With lime softening well established in areas of the US with hard groundwater, theintroduction of membrane softening represented a fundamental shift in the approach toprovide soft, potable water. Nevertheless, the rapid acceptance of membrane softening inthe early 1990s was driven by several factors including the ability to produce a membranespecifically suited for softening, with associated low pressures, and due to the aging limesoftening infrastructure around the country. The need for new or expanded softeningwater treatment plants and the introduction of a lower pressure diffusion controlledmembrane made for perfect timing.In the past, traditional membrane fabrication had focused on maximizing salt rejection tomeet the needs of seawater and brackish water applications. While significant efforts hadbeen extended toward reducing net driving pressure, this was achieved only within thebounds of maintaining high monovalent ion rejection. In the 1980s, Stuart McClellanwith Dow/Filmtec and others began evaluation of the needs of communities treating freshgroundwaters with high total hardness. These sources did not require reduction in totaldissolved solids (TDS) but did require removal of divalent ions such as calcium andmagnesium. Traditional reverse osmosis membranes produced high quality finishedwater but at pressures over 200 psi. With modifications to the fabrication process, themembranes were essentially "loosened" to allow higher salt passage at lower pressures.Given the higher charge on divalent ions such as calcium, rejection remained high forthese constituents. The resulting pressures, well below 200 psi, represented a reducedO&M cost and a new opportunity for membrane applications.Concurrently, Dr. Jim Taylor with the University of Central Florida and others beganfocusing on the ability of diffusion controlled membranes to remove disinfection byproductprecursors. This work clearly showed that precursors could be effectivelyremoved even by the new, lower pressure, softening membranes. This ability to softenwater as well as remove precursors represented a multi-contaminant removal capabilityof significant value to many utilities.Without a doubt, Florida led the country in the early 1990s in the implementation of largemembrane softening facilities. With a large number of lime softening facilities and aburgeoning population, replacement or expansion of lime softening plants was common.Combined with the ability to soften and remove organic material at lower pressures thanever, membrane softening met the needs and was widely embraced by the drinking watercommunity. Based on a 2001 survey by Reiss Environmental of all demineralizationfacilities in Florida greater than 0.1 MGD, only one membrane softening facility wasconstructed prior to 1990. In the 1990s, 14 facilities were constructed. Running at asimilar pace, three facilities have been constructed since 2000.Well established and accepted by the drinking water community, membrane softening hasprovided its capabilities, defined by its advent and the resulting performance of thepioneering facilities installed in the early 1990s.
展开▼
