Chemical Processes Used to Purify Water

Chemical Water Purification Processes may be used in any of the primary treatment steps:
Sedimentation, Filtration and Disinfection.
In Sedimentation, chemicals are added to make impurities in water clump together and sink.
In Filtration, chemicals improve the ability of filter materials to trap impurities.
In Disinfection, chemicals may be used to kill or oxidize waterborne pathogens (disease-carrying organisms).

  1. Source Water Chemical Treatments

    • While ground water (water from wells and aquifers) have few organic threats, surface water (rivers, lakes, reservoirs) will often be treated to reduce algae. Copper sulfate is a common chemical treatment to reduce or curtail algae blooms. Alum or potassium sulfate may be used to prevent a surface water source from becoming too contaminated to use.


    • Coagulants are used to cause impurities in water to clump together. Ferric sulfate, polymers and other chemical additives may be used to form organic and non-organic impurities to adhere and sink to the bottom of holding tanks or basins.
      This clumping and gravity work together to pull heavier-than-water materials out of the source water.


    • Filters, whether they are tanks with layers of coal, gravel and sand or membranes screening out particles measured in angstroms, often use chemicals to enhance their ability to trap particles.
      The effectiveness of a filter is measured by the smallest particles it will capture.
      Chemicals will also be used to wash and disinfect filters for reuse, adding to their cost-effectiveness and useful lifetime.


    • The key step in producing safe drinking water is the ability to kill waterborne pathogens.
      In most US treatment plants, chlorine (either as diffused gas or a powder mix) is mixed with the raw water.
      The combination of disinfectant concentration and contact time (CT) between the disinfectant and the treated water is used to determine if the water has been disinfected.
      Other chemical treatments include diffusing ozone (O3) into water to oxidize waterborne pathogens. This has the advantage that as ozone breaks down, it reconverts into oxygen rather than leaving a residue in the water.

    Other Chemical Treatments

    • Carbon Dioxide gas or Calcium Carbonate slurry may be added to water to lower or raise the water's pH (level of alkalinity).
      Carbon may be added to the water to improve the taste by counteracting the effects of algae in the source water.
      Phosphates are sometimes added late in the process to reduce levels of lime buildup in consumer water lines.

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