Environ. Sci. Technol. 1997, 31, 1512-1517
Unidirectional Freezing of Waste-Activated Sludge: The Presence of Sodium Chloride C. P. CHU, W. H. FENG, Y. H. TSAI, AND D. J. LEE* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, 10617, Republic of China
This paper experimentally addressed the discrepancies in sludge freezing literature regarding the effect of electrolyte (sodium chloride) on the subsequent freeze/thawed sludge dewaterability. Waste-activated sludge is vertically frozen at fixed freezing speeds with sodium chloride concentration as a parameter. Results obtained herein demonstrate that if the sludge is completely frozen, regardless of the electrolyte, the sludge filterability can be markedly improved. However, the associated sludge settleability and the final sediment height do not alter much except when flocs gross migration occurs. Adding NaCl retards the flocs gross migration, thereby reducing the corresponding critical freezing speed. We speculate that the transition-layer freezing point suppression, the double-layer compression, and the initiation of wavy ice front are possible factors for the electrolyte effects. However, whether the double-layer compression is an influential mechanism in freeze/thaw treatment still remains unclear.
Introduction Freeze/thaw treatment, an effective sludge dewatering technique, has received considerable attention [see review by Vesilind and Martel (1)]. Freeze/thaw treatment can significantly improve certain sludge dewatering characteristics, transform the floc structure into a more compact form (e.g., see ref 2), and reduce the sludge bound water content (3). During freeze/thaw treatment, the sludge dewatering efficiency is generally believed to decrease with an increasing freezing rate; however, a long freezing time is economically unfavorable (4-6). Obtaining further information about freeze/thaw treatment with a high freezing speed and sufficient effectiveness is highly desired. Lee and Hsu (3) investigated the freeze/thaw treatment on sludge at an average freezing speed up to 40 mm/h (11.1 µm/s). They found reductions in both bound water content and resistance to filtration (and drying as well) due to such a “fast” treatment. Hong et al. (7) investigated the chemical compositions of sludge changes on freeze/thaw conditioned sludge. Hong et al. proposed that the freeze/thaw treatment releases some extracellular polymers (ECPs) from the sludge body. Most sludge particles in a high-speed freezing test would become trapped in the growing ice. Such a trapping effect is referred to as the “micromigration” process (4). Particles with a sufficiently low freezing speed are mostly rejected by the ice front and concentrated at the far-end (top) section of the frozen sample. This concentration is referred to as the “gross migration” process. Hung et al. (8) employed a vertical freezing apparatus to examine the freezing speed effects on * Corresponding author fax: 886-2-362-3040; e-mail: djlee@ ccms.ntu.edu.tw.
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sludge performance, including filterability, zone settling, and floc size versus density relationship. Sludge filterability is significantly improved at a high freezing speed at which no global floc migration occurs. However, the floc density and the floc morphology are slightly affected. With a low freezing speed, the flocs in sludge have almost entirely concentrated to the top section. Consequently, not only is the sludge filterability significantly improved but also the floc density and the morphology are improved as well; hence the settleability changes considerably. An intermediate freezing speed creates a situation in which part of the flocs are rejected; others are trapped in the growing ice layer. The weightaverage sludge characteristics are observed, which is due to an average behavior of the constituent flocs. Sludge freezing literature concerned with the presence of the electrolyte, however, reveal certain discrepancies. Vesilind et al. (2) examined the CSTs for freeze/thawed sludges with an addition of sodium chloride up to 2%. They found no significant sludge filterability improvement, concluding that double-layer compression is not an influential mechanism in freeze/thaw treatment. Recently, Kawasaki and Matsuda (9) examined sodium chloride-induced changes in filterability, settleability, and residual moisture of a freeze/thawed sludge using a radial freezing test. (Note: The so-called “radial freezing” here implies that the sludge sample was placed in a cylindrical container with its outer wall surface cooled by a coolant. Therefore, the ice front would develop from the rim region toward the central region along the radial direction with a decreasing freezing speed.) Nevertheless, Kawasaki and Matsuda found a profound sludge performance change when NaCl was added. They also demonstrated that the NaCl retarded the occurrence of gross floc migration in their tests. Instigated by the pioneering works of Vesilind et al. (2) and Kawasaki and Matsuda (9), this research investigates the effects of adding NaCl on the original and freeze/thawed sludge characteristics, including filterability, settleability, final sediment height, and the floc size/density/morphology relationship. The vertical freezing test proposed by Hung et al. (8) is employed herein. This work concentrates primarily on the following: (i) to clarify the discrepancies between Vesilind et al. (2) and Kawasaki and Matsuda (9) and (ii) to further discuss the promising role of electrolytes on freeze/ thaw treatment.
Experimental Section Samples. Three independent waste-activated sludge samples were taken from the wastewater treatment plant of the Neili Bread Plant, Presidental Enterprise Co., Taoyuan, Taiwan. The testing started within 2 h after sampling to prevent subsequent sludge changes. (Note: Since the experimental results from the three sludge samples are quite similar, only one of the samples is thoroughly discussed in this report.) The chemical oxygen demand (COD), suspended solids (SS), and turbidity data were for the supernatant drawn from the sludge, as determined using EPA Taiwan standard methods. The results read as follows: 31.2 mg/L (COD), 8.2 mg/L (SS), and 5.5 NTU (turbidity). The original sludge sediment was employed as the freezing sample, thereby largely eliminating the sedimentation effects. The original sediments weight percent was 1.5% w/w. Particle size distribution (PSD) was determined via a particle size analyzer (Coulter LS230). The floc average diameter of the original sludge was approximately 120 µm. Sodium chloride was added to produce samples containing 0.5, 1.0, and 2.0 % w/w of NaCl (based on the sludge sample), respectively. The solution was mildly stirred for 3 min to facilitate NaCl dissolution. The sludge’s appearance did not apparently change after adding NaCl.
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1997 American Chemical Society
Freezing Apparatus and Test. The vertical freezing apparatus was equipped with a computer-controlled stepping motor and a freezing pool and was the same as that used by Hung et al. (8). Sludge samples were placed in 3-6 glass tubes having a diameter of 1 cm and a height of 15 cm. The sample tubes were then fixed to a rod driven vertically by the stepping motor. By adjusting the stepping motor and the gear set required, the sample tubes were vertically immersed into the freezing pool at a constant speed ranging from 0.72 to 13.6 µm/s. The freezing chamber’s temperature was maintained at -17 °C. The frozen sample was cut along its axial direction into two sections: a top section of one-fourth and a bottom section of three-fourths of the total length, respectively. The ratio of the sludge solids concentration (excluding the sodium chloride) in the top section (CT) to that of the original sludge (C0) is defined as the degree of gross migration (RM). The range for index RM thereby ranges from 1 (no gross migration) to 4 (complete gross migration by definition), as suggested by Hung et al. (8). The weights of the sludge solids in the upper and lower sections of the frozen samples were calculated on a mass balance basis. Sludge Characteristics. A capillary suction apparatus as described by Lee and Hsu (10, 11) was employed herein to estimate the sludge filterability. The inner cylinder radius was 0.535 cm, while the time required for the filtrate to invade from 1.5 to 3.0 cm was defined as the capillary suction time (CST). Whatman No. 17 paper was used as the filter paper. The CSTs for the originally settled sludge with or without adding NaCl were all close to 140-150 s. From the CST data, the coefficient of dewaterability, χ, originally proposed by Vesilind (12) can be subsequently obtained. Hindered settling tests were directly performed in the freezing tubes after sample thawing. Strong wall effects presented in these tests. However, since only the relative differences between sludge samples were of interest, the wall effects were expected to largely cancel out each other between tests. The ZSV (zone settling velocity) was obtained by linear regression of the interface height versus time data for the constant rate period with a regression coefficient exceeding 0.98. Chen et al. (13, 14) provide other experimental details. The ZSV for the original settled sludge was essentially zero. The equilibrium sludge height after 24 h of settling was recorded. For the originally settled sludge, only a very slight settling took place (less than 2%). The addition of NaCl only slightly influences the final interface height. A glass cylinder (6 cm in diameter and 50 cm in height), sectioned on a side with an attaching plane view glass, was used for the free-settling test. A video camera (Model grM7pro, JVC, Japan) equipped with a close-up lens was used to record the floc diameter (normal to the vertical direction) and the terminal velocity data. Other experimental details can be found elsewhere (15). Although uncertainties still arise in applications, the modified Stokes’ law proposed by Tambo and Watanabe (16) is employed herein to construct the floc density versus size relationship. From the log-log plot of the size-density data, the slope (S) can be calculated via linear regression analysis, from which the fractal dimension (D) is evaluated by D ) 3 + S.
Results and Discussion Floc Migration. The freezing sample is the settled portion of the original sludge. Consequently, any global particle migration after sample freezing is due to the formation and movement of the ice front. The RM values (shown in Figure 1) for the original sludge are 1.3 and 3.7 at a freezing speed of 13.6 and 0.72 µm/s, respectively, corresponding to the visual observation that the former causes nearly no gross migration, while for the latter nearly complete migration occurs. For the 2% w/w sludge, however, the corresponding RM values
FIGURE 1. Degree of gross migration versus freezing speed.
FIGURE 2. Ratio of floc type II to floc type I. The arrow corresponds to the original sludge sediment. No. 1, without NaCl addition; no. 2, 0.5% w/w NaCl; no. 3, 1.0% w/w NaCl; no. 4, 2.0% w/w NaCl. became 1.03 and 1.49, respectively. The RM value gradually increased with a decreasing freezing speed from 13.6 to 0.72 µm/s. At a fixed freezing speed, a higher NaCl concentration caused a lower RM. That is, sodium chloride markedly retarded floc gross migration, thereby corresponding to Kawasaki and Matsuda’s findings. Floc Characteristics. Hung et al. (8) found in their vertical freezing test that the floc morphology in the top (concentrated) section markedly differed from those trapped in the bottom (ice) layer. The original and those trapped in the bottom ice layer were of loose, fractal-like shapes, referred to as type I flocs; meanwhile, those in the top concentrated layer were of compact, round-like shape, termed as type II flocs. The two mentioned flocs types were observed in this research as well. The microphotos resemble those reported in Hung et al. (8) and are not shown here for brevity sake. The ratios between type II to type I flocs (II/I) for the original settled sludge with or without adding NaCl were all approximately 0.04. This finding suggests that the major type of the constituent flocs has a loose, fractal-like structure. Figure 2 plots the results for frozen samples versus freezing speed with NaCl concentration as a parameter. The II/I ratio for the case without adding NaCl markedly increases with a
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FIGURE 3. Floc diameter versus freezing speed. The arrows correspond to the original sludge sediment. No. 1, without NaCl addition; no. 2, 0.5% w/w NaCl; no. 3, 1.0% w/w NaCl; no. 4, 2.0% w/w NaCl. decreasing freezing speed. Such an occurrence confirms the observations of Hung et al. (8) that the originally loose flocs (type I) once rejected by the growing ice layer are largely transformed into more compact, round-shape flocs (type II). Similar treads have been observed for the sludges with NaCl addition. However, the more NaCl added implies the less II/I ratio. In the 2% w/w tests, this ratio only slightly increased (from 0.04 to approximately 0.3) as the freezing speed reduced from 13.6 to 0.72 µm/s, as compared with that for tests without adding NaCl (from 0.04 to 4.7). That is, the floc shape transformation during gross migration was significantly reduced when NaCl was present. Figure 3 depicts the mean floc diameters for the original sludge and the conditioned sludge, which were calculated on the basis of the measured PSD. The mean floc size under a high freezing speed only slightly changed from the original sludge; however, it increased substantially with a low freezing speed. For instance, the original sludge exhibited a mean floc diameter of 123 µm, which became 400 µm at a freezing rate of 0.72 µm/s when NaCl was not present. The floc diameter before freezing decreased as the NaCl concentration increased. At a concentration of 0.5%, 1.0%, and 2% w/w, the flocs diameters became 83, 71, and 59 µm, respectively. Following freezing, the floc size of the NaCl-containing sludges increased as well. However, the degree of increase decreased with an increasing NaCl concentration, as evidenced by the following ratios for floc size after 0.72 µm/s freezing of the corresponding sludges: 3.32 (0%), 2.51 (0.5%), 1.71 (1.0%), and 1.67 (2%). Figure 4 illustrates the CST data. Arrows in the figures denote the results for the nonfrozen sludges. With a 2% w/w NaCl concentration, the sludge filterability slightly increased due to a smaller floc size and a higher filtrate viscosity. The corresponding coefficients of dewaterability were calculated and plotted in Figure 5. (The corresponding instrument constant, φ, was approximately 0.06 for the present work.) Notably, if the sample was completely frozen, the filterability significantly improved, as evidenced by the increase in χ from approximately 6.4 × 10-6 for the original sludge to 1.3-2.2 × 10-5 kg2/s2‚m4 for the freeze/thawed sludges. The presence of NaCl reduced the filterability, however, except for tests of high NaCl concentration (2% w/w in this work) in which its effect was secondary. Such a result partially supports the observation by Vesilind et al. (2) that adding NaCl had virtually no effect on sludge dewaterability (filterability) improvement.
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FIGURE 4. CST versus freezing velocity. The arrows correspond to the original sludge sediment. No. 1, without NaCl addition; no. 2, 0.5% w/w NaCl; no. 3, 1.0% w/w NaCl; no. 4, 2.0% w/w NaCl.
FIGURE 5. χ coefficient versus freezing velocity. The arrows correspond to the original sludge sediment. No. 1, without NaCl addition; no. 2, 0.5% w/w NaCl; no. 3, 1.0% w/w NaCl; no. 4, 2.0% w/w NaCl. Figures 6 and 7 illustrate the corresponding ZSV and final interface height data, respectively. Notably, a strong correlation arises between the sludge settleability, the freezing speed, and the NaCl concentration. With a high freezing speed (depending on the NaCl concentration), the freeze/ thawed sludge exhibited an extremely low ZSV and a small reduction in the final interface height. With a low freezing speed (still depending on the NaCl concentration), the ZSV increased markedly, and the final interface height significantly decreased as well. With an intermediate freezing speed, the ZSV and final interface height gradually changed. With NaCl, the increase in ZSV became less significant. For instance, at a freezing speed of 0.72 µm/s, the ZSV for 2% w/w sludge was 20 µm/s, i.e., a value less than one-tenth of that without adding NaCl (225 µm/s). The corresponding final interface height ratio decreased from 0.8 to approximately 0.4 as well. Clearly, the sludge settleability and final interface height were markedly affected by the NaCl concentration. This conclusion also supports the conclusions drawn by Kawasaki and Matsuda (9). Figures 8 and 9 depict the effective floc density versus floc diameter (df) relationships for the sludges without and
FIGURE 6. Zone settling velocity (ZSV) versus freezing speed. The ZSV for original sludge sediment is essentially zero.
FIGURE 7. Hf /Hi versus freezing speed. The arrow corresponds to the original sludge sediment. No. 1, without NaCl addition; no. 2, 0.5% w/w NaCl; no. 3, 1.0% w/w NaCl; no. 4, 2.0% w/w NaCl. with adding NaCl, respectively. As Figure 8 reveals, the floc density versus diameter curves shift upwards with a decreasing freezing speed. The comparisons clarify that at a freezing speed of 0.72 µm/s, the effective floc density can be 2.5 times that of the original sludge. This finding corresponds to a previous report (8). With 2% w/w NaCl in sludge, the effective floc density prior to freezing became two times that of the original sludge. However, freeze/thaw treatment did not significantly alter the sludge density. The sludges having an intermediate NaCl dosage amount exhibited a behavior that was an average of those shown in Figures 8 and 9. In summary, regardless of whether NaCl was added or not, the changes in floc characteristics after freezing were similar: (i) if the sample was completely frozen, the sludge filterability was markedly improved; and (ii) if floc gross migration occurred, the sludge settleability was greatly enhanced (Figure 6), the final interface height was reduced (Figure 7), and the floc size/density/morphology was altered (Figures 2, 3, 8, and 9). The correlation between the sludge characteristic changes after freezing and the corresponding RM becomes clear by comparing the shapes of the curves in Figures 2-7 with Figure 1. Hence, sufficiently improving
FIGURE 8. Effective floc density versus size relationships for the original and the freeze/thaw conditioned sludges. No NaCl addition.
FIGURE 9. Effective floc density versus size relationships for the original and the freeze/thaw conditioned sludges. 2% w/w NaCl. sludge performance heavily depends on gross floc migration (except for the sludge filterability, whose improvement can be achieved by simply complete freezing). This conclusion corresponds to our previous investigation (8). The hypothesis proposed by Hong et al. (7) possibly accounts for why such a strong correlation arises between sludge characteristic changes and the gross migration. They speculated that when a growing ice front rejects a floc, some cellulose-based ECPs are released, thereby causing subsequent changes in the floc structure and improving dewaterability. Our results demonstrate that NaCl retards gross migration of flocs and, ultimately, heavily influences the conditioned sludge settleability and the final interface height. However, the basic dewatering mechanism remains unchanged. Such a conclusion, however, markedly differs from the generally accepted concept of a specific freezing speed (the so-called critical freezing speed), below which the freezer should be designed (17-19). For instance, Uhlman et al. (20) obtained a critical freezing speed of 5 µm/s for inorganic slurries; Logsdon and Edgerley (4) suggested a speed of 1218 µm/s; while Hung et al. (8) concluded a lower than 3 µm/s freezing speed was sufficient to improve activated sludge dewaterability. Martel (21-23) provided design equations while considering the freezing and thawing speeds in the freezing bed. However, as this study reveals, the operational
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parameters, e.g., as the sodium chloride concentration in sludges, may substantially change the corresponding critical freezing speed. A freezer designed on the basis of some prescribed critical freezing speed may frequently fail if the influent sludge characteristics change. Vesilind et al. (2) preliminarily discussed the mechanism of gross migration. However, to our knowledge, no quantitative criterion for the gross migration considers all the important factors encountered in practice. Therefore, we predict a design criterion based on the occurrence of gross floc migration rather than on some prescribed critical freezing speed. Furthermore, our results have resolved the discrepancies between Vesilind et al. (2) and Kawasaki and Matsuda (9). Vesilind et al. (2) investigated sludge filterability. As a result, adding sodium chloride only slightly influenced the sludges, corresponding to the results presented herein. On the other hand, beside sludge filterability, Kawasaki and Matsuda (9) also examined sludge settleability and final settled interface height. (In their filterability test on the unstirred sludges, denoted as the open symbols in their Figure 6, the sodium chloride effect was found to be secondary as well.) Kawasaki and Matsuda concluded that adding NaCl significantly influences the sludge dewaterability on the basis of the settleability and the residual moisture measurements. Their conclusion also corresponds to our experimental findings. Apparently, a discrepancy arose due to the focus on different aspects of sludge dewaterability. (Note: Ref 24 discusses the term dewaterability usually referred in previous literature). Role of Electrolytes. The mechanism by which NaCl retards floc gross migration has not yet been fully explored. For a particle to migrate, a water layer [also called a transition layer, see Vesilind et al. (2)] should always exist between the particle and the growing ice layer (25). A lower freezing speed and smaller particle size are thereby favorable to gross migration (26). The electrolyte can affect the sludge body in several ways. First, it could enhance the salting out effect. That is, the moisture in the original sludge floc interior tends to move to the bulk solution, as evidenced by the smaller floc diameter data (nos. 2-4) in Figure 3. In addition, smaller flocs in sludges prior to freezing are favorable to gross floc migration (26). Second, the electrolyte may tend to concentrate in the transition layer between the particle and the ice front. The concentrated electrolyte would not only effectively suppress the freezing point of the transition layer but also cause a higher osmosis pressure. For the former, because the liquid surrounding the particle contains less electrolytes (higher freezing temperature) and is ready to freeze, a nonporous particle would be easily trapped in the growing ice layer when an electrolyte is added. For the latter, with a highly porous floc having a mass of internal water, the intrafloc moisture would tend to flow into the transition layer rather than to the surrounding bulk liquid (since the latter has exhibited a lower osmotic pressure.) This could replenish the transition water layer and, subsequently, cause the underneath ice layer to grow, which would dehydrate and reject the floc. For a floc located just behind a growing ice layer, the dehydration and push-out actions act simultaneously and continuously. That is, a competition results. When a floc that originally could be pushed out by the ice front has been dehydrated to some extent, the interior moisture might fail to sufficiently replenish the transition layer, and the floc may thereby be trapped. Third, the electrolyte can increase the solution ionic strength which, subsequently, could compress the electrical double layers of the constituent particles. When two smaller particles are located in front of an ice layer, the concentrated electrolyte can facilitate their coagulation. As discussed in Vesilind et al. (26), larger particles are not favorable to gross floc migration.
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Fourth, the electrolyte can influence the stability characteristics of the growing ice front. When an ice front freezes and continuously moves in a solution, the initially flat front shape loses its orderliness when the freezing speed is sufficiently high. A complicated front morphology, such as dendrite or dense fingers, may appear due to interfacial instability. (Note: Such an instability is typically caused by competition between diffusion of heat and electrolytes close to the freezing front. A higher freezing speed facilitates the formation of a wave-like ice front shape.) When a wavy ice front forms in the sludge with a wavelength of several hundred micrometers, the flocs in the valley tend to be trapped in the sludge. Theoretical and experimental investigations reveal that the interfacial wavelength is controlled by the group (V/ σD)1/2, where V, σ, and D denote the front speed, interfacial tension, and binary diffusivity, respectively (27, 28). Therefore, an increasing σ would have a similar effect as a decreasing V. Moreover, an electrolyte can reduce the interfacial surface tension (the so-called Jones-Ray effect) (29), thereby facilitating the interfacial instability, which is unfavorable to gross floc migration. Results in this work and of Kawasaki and Matsuda (9) both confirm that the NaCl retards the gross migration and is unfavorable to effective dewatering. That is, those factors retarding gross floc migration (e.g., transition-layer freezing point suppression, double-layer compression, and wavy front formation) dominate. Further research would clarify the detailed mechanism. Furthermore, because except for sludge filterability adding NaCl substantially affects sludge characteristics, double-layer compression cannot be excluded as a possible dominating mechanism during freeze/thaw treatment, as suggested by Vesilind et al. (26). This work has experimentally examined the effects of adding sodium chloride on the freeze/thaw conditioned activated sludge dewaterability. Discrepancies in previous literature have been resolved. Results obtained for sludges with and without adding NaCl reveal that if the sludge is completely frozen, sludge filterability is markedly improved. Also, with gross floc migration, the sludge settleability and the final interface height can be effectively improved. Clearly, NaCl retards the gross migration, thereby reducing the critical freezing speed. The promising role of the electrolyte is also discussed. Nevertheless, whether or not double-layer compression is essential to sludge conditioning by freeze/thaw treatment remains unclear.
Acknowledgments The authors are grateful to Neili Bread Plant, Presidental Enterprise Co., Taoyuan, Taiwan, for providing activated sludge samples. This work was supported by National Science Council, Republic of China.
Literature Cited (1) Vesilind, P. A.; Martel, C. J. J. Environ. Eng., ASCE 1990, 116, 854. (2) Vesilind, P. A.; Wallinmaa, S.; Martel, C. J. Can. J. Civ. Eng. 1991, 18, 1078. (3) Lee, D. J.; Hsu, Y. H. Environ. Sci. Technol. 1994, 28, 1444. (4) Logsdon, G. S.; Edgerley, E., Jr. J. Am. Water Works Assoc. 1971, 63, 734. (5) Ezekwo, G.; Tong, H. M.; Gryte, C. C. Water Res. 1980, 14, 1079. (6) Randall, C. W.; Khan, M. Z.; Stephens, N. T. Water Res. 1975, 9, 917. (7) Hong, S. G.; Young, Y. D.; Chen, G. W.; Chang, I. L.; Hung, W. T.; Lee, D. J. J. Environ. Sci. Health A 1995, 30, 1717. (8) Hung, W. T.; Chang, I. L.; Lin, W. W.; Lee, D. J. Environ. Sci. Technol. 1996, 30, 2391. (9) Kawasaki, K.; Matsuda, A. Kagaku Kogaku Ronbunsyu 1995, 21, 859. (10) Lee, D. J.; Hsu, Y. H. Ind. Eng. Chem. Res. 1992, 31, 2379. (11) Lee, D. J.; Hsu, Y. H. Ind. Eng. Chem. Res. 1993, 32, 1180. (12) Vesilind, P. A. Res. J. Water Pollut. Control Fed. 1988, 60, 215. (13) Chen, G. W.; Chang, I. L.; Hung, W. T.; Lee, D. J. Water Res. 1996, 30, 1844.
(14) Chen, G. W.; Chang, I. L.; Hung, W. T.; Lee, D. J. J. Environ. Sci. Health A 1996, 31, 521. (15) Lee, D. J.; Chen, G. W.; Liao, Y. C.; Hsieh, C. C. Water Res. 1996, 30, 541. (16) Tambo, N.; Watanabe, Y. Water Res. 1979, 13, 409. (17) Clements, G. S.; Stephenson, R. J.; Regan, C. J. J. Inst. Sewage Purif. 1950, 4, 318. (18) Cheng, C.-Y.; Updergraff, D. M.; Ross, L. W. Environ. Sci. Technol. 1970, 4, 1145. (19) Katz, W. J.; Mason, D. G. Water Sewage Works 1970, 110. (20) Uhlman, D. R.; Chalmers, B.; Jackson, K. A. J. Appl. Phys. 1963, 35, 2986. (21) Martel, C. J. J. Cold Regions Eng., ASCE 1988, 2, 145. (22) Martel, C. J. Res. J. Water Pollut. Control Fed. 1989, 61, 237. (23) Martel, C. J. J. Environ. Eng., ASCE 1989, 115, 799. (24) Chen, G. W.; Lin, W. W.; Lee, D. J. Water Sci. Technol. 1996, 34, 443.
(25) Corte, A. E. J. Geophys. Res. 1962, 67, 1085. (26) Vesilind, P. A.; Hung, W.; Martel, C. J. J. Cold Region Eng., ASCE 1991, 5, 77. (27) Mullins, W. W.; Sekerka, R. F. J. Appl. Phys. 1964, 35, 444. (28) Langer, J. S. Rev. Mod. Phys. 1980, 52, 1. (29) Hunter, R. J. Foundations of Colloid Science, Vol. I; Oxford University Press: New York, 1989; pp 423-424.
Received for review September 10, 1996. Revised manuscript received December 23, 1996. Accepted January 3, 1997.X ES960781X X
Abstract published in Advance ACS Abstracts, March 15, 1997.
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