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.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 190 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 2 |
| Published : | 11/01/2002 |
