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With lime softening well established in areas of the U.S. with hard groundwater, the introduction of membrane softening represented a fundamental shift in the approach to provide soft, potable water. Nevertheless, the rapid acceptance of membrane softening in the early 1990s was driven by several factors including the ability to produce a membrane specifically suited for softening, with associated low pressures, and due to the aging lime softening infrastructure around the country. The need for new or expanded softening water treatment plants and the introduction of a lower pressure diffusion controlled membrane made for perfect timing. In the past, traditional membrane fabrication had focused on maximizing salt rejection to meet the needs of seawater and brackish water applications. While significant efforts had been extended toward reducing net driving pressure, this was achieved only within the bounds of maintaining high monovalent ion rejection. In the 1980s, Stuart McClellan with Dow/Filmtec and others began evaluation of the needs of communities treating fresh groundwaters with high total hardness. These sources did not require reduction in total dissolved solids (TDS) but did require removal of divalent ions such as calcium and magnesium. Traditional reverse osmosis membranes produced high quality finished water but at pressures over 200 psi. With modifications to the fabrication process, the membranes were essentially "loosened" to allow higher salt passage at lower pressures. Given the higher charge on divalent ions such as calcium, rejection remained high for these constituents. The resulting pressures, well below 200 psi, represented a reduced O

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Edition: Vol. - No. Published: 11/01/2002 Number of Pages: 2File Size: 1 file , 190 KB