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Adsorption and Ion Exchange Systems

Adsorption systems treat water by adding a substance, such as activated carbon or alumina, to the water supply. Adsorbents attract contaminants by chemical and physical processes that cause them to ‘stick’ to their surfaces for later disposal.

By far, the most commonly-used adsorbent is activated carbon—a substance similar to common charcoal but extremely porous. Powdered activated carbon is often used when temporary quality problems arise; it can simply be added to the water and discarded with waste sludge. Granular activated carbon is often arranged in a bed through which source water is slowly passed or percolated.

Activated alumina treatment is used to attract and remove contaminants, like arsenic and fluoride, which have negatively charged ions. However, this option can be expensive and may require complicated system maintenance. Also, the water may require pH adjustment prior to the adsorption column, and a significant aluminum residual is a common problem. Both acids and bases are required for regeneration.

Ion exchange uses a resin that removes charged inorganic contaminants like arsenic, chromium, nitrate, radium, uranium, and excess fluoride by exchanging them for harmless charged ions on its surface. It works best with particle-free water and can be scaled to fit any size treatment facility. Ion exchange is most often used to remove hardness (cation resin) or nitrate (anion resin). In both instances, it can be regenerated with salt water. The use of ion exchange to remove radionuclides is complicated by the fact that these materials accumulate in the resin and occur at high levels in the regenerant, greatly complicating operations.

Activated carbon is generally preferred for removing organic contaminants, whereas ion exchange is often best for removing inorganic soluble molecules.

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