Point-of-Use Water Treatment

In most communities, the water that reaches our homes, businesses, and industries is clean and free from impurities that detract from the water's suitability for human use. However, water-treatment facilities in some communities do not adequately treat the water to make it safe to drink and appropriate for other uses. And even when the public water supply is considered good by most people, some users have special requirements that necessitate further treatment of the water. In these situations, special point-of-use treatment procedures are employed. Point-of-use water treatment includes any treatment of the water that occurs at the location where the water is to be used (e.g., in the home or at an industrial site, as opposed to in a community plant from which the water will be distributed to many locations). Two of the most common types of point-of-use treatment are water softening and adsorption filtration.

Point-of-Use Water Softeners

Household water softeners (Figure 10) typically use a different process from the precipitation reaction described above ("Treating Water Hardness" section), known as ion exchange. Ion-exchange devices consist of a bed of plastic (polymer) beads covalently bound to anion groups, such as -COO^-. The negative charge of these anions is balanced by Na⁺ cations attached to them. When water containing Ca²⁺ and Mg²⁺ is passed through the ion exchanger, the Ca²⁺ and Mg²⁺ ions are more attracted to the anion groups than the Na⁺ ions. Hence, they replace the Na⁺ ions on the beads, and so the Na⁺ ions (which do not form scummy residues) go into the water in place of the Ca²⁺ and Mg²⁺.

Unfortunately, many people with high blood pressure or other health problems must restrict their intake of sodium. Because water softened by this type of ion exchange contains many sodium ions, people with limited sodium intakes should avoid drinking water that has been softened this way. Several new techniques for softening water without introducing sodium ions are beginning to appear on the market.

Point-of-Use Adsorption Filters

Many of the contaminants that make our drinking water unsafe or unpleasant to drink, such as lead (which may be leached into the water from lead pipes) or organic molecules producing offensive odors and tastes, can be removed by adsorption-filtration devices installed at the tap. These devices have filters containing powdered activated carbon, which adsorbs the offending contaminants in the water. The PAC (with the unwanted contaminants attached) is strained out of the water exiting the device (for consumption) by the filter. Periodically, the filter must be replaced so that it does not become clogged and ineffective.

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