AWWA WQTC57022

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&M cost and a new opportunity for membrane applications. Concurrently, Dr. Jim Taylor with the University of Central Florida and others began focusing on the ability of diffusion controlled membranes to remove disinfection byproduct precursors. This work clearly showed that precursors could be effectively removed even by the new, lower pressure, softening membranes. This ability to soften water as well as remove precursors represented a multi-contaminant removal capability of significant value to many utilities. Without a doubt, Florida led the country in the early 1990s in the implementation of large membrane softening facilities. With a large number of lime softening facilities and a burgeoning population, replacement or expansion of lime softening plants was common. Combined with the ability to soften and remove organic material at lower pressures than ever, membrane softening met the needs and was widely embraced by the drinking water community. Based on a 2001 survey of all demineralization facilities in Florida greater than 0.1 MGD, only one membrane softening facility was constructed prior to 1990. In the 1990s, 14 facilities were constructed. Running at a similar pace, three facilities have been constructed since 2000. Well established and accepted by the drinking water community, membrane softening has provided its capabilities, defined by its advent and the resulting performance of the pioneering facilities installed in the early 1990s. Includes extended abstract only.

Product Details

Edition:

Vol. - No.

Published:

11/01/2002

Number of Pages:

2

File Size:

1 file , 190 KB