Compact Accelerated Precipitation Softening (CAPS) with Submerged

In this study, the concept of submerged filtration is applied to the CAPS process. The former technology has been successfully used in the treatment o...
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Ind. Eng. Chem. Res. 2002, 41, 5308-5315

Compact Accelerated Precipitation Softening (CAPS) with Submerged Filtration: Role of the CaCO3 “Cake” and the Slurry Y. Oren,* V. Katz, and N. C. Daltrophe The Institutes for Applied Research, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel

A new concept for compact accelerated precipitation softening (CAPS), comprising in-tank mixing and filtration, is presented. Softening with CAPS is both fast and efficient, because CaCO3 precipitation is achieved in a well-stirred calcite slurry and in a compact cake formed on the filter elements. Other advantages of the CAPS process are its compactness, technical simplicity, and low cost. In this study, laboratory CAPS units were run continuously (up to 600 h) for tapwater softening. The effects of the initial concentration of the CaCO3 slurry, the pumping speed, and the pH and the individual effects of the slurry and the cake and of the filter cake load were analyzed in terms of the reduction of the saturation index (SI) and the precipitation capacity. It was found that calcium reduction in the cake was 10-100 times faster than that in the slurry. In addition, turbidity was reduced significantly because of the microfiltration capability of the cake. CAPS can be used as a stand-alone water-treatment process or in conjunction with pressureor electricity-driven membrane processes (UF, NF, RO, ED) as an effective pretreatment routine for increasing recovery and decreasing fouling rates. Introduction The increased demand for higher recovery in membrane water-treatment processes calls for new ideas for improving feed or brine treatments. In both osmosis (RO) and electodialysis (ED), scaling and fouling rates of the membranes are increased as the brine concentration is increased. These phenomena limit water recovery, increase energy consumption, shorten membrane life, and increase demand for chemicals (antiscalants and acids). As a result, water-treatment costs increase. It is thus generally accepted that there is a need to decrease scaling and fouling rates, and accordingly, new pretreatment procedures are being sought. To increase the concentration factor and hence the recovery ratio, the feedwater should be pretreated to remove scaling and fouling factors. For this purpose, a compact and environmentally compatible softening process is required. However, the commonly used softening technologies, namely, lime softening, ion exchange, and membrane softening, do not satisfy these requirements. Lime softening is slow, requires extensive space,1 and generates a sludge that is difficult to handle. Ion exchange requires salt for regeneration and thus increases environmental salinization. The use of nanofiltration to retain Ca2+ in preference to Na+ has not produced the required degree of recovery.1 A new technology, designated compact accelerated precipitation softening (CAPS), designed to be both compact and environmentally compatible, was first investigated by Kedem and Ben-Dror2 and Kedem and Zalmon.3 In this process, very fast equilibrium was attained when a supersaturated CaCO3 solution was filtered through a cake of CaCO3 particles. Because no significant changes in the particle size distribution occurred upon repeated filtration-crystallization through the same cake, it was concluded that the fast process * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: 972-7-6477167. Fax: 972-76472960.

comprised both secondary nucleation and crystal growth.4 Within the dense cake structure, the removal of low calcium concentrations is possible within short contact times because of the enhanced mass transfer rates resulting from large solution velocities within the narrow pores and a much larger surface-to-volume ratio. In a later study, Massarwa et al.5 investigated the possibility of simultaneous CaCO3 and silica removal with CAPS. Gilron et al.6 studied the possibility of applying CAPS for reducing SDI, organics, and hardness to levels satisfactory for prolonged RO treatment. In that work, a CaCO3 slurry was circulated under pressure through a microfiltration unit in which the filter element was coated with a cake. In CAPS, reduction of calcium and carbonate alkalinity from tap water is achieved by accelerated CaCO3 nucleation and growth in two consecutive steps: (1) in a pre-prepared slurry of small calcite particles and (2) in a CaCO3 layer (cake) formed on the top of the filter through which softened water is pumped out. Although most of the precipitation occurs in the slurry, the cake process is a polishing step in which calcium concentration is further reduced. In this study, the concept of submerged filtration is applied to the CAPS process. The former technology has been successfully used in the treatment of various types of water, such as industrial effluents,7-9 wastewater,10,11 and drinking water.12 In the CAPS device tested in this work, stirring and filtration were performed in the same tank through a submerged filtration module. This setup makes CAPS particularly attractive for the pretreatment of water for membrane processes: the process is compact and simple, and therefore, it becomes more feasible in terms of cost. Experimental Section CAPS Unit. A schematic representation of the new laboratory CAPS setup is shown in Figure 1. The setup consists of a 25-L vessel, in which a filtering module made of microporous filtration elements having a nomi-

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Figure 1. Schematic description of the CAPS experimental setup. Table 1. Feed Composition (Typical) component

concentration (ppm)

pH Ca2+ Mg2+ HCO3CO32ClSO42NO3Na+ TOC

7.5-8.5 65-80 35-40 210-260 0 150-190 60-70 10-14 120-140 3-5

nal pore diameter of 10-13 µm is vertically submerged in the slurry. The total surface area is 197 or 394 cm2 (depending on the number of filtration elements installed). The setup also contains a stirring unit capable of maintaining a stable and homogeneous suspension throughout an experimental run. The water volume in the tank is kept at 17 L by a level switch-controlled solenoid valve. Water is pumped through the filter module at a controlled flow rate, and thus, the pressure drop changes during the experiment. A pH controller, a metering pump, and a glass electrode on the permeate line maintain the permeate pH at a preset value in the range of 8.5-10.5 by injection of a NaOH solution. The upper pH limit used in this study was 10.5 so as to avoid excessive precipitation of Mg(OH)2, which would quickly clog the filters. Transfilter pressure, water flow rate, product and in-tank pH values, and temperature are continuously monitored by appropriate transmitters, and the data are recorded on a personal computer. Experimental Conditions. In all experiments, tap water with the typical composition listed in Table 1 served as the feed. Prior to the start of each experiment, a slurry of 0.1-5% CaCO3 in the form of calcite (as verified by XRD) was prepared in the tank. The average particle diameter was 20 µm. The powder was thoroughly mixed with the water contained in the vessel, and slurry homogeneity was verified by sampling the slurry at different heights. Both short (0.5-3-h) and long (up to 600-h) experiments were conducted to determine the effects of the permeate flow rate, pH, CaCO3 concentration, and

operation time on the pressure drop, cake load, particle distribution, and water quality. In the long experiments, the filter module was backwashed with air every 3060 min by the application of 2-5-s pulses. The short experiments were followed by backwashing at the end of each run. The amount of CaCO3 adhering to the filters and forming the cake was determined by sampling the suspension just before and immediately after the backwash. The samples were then dried at 100 °C for 12 h and weighed. Cake load was calculated from the difference between the weights, assuming that backwash returned all of the cake material to the suspension. In the long experiments, CaCO3 accumulated in the tank, and its concentration was determined throughout the process by sampling of the slurry. This facilitated the correlation of cake load, pressure drop across the filter module, water content, and particle size distribution with slurry concentration as the experiment proceeded. Calcium and magnesium contents were determined by colorimetric titration and atomic absorption. Carbonate and bicarbonate contents were determined by potentiometric titration. The CaCO3 concentration distribution in the vessel was determined by sampling the suspension at the top, middle, and bottom of the tank. Permeate turbidity was measured with a Hach model 2100A turbidimeter. The particle size distribution was obtained with a Galai (Israel) particle analyzer (model CIS-1). XRD and SEM analyses were performed on dried samples of the suspension taken during the long experiments. The apparent specific gravity of the CaCO3 forming the cake was determined as follows: Fifty milliliters of distilled water were thoroughly mixed with 23.64 g of calcite primer, and the mixture was transferred to a graduated glass cylinder. The particles were allowed to settle for a few days, at the end of which the volume occupied by the solid was measured. Results and Discussion Quality of the CAPS Product. Water quality at different stages of the CAPS process can be expressed in two different ways: (A) Water quality can be expressed in terms of the saturation index (SI)13

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SI ) pH - pHS

(1)

where pH is the measured value and pHS is the equilibrium value determined by

pHS ) -log(HCO3-) - log(Ca2+) + pK2 pKSP + log(fD) (2) where fD is an activity factor for divalent ions and -log(fD) is given by14

-log(fD) ) 2.04[xI/(1 + xI) - 0.3I]

(3)

In eq 3, I is the ionic strength, and

KSP ) (Ca2+)(CO32-)

(4)

(H +)(CO32-) fD (HCO3- )

(5)

K2 )

SI is the traditionally used measure of the degree of supersaturation with respect to CaCO3. Specifically, for SI ) 0, the solution is saturated; for SI > 0, the solution is supersaturated; and for SI < 0, more CaCO3 can still dissolve. (B) In some cases, particularly those in which pH adjustment for the product is not required and thus water can still be supersaturated with respect to CaCO3, it is important to know the precipitation capacity (PC) of the softened water, i.e., the maximum amount of CaCO3 per unit volume expected to precipitate if, in some way (e.g., by seeding), water is brought to equilibrium. As CaCO3 precipitates, the equilibrium pH changes. Therefore, PC should be determined by a stepwise calculation. In this study, this was achieved using the MINTEQA2 algorithm.15 In Figure 2, the SI for the permeate and the water in the vessel is shown as a function of the in-tank CaCO3 concentration. The feed SI is also given for purposes of comparison. In calculating these SI values, it was assumed that the treated water was re-acidified to the feed pH, leaving the concentration of HCO31- unchanged. The latter assumption is reasonable, because calculation of the ratio [HCO31-]/([H2CO3] + [HCO31-] + [CO32-]), taking into account the carbonic acid dissociation constants, showed that [HCO31-] did not change significantly when the pH was lowered from 9.5-10 to 7.5-8. From Figure 2, it is evident that both the in-tank and permeate SI values decreased significantly as the CaCO3 concentration increased, and both were much smaller than the feed values, indicating the CAPS softening effect. The most striking observation to emerge from this figure is the difference between the in-tank and permeate values, indicating the additional effect (15-20%) of the CaCO3 filter cake on the reduction of the calcium concentration. In Figure 3, SI values for the permeate, adjusted for the feed pH, are presented as a function of the in-tank CaCO3 concentration for different output flow rates. The effect of the slurry concentration is similar to that shown in Figure 2. It is evident that the flow rate affects the water quality as expected; that is, because of the increasing contact time between the water and the CaCO3 particles, the SI decreased with flow rate, reaching negative values (complete supersaturation relief) at the smallest applied flow rates.

Figure 2. Corrected saturation index (SI) for the product and the water in the vessel as a function of the in-tank CaCO3 concentration.

Figure 3. Corrected saturation index (SI) as a function of the in-tank CaCO3 concentration for the product and the feed at various flow rates.

Results of the PC calculation as a function of the CaCO3 in-tank concentration for different flow rates are shown in Figure 4, together with the average value calculated for the feed. It is evident that increasing the CaCO3 concentration resulted in a decrease of the PC. However, the increasing flow rate had an adverse effect; that is, the PC increased because of higher Ca2+ concentrations remaining in the water as the contact time decreased. Cake load values, that is, the amount of CaCO3 adhering to a unit area of filter module, as obtained at the end of the softening-backwash cycles for different flow rates and in-tank CaCO3 concentrations, are shown in Figure 5. It is apparent that both parameters significantly affected the amount of material attached to the filter module. The cake load is formed by particles dragged from the slurry by the water flowing through the filter; that is, the number of particles per unit time and area hitting the filter surface is a function of the

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Figure 4. Precipitation capacity (PC) as a function of the in-tank CaCO3 concentration for the product and the feed at various flow rates.

Figure 5. Cake load as a function of the in-tank CaCO3 concentration for various flow rates.

water flow rate and concentration. However, because stirring of the slurry in the vessel was intensive, it should be assumed that shear forces applied to the cake surface reduced the amount of material attached to it. Thus, at each specific time in a continuous operation, the cake load is a function of the balance between the drag forces of the flowing water and shear forces of the stirring action. An additional aspect of CaCO3 precipitation in CAPS is shown in Figure 6a and b. As mentioned above, the CaCO3 primer is in the calcite form. As shown in Figure 6a, these particles are randomly distributed in terms of shape, with an average diameter of 20 µm, as verified by size distribution analysis. BET analysis gave a specific surface area of 0.1 m2/g, as was also calculated by assuming nonporous spherical particles with a diameter of 20 µm. As the experiment proceeded, the seeding calcite particles became covered with smaller needlelike aragonite crystallites, as shown in Figure 6b and verified by XDR analysis.

Figure 6. (a) SEM photograph of calcite primer (×600); (b) SEM photographs of aragonite crystallites on calcite (×600 and ×10 000).

Presenting a kinetically favorable crystalline form, aragonite was expected to be the major component precipitating throughout the fast supersaturation release in CAPS. However, coprecipitation of calcite should not be ruled out. Yet, in light of the findings that major retardation of calcite precipitation occurs in the presence of magnesium ions, calcite is expected to precipitate in the form of microcrystallites. Aragonite precipitation, on the other hand, is not affected by the presence of magnesium,16 and thus larger crystallites are formed.

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Figure 7. Mean particle size as a function of the in-tank CaCO3 concentration at different flow rates.

Particle size distribution analysis revealed a constant shift in particle diameter toward larger values as an experiment proceeded, as shown in Figure 7. In Figure 8, variations with experiment duration in the percent volume density of the particles in different size ranges are shown. Evidently, the main precipitation process in the slurry is crystallization (rather than nucleation), as shown by the tendency of the smaller particles to disappear and of the larger particles to grow as the experiment progresses. In contrast, as mentioned above, it was found that, in the cake, both crystal growth and secondary nucleation take place.3 The results shown in Figures 2-4 should be interpreted in terms of the water retention time within the vessel, the contact time with the cake, and the crystalline shape and size distribution of CaCO3 deposited in the tank, because these parameters determine the rate at which the Ca2+ concentration is reduced in each CAPS region. Reaction rates in the slurry at constant CaCO3 concentration, kslurry were calculated from

kslurry )

(Cfeed - Cvessel)Q V

(6)

where Cfeed, Cvessel, Q, and V are Ca2+ the feed and intank concentrations, the volumetric water flow rate (cm3 s-1), and the water volume in the tank, respectively. The corresponding reaction rate values in the cake, kcake, were calculated as follows

kcake )

(Cvessel - Cproduct)QF AΘ

Figure 8. Particle volume fraction as a function of the duration of the experiment and the in-tank CaCO3 concentration for different ranges of particle size.

Figure 9. Ca2+ concentration reduction rates in the slurry and cake as a function of the in-tank CaCO3 concentration at different flow rates.

(7)

where Cproduct, F, A, and Θ are Ca2+ the concentration in the product, the CaCO3 apparent specific gravity, the filter module total surface area, and the cake load (g cm-2), respectively. In the calculations performed according to eq 7, it was assumed that cake CaCO3 particles were tightly packed, having an apparent specific gravity of 1.244 g cm-3, as found by the procedure outlined in the Experimental Section. It was further assumed that the cake content was distributed evenly around each of the filter elements. Although the first assumption is close to reality, the second is a rough approximation of the distorted real cake shape resulting from uneven slurry stirring and

flow distribution around the filter module elements. In other words, it was found that, in the parts of the filter elements not facing the stirrer, the cake was thinner than in those facing the stirrer. Consequently, kcake is an average of all local values. The results of reaction rate calculations are summarized in Figure 9 for the CaCO3 concentration in the vessel and the water flow rate. The most striking finding is the large difference between the reaction rates in the cake and in the slurry, with the rates in the cake being 10-100 times greater than those in the slurry over the entire CaCO3 concentration range. The small increase in reaction rate in the slurry with increasing CaCO3 concentration can be justified in

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Figure 11. Transfilter pressure as a function of time within the cycle for different in-tank CaCO3 concentrations. Figure 10. Turbidity of product (normalized to feed) as a function of time within a cycle for different in-tank CaCO3 concentrations.

terms of the increasing reaction surface area as precipitation proceeds. However, at this stage, we have no definitive explanation for the significant decrease in reaction rates in the cake as CaCO3 accumulates in the reaction vessel. A possible explanation is that, as the cake becomes thicker (as shown in Figure 5), its hydraulic resistance increases, and as discussed above, it might also be less homogeneously distributed around the filter elements. In this case, a decreasing fraction of the cake might be available to the crystallization process because of the water flow favoring those parts with smaller hydraulic resistance. Nevertheless, this phenomenon has to be studied further under bettercontrolled conditions. Another aspect of the CAPS product quality is depicted in Figure 10, namely, variations in the turbidity (normalized with respect to that of the feed) during a cycle and as a function of the slurry concentration. It is obvious that turbidity lessened with increasing time within a cycle and with increasing slurry concentration, resulting in less turbid water as compared to the feed. This clearly resulted from the improving filtration capability of the filtration elements in the developing cake. At this point, it should be stressed that, in this sense, the cake serves as a microfilter, rejecting not only the CaCO3 particles, but also other contaminants in the micron range. Hydrodynamic Characteristics of the Cake. In a continuous prolonged operation, the cake load and filtration characteristics change both within each cycle and between cycles. These changes are important in the sense that they provide the information necessary to make decisions on the time gap between backwashes, the duration of the backwash, and the stage at which the process should be terminated for thorough cleaning of the filtration module, e.g., with acid. For this purpose, it is necessary to follow the transfilter pressure changes under constant flow conditions. An example of transfilter pressure variations between backwashes and as a function of the slurry concentration is given in Figure 11. Within the first 10-15 min after backwash, the pressure drop increased sharply. This was followed by a mild and prolonged, nearly plateau-shaped pressure

Figure 12. Transfilter pressure at the beginning and the end of a cycle as a function of the experiment duration and the in-tank CaCO3 concentration.

growth. This behavior is an indication of fast bulk cake buildup immediately after backwash, followed by slow precipitation in the cake pores that results in an additional gradual increase of the hydraulic resistance of the cake. In filtration processes to which backwash should be applied to revive the filter unit after a prolonged operation, the question of backwash efficiency always arises. Following the transfilter pressure drop that develops immediately after backwash at constant flow rate provides the necessary information answer to this question. In Figure 12, these pressures are shown, together with those at the end of the pumping cycles, as a function of the duration of the experiment. As discussed above in conjunction with Figures 5 and 10, the pressure drop at the end of the cycle is determined mainly by the CaCO3 in-tank concentration and cake load, which vary with the duration of the experiment. The transfilter pressure at the end of the cycle is

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Figure 13. NaOH consumption and amount of CaCO3 precipitated as a function of the flow rate. Table 2. Comparison of [H2CO3*]product/[H2CO3*]feeda Obtained under Different Conditions case h-1

this study, flow 1.21 m this study, flow 1.82 m h-1 this study, flow 2.58 m h-1 Gilron et al.6 Kedem and Zalmon3 MINTEQA2 simulationb

pH

[H2CO3*]product/[H2CO3*]feed

9.1-9.3 9.1-9.3 9.1-9.3 8.6 9.0 9.3

0.89 0.87 0.89 0.485 0.474 0.883

a [H CO *] ) [HCO -] + [CO 2-]. b Assuming equilibrium with 2 3 3 3 atmospheric CO2 and complete CaCO3 precipitation.

therefore expected to increase with the duration of the experiment, as shown in Figure 12. However, an increase in the initial pressure with time was not obvious, and there was evidence of an irreversible plugging process that could not overcome by backwashing. This process could have been caused by precipitation of CaCO3 in the filter pores and by organic material originally present in the feedwater. It should be noted that a constant increase in pressure was also observed in those experiments in which the slurry concentration was kept constant by the periodic withdrawal of solid from the tank. It is therefore desirable to terminate the process periodically for thorough cleaning. In this study, the process was stopped after 500-600 h of continuous operation, and all of the reactor parts were washed with a solution of citric acid. After this procedure, the filter characteristics were completely restored. NaOH Consumption. According to the stoichiometry of the reaction between Ca(HCO3) 2 and NaOH, 1 mol of base is required for the precipitation of 1 mol of CaCO3. However, this was not the case in the experiments performed in this study, as shown in Figure 13,

in which the number of moles of CaCO3 precipitated from each cubic meter of water treated and the corresponding NaOH consumption are expressed as a function of the flow rate. From the regression lines, it is evident that the ratio [NaOH]/[CaCO3] is 1.78, irrespective of the flow rate. At this stage, we do not have a suitable explanation for this large discrepancy. It might be related to the very intensive stirring applied to the slurry and to the fact that the slurry was exposed to the open atmosphere and to backwash with air. As a result, the water in the reaction vessel was always at or near equilibrium with atmospheric CO2, and therefore, additional base was required to bring it to the preset pH. In Table 2, the ratio of the sum of carbonic acid components in the CAPS-treated water to that in the feed is given for different cases. The large differences between the values calculated in this study and those calculated from the data given by Kedem and Zalmon3 and by Gilron et al.6 are evidently due to the differences in experimental conditions: In both CAPS devices used in the above-described studies,3,6 the slurry was not exposed to the atmosphere as intensively as in the current study (closed systems were used), and stirring was much milder. (In fact, in both studies, the slurry was circulated with a pump, and no additional stirring was applied.) These conditions resulted in less efficient absorption of atmospheric CO2 and, hence, the small values shown in Table 2. Unfortunately, no data on NaOH consumption was provided in the previous studies.3,6 The agreement between the actual experimental ratio values measured in this study and the ratio calculated from the simulation, imposing equilibration with atmospheric CO2 (partial pressure of 3.5 × 10-4 atm) and the relevant pH, strengthens the explanation presented above. Because of the adverse effects of surplus NaOH consumption, namely, increasing process costs and excessive and unnecessary addition of Na+ ions to water, it is extremely important to study this problem further and to find means of closing the gap between the actual base consumption and the theoretical requirement. Summary and Conclusions In this study, the basic features of a new concept for the CAPS process were investigated. It was shown that precipitation in the CaCO3 cake formed around the submerged filter unit was 10-100 times faster than that in the slurry. However, the slurry was more effective in reducing the Ca2+ concentration because of highly efficient stirring and much longer retention times. Thus, the process in the cake can be considered a polishing step. Another important feature of CAPS is that the cake acts as a microfilter capable of rejecting CaCO3 particles as well as other contaminants in the micron

Table 3. Comparison between CAPS and Other Softening Processes water recovery unit core size (retention time) treatment cost environmental impact posttreatment remarks

lime softening1

ion exchange

nanofiltration1

fluidized bed17

CAPS

99 0.08-0.15 m3/m3/h (5-9 min) ∼10 ¢/m3 wet, compact solid disposal acidification (not always) low turbidity (microfiltration), simple

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range. A comparison of some of the basic operational characteristics of the improved CAPS process with those of other methods used for water softening is presented in Table 3. In this table, CAPS is evaluated with respect to lime softening, ion exchange, fluidized bed softening,17 and nanofiltration. The fluidized bed, pelletsoftening process17 is fast and efficient but restricted to high flow rates and large facilities. In contrast, ion exchange, nanofiltration, and CAPS are compact processes. In addition to compactness, the unique advantages of CAPS lie in the very high water recovery, the compact solid generated for disposal, and the fact that it does not impose serious environmental problems, as does ion exchange. CAPS can thus be used as a compact stand-alone water-treatment process or in conjunction with pressure- or electricity-driven membrane processes [ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), electodialysis (ED)] as an effective pretreatment routine for increasing recovery and decreasing fouling rates. Acknowledgment The authors are indebted to Prof. O. Kedem and Dr. J. Gilron for helpful discussions. This study was supported by the Magneton Program of the Israel Ministry of Industry & Trade, by Nitron Ltd. (Israel), and by the Charles Wolfson Charitable Foundation. Literature Cited (1) Bergman, R. A. Desalination 1995, 102, 11. (2) Kedem, O.; Ben-Dror, J. Water Softening Process. U.S. Patent 5,152,904, 1992. (3) Kedem, O.; Zalmon, G. Desalination 1997, 113, 65-71. (4) Peters, R. W.; Chen, P. H.; Chang, T. K. In Industrial Crystallization 84; Jancic, S. J., de Jong, E. J., Eds.; Elsevier: Amsterdam, 1984.

(5) Massarwa, A.; Meyerstein, D.; Daltrophe, N.; Kedem, O. Desalination 1997, 113, 73. (6) Gilron, J.; Chaikin, D.; Daltrophe, N. Desalination 2000, 127, 271-282. (7) Mallon, D.; Steen, F.; Brindle, K. Spec. Publ. R. Soc. Chem. 2000, 249, 226-232. (8) Silva, M. C.; Reeve, D. W.; Woodhouse, K.; Husain, H.; Behmann, H. In International Environmental Conference and Exhibition; TAPPI Press: Atlanta, GA, 1998; Book 3, pp 10351044. (9) Hermann, A. P.; Funfrocken, E.; Wefringhaus, E.; Janke, H. D. Wasserwirtsch. Abwasser, Abfall 2000, 47 (7), 1001-1004, 1006-1013. (10) Martyn, H.; Graham, N.; Day, M.; Cooper, P. Presented at MBR2: Membrane Bioreactor Wastewater Treatment, 2nd International Meeting, Cranfield, U.K., June 2, 1999. (11) Carriere, J.; Mourato, D. Vecteur Environ. 2000, 33 (4), 19-24. (12) Johnson, W. T. In Proceedings of the Annual Conference of the American Water Works Association; American Water Works Association: Denver, CO, 1999; pp 951-962. (13) Clark, J. W.; Viessman, W.; Hammer, M. J. Water Supply and Pollution Control. International Textbook; International Textbook Company: Scranton, PA, 1971. (14) Wiechers, H. N. S.; Sturrock, P.; Maris, G. V. R. Water Res. 1975, 9, 835. (15) MINTEQA2: Metal Speciation Equilibrium Model for Surface and Ground Water, version 3.11; Center for Exposure Assessment Modeling (CEAM), Office of Research and Development, Environmental Research Laboratory, U.S. Environmental Protection Agency: Athens, GA, 1991. (16) Berner, R. A. Role of magnesium in the crystal growth of calcite and aragonite from sea water. Geochim. Cosmochim. Acta 1975, 39, 489-504. (17) van der Veen, C.; Graveland, A. Central Softening by Crystallization in a Fluidized Bed Process. J. Am. Water Works Assoc. 1988, 80, 51.

Received for review September 5, 2001 Revised manuscript received April 19, 2002 Accepted April 19, 2002 IE010740W