into the second tank to repeat the process and settle out more suspended particles.
In most operations, no chemicals are added during the sedimentation step itself, although measurements of water clarity (turbidity) and pH (level of acidity) are made on the water leaving the sedimentation tanks to determine what further treatment is required.

Filtration

This step "polishes" the water leaving the sedimentation tanks. Filtration can include passing treated water through layers of carbon, sand, gravel, activated carbon or biologically active filtering media. Like the sedimentation step, this is primarily a mechanical rather than a chemical process. Another type of filtration is filtration through membrane filters to trap microscopic particles. The finest filters used at the municipal level are reverse-osmosis (RO) filters which trap particles larger than one-one thousandth of a micron in size.

Disinfection

In the United States, most disinfection is done by mixing chlorine into the filtered water. Contact Time (CT) is measured as a way to calculate effectiveness of the disinfection process. To ensure a longer life for the disinfectant, free (pure) chlorine is often mixed with ammonia to create chloramines which will stay in the water through the distribution system and all of the way to the customer's tap.
Another popular disinfectant, particularly in Europe, is ozone. The unstable ozone (O3) molecule is generated near the treatment stage and diffused through the water, oxidizing organic particles and some colloidal metals and passing through as (mostly) free oxygen.
Disinfection may be affected by the pH or other factors of the incoming water. The pH may be dropped (made more acidic) by diffusing carbon dioxide or other acid-generating gasses. The pH may be raised (made less acidic) by adding caustics.

Storage

The treated water is then moved to storage, usually tanks or clear wells. The water is monitored for turbidity, pH and the level of remaining disinfectant.
Testing for coliform bacteria and other potentially harmful indicators the water has not been properly treated are done. In most treatment operations, the tests are made at the treatment site and at points throughout the distribution system to ensure safe drinking water.

References

• Lenntech: Water Treatment Chemicals
• EPA: Water Treatment Process Graphic