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Newsletter and Technical Publications 2.2 Water Softening The suspended materials contained in water consist of minerals and organic materials. The positive- and negative-charged mineral ions are the most important of these materials (in addition to some special ions, such as iron, heavy metals, nitrate and phosphate, that prevent some water uses). These mineral ions are removed using ion exchange technology, which is categorized into three types, including (1) softening, (2) carbonate removal, and (3) total deionization. Technology Description This technology is based on removing certain ions from treated water, using special materials that have the property of attracting specific types of loose ions in the water, while at the same time releasing other ions that were originally attached to it. These materials can be natural or artificial, composed of particles carried in its outer layers. These materials can exchange or replace their original ions with loose ions in the water, without changing their physical character. The negative ions in the material exchange with the loose negative ions in the water, while the positive ions exchange with positive ions in the water. Some artificial materials are known as resin. The calcium ion is the basic ion for forming calcium carbonate in water. Excessive quantities of calcium can interfere with processes such as the dissolution of detergents, and such water is typically called “hard” water. Accordingly, with the use of this technology, the calcium ion is exchanged (replaced) in a solution of sodium ions in accordance with the following chemical reaction: R.Na2 + Ca2+ à R.Ca + 2 Na+ The sodium chloride used in this reaction allows all the calcium and magnesium salts contained in the water to be transformed into sodium salts. This reaction removes the calcium (and the water hardness), but does not affect the pH of the water or the carbonate content of the water. This process is known as water “softening.” Due to its limited operational life, the resin must be periodically regenerated to remove the calcium ions deposited on it. The regeneration process replaces the calcium ion with new sodium ions, thereby reactivating the resin. For optimal use of this resin-based exchange technology, therefore, it is necessary to determine the optimal exchange capacity for each ion exchanger. The process of softening and carbonate removal is illustrated in Figure 49. Optimal use of this technology also requires that the water to be treated be free of all suspended materials, oils or similar materials, as well as gas. Extent of Use This technology is typically used on an industrial scale to prevent the deposition of calcium in heating or hot water circuits, which can block the pipes and corrode them over time. It is rarely used for water treatment, other than occasionally on a local scale for residences provided with hard potable water, as a means of preventing damage to heating and other household systems. It also is used for water treatment for small residential populations that have only very hard water available for drinking. This is because provision of water of proper quality, and its conveyance over long distances, may be too costly for a limited number of residents. In such cases, water softening is used on a very limited scale, prior to pumping the water to the residences.
Figure 49. Carbonate removal by ion exchange resins Operation and Maintenance Ion exchangers are not used until the feed water is free of all suspended solids, organic materials and residual chlorine. On the other side, every ion exchanger has its specific properties, dependent primarily on its ion exchange capacity. This capacity defines the maximum weight of ions that can be exchanged, and is specific for each type of resin. There also is the usable capacity, which is the part of the resin layer that is actually used, and it is dependent on the water pressure and the chemical conditions in each case. This capacity also is referred to by other expressions, including (1) bed volume – the percentage of water to be treated in one hour compared to the resin volume, and (2) ion volume – the bed volume multiplied by the water salinity. Water flows from the top of the ion exchange cylinder containing the resin. There typically is a space left above the resin to allow for its expansion, which can range between 30-100% of the compressed volume, depending on the type of resin. Treatment of hard water can be done after the process of calcium clarification. This process decreases the carbonate in the water by removing it from the solution. This produces soft, carbonate-free water from the exchange process. In regard to required maintenance, it is necessary to ensure proper operation of the technology prior to introducing the water to the exchanger. It also is important to reactive the resin when the exchange process is not proceeding efficiently, as determined by measuring the degree of hardness that reflects the quantity of calcium carbonate in the water. If there is evidence of persistent inefficient ion exchange after the reactivation process, the resin must be replaced. Level of Involvement Interest in this technology is limited primarily to industrial foundations that use hot water as a raw product, as well as in their heating systems. Use of this technology in potable water is very limited, typically restricted to some small residential concentrations with water hardness exceeding allowable limits. Its limited use is due primarily to the fact that treating large quantities of water usually requires continuous replacement of the resin columns, which is not economically feasible. Costs There are no construction costs, except for allocation of a special chamber for the ion exchanger. The primary cost is the price of the exchanger itself, in addition to the cost of resin. It also requires allocation of a technician for the monitoring and reactivation process. Effectiveness of the Technology This technology is considered very effective for softening water. Thus, it works to protect pipes and equipment from corrosion due to calcium deposition. The latter condition typically requires regular reactivation of the resin. Suitability This technology is beneficial for treating water used in hot water systems, thereby preventing calcium carbonate deposition. It can be used in any location with available electricity and a supply of resin for regular reactivation of the ion exchanger. Advantages The advantages of this technology are as follows:
Disadvantages The disadvantages of this technology are as follows:
Cultural Acceptance This technology has been known for a long time, and is still used on a large scale. To soften water, residents used to, and still resort to, boiling water before using it for such purposes as making tea. Boiling causes the deposition of calcium carbonate on the sand bottom of the bowl, and the boiled water is almost free of calcium that could cause turbidity in the boiled water. Further Development of the Technology Although this technology has been known for a long time, it is still necessary to improve the types and efficiency of resins, in order to sustain a large-scale ion exchange process and reduce the costs and required maintenance. Information Sources Contacts Fouad Abu Samrah Marawan Haddad References Degremont. 1979. Water Treatment Handbook, 5th Edition.
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