Reuse of softening pellet for remineralizing desalinated water

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Abstract

Desalinated water produced by reverse osmosis can generally be characterized as aggressive and corrosive water due to its low pH, low bicarbonate concentration and low hardness. Post treatment is required to remineralize desalinated water to comply with the Drinking Water Guidelines. Softening pellets, a “waste” product from the pellet softening process, have been investigated to be reused for the remineralization process due to its main composition of calcium carbonate. The remineralization process can include two sub-processes: (1) a dissolution process and (2) a filtration process. The implementation of dissolution process is considered as practical complement to the existing limestone filtration process. In the dissolution process the softening pellets are dissolved by carbon dioxide acidified water to make a chemical solution with high concentration of calcium, bicarbonate and carbon dioxide. This chemical solution can be mixed with desalinated water in a ratio of 1:50 to 1:100. In the filtration process the aggressive water is filtered over a bed filled with softening pellets. Theoretical calculations showed that softening pellets can be used to make chemical solution with high concentration of calcium, bicarbonate and carbon dioxide; a pressure of 1bar carbon dioxide gas was assumed as an applicable operational parameter in dissolution process. The results of dissolution experiments confirmed this assumption. The results of filtration experiments showed that softening pellets can be applied as a filtration medium for remineralization process; the reaction constant of the softening pellets filtration process was approximately 0.35min-1 which was comparable to limestone filtration. However, difficult removal of nuclear sand and impurities of softening pellets in filtration process limited this application.