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What Chemicals Are Used to Purify Water?

Sewage from cities and towns must be treated and cleansed before it's discharged into the environment. Water from sources like rivers or reservoirs often contains microorganisms, organic compounds and chemical contaminants released by human activity. Water treatment plants use a variety of chemicals to disinfect water and assist in these purification processes.

  1. Chlorine

    • According to the World Health Organization's "Guidelines for Drinking-Water Quality" (2004), chlorine is the most commonly used primary disinfectant. It reacts with organic compounds to kill microorganisms like bacteria in the water. Although chlorine is effective, it's not as effective as chlorine dioxide, and it can also generate unwanted byproducts like trihalomethanes that must be removed from the water.

    Chlorine Dioxide

    • Chlorine dioxide is a potent disinfectant that can kill microorganisms at concentrations as low as 0.1 parts per million; moreover, it remains active even at widely varying pH levels. Chlorine dioxide can break down to release chlorite; water treatment plants must also control the level of chlorite in the water.

    Ozone

    • In contrast to oxygen gas, each molecule of which contains two oxygen atoms, a molecule of ozone contains three oxygen atoms. Ozone is another powerful disinfectant that reacts with organic compounds in viruses or bacteria to destroy them. Ozone has a short lifespan and does not leave behind any harmful compounds or residues. Like many other disinfectants, it's also highly reactive and hence requires special conditions like corrosion-resistant metals for application in water treatment.

    pH Adjusters

    • Hydrochloric acid and sodium hydroxide are two chemicals often used to adjust the pH of water before treatment. Sodium hydroxide is a strong base that will raise the pH of the solution, while hydrochloric acid is a strong acid that will reduce pH.

    Coagulants

    • Compounds like ferric sulfate and aluminum sulfate can act as coagulants---chemicals that cause particles in suspension in the water to clump up or aggregate. The resulting precipitate is easier to filter or remove from the water.

    Scale Inhibitors

    • Poorly soluble compounds like calcium carbonate can precipitate to form scale on surfaces that come in contact with the water. Water treatment plants may use scale inhibitors like phosphoric acid to prevent scale formation.

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