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Ind. Eng. Chem. Res. 2009, 48, 3126–3135
Impact of Conventional Water Treatment Coagulants on Mineral Scaling in RO Desalting of Brackish Water Myung-man Kim,† James Au,† Anditya Rahardianto,† Julius Glater,† Yoram Cohen,*,† Fredrick W. Gerringer,‡ and Christopher J. Gabelich‡ Water Technology Research (WaTeR) Center, Chemical and Biomolecular Engineering Department, UniVersity of California, Los Angeles, Los Angeles, California 90005-1592, and Metropolitan Water District of Southern California, 700 Moreno AVenue, La Verne, California 91750
The potential impact of coagulants on mineral scaling in reverse osmosis (RO) feed treatment of brackish water was assessed experimentally, with respect to calcium sulfate and barium sulfate scaling potential, via a bulk crystallization and membrane scaling tests. The scale suppression effectiveness of six commercial antiscalants was first ranked based on measured observed bulk crystallization induction times. Bulk crystallization tests with three standard coagulants (ACH, FeCl3, and polyDADMAC), when used independently, demonstrated retardation of the observed crystallization induction time. However, along with an antiscalant dosing, antiscalant scale suppression was significantly reduced and scaling occurred at the same or greater severity relative to the additive-free feed solution. RO membrane scaling tests also indicated that, when the coagulants were present, antiscalant effectiveness was significantly reduced, consistent with the expectation based on bulk crystallization tests. Although the coagulants demonstrated a slight scale suppression quality in bulk crystallization tests at low dosages ( AS3 > AS5 > AS1 > AS6 > AS4. The above ranking, however, does not reflect the true differences in antiscalant effectiveness since the active ingredient concentration may vary among the various formulations. In fact, the crystallization induction time appeared to correlate with the antiscalant concentration (Figure 6) on the basis of its residual solids content (i.e., milligrams of residual solids of the antiscalant formulation in 1 L). The above behavior suggests that scale suppression effectiveness of the different antiscalants may be linked to similar active ingredients. This hypothesis is supported by the
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Figure 11. Effect of antiscalant AS2 in reducing flux decline due to mineral scaling in the presence of different coagulants (ACH, FeCl3, and polyDADMAC) and for coagulant addition alone and without additives (NO AD), all at dosages of 3 mg/L, using model solution B (Table 2).
FTIR spectra shown in Figure 3. As a further evaluation of the above hypothesis, a bulk crystallization test was carried out with antiscalants AS1 and AS2 at the same dosage of ∼1 mg/L residual solids content (i.e., equivalent to a total formulation dosage of 6.3 and 2 mg/L for AS1 and AS2, respectively). The results, shown in Figure 7, indicate that the crystallization induction times for AS1 and AS2 were nearly the same at the same dosage (on the basis of their residual solids content), thus supporting the hypothesis that, for the present set of antiscalants, the effectiveness of scale suppression was indeed similar when compared on the basis of their residual solids content. 3.2. Effect of Coagulants on Bulk Crystallization. In order to demonstrate the potential impact of the selected coagulants (ACH, ferric chloride, and polyDADMAC) on antiscalant performance, bulk crystallization induction times were determined in a series of experiments with AS2 which was found to be most effective for scale suppression for the present water chemistry. In addition, the combined effects of ferric chloride and polyDADMAC were also evaluated for different dosage ratios of (ferric chloride to polyDADMAC, i.e., F:P ratio) in the presence of 2 mg/L dosage of AS2 (Figure 8). Both of the above coagulants retarded the onset of mineral crystallization in the crystallization tests using model solution A (Table 2). However, the crystallization induction time decreased from about 44.0 min at F:P ratio of 10 to 26 min at F:P ratio of unity. This behavior was likely to be a direct result of higher degree of flocculation with an increase of polyDADMAC dosage14 that could have accelerated the onset of mineral salt precipitation. Once the rapid crystallization stage was reached (Figure 8), the rate of turbidity rise was similar in all cases (150-1000 NTU). However, it is noted that, in the presence of ferric chloride, the rate of turbidity rise was lower than for the solutions that also contained polyDADMAC (Figure 8). This behavior appears to suggest, consistent with previous work,14 that polyDADMAC enhanced the coagulation efficiency of ferric chloride, thereby increasing the rate of mineral salt precipitation. In order to evaluate the added effect of ACH on antiscalant effectiveness, when used in combination with FeCl3 and polyDADMAC, a series of bulk crystallization tests were carried out with model solution A, for a fixed ratio of 1 mg/L FeCl3: 0.5 mg/L polyDADMAC (i.e., F:P ratio of 2) along with antiscalant AS2 dose of 2 mg/L (i.e., equivalent to an AS2 dosage of 1.1 mg/L of residual solids). As shown in Figure 9,
dosing with ACH, in addition to the FeCl3/polyDADMAC combination, decreased the induction time (i.e., decreased antiscalant suppression effectiveness) relative to that which was obtained only with ACH. It has been suggested that aluminum ions in solution can form a positively charged complex that has a greater tendency to associate with antiscalant than with the mineral salt crystals (gypsum and/or barite),4,15,16 thereby reducing the antiscalant effectiveness. As the ACH dosage was reduced from 2 to 0.01 mg/L, the crystallization induction time for AS2 increased by about a factor of 11. At the higher ACH dosage of 2 mg/L (with or without FeCl3 and polyDADMAC at antiscalant dosage of 2 mg/L), the crystallization induction time for AS2 was in the range of about 20-250 min, respectively, relative to 14.7 min in the absence of any additives. However, when only ACH was added, along with the antiscalant, the observed crystallization induction time was lower relative to the case of using AS2 (Figure 9). The impact of each of the coagulants (ACH, FeCl3, and polyDADMAC) on mineral salt crystallization was also determined (in the absence of antiscalant addition) via bulk crystallization tests. Each one of the coagulants (when used on their own) displayed a crystallization suppression quality, which was quantified by a crystallization induction time (Figure 10) which was longer than that obtained in the absence of any additives (Figures 4 and 5). For all three coagulants, the observed crystallization induction time increased with the additive dose up to a maximum reached at critical concentrations of 5, 2, and 10 mg/L for ACH, FeCl3, and polyDADMAC, respectively, and then decreased somewhat from the maximum (by about 70-87% relative to the maximum) with further coagulant dosage increase. The scale suppression capability of the coagulants increased with their dosage at low concentrations, but decreased beyond a critical dosage. It is plausible that their flocculation tendency was regained,17 thus accelerating the crystallization process from the supersaturated solution. 3.3. Effect of Coagulants on Membrane Surface Crystallization. Previous studies have suggested that the relative ranking of antiscalant effectiveness and the impact of chemical additives on mineral salt crystallization would demonstrate similar relative ranking in heterogeneous (e.g., at the membrane surface) and bulk crystallization18 tests. It is well-known, however, that heterogeneous mineral salt scaling on RO membranes19,20 occurs at lower mineral salt supersaturation
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Figure 12. (a-d) Optical images of the TFC-ULP membrane surface at three different times during the membrane scaling runs (Figure 11): (a) without additives, and with the addition of (b) AS2 (3 mg/L) + ACH (3 mg/L), (c) AS2 (3 mg/L) + FeCl3 (3 mg/L), and (d) AS2 (3 mg/L) + polyDADMAC (3 mg/L). (Note: in the presence of AS2 only (3 mg/L) the membrane surface appeared as in the unscaled areas shown in Figure 12a-d).
levels compared with homogeneous crystallization. Therefore, selection of antiscalants and assessment of the impact of added coagulants must be verified via RO membrane scaling tests which, in the present study, were carried out with antiscalant AS2 that ranked highest in the bulk crystallization tests (Figure 4). The initial level of solution supersaturation at the membrane surface, with feed solution B (Table 2), was identical for all of the scaling experiments in order to appropriately compare the impact of the various additives and antiscalants on surface mineral scaling.3 In the absence of any additives, the permeate flow rate declined by about 31% by the end of the 24 h test period (Figure 11). Membrane scaling proceeded as the result of both nucleation of surface crystals and their growth as illustrated in the sequence of surface images shown in Figure 12a. Antiscalant AS2 which was effective in suppressing bulk crystallization (Figure 4) was also effective in suppressing membrane scaling, as implied by the suppression of flux decline as shown in Figure 11 for 3 mg/L dosage of AS2 (relative to the no-additives case) with no
observable mineral crystals on the membrane surface. Upon the addition of 3 mg/L of ACH (along with 3 mg/L of AS2), severe scaling was observed as shown in Figure 12b, resulting in flux decline that was more severe than in the absence of any additives (reaching about 49% at the end of the 24 h scaling test; Figure 11). FeCl3 and polyDADMAC also had a negative impact on the antiscalant scale suppression effectiveness (Figure 12, c and d, respectively) with flux decline of 37% and 26% (in the presence of 3 mg/L AS2), respectively, at the termination of the scaling test (i.e., t ) 24 h). It appears that the surface number density of crystals was higher in the presence of FeCl3 relative to scaling in the presence of polyDADMAC, followed by ACH. Apparently, coagulant addition reduced the antiscalant effectiveness, thereby allowing for nucleation of gypsum crystals over the course of the scaling test. An evaluation of the impact of the three coagulants (at a dosage of 3 mg/L) on flux decline (without antiscalant dosing) was also carried out with the results shown in Figure 11. Flux decline at the end of the 24 h period was highest for FeCl3
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Figure 13. Optical images of the TFC-ULP membrane at t ) 24 h for scaling tests with model solution B (Table 2) with coagulant dose levels of (a) 3.0 mg/L ACH, (b) 3.0 mg/L FeCl3, and (c) 3.0 mg/L polyDADMAC.
MAC), were consistent, within the level of discrimination of the present analysis, with the above flux decline behavior. Although the impact of ACH and polyDADMAC, at the dosage of 3 mg/L, on mineral scaling was not adverse (i.e., about the same level of flux decline due to scaling), relative to the case of no additives (Figure 10), it is clear that all of the three additives had an adverse effect on antiscalant effectiveness (Figure 11). Previous studies have suggested that flux decline due to mineral scaling is primarily due to surface blockage21 such that the fractional flux decline, FD ()J/J0, where J and J0 are permeate fluxes with and in the absence of mineral scaling), is given by
(
FD ) 1 -
Figure 14. Correlation of percent surface area covered by optically observed mineral scale with measured percent flux decline for various scaling with and without additives (No AD) tests with solution B (Table 2) at the end of the scaling runs (t ) 24 h).
(∼50%), followed by polyDADMAC (∼32%) and ACH (28%). Correspondingly, images of the membrane surface near the exit region (Figure 13) showed that when FeCl3 was added to the feed, a greater degree of scale coverage (53%) was observed, followed by polyDADMAC (32%) and ACH (28%). Image analysis of the complete membrane surface revealed that the corresponding percentages of surface scale coverage for the three coagulants, 53% (FeCl3), 24% (ACH), and 32% (polyDAD-
)
Lp,S Φ Lp,f
(3)
in which the fractional area of the membrane covered by scale is given as Φ ) As/A0, where A0 and As are the initial clean and scaled membrane surface areas, respectively, in which Lp,f and Lp,S are the average permeabilities of the scale-free and scaled membrane areas, respectively. Since both FD and Φ are measured independently, a linear plot of the data according to eq 3 should yield a straight line (with a positive slope since Lp,f > Lp,S), provided that the scaled areas (i.e., those covered by mineral scaled crystals) are of the same permeability irrespective of their location along the membrane.3 It is interesting to note that for scaling tests for the no-additives and for AS2 + ACH and FeCl3 cases the percent flux decline was lower than the percent of scaled membrane surface area (Figure 14). Such a
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Figure 15. Scaling tests with ACH, polyDADMAC + AS2, and AS2 + FeCl3 all revealed the presence of barite crystals on the membrane surface of the form depicted in the SEM images of (a) and in (b) also showing a portion of the structure of a gypsum rosette.
Figure 16. SEM image TFC-ULP membrane showing barite crystals and the appearance of a film layer associated with ACH (scaling test with model solution B with 3 mg/L ACH). (b) Zoomed in portion of image (a).
behavior was also previously reported3 for gypsum scaling (the correlation from ref 3 based on eq 3 is shown in Figure 14), with and without the presence of antiscalants, suggesting partial permeation through the gypsum rosette structures. On the other hand, tests with ACH, polyDADMAC + AS2, and AS2 + FeCl3 revealed percent flux decline that was greater by about 14%, 70%, and 14%, respectively, than would be expected based on a surface blockage by the observed scale coverage (Figure 14). High-magnification SEM images of the surface of the scaled TFC-ULP membrane, for scaling tests with ACH, polyDADMAC + AS2, and AS2 + FeCl3, revealed the presence of mineral crystals resembling the shape of barite crystals (Figure 15). EDS spectra of the scaled membrane areas for tests in which the coagulants were added, alone or along with the antiscalant, confirmed the presence of calcium, barium, sulfur, and oxygen, suggesting the presence of calcium sulfate and barium sulfate (barite). Barite, however, could not be detected at the optical level of magnification of the present imaging system and thus its contribution to the area covered by scale could not be ascertained by direct optical imaging. In such cases, the reported scaled area was lower than the percent flux decline (Figure 14). It is also noted that EDS spectra of the membrane surface, from scaling tests in which ACH was added, also indicated the presence of aluminum, possibly associated with ACH which may have complexed with or adsorbed onto the membrane surface and/or other formed surface minerals. When the coagulant ACH was utilized, there were scaled regions (of
the type shown in Figure 16) that seemed to consist of a surface film (possibly associated with adsorbed or surface complexed ACH) with dispersed rhomboid-like crystals suggestive of the presence of barite as also confirmed by EDS analysis. The above results confirmed that the observed flux decline, beyond that which was observed in optical surface imaging, was most likely due to both barite and the added coagulant both which were could not be observed at the level of magnification offered by the present method of optical imaging. 4. Conclusions The potential impact of chemical coagulants, used in conventional water pretreatment, on mineral scaling in brackish water RO desalting was investigated in both bulk crystallization and membrane scaling tests. Membrane scaling tests were carried out with the coagulants alone and in combination with antiscalant AS2 (ranked as most effective based on bulk crystallization tests). Bulk crystallization studies with AS2 showed that the addition of coagulants ACH, FeCl3, and polyDADMAC had an adverse effect on antiscalant effectiveness in both bulk crystallization and membrane scaling tests. When used alone, each coagulant displayed a slight scale suppression capability in retarding bulk crystallization. In membrane scaling tests, however, the presence of coagulants in the RO feed (without antiscalant addition) resulted in mineral scaling (quantified in terms of flux decline) that was greater for
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FeCl3 and polyDADMAC and lower for ACH, relative to scaling without additives. In all cases, however, the addition of the three coagulants (with and without antiscalant addition) resulted in scaling that was more severe relative to the case in which only the antiscalant was used and for which scaling was completely suppressed. In essentially all cases, the increased level of surface scaling was linked to a higher surface number density of mineral crystals. Moreover, the percent flux decline increased with increasing percent of scaled membrane area. Flux decline was found to be lower than was suggested by the percent area scaled for a few cases (scaling tests without additives, FeCl3 and with ACH + AS2), possibly due to partial permeation through the large gypsum rosette structures as reported in previous work.3 Higher flux decline was also observed relative to the scaled surface area (scaling tests with AS2 + polyDADMAC, ACH, and AS2 + FeCl3) in scaling tests that also showed significant scaling by barium sulfate crystals which could not be observed at the present level of optical magnification, but detected by SEM imaging with the presence of barium also verified by surface elemental analysis. Complexation of the coagulants and antiscalants and their adsorption onto the membrane surface are also a possibility that merits further study. The results of the study suggest that when feed treatment includes the use of coagulants, the selection of the antiscalant should be optimized to minimize the impact on membrane scaling and that excess antiscalant could be needed to offset the adverse impact of the coagulants on antiscalant effectiveness. Acknowledgment The present work was funded, in part, by the Metropolitan Water District of Southern California, the California Department of Water Resources, and the California Water Resources Center. A UCLA graduate research fellowship to J.A. and scholarship support to A.R. from the International Desalination Association are also gratefully acknowledged. Literature Cited (1) Barnett, J. A.; Henley, T. J. Review: Water quality standards for salinity; Colorado River Basin Salinity Control Forum, 2005. (2) Characklis, G. W. Economic decision making in the use of membrane desalination for brackish supplies J. Am. Water Resour. Assoc. 2004. (3) Rahardianto, A.; Shih, W.-Y.; Lee, R.-W.; Cohen, Y. Diagnostic characterization of gypsum scale formation and control in RO membrane desalination of brackish water. J. Membr. Sci. 2006, 279 (1-2), 655–668.
(4) Allenby, M. N. So little aluminum--so much of a problem for the RO plant. Presented at the IDA World Congress of Desalination and Water Reuse, Paradise Island, Bahamas, 2003. (5) Faust, S. D.; Aly, O. A. Chemistry of Water Treatment, 2nd ed.; Ann Arbor Press: Ann Arbor, MI, 1998. (6) Metropolitan Water District of Southern California and U.S. Department of Interior, B. o. R. Salinity Management Study; Final Report; Bookman-Edmonston Engineering: Sacramento, CA, 1998. (7) Green, J. F.; Gabelich, C. J.; Yun, T. I.; Bruno, J.-M.; Beuhler, M. D.; Leslie, G. L. In Metropolitan’s Desalination Research and Innovation Partnership (“DRIP”); AWWA Annual Conference & Exposition, Dallas, TX, 1998. (8) Gabelich, C. J.; Yun, T. I.; Coffey, B. M.; Suffet, I. H. M. Effects of aluminum sulfate and ferric chloride coagulant residuals on polyamide membrane performance. Desalination 2002, 150 (1), 15–30. (9) Shih, W.-Y. “Formation and Control of Calcium Sulfate Dihydrate (Gypsum) Crystallization”. Ph.D. Thesis, University of California at Los Angeles, Los Angeles, 2007. (10) Shih, W.-Y.; Albrecht, K.; Glater, J.; Cohen, Y. A dual-probe approach for evaluation of gypsum crystallization in response to antiscalant treatment. Desalination 2004, 169 (3), 213–221. (11) OLI Systems, 2.0; OLI Analyzer: Morris Plains, NJ, 2005. (12) Lyster, E.; Cohen, Y. Numerical study of concentration polarization in a rectangular reverse osmosis membrane channel: Permeate flux variation and hydrodynamic end effects. J. Membr. Sci. 2007, 303 (1-2), 140–153. (13) Smith, B. C. Fundamentals of Fourier Transform Infrared Spectroscopy; CRC Press: Boca Raton, FL, 1996. (14) Gabelich, C. J.; Gerringer, F. W.; Franklin, J.; Gao, J.; Cohen, Y.; Suffet, I. H. In Reverse osmosis pretreatment: Challenges with conventional treatment; AWWA ACE, Orlando, FL, 2004. (15) Gabelich, C. J.; Ishida, K. P.; Gerringer, F. W.; Evangelista, R.; Kalyan, M.; Suffet, I. H. Control of Residual Aluminum from Convention Treatment to Improve Reverse Osmosis Performance. Desalination 2006, 190 (1-3), 147–160. (16) Gabelich, C. J.; Chen, W. R.; Yun, T. I.; Coffey, B. M.; Suffet, I. H. M. The role of dissolved aluminium in silica chemistry for membrane processes. Desalination 2005, 180, 307–319. (17) Tjipangandjara, K. F.; Huang, Y.-B.; Somasundaran, P.; Turro, N. J. Correlation of alumina flocculation with adsorbed polyacrylic acid conformation. Colloids Surf. 1990, 44, 229–236. (18) Shih, W.-Y.; Gao, J.; Rahardianto, A.; Glater, J.; Cohen, Y.; Gabelich, C. J. Ranking of antiscalant performance for gypsum scale suppression in the presence of residual aluminum. Desalination 2006, 196 (1-3), 280–292. (19) Lee, R.-W. “Low pressure reverse osmosis membrane treatment of agricultural drainage water and surface water”. M.S. thesis, University of California at Los Angeles, Los Angeles, CA, 2003. (20) Semiat, R.; Sutzkover, I.; Hasson, D. Scaling of RO membranes from silica supersaturated solutions. Desalination 2003, 157, 169–191. (21) Gilron, J.; Hasson, D. Calcium sulphate fouling of reverse osmosis membranes: Flux decline mechanism. Chem. Eng. Sci. 1987, 42 (10), 2351– 2360.
ReceiVed for reView June 14, 2008 ReVised manuscript receiVed December 14, 2008 Accepted January 5, 2009 IE800937C
