FOR HARDNESS REMOVAL. Integrating Membrane Filtration and a Fluidized-bed Pellet Reactor

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1 Fluidized-bed pellet reactor technology was developed to remove water hardness with minimal sludge production. Operation procedures of this process include adding pellets into a column reactor and directing raw water and chemicals through the bottom of the column, keeping pellets fluidized to prevent them from being coagulated. In this study a process combining an ultrafiltration (UF) membrane with outside-in flow configuration and a fluidizedbed pellet reactor was proposed for hard water softening. The advantages of such a process include a compact reactor footprint, high treatment efficiency, and excellent effluent quality. Three different pellets of various sizes were used: quartz sand, beach sand, and heated iron PHOTO: YI-MING CHEN oxide particles (HIOPs). Results show that with a removal efficiency of less than 16%, a UF membrane alone is not an effective process for removing hardness due to the slow reaction kinetic of calcium carbonate precipitation without the presence of pellets. When three types of pellets were added separately, the removal efficiency of the integrated UF/pellet process was more than 60% at a ph level of 9.0. Hardness removal efficiency improved with increasing ph and pellet surface area, and no significant membrane fouling was observed during the experiments. Integrating Membrane Filtration and a Fluidized-bed Pellet Reactor FOR HARDNESS REMOVAL BY CHI-WANG LI, JAU-CHENG JIAN, AND JUNG-CHIH LIAO W ater softening by raising solution ph levels to approximately 9 10 in which cations that cause hardness, such as calcium ion (Ca 2+ ) and magnesium ion (Mg 2+ ), are removed by chemical precipitation can be very effective. However, the high cost associated with treating the sludge produced from the chemical precipitation process may not be economically viable for locations with high population density and limited land space. Fluidized-bed pellet reactor technology was developed to remove water hardness with minimal sludge production. Operational procedures include adding pellets (usually silica sand) into a column reactor and directing raw water and chemicals (such as lime or sodium hydroxide [NaOH]) through the bottom of a column, keeping pellets fluidized to prevent them from being coagulated. At an elevated ph level, Ca 2+ or Mg 2+ ions will precipitate, or crystallize, on the pellet surfaces. Kinetics of precipitation or crystallization depend on the surface area of the pellets inside the reactor (Snoeyink & Jenkins, 1980). Because of crystallization, pellets will grow and settle at the bottom of the column. To maintain a high area of pellets inside the reactor, large pellets that may have settled at LI ET AL PEER-REVIEWED 96:8 JOURNAL AWWA AUGUST

2 TABLE 1 Characteristics of pellets used in this study Pellet Characteristics Quartz Sand Beach Sand HIOPs* Diameter mm Density g/cm Specific area m 2 /m 3 1E4 2.02E4 1.2E6 Surface area per gram of pellet m 2 /g 3.72E E E-1 * TABLE 2 Characteristics of UF* membrane used in this study Parameter Manufacturer s Specifications Type ZeeWeed 1 Nominal pore size 0.04 µm Outside diameter 2 mm Effective area m 2 (0.5 sq ft) Operating pressure 4 bar Operating flow rate 5 to 25 ml/min Operating ph 7 10 Properties Hydrophilic *UF ultrafiltration Zenon Environmental Inc., Burlington, Ont., Canada TABLE 3 Data for pellets tested at ph 9.0 Type of Pellets Quartz Sand Beach Sand HIOPs* Pellet added g 3,485 1, Pellet-to-reactor volume ratio % Pellet surface area m 2 /L Average hardness removal efficiency % * TABLE 4 Effect of ph on hardness removal using HIOPs* (0.14 m 2 /L) or quartz sand (1.85 m 2 /L) ph HIOP System Quartz Sand System 8.5 (21) (65) (188) * Numbers in parentheses are the theoretical saturation index for calcium carbonate. Percent hardness removal efficiency the bottom of the column must be expelled and new pellets added (Graveland et al, 1983). A pellet reactor is usually operated at an upflow velocity of around 70 to 100 m/h (230 to 328 fph). The diameter of the pellets and the hydraulic residence time are mm and 8 10 min, respectively (Harms & Robinson, 1992). It is clear that the smaller the pellet size, the bigger the specific surface area available for precipitation. However, the choice of pellet size is limited by the upflow velocity of the process. When small pellets are chosen, high upflow velocity should be avoided to prevent the pellets from being carried out of the reactor. One way to overcome this problem is to use pellets that have a high density, such as steel grit or marble chip (Van der Veen & Graveland, 1988). Another problem is that improper design and improper operation of the fluidizedbed reactor may cause a high content of suspended solid to break through the reactor. In the United States, certain state governments require a sedimentation tank to be installed after a fluidized-bed reactor (Harms & Robinson, 1992). Sluys et al (1996) proposed an inside-out hollow-fiber membraneassisted crystallizer (MAC) for removing hardness. Sluys and colleagues investigated the effect of a pellet s size on the rate of CaCO 3 crystallization, reporting that the rate of CaCO 3 crystallization increased as the size of the pellets decreased and as increasing amounts of pellets were added. Removal efficiency of the MAC for hardness was approximately 98% compared with 70% for traditional pellet reactors. However, the authors did not specify the operation ph level of these systems. Sluys et al (1996) also investigated the effect of cross-flow velocity on the flux of membrane, reporting that increasing cross-flow velocity significantly enhanced membrane flux until a critical cross-flow velocity of approximately 5.0 m/s (16.4 fps) was reached. Although excellent hardness removal efficiency can be achieved, the MAC process presents potential disadvantages: the membrane may be fouled by either crystallization of hardness on the membrane surface, or the membrane lumen or pores may be blocked by the particles. BACKGROUND For this research, the authors studied hardness removal using a process combining an outside-in hollow-fiber 152 AUGUST 2004 JOURNAL AWWA 96:8 PEER-REVIEWED LI ET AL

3 membrane and fluidized-bed pellet reactor. Effects of operation ph level; membrane operation conditions; and pellet size, type, and quantity on hardness removal efficiency and membrane fouling were investigated. Applying an outside-in hollow-fiber membrane instead of an inside-out configuration in a crystallizer can avoid possible blockage of membrane fiber lumen by pellets as indicated by Sluys et al (1996). Also, although it is necessary to provide cross flow through an inside-out membrane to reduce the extent of membrane fouling, it is not necessary to provide cross flow through an outside-in hollow-fiber membrane, which reduces the amount of energy needed (Sluys et al, 1996). MATERIAL AND METHODS Three types of pellets were tested: quartz sand, beach sand (obtained from a local beach in Taipei, Taiwan), and heated iron oxide particles (HIOPs). Characteristics of the pellets, which were chosen based on their varying diameters and surface areas, are listed in Table 1. For example, 1 g of beach sand and 1 g of HIOPs have surface areas of 3 and 117 times of that of 1 g of quartz sand, respectively. After the sample of beach sand was cleaned with deionized (DI) water to remove salts and dried at 110 o C, the sample was put through a mesh sieve. Those particles that passed through a mm sieve and stayed at a mm sieve were selected. The diameter of the beach sand was reported at mm as listed in Table 1. HIOPs were manufactured in a lab following the procedures reported by Chang and Benjamin (1996) at a diameter of approximately mm. In this study, the HIOPs were chosen to demonstrate the ability of the proposed system to accommodate the tiny pellet size and to demonstrate the idea that the smaller the size of the pellets, the smaller the size of the process footprint. Pellets of any size can be applied in the proposed system; however, selection of a pellet size should take into account the volume of the reactor needed to house the membrane, the ease of pellet handling, and the total cost of the pellets. An outside-in membrane, the characteristics of which are shown in Table 2, was submerged either in a reactor with an acrylic column 7 cm (2.75 in.) in diameter (reactor A for sand pellets) or in a rectangular reactor with a length of 38.5 cm (15.2 in.), width of 18 cm (7.1 in.), and height of 20 cm (7.9 in.) (reactor B for HIOPs), depending on the type of pellets tested. Except for the tests to investigate the effect of flow rate on the proposed process, the UF membrane was operated with or without pellets inside the reactor at a fixed flow rate of 20 L/h/m 2 (15.5 ml/min) as suggested by the manufacturer. With a reactor volume of 7 L, a flow rate of 15.5 ml/min corresponds to a detention time of 7.53 h, which is much longer than what is encountered when using a conventional fluidized-bed reactor (Sluys et al, 1996; Harms & Robinson, 1992). Long detention time in this study does In the study, three types of pellets were tested: quartz sand, beach sand (obtained from a local beach in Taipei, Taiwan), and heated iron oxide particles. not hinder the comparison of hardness removal performance of the current system with others because it is reported that hardness removal reaches a steady state within minutes, i.e., detention time longer than a few minutes does not further enhance hardness removal (Sluys et al, 1996). Because of the low flow rate, a circulation pump (for reactor A) or mixing device (for reactor B) was used to maintain the upflow velocity (providing 150% of bed expansion), as shown in Figure 1. It should be noted that in the scaled-up system, continuous bubbling up of compressed air will be used to scour the membrane fibers and remove solids from the surface of the membrane according to the membrane manufacturer. Therefore, the mixing device used in the current study might be eliminated. The membrane was operated at either 30- or 60-min filtration cycles. After each filtration cycle, the membrane was backwashed with DI water containing approximately 10 mg/l of Cl 2 of free chlorine for 6 s at 10 psi (69 kpa) (or 12 s for a 60-min filter cycle). This corresponds to a recovery ratio of approximately 92±1%. Membrane fouling was evaluated by continuously monitoring operating PHOTO: YI-MING CHEN LI ET AL PEER-REVIEWED 96:8 JOURNAL AWWA AUGUST

4 FIGURE 1 5 NaOH FIGURE 2 Pt/P Schematic of the experimental setup of the fluidized-bed pellet reactor Influent Effluent Deionized water 8 1. Membrane 2. Influent pump 3. Effluent pump 4. Recirculation pump 5. Chemical dosing pump 6. Auto ph control and temperature measurement unit 7. Backwashing reservoir 8. Two-way valve 9. Backwash reservoir refilled pump 10. Nitrogen tank 11. Pellets 12. Fluidized-bed reactor Pressure profile for systems with different types of pellets added at ph Quartz sand Beach sand HIOPs P 0 membrane pressure at the beginning of the test, P t pressure recorded during the filter run pressure, which was recorded with a pressure sensor 1 that ranged from 1 to 3 bar ( 14.5 to 43.5 psi). The pressure sensor continuously outputs voltage that is digitalized 2 and recorded onto personal computers through an RS-232 port. Pressure was calibrated for the temperature recorded during the test to 20 o C. System ph was controlled with an automatic ph controller consisting of a ph sensor and pump. After each test, the membrane module was cleaned by filtering citric acid with a ph level of 4.0 for 30 min (in an outside-in flow pattern), followed by filtering the membrane with DI water for 30 min. DI water filtration pressure was recorded to evaluate the effectiveness of the cleaning process. In some cases, for example for the membrane operated at a 60-min filtration cycle, sonication treatment in acidic and basic solution was needed. Raw water was prepared by adding CaCl 2 and Na 2 CO 3 to tap water that contained a hardness level of mg/l as CaCO 3 and less than 1 mg/l of dissolved organic carbon (DOC). The final hardness level of the solution was 300 mg/l as CaCO 3 (0.003 M) and contained an additional total carbonate concentration of M. The final ph level of the solution was adjusted to 7.0 with H 2 SO 4 and NaOH. Iron content of the HIOPs was digested with strong nitric acid and then analyzed following the Fe D Phenanthroline methods listed in Standard Methods for the Examination of Water and Wastewater (1995). Ca concentration was analyzed either by ethylenediaminetetraacetic acid (EDTA) titration (2340C EDTA titrimetric method) or using a Ca ion selective electrode. 3 The two analytical methods gave comparable results. RESULTS AND DISCUSSION Effect of pellet type on hardness removal and membrane fouling. Three pellets were tested for enhancing hardness removal in an integrated membrane and fluidizedbed pellet reactor process. Reactor B used an HIOP system with a stirring device, which was installed to provide a completely mixed condition because of the small volume of HIOPs added. The results of the three types of pellets tested are listed 154 AUGUST 2004 JOURNAL AWWA 96:8 PEER-REVIEWED LI ET AL

5 Removing water hardness using an integrated membrane and fluidized-bed pellet reactor process was shown to be very effective. in Table 3. The surface areas of pellet per litre of reactor volume, which is one of the most important parameters controlling hardness removal for a fluidized-bed pellet reactor, are 1.85 m 2 /L for quartz sand, 2.03 m 2 /L for beach sand, and 2.00 m 2 /L for HIOPs. The surface area of the HIOPs was calculated by assuming a sphere shape with a density of 2.75 g/cm 3 and a diameter of 5 µm. Quartz sand, which has the largest diameter among the three types of pellets, had the highest pellet-to-reactor volume ratio of 18.5%, followed by 10.1% and 0.17% for beach sand and HIOPs, respectively. At a ph level of 9.0, hardness removal efficiencies for the three systems were all higher than 60%. In the HIOP system, the value was as high as 72.8%. Counting possible experimental errors and error on assumptions for calculating the surface area of pellets, the type of pellet might not be a strong factor affecting hardness removal. The pressure profiles for all three systems are shown in Figure 2. The fouling pattern was evaluated by comparing pressure with the pressure of the membrane at the beginning of the test. For the HIOP and beach sand systems, pressures were steady no significant fouling was reported during 40- and 60-h runs, respectively. However, in the quartz sand system, pressure gradually increased (less than 10%) during the first 10 h of testing and remained fairly constant for next 10 h. Effect of ph on hardness removal and membrane fouling. The effect of ph level on hardness removal and membrane fouling was investigated in systems containing HIOPs or quartz sand at ph levels of 8.5, 9.0, and 9.5. The surface FIGURE 3 Ca Concentration (mg/l as CaCO 3 ) or Saturation Index areas of the pellets added per litre of reactor volume were 0.14 m 2 /L for the HIOP system and 1.85 m 2 /L for the quartz sand system. Table 4 summarizes the average hardness removal for two systems at the three ph levels. As expected, hardness removal increased when the ph level increased. At a ph as low as 8.5, which has a theoretical saturation index of 32 (i.e., oversaturation), both systems removed approximately 50% of hardness. At ph levels of 9.0 and 9.5, raw waters were oversaturated, with a saturation index of 98 and 282, respectively. Removal efficiencies for hardness were more than 62 and 75% for both systems at these two ph levels. After softening, the effluent was still oversaturated with Ca with an index of 5 for a system at a ph level of 8.5. However, saturation indexes for effluent samples in ph 9.0 and 9.5 systems were 8 and 11, respectively, indicating that Ca precipitation was kinetically limited after most of Saturation index and Ca concentration as a function of time for a system with HIOPs (0.017 m 2 /L) at ph 9.5 Saturation Ca PHOTO: YI-MING CHEN LI ET AL PEER-REVIEWED 96:8 JOURNAL AWWA AUGUST

6 FIGURE 4 P t /P Pressure profile for an HIOP system at different ph levels ph 8.5 ph 9.0 ph P 0 membrane pressure at the beginning of the test, P t pressure recorded during the filter run FIGURE 5 Hardness Removal Efficiency % TABLE 5 Hardness removal efficiency versus time for systems with varying HIOP surface areas added at ph Surface Area m 2 /L Relationship between surface area of HIOPs* and average hardness removal efficiency at ph 9.0 * Average Hardness Removal Efficiency % the Ca was removed. To further explore this possibility, precipitation kinetics were evaluated in a 1-L beaker containing 500 ml of raw water and an HIOP concentration high enough to provide a surface area of m 2 /L. After adjusting the solution to a ph level of 9.5, samples were taken at various times and hardness was analyzed. Figure 3 shows the Ca concentration as a function of time. Ca concentration decreased rapidly within 1 h and remained steady for the next 16 h. The same trend was observed for the saturation index (also shown in Figure 3). It can be seen that the saturation index of a solution decreased rapidly from 115 to 12 in 1 h and then decreased very slowly. At the end of the test at a reaction time of 17 h the solution was still oversaturated, with an approximate saturation index of 7. Figure 4 illustrates the pressure profiles for the HIOP system at the three ph levels. During a 110-h continuous operation, there was no sign of membrane fouling. The same conclusion was found for the system that used quartz sand (data not shown). Effect of pellet surface area on hardness removal and membrane fouling. The effect of pellet surface area on hardness removal and membrane fouling was tested at a ph level of 9.0 using HIOPs with areas that varied from 0 to 10.0 m 2 /L. Figure 5 illustrates the hardness removal efficiency as a function of time for systems with different surface areas, and Table 5 lists the results of the tests. The data shown indicate that increasing the amount of pellets added, i.e., increasing the surface area, increases hardness removal efficiency. At a surface area of 10 m 2 /L (which corresponds to 161 g of HIOPs added, or 2.3 wt%), on average more than 85% of hardness was removed, whereas less than 16% of hardness is removed in a system that contains UF membrane alone (no pellets added). More than 19 kg (42 lbs) of quartz sand with a diameter of 0.6 mm has to be added to reach a surface area of 10 m 2 /L, resulting in a pellet-to-reactor volume ratio of 100% (the whole reactor would be packed with sand). In the current system, the HIOPs only occupy 0.83% of the reactor volume. No membrane fouling was observed in all four systems (data not shown). It is interesting to note that in a system that contains UF membrane alone, no membrane fouling was observed during a 20-h operation. As indi- 156 AUGUST 2004 JOURNAL AWWA 96:8 PEER-REVIEWED LI ET AL

7 FIGURE 6 cated by Sluys et al (1996), fouling in the membrane softening process might be caused by the direct crystallization of hardness onto the membrane surface or by CaCO 3(s) precipitates blocking the membrane pores. However, the addition of pellets decreases direct crystallization of hardness onto the membrane surface and the precipitation of CaCO 3(s), therefore decreasing membrane fouling. In the current study, an outside-in membrane was used. This type of membrane has a much lower membrane surface area-to-reactor volume ratio ( m 2 /L) compared with a typical inside-out membrane (~2 m 2 /L) used by others (Chang & Benjamin, 1996). Therefore, direct crystallization of hardness on the membrane surface might not be as serious as that on the inside-out membrane even without pellets added. Indeed, in the study by Sluys et al (1996) in which an inside-out membrane was used, even without adding pellets, the membrane alone can still have the same level of hardness removal as the same system with pellets, although the hardness removal process takes longer. For example, in a system that contains a UF membrane alone, the hardness removal process takes 6 min. In a system with 0.4 wt% of pellets added, it takes less than 1 min for the system to reach the same level of hardness removal efficiency, indicating the large surface area in an inside-out membrane. In contrast, as demonstrated in this study, a system that contains an outside-in UF membrane alone can only remove 16% of hardness with a detention time of 7.53 h. Effects of flow rate and filtration cycle length on hardness removal and membrane fouling. Tests were conducted at two flow rates to explore the effects of flow rate on hardness removal efficiency and membrane fouling. HIOP (with a surface area of 2.0 m 2 /L of reactor volume) was tested at a ph level of 9.0. The system was operated at a 15.5-mL/min flow rate for approximately 68 h and then increased to 30 ml/min for another 68 h, as shown in Figure 6. Both systems have a production cycle of 30 min before backwashing. The pressure for the system operated at a flow rate of 30 ml/min was approximately 15% more than the pressure for the system operated at a flow rate of 15.5 ml/min. Although the flow rate of 30 ml/min was P t / P Effect of flow rate on the pressure profile of systems with HIOPs (2.0 m 2 /L) at ph ml/min 30 ml/min P 0 membrane pressure at the beginning of the test, P t pressure recorded during the filter run FIGURE 7 P t / P Effects of cycle length on the pressure profile of systems with HIOPs (0.5 m 2 /L) at ph 9.0, 300 mg/l as CaCO 3, and 5 mg/l dissolved organic carbon of Aldrich humic acid 60 min 30 min Time days P 0 membrane pressure at the beginning of the test, P t pressure recorded during the filter run about two times that suggested by the membrane manufacturer, membrane fouling was not significant during a 68-h operation. Although a critical flux concept for membrane filtration has been suggested by several researchers (Field et al, 1995), experimental data show that operating at a flow rate of 30 ml/min (corresponding to flux of 39 L/h/m 2 ) might still be lower than the critical flux for this membrane. Data for hardness removal indicated that both systems can remove more than 70% of hardness and that fil- LI ET AL PEER-REVIEWED 96:8 JOURNAL AWWA AUGUST

8 tration cycle length was not as strong a function of flow rate as expected. However, although increasing the flow rate to 30 ml/min decreases the detention time to 3.76 h, the detention time is still much longer than minutes, which is the time required for hardness removal in a fluidizedbed reactor suggested by several researchers (Sluys et al, 1996; Harms & Robinson, 1992). Tests were also conducted to investigate the effects of filtration cycle lengths (30 and 60 min) on hardness removal and membrane fouling. For the 60-min filter cycle, 12 s of backwashing was conducted as opposed to 6 s of backwashing for the 30-min filter cycle; therefore, both tests have the same water recovery ratio of 92±1%. Results indicated that hardness removal efficiency was not a strong function of cycle length as expected and was higher than 75% for both systems for treating raw water containing 300 mg/l as CaCO 3 hardness and 5 mg/l as DOC of a humic acid 4 at a ph level of 9.0. However, membrane fouling was severe for the system operated at a 60-min cycle. As shown in Figure 7, after operating the 60-min cycle system for 20 days, operating pressure increased more than 2.2 times from that at the beginning of the test. For the 30-min cycle system, membrane pressure increased less than 20% of that at the beginning of the test after operating the system for 23 days. From these results, it can be concluded that frequent backwashing is the best way to eliminate the possibility of membrane fouling. CONCLUSIONS Removing water hardness using an integrated membrane and fluidized-bed pellet reactor process was shown to be very effective. Among the three types of pellets tested, HIOPs are the most economic because of their small radius. To provide 10 m 2 of pellet surface area per L of reactor, HIOPs only occupy 0.83% of reactor volume, which allows a very compact treatment system to be designed. Furthermore, integration of membranes in the fluidized-bed pellet reactor eliminates the need for postsoftening sedimentation and filtration processes, which are required in most conventional softening processes. Removal efficiency for hardness was shown to be a strong function of ph and pellet surface area and less dependent on flow rate and filter cycle length. It was shown that increasing ph and pellet surface area increases hardness removal. Operating pressure of the membrane strongly depended on the flow rate and the length of the filtration cycle. Membrane fouling can be greatly reduced by backwashing every 30 min. Increasing the filtration rate from 15.5 to 30 ml/min while keeping the membrane cycle length at 30 min did not show significant membrane fouling during a 68-h operation, although membrane operation pressure was 15% higher than that running at a flow rate of 15.5 ml/min. ACKNOWLEDGMENT The authors would like to thank the National Science Council of Republic of China for providing financial support (NSC E and NSC E ) and Zenon Environmental Inc. in Burlington, Ontario, for providing membranes for use in the study. ABOUT THE AUTHORS: Chi-Wang Li 5 is an assistant professor in the Department of Water Resources and Environmental Engineering at TamKang University in Taipei, Taiwan; (886) ; chiwang@mail.tku.edu.tw. Chi-Wang has more than 12 years experience in drinking water research and has been published in JOURNAL AWWA, Environmental Science & Technology, and Water Science & Technology. Chi-Wang received a bachelor s degree from TamKang University in Taipei, Taiwan, and a master s degree and PhD from the University of Washington in Seattle. Jau-Cheng Jian and Jung-Chih Liao received master s degrees from the Department of Water Resources and Environmental Engineering at TamKang University in Taipei, Taiwan, and were graduate students at the time of this study. FOOTNOTES 1 Huba Control, Switzerland. 2 ADAM-4520 and ADAM-4017, sensor-to-computer interface module, Advantech Co., Ltd., Taiwan. 3 Orion 93-20, Thermo Electron, Corp., Waltham, Mass. 4 Aldrich Chemical, Milwaukee, Wis. 5 To whom correspondence should be addressed. If you have a comment about this article, please contact us at journal@awwa.org. REFERENCES Chang, Y.J. & Benjamin, M.M., Iron Oxide Adsorption and UF to Remove NOM and Control Fouling. Jour. AWWA, 88:12:74. Field, R.W. et al, Critical Flux Concept for Microfiltration Fouling. Jour. Membr. Sci., 100:3:259. Graveland, A. et al, Developments in Water Softening by Means of Pellet Reactors. Jour. AWWA, 75:12:619. Harms, W.D., Jr. & Robinson, R.B., Softening by Fluidized Bed Crystallizers. Jour. Envir. Engrg., 118:4:513. Sluys, J.T.M.; Verdoes, D.; & Hanemaaijer, J.H., Water Treatment in a Membrane-assisted Crystallizer (MAC). Desalination, 104:1:135. Snoeyink, V.L. & Jenkins, D., Water Chemistry. John Wiley & Sons, New York. Standard Methods for the Examination of Water and Wastewater. APHA, AWWA, and WEF, Washington (19th ed.,1995). Van der Veen, C. & Graveland, A., Central Softening by Crystallization in a Fluidizedbed Process. Jour. AWWA, 80:6: AUGUST 2004 JOURNAL AWWA 96:8 PEER-REVIEWED LI ET AL