Some water treatment systems use resin beads to purify water by removing minerals. Partial mineral removal is called softening, and complete mineral removal is called demineralization. Most resins are made of a copolymer called styrene-divinylbenzene, although some are acrylic. Water flows through the resin beads, where resin ions exchange for mineral ions, thus removing minerals from the water.
Function
Resins remove calcium, magnesium and traces of iron and manganese from water. The resin is supersaturated with sodium or acid ions, and as water passes through the resin, charged calcium and magnesium ions exchange for sodium or hydrogen ions. Resins have different numbers of cross-links that give the resin strength. Higher cross-linked resins--10 percent--is used for water with high chlorine content and high temperature, which tend to break down resins faster. An 8 percent cross-linked resin is most common.
Regeneration
When the resin becomes saturated with minerals, it needs regeneration. The frequency of regeneration depends on how much water is used and how high the mineral content is. Regeneration is accomplished by passing a high sodium or acid brine solution through the resin and completing a second ion exchange. The unwanted minerals wash away as waste water.
Resin Types
Individual households use strong acid cation resins most often. These can either soften water or totally demineralize it, depending on the type of ion system used. The sodium form softens, and the hydrogen form demineralizes. Weak acid cation resins remove only the ions associated with highly alkaline water. Strong acid anion resins complete both demineralization and dealkalization, and weak base anion resins absorb acid by removing anions of strong mineral acids, such as sulfate, chloride and nitrate, while allowing silica to pass through.
Advantages
Ion exchange resins have a low running cost. Little energy is needed to run the system and well-maintained resins last for many years. By softening water, resins eliminate hard-water discolorations of laundry, appliances and hair. Soft water increases soap foaming and cleaning ability, thus reducing detergent costs and cleaning time. Also, it eliminates the clogging of pipes and water-heater coils with mineral buildup.
Disadvantages
Resin-softened water contains sodium, which is not recommended for diets requiring low sodium intake. Regeneration of resins dumps sodium waste water into the environment, and the large water volume needed is inefficient. Ion exchange resins do not remove organic compounds or biological contaminants. If resin is not sanitized or regenerated regularly, bacterial colonies proliferate on resin surfaces and can contaminate drinking water.
Considerations
A complete water analysis should be performed prior to resin-system installation to enable a proper system choice. The amount of minerals and the type of minerals in the water justify different resins. Second, correct resin choice depends on climate and system purpose. A house in Africa, with alkaline water containing silica, requires different resin than a house in Canada with high magnesium water content. Thirdly, daily water usage will help determine the amount and size of resin needed.
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