TOC Reduction Using Membrane Technology
By Dean Schwarz
Honeywell's Solid State Electronics Center (SSEC) in Plymouth, Minn., had been trying to reduce the level of total organic carbon (TOC) in its high-purity deionized water system for many years. They had always experienced periods of relatively low TOC (less than 10 parts-per-billion [ppb]) but always had periods of high TOC (greater than 50 ppb). These high periods had usually been short periods of time and they could not directly correlate the TOC to any contamination on the product. Their water source was from a well via the Plymouth municipal water supply.
In April 1997 everything changed. For the first time, SSEC had a TOC excursion that was long-term. What made the matter more critical is that SSEC could correlate the TOC to contamination of the product. These two things were enough to convince everyone that SSEC had to find a solution that would ensure consistently low TOC levels.
Sequence of Events
The initial response to this problem was to systematically replace all of the consumables that might have been causing the problem. The SSEC brined and burned the two-bed ion-exchange units, regenerated the mixed-bed ion-exchange units, and replaced the activated carbon resin. Ultraviolet (UV) lamps and quartz sheaths in the UV TOC reduction units were also replaced. The SSEC then proceeded by sanitizing the reverse osmosis (RO) system and replacing the polisher mixed-bed resin with virgin resin. Unfortunately, none of this had any measurable effect on the TOC levels, as indicated in Figure 1.
The Plymouth municipal water source normally has a TOC level of about 1.5 parts per million (ppm) and the pH level is normally about 7.1. The SSEC did extensive chemical analysis of the city water supply as well as different sampling points in the building and in the supply lines. These initial samples came back inconclusive. The SSEC followed this up by having some samples concentrated and analyzed. These samples came back showing some chloroform and m,p-xylene detected at a concentration below the estimated concentration limit but above the detectable limit. This was the first measurable indication of the source of a problem….
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