Ind. Eng. Chem. Res. 2005, 44, 5465-5471
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Comparison between Compact Accelerated Precipitation Softening (CAPS) and Conventional Pretreatment in Operation of Brackish Water Reverse Osmosis (BWRO) J. Gilron,†,‡ N. Daltrophe,† M. Waissman,† and Y. Oren*,†,‡ Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, and Unit of Environmental Engineering, Ben Gurion University, P.O. Box 653, Beer Sheva 84105, Israel
Preliminary tests have previously been conducted on compact accelerated precipitation softening (CAPS) as a pretreatment for membrane processes operating on various natural and industrial waters. In the present study, we compare CAPS efficacy for desalting brackish groundwater from the Negev region of Israel (original pH ) 7.8) to that for conventional pretreatment, namely, dual media filtration (DMF) with acid or antiscalant. Samples of groundwater (400 L) were pretreated alternately by CAPS and conventional means and concentrated >8-fold in a batch desalination experiment with 2.5 in. x 40 in. spiral low-pressure reverse osmosis (RO) elements. The pure water flux of the membrane element was preserved in an experiment reaching nearly 90% recovery from CAPS-treated water adjusted to pH 7.6, whereas it declined to 30% of its original value after desalting the brackish groundwater treated by dual media filtration and adjusted to the same pH. In runs comparing CAPS pretreatment and conventional pretreatment with the supplemental use of antiscalant or acid, membrane processes operated with a CAPStreated feed stream required less antiscalant and acid than those operated with conventionally pretreated water. In addition, the membrane modules required no cleaning at the end of the experiments, whereas membranes operated with conventionally pretreated water required significant cleaning to restore the pure water flux obtained at the beginning of each run. A comparison is made of the chemical requirements and brine disposal cost associated with each type of operation. Introduction In the application of membrane desalination processes (electrodialysis (ED), nanofiltration (NF), and reverse osmosis (RO)) to the development of new sources of potable water, concentrate disposal at the end of the process is a pressing problem. The method of disposal can conceivably have negative environmental impact and is regulated in many Western countries. The expense of such concentrate disposal can contribute significantly to the water production cost. This issue has been addressed in a number of studies and policy reviews.1,2 Disposal options are going to involve costs directly related to the volumes of concentrate for disposal. It is, therefore, of immediate interest to develop technologies which will allow further reduction of concentrate volumes. To achieve this goal, it is necessary to increase the recovery of the water treatment process. Recovery, Y, is defined as the fraction of the feed flow rate, Qf, that is recovered as product water in the permeate (flow rate Qp), namely, Y ) Qp/Qf. It is well-known that increasing the recovery results in a reduction of the pretreatment cost and energy per unit of water produced. However, as the concentrate volumes become smaller while higher fractions of the feedwater are recovered in the permeate, concentrations of the chemical species in the rejected * Corresponding author. † Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research. ‡ Unit of Environmental Engineering, Ben Gurion University.
stream become higher. The upper limits of recovery in both RO and ED are often determined by the onset of scaling and fouling of the membranes. The feedwater must be pretreated to prevent such scaling and fouling phenomena. Scaling is usually prevented by acidifying the feedwater to prevent the precipitation of carbonates and by the use of antiscalants to prevent the precipitation of sulfates of Ca2+, Ba2+, and Sr2+.3 Eventually, a recovery limit of 80-90% is reached even in the presence of antiscalant additives, because the supersaturation ratio becomes too great (e.g., up to 400% of calcium sulfate). Cases have also been reported in which the antiscalants themselves can contribute to the fouling of membranes.4 Since the supersaturation ratio will be dictated by the load of precipitating cations in the feedwater, partial softening before desalting will allow a much higher concentration factor before the same supersaturation ratios are reached. In addition, concentrate volumes may conceivably be reduced by secondary softening of the concentrate to allow further recovery. A new process called HEROR completely softens the feed to RO to allow operation at high pH, where very high silica levels can be maintained before precipitation is observed.5 Evidently, partial softening of the water can have two immediate economic consequences: (1) reducing the need for antiscalant and (2) increasing the recovery ratio. Reduction of the pretreatment and concentrate disposal costs are two of the economic incentives for developing a compact partial softening process. Previous publications6-10 have discussed the development of compact accelerated precipitation softening (CAPS) as a method to reduce calcium carbonate hard-
10.1021/ie050002y CCC: $30.25 © 2005 American Chemical Society Published on Web 06/14/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 Table 1. Typical Composition of Brackish Groundwater at Mashabe Sadeh in the Negev Region of Israel before and after CAPS Pretreatment concentration, mg/L species Ca2+
Figure 1. Schematic presentation of the CAPS pretreatment module.
ness. The conceptual scheme of CAPS is illustrated in Figure 1. It consists of a mixing reaction tank, a microfilter media, and a pump for recycling between the reaction tank and the filter media and for pumping the reaction suspension across the filter media. The reaction suspension in the tank consists of water and 1-3% (w/w) of calcium carbonate solids. Raw water is fed to the feed tank along with the base to increase the level of supersaturation with respect to calcium carbonate. The reaction of the base with carbonate alkalinity in the water generates supersaturation with respect to calcium carbonate:
Ca2+ + OH- + HCO3- f Ca2+ + CO32- + H2O Ca2+ + CO32- f CaCO3 (s) Supersaturation is released by the precipitation of calcium carbonate at the following sites: (1) in the slurry due to crystal growth on the suspended particles and (2) in the filter cake due to (a) secondary nucleation and (b) crystal growth on the pore wall of the cake. The processes occurring in the cake are the most rapid where supersaturated solution-cake contact times of a few seconds were sufficient to significantly relieve the supersaturation. Apparently, the relatively small dimensions of the cake pores and the relatively high shear rate along the pore axis lead to extremely rapid mass transfer. Indications for such rapid mass transfer are found in the results of Massarwa et al.,9 in which relief of supersaturation is obtained in a matter of seconds on passing a supersaturated calcium carbonate solution through a preformed cake of calcium carbonate. Further work10 has demonstrated the potential of CAPS to remove heavy metals and silica, which are also potential membrane foulants, while work on actual pond water showed CAPS could reduce level of organics and bacteria.11 The studies on fish pond water11 and paper mill acid rinse water12 showed how the CAPS-treated water reduced fouling relative to the case of untreated water. The present study makes a quantitative comparison between CAPS and a conventional pretreatment (dual media filtration with acid and/or antiscalant) in terms of their efficacy for preventing fouling during reverse osmosis of a brackish groundwater from the Negev region of Israel. Experimental Section Brackish water from wells in the Mashabe Sadeh region of the Negev desert was chosen for pretreatment
Mg2+ Fe3+ Na+ K+ NH4+ CO32HCO3SO42ClSiO2 TOC
raw
CAPS
188-274 72-109 0.01-0.15 610-710 17.5-22 0.3 0.3-11 218-324 420-520 1030-1140 8.7-17.5 2-10
10-15 91 8). (In RO10, the final value of pure water permeability (PWP) after operation, Lpf, being higher than the initial value of PWP is probably due to inadequate conditioning of the new ESPA 2540 module. The other values of Lpo seem to indicate that the original Lpo should be taken as 4.6.) Parallel treatments with a dual media filter (RO8 and RO11) showed significant drops in Lp at VCF values >4. If the CAPS-treated feed was acidified to a pH slightly below the raw feed pH (7.6 in RO10), the Lp values were even more steady. Conventionally pretreated brackish water required the combined use of acid and antiscalant (RO12) and an initial pH of 6 in order to achieve the same level of stability in an RO operation at high recoveries. These results are corroborated by the summary of the pure water flux before and after the concentration run and after cleaning. The Lp values were higher at the end for CAPS-treated water, and the flux recovery after cleaning was better for CAPS (RO7 vs RO8). The difference between the CAPS-treated and conventionally treated runs is also seen in the change of pH and Ca2+ levels as a function of VCF. Figure 3 shows the results for the run with RO8 (conventional pretreatment at raw water pH). Figure 3 shows that, whereas the monovalent ion concentration increases monotonically with the volume concentration factor, the concentration of calcium breaks at VCF ) 4, indicating the onset of calcium carbonate precipitation. Normally, the membranes are selective for HCO3- and H2CO3 over CO32-. As a result, the pH will tend to increase with recovery in normal RO operations. In the case of RO8, the pH actually decreases instead of increasing. This is another sign of the
Figure 4. Plot of concentrate composition as a function of the volume concentration factor for run RO7. Table 4. Summary of Cleaning Conditions Used on RO Membrane Elements to Restore Original Pure Water Flux experiment description
cleaning conditions
RO7 RO8
CAPS, pH 7.91 DMF, pH 7.8
RO10 RO11
CAPS, pH 7.6 DMF, pH 7.89
RO12
DMF, pH 6 + antiscalant
0.1% EDTA at pH 11 0.1% EDTA at pH 11 followed by pH 3 (0.096 citric acid/m2), 0.1% EDTA, and 0.03% SDS at pH 11, followed by pH 2.3 (0.38 kg citric acid/m2) none 0.1% EDTA at pH 11, followed by pH 2.3 (0.045 kg HCl/m2) none
occurrence of calcium carbonate precipitation, as shown by the following reaction equation:
Ca2+ + 2HCO3- f CaCO3 + H2CO3
(3)
In contrast, in Figure 4 the run for CAPS-treated water shows that both Ca2+ and pH increase monotonically over the entire concentration run. This indicates that, even at high recoveries (VCF > 8, recovery > 87%), calcium carbonate deposition is not a major problem. The cleaning conditions for the various experiments are summarized in Table 4. In runs RO10 and RO12, no cleaning was necessary because the pretreatment was adequate to prevent all fouling (see PWP ratio before cleaning in Table 3). Among the three other runs (RO7, RO8, and RO11), RO7 required, by far, the least extensive cleaning conditions. For run RO7, only a basic rinse was required. By contrast, in run RO8, the base rinse and then an acid cleaning cycle had to be done twice, and even then, it was not completely effective. 3. Economic Comparison of Treatment Alternatives. In comparing the relative economic advantage from the different pretreatment procedures, the following cost data have been used: (1) Chemical costs: caustic soda prices$370/mt; hydrated limes$80/mt; H2SO4s$150/mt; and antiscalant PC191s$2/kg
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5469 Table 5. Treatment Cost Projections Comparing Pretreatment with CAPS or Dual Media Filtration (DMF) CAPS dose, g/m3 recovery base, NaOH acid, H2SO4 antiscalant brine costs total
DMF material, cost $/kg
Case 1a. CAPS: As Is 0.8 202 0.37 29. 6 0.12
treat, cost ¢/m3
dose, g/m3 recovery
7.5 0.4 37.5 45.4
Case 1b. CAPS: No CO2 Absorption recovery 0.8 base, Ca(OH)2 111 0.085 0.9 acid, H2SO4 brine costs 37.5 total 38.4 Case 3a. CAPS: Acid Addition recovery 0.88 base, NaOH 202 0.37 7.5 acid, H2SO4 88.7 0.12 1.1 antiscalant brine costs 20.5 total 29.1 Case 3b. CAPS: Acid Addition, No CO2 Absorption recovery 0.88 base, Ca(OH)2 111 0.085 0.9 acid, H2SO4 9.6 0.12 0.1 brine costs 20.5 total 21.5
(2) Costs for brine disposal:14 (BC)s$1.50/(m3 of brine). To calculate brine disposal costs, the recovery for each pretreatment case must be known. The brine disposal costs will then be given by
Brine disposal costs ($ /(m3 of product)) ) (1 - Y)BC (4) Y where Y is the fractional recovery and BC is the brine costs/(m3 of brine). To that end, we took the maximum recovery found in the concentration runs that gave a value of specific flux (Lp) that was 95% of its initial value (see Figure 2). To calculate the chemical consumptions, we used the amounts found in the actual experiments (see Table 2). Given that the base consumption for CAPS was found to be 202 mg/L of NaOH and that not all of the alkalinity was removed, the present chemical costs for CAPS pretreatment are much higher than they would be for an optimized CAPS process. Also, given the very high present costs of NaOH, the option of using hydrated lime (Ca(OH)2) should also be considered. However, upon using this option, one should keep in mind that the amount of sludge is significantly increased. The following cases are considered and described in Table 5, along with the corresponding cost estimations. Case 1: CAPSsNo Additional Treatment Prior To RO. Case 1a. CAPS is operated with raw water using 202 g/m3 (mg/L) of caustic soda. This allows a recovery of 80%. To operate this way requires that CAPS product water at pH ) 9 must be reacidified to the raw water pH (7.8) using 49.8 g/m3 of pure H2SO4. Case 1b. CAPS is operated with raw water, using 111 g/m3 of calcium hydroxide. The amount of bicarbonate left will be minimal (calcium in excess of bicarbonate); thus, acid dosing to return to feed pH will be minimal.
acid antiscalant brine cost total
material, cost $/kg
treat, cost ¢/m3
Case 2. DMF: As Is 0.72 0 0
0.12 2
0.0 0.0 58.3 58.3
Case 4. DMF: Acid and Antiscalant Addition recovery 0.88 acid antiscalant brine cost total
164 1.7
0.12 2
2.0 0.3 20.5 22.8
In this case, too, water recovery will be only 80% because of the remaining calcium and bicarbonate and the experimental result. Case 2: DMFsNo Additional Treatment Prior To RO. Raw water is treated with a dual media filter followed by RO. Using the data from the experiment, only a 72% recovery can be reached before the effective water permeability of the membrane Lp (or flux for a constant set of conditions) will drop below 95% of the original because of fouling. Case 3: CAPS with Treatment Prior To RO. Case 3a. For the CAPS with acid addition case, enough acid is added to the CAPS-treated feed to drop the feed pH to 7.6 (instead of 7.8) and to allow recoveries >88% without Lp dropping (within 5% of initial). In this case, we use 202 g/m3 of caustic soda and allow absorption of atmospheric CO2. The acid dose required to return the CAPS product from pH 9 to pH 7.6 is 88.7 g/m3 of pure H2SO4 (based on the experimental result). The recovery of >88% is also an experimental result. Case 3b. In this case, as in case 1b, the CAPS treatment is done with lime (Ca(OH)2), but care is taken to prevent reabsorption of CO2. Only 111 g/m3 of lime are needed (as in case 1b), and only 9.6 g/m3 of acid (H2SO4) are needed to reduce the pH to 7.6 (calculated using the projected composition of the CAPS product from Minteqa215a and then using the integrated membrane system (IMS) program of Hydranautics15b to calculate the acid dose needed to reach pH 7.6 in the CAPS product). Again, by reducing the calcium and bicarbonate levels and the slight reduction of pH (to 7.6), the recovery should be at least as good as in case 3a). In actuality, it should be possible to go to even higher recoveries since the overall carbonate level is lower than in case 3a. Case 4: DMFsWith Treatment Prior To RO. This case is based on actual experimental results using dual media filtration and the dosages (in g/m3) of acid (H2SO4) and antiscalant (Permeatreat 191) actually used.
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The dosages were originally projected by Genesys software15c in order to allow recovery of >80%. The dose of acid was that required for pH ) 6.1. The actual recovery of >88% is also based on the experiment (RO12). As can be seen, in the case of no additional treatment, CAPS as presently practiced (CAPS case 1a) is still more economical than DMF filtration (DMF case 2), because significantly higher recoveries are attainable with CAPS (0.8 vs 0.72). CAPS’s advantage is even greater when the optimized (with respect to CO2 absorption) version is used (38 cents/m3 instead of 45 cents/m3). On the other hand, if optimum chemical treatments are used (case 3 and case 4), CAPS treatment as now practiced (case 3a) is more expensive than DMF treatment with acid and antiscalant. This is because of the high cost of caustic presently used with CAPS. When lime is used and the CAPS process is optimized to prevent absorption of carbon dioxide resulting in near-stoichiometric chemical consumption to remove carbonate alkalinity, the base and acid consumption costs drops considerably (case 3b: 21.5 cents/m3 product water), and then CAPS is less expensive than DMF with acid and antiscalant (case 4: 22.8 cents/m3 product water). Overall, this analysis shows that it pays to use more chemical pretreatment (cases 3 and 4) if it will allow higher recovery, because the costs of brine disposal are so high. Conclusions Compact accelerated precipitation softening will allow stable operation at higher recoveries than media filtration with less need of acid and antiscalant. The higher cost of caustic required by the CAPS operation can be offset if significantly higher recoveries can be maintained compared to those for conventional pretreatment. Less cleaning was required to restore the original pure water fluxes of membranes operated on the CAPStreated feed as compared to the feed pretreated with media filtration. CAPS will only be economical relative to conventional chemical dosing (acid and/or antiscalant) if CO2 absorption is prevented and low cost sources of base are used (e.g., lime). Acknowledgment The Israel Ministry of Science and Technology (Project No. 01-01-01208), Samuel Lunenfeld Charitable Foundation of Toronto, Barrie Rose and the Brooke Foundation of Ontario, and the Jacob Blaustein Foundation for Desalination provided financial support for this work. The authors are indebted to Prof. O. Kedem for helpful discussions and to the Mekorot Southern District staff for providing the field site and for their assistance in the CAPS operation. Notation A ) membrane area, m2 C ) solute concentration Lp ) effective water permeability of membrane as measured in the process fluid, L/m2‚h‚bar PWP ) pure water permeability of membrane as measured in deionized water or synthetic NaCl solution, L/m2‚h‚bar
CAPS ) compact accelerated precipitation softening VCF ) volume concentration factor ∆P ) transmembrane pressure, bar P ) pressure, bar Qp ) permeate flow rate, L/h Qf ) feed flow rate, L/h Y ) membrane recovery, equal to Q0/Qf φ ) osmotic coefficient π ) osmotic pressure, bar Subscripts a ) after RO experiment ac ) after cleaning b ) before RO experiment f ) feed o ) initial value at start of RO experiment p ) permeate Literature Cited (1) Truesdal, J.; Mickley, M.; Hamilton, R. Survey of membrane drinking water plant disposal methods. Desalination 1995, 102, 93-105. (2) Kimes, J. K. The regulation of concentrate disposal in Florida. Desalination 1995, 102, 87-92. (3) Wilf, M.; Ricklis, J. RO desalting of brackish water oversaturated with CaSO4. Desalination 1983, 47, 209-219. (4) Richards, A.; Suratt, W.; Winters, H.; Kree, D. Solving membrane fouling for the Boca Raton 40-mgd membrane water treatment plant: the interaction of humic acids, pH and antiscalants with membrane surfaces. In Membrane Technology Conference: The Future of Purer Water, Proceedings, San Antonio, TX, Mar 4-7, 2001; American Water Works Association: Denver, CO, 2001; pp 1524-1532. (5) Mukhopadhyay, D. Method and apparatus for high efficiency reverse osmosis operation. U.S. Patent 6,537,456, Mar 2003. (6) Kedem, O.; Zalmon, G. Compact accelerated precipitation softening (CAPS) as a pretreatment for membrane desalination. I. Softening by NaOH. Desalination 1997, 113, 6572. (7) Oren, Y.; Katz, V.; Daltrophe, N. C. Improved Compact Accelerated Precipitation Softening (CAPS). Desalination 2001, 139, 155. (8) Oren, Y.; Katz, V.; Daltrophe, N. C. Compact Accelerated Precipitation Softening (CAPS) with Submerged Filtration: Role of the CaCO3 Cake and the Slurry. Ind. Eng. Chem. Res. 2002, 41, 5308. (9) Massarwa, A.; Meyerstein, D.; Daltrophe, N.; Kedem, O. Compact accelerated precipitation softening (CAPS) as pretreatment for membrane desalination. II. Lime softening with concomitant removal of silica and heavy metals. Desalination 1997, 113, 73. (10) Gilron, J.; Daltrophe, N. Silica removal from water sources. Final Report for 1997; Ministry of Trade and Industry: 1998; Contract No. 86650101. (11) Gilron, J.; Chaikin, D.; Daltrophe, N. Demonstration of CAPS pretreatment of surface water for RO. Desalination 2000, 127, 271. (12) Manttari, N.; Daltrophe, N.; Oren, Y.; Gilron, J.; Nystrom, M. Treatment of Paper Mill Process Water with Lime Treatment, MF and NF. Proceedings of Engineering with Membranes, Vol. 1, Granada, Spain, June 3-6, 2001; p I295. (13) Pitzer, K. S. Ion Interaction Approach: Theory and Data Correlation. In Activity Coefficients in Electrolyte Solutions, 2nd ed.; Pitzer, K. S., Ed.; CRC Press: Boca Raton, FL, 1991; Chapter 3. (14) Glueckstern, P. Design and operation of medium and small size desalination plants in remote areas: New perspective for
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5471 improved reliability, durability and lower costs. Desalination 1999, 122, 123-140. (15) (a) MINTEQA2: Metal Speciation Equilibrium Model for Surface and Ground Water, Version 3.11; Center for Exposure Assessment Modeling (CEAM), U. S. Environmental Protection Agency, Office of Research and Development, Environmental Research Laboratory: Athens, GA, 1991. (b) Membrane Master II, Version 1.1.0.1; Genesys International Ltd.: Burnham, U.K.,
2004. (c) Integrated Membrane System Design (IMSdesign); Hydranautics: Oceanside, CA; http://www.membranes.com/design/ imsd_memo.htm.
Received for review January 2, 2005 Revised manuscript received April 17, 2005 Accepted May 10, 2005 IE050002Y
