Drinking Water Softening/Scale Prevention Technology Assessment and Performance of Template Assisted Crystallization

Hard water typically has a hardness concentration over 120 mg/L CaCO3. Hardness is not a
\nregulated drinking water parameter and does not have severe health effects. However, hard water
\ncauses more soap and detergent consumption and can cause scaling problems on household
\nheating appliances, distribution pipes, and industrial cooling equipment.
\nThere are various approaches to soften hard water or prevent scale formation at the
\ncentralized and the household scale. In order to choose the best treatment for a specific set of
\nconditions, an appropriate technology evaluation is necessary. Prior research have tested single
\ntechnology or compared two or three technologies with respect to their performance. Not many
\npapers have compared all available technologies using the same assessment criteria.
\nAt the household scale, point-of-entry (POE) devices are commonly used. Among these, ion
\nexchange is the most widely applied POE device in Canada, though it has two major
\ndisadvantages: high sodium concentration in softened water and high chloride content in the
\nbrine which is often discharged into the sewer. Hence, there is an increasing interest in adopting
\nsalt-free treatment technology. Template assisted crystallization (TAC) is a relatively new
\nhousehold scale prevention technology. TAC media transforms free calcium (Ca2+) and
\nmagnesium (Mg2+) ions into insoluble microcrystals. TAC technology has the potential to be an
\nalternative to ion exchange, but there is very little published journals about this technology.
\nTherefore, this study had two objectives: 1) to assess and rank currently available softening
\nand scale prevention technologies at both the centralized and household level, and 2) test the
\nperformance of the TAC technology using two source waters.
\nThe multi-criteria assessment (MCA) method was utilized to evaluate centralized
\ntechnologies (lime softening, pellet softening, nanofiltration, and ion exchange) as well as
\nhousehold technologies (TAC, ion exchange, nanofiltration, electrically induced precipitation,
\nmagnetic water treatment, and capacitive deionization). Criteria that were chosen in this
\nassessment were: waste disposal, energy requirement, life-cycle cost, efficiency, subsequent
\ntreatment needed for finished water, chemical addition, easy to use, and validated technology.
\nThe initial assessment assigned a higher weight to the first four criteria listed. A sensitivity
\nanalysis (SA) was done by changing the weight assignment of different criteria. Three cases
\nwere selected: more focus on waste disposal and energy requirement; more emphasis on cost;
\niv
\neach criterion shared equal importance. For the centralized technologies assessment, pellet
\nsoftening had the highest score, followed by ion exchange, lime softening, and nanofiltration. SA
\nresults showed that although the total score of each technology varied, the final rank did not
\nchange. For the household technologies assessment, TAC had the highest score, followed by ion
\nexchange, magnetic water treatment, electrically induced precipitation, nanofiltration, and
\ncapacitive deionization. SA results showed that the total score varied, but the final rank did not
\nchange.
\nThe performance of TAC technology was assessed using four tests which compared
\nuntreated and treated water samples of two selected source waters. The first test measured the
\nreduction of free Ca2+ by a Ca2+ selective electrode after being treated by TAC in two source
\nwaters and, test results did not show a lot of reduction with percentage reductions ranging from
\n4.0% to 5.0% for both locations. The reductions were statistically significant but not large
\nenough to be of much practical value. The second test was to measure the change in total Ca2+
\nand Mg2+ concentration after TAC. The changes were relatively small in both source waters,
\nwith percentage reductions ranging from 2.7% to 4.4% for Ca2+ and 4.0% to 6.9% for Mg2+.
\nAgain, the reductions were statistically significant but not large enough to be of much practical
\nvalue. The third test was a sequential ultrafiltration test utilizing membranes (3000 Da, 1000 Da,
\nand 500 Da) to identify the microcrystal size. This test was not able to isolate substantial
\namounts of microcrystal, nor did it identify the approximate microcrystal size. The last test was
\ndeveloped as a simplified scale test. Results showed that treated water forms somewhat less scale
\nthan untreated water for both source waters. Scale formation potential indices: Langelier
\nSaturation Index (LSI) and Calcium Carbonate Precipitation Potential (CCPP) were also
\ncalculated, and results showed that there was essentially no change in both indices after the TAC
\ntreatment.
\nOverall, assessment results showed that using this study’s criteria, pellet softening and TAC
\nwere the two most suitable technologies to be applied at the centralized and household level,
\nrespectively. The TAC performance tests did not establish a substantial reduction in free calcium
\nions, nor were any crystals isolated. The scale test only showed relatively small differences
\nbetween untreated and treated water. Future research could construct a flow-through system to
\ntest the performance of TAC technology and should also conduct some tests on new and used
\nmedia.

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