Unidirectional Freezing of Waste-Activated Sludges: Effects of

No precise definition for the critical freezing speed can thereby be defined. If improvement of sludge filterability is the only concern, a high freez...
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Environ. Sci. Technol. 1996, 30, 2391-2396

Unidirectional Freezing of Waste-Activated Sludges: Effects of Freezing Speed W. T. HUNG, I. L. CHANG, W. W. LIN, AND D. J. LEE* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, 10617 Republic of China

In this work, we employ unidirectional freezing of waste-activated sludge to investigate the effects of freezing speed on the sludge performance including filterability, zone settling, and floc density versus size relationships. With a high freezing speed, no global particle migration occurs, but the sludge filterability is markedly improved. However, the floc density and the floc morphology are only slightly changed; therefore, the zone settling characteristics are those of the original sludge. With a low freezing speed, however, global particle migration occurs. Consequently, both the sludge filterability and settleability are enhanced. The floc density and the morphology are changed significantly. With an intermediate freezing speed, a gradual transition of sludge characteristics occurs between the two extremes, resulting from an average behavior of the constituting flocs. No precise definition for the critical freezing speed can thereby be defined. If improvement of sludge filterability is the only concern, a high freezing speed is acceptable. Furthermore, a low freezing speed is necessary if both sludge settleability and bound-water content must be changed.

Introduction Freeze/thaw treatment, an effective sludge dewatering technique, has received extensive attention (1-25). Employing freeze/thaw treatment can significantly improve certain sludge dewatering characteristics, irreversibly transform the floc structure into a more compact form (e.g., see ref 19), and reduce the corresponding bound-water content (14-16, 21, 24, 25). In freeze/thaw treatment, freezing time is critical, and instant freezing is deemed inadequate (6). The sludge dewatering efficiency is generally accepted to decrease with an increasing freezing rate. However, a long freezing time is economically unfeasible (7, 27). Therefore, obtaining information about an effective freeze/thaw treatment with a high freezing speed is highly desired. Lee and Hsu (24) performed freeze/thaw treatment on sludges at an average freezing speed up to 40 mm/h (11.1 µm/s), indicating that * Corresponding author fax: 886-2-362-3040; e-mail address: [email protected].

S0013-936X(95)00889-3 CCC: $12.00

 1996 American Chemical Society

TABLE 1

Properties of Original Activated Sludgesa sample

COD (mg/L)

SS (mg/L)

turbidity (NTU)

wt %

s t

14.1 18.8

0.6 1.0

0.70 0.73

1.17 1.41

a COD, SS, and turbidity are for supernatants, while weight percents are for sediments.

such a “fast” treatment can not only reduce the sludge bound-water content to 50% but also largely decrease the resistance to drying and filtration. In most conventional freezing apparatuses, the effects of freezing speed are difficult to isolate due to the complicated interactions between multidimensional heat conduction and the moving ice fronts. A previous work employed the unidirectional freezing test, as the sludge is frozen along one direction by immersing the sample vertically into a freezing pool under specific speed, to examine in detail the interactions between the ice front and the sludge flocs (8). Notably, when the freezing speed is high, most of the particles are trapped in the ice, and the sludge dewatering improvement is insignificant. However, for a completely frozen sample, the ice crystals penetrate into the trapped flocs, thereby resulting in alternating layers of ice crystals and sludges in the frozen sample, which is referred to as the “micromigration” process (6). Nevertheless, with a low freezing speed, particles are rejected by the ice/water front and tend to concentrate at the top section of the frozen sample. The dewatering of sludge is mainly achieved by diffusion of moisture from unfrozen bulk sludge to the ice front. This event is referred to as the “gross migration” process (6). Gross migration seems to be a prerequisite to effectively improve sludge dewatering. The role of the freezing speed has not yet been fully explored. In this work, we focus primarily on the effects of freezing speed toward sludge dewaterability. Unidirectional freezing tests are first performed, from which the degree of particle migration and the corresponding sludge characteristics are measured. Next, capillary suction time (CST) data are used to evaluate the change in sludge filterability, while the zone settling velocity and the final settling height are for the sludge settleability and the sludge bound water. Finally, the critical freezing speed below which the sludge dewaterability can be largely improved is discussed.

Experimental Section Samples. Waste-activated sludge samples were taken from a large wastewater treatment plant in Neili Bread Plant, Presidential Enterprise Co., Taoyuan, Taiwan. Chemical oxygen demand (COD), suspended solid (SS), and turbidity data listed in Table 1 were for the supernatant drawn from the sludge, as determined by EPA Taiwan standard methods. Weight percent data are for sediments, as determined by weighing and drying. Freezing Tests. A unidirectional freezing apparatus (schematically shown in Figure 1) equipped with a computer-controlled stepping motor and a freezing pool was installed and used for all subsequent freezing tests. The sludge sample was placed in three to six glass tubes of 1.3 cm diameter and 15.0 cm height. The sample tubes were

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FIGURE 1. Schematics of the unidirectional freezing apparatus.

then fixed to a rod driven vertically by the stepping motor. By adjusting the stepping motor and the gear set required, the sample tubes can be immersed vertically into the freezing pool with a constant speed ranging from 0.1 to 3000 µm/s. The freezing temperature was fixed at -16 °C (a compromise between the high summer temperatures during the tests, usually above 30 °C, and the refrigerator compressor capacity). Other freezing temperatures were tested; however, no significant changes in experimental results were observed. This finding corresponds to the dilatometry works by Prof. Vesilind and co-workers at Duke University (28). They found that the moisture amount frozen over a wide temperature range is almost the same. Freezing speed range in the present work is from 1.41 to 72.6 µm/s, i.e., much wider than previous works (up to 10 µm/s). Also, a freezing speed higher than 72 µm/s causes incomplete freezing of the sample. When the sample tubes were gradually immersed in the freezing pool (Figure 1), an ice layer develops from the bottom and grows upward as the sample tube is immersed further into the cooling bath. Since the freezing speed is rather slow in this work, the freezing is observed as onedimensional (vertically); meanwhile, the radial dependence of freezing can be safely neglected. The sediment for the original sludge after 36-48 h of settling under 5 °C was employed as the freezing sample. The sedimentation effects can thereby be largely eliminated. No stirrer was used in this work to suspend the sludge as suggested by Halde (8). Sludge Characteristics. A capillary suction apparatus, as described in refs 29 and 30, was employed to estimate the sludge filterability. The inner cylinder radius was 0.535 cm, while the time for filtrate to invade from R ) 1.5-3.0 cm was referred to as the capillary suction time (CST). Whatman No. 17 paper was the filter paper. The coefficient

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of dewaterability, χ, as originally proposed by Vesilind (31), was calculated from the CST data. Hindered settling tests were performed directly in the sample tubes after sample thawing. Strong wall effects should arise in the tests. However, since only the relative differences between sludge samples are of interest, the wall effects were expected to largely cancel out between tests. Ramalho (32) suggested that stratification might occur in an unstirred settling sludge. This phenomenon was, however, not observed in these sludge samples. The ZSV (zone settling velocity) could be obtained by linear regression of the interface height versus time data for the constantrate period with a regression coefficient higher than 0.98. Other experimental details can be found in ref 33. Since the original sludge sediment was used as the freezing sample, any further reduction in final sediment height can function as an index of the corresponding boundwater content. (Note: Ref 34 provides a detailed discussion of bound water in a sludge.) The equilibrium sludge height after 24 h of settling was recorded. A glass cylinder (6 cm in diameter and 50 cm in height), sectioned on a side with an attached plane view glass, was used for the free-settling test. A JVC camera equipped with a close-up lens was used to record the floc diameter (normal to vertical direction) and the terminal velocity data. Other experimental details can be found elsewhere (35). Although uncertainties still exist in applications (35), the modified Stokes’ law proposed by Tambo and Watanabe (36) was employed 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 with a correlation coefficient up to 0.93, from which the fractal dimension (D) is evaluated by D ) 3 + S. Fractal dimension, ranging from 1 to 3, accounts for the manner in which the constituting particles pack the floc’s interior space (37). A floc with a fractal dimension close to 3 indicates a close interior packing. If D is close to 1, an extremely loose packing feature appears.

Results and Discussion Particle Migration. Since the freezing sample is the sediment of the original sludge, any global particle migration after sample freezing should be caused by the formation and movement of the ice front. The frozen sludge is cut into four sections of equal length. The ratio of the solid concentration (CT) in the top section to that in the original sludge (C0) is defined as the degree of gross migration (RM). Clearly, if there is no gross migration, RM ) 1. A situation in which all solids contents are pushed up to the top section, whose volume is one-fourth of the total volume, CT ) 4C0, would yield RM ) 4. Figure 2 summarizes the experimental results. When the freezing velocity is higher than 22 µm/s, RM was all close to unity, implying that nearly no gross migration occurred under a high freezing speed. Visual observation also revealed no macroscopic inhomogeneity in the sludge samples. When the freezing speed was lower than approximately 3 µm/s, however, RM was 3 or higher. That is, the sludge concentration in the upper section was 10 times higher than that in the bottom section. (Notably, a value of RM ) 3 indicates that CT ) 3C0. Based on mass balance, the solids concentration in the bottom sections becomes CB ) C0/3, thereby yielding CT ) 9CB.) Most of the particles have therefore been pushed up to the top section to form a highly concentrated sludge phase with a nearly transpar-

FIGURE 2. CT/C0 versus freezing speed plot.

FIGURE 4. Coefficient of dewaterability, χ, versus freezing speed plot.

FIGURE 3. Capillary suction time versus freezing speed plot. R0 ) 0.535 cm. Sampling interval: R ) 1.5-3 cm.

FIGURE 5. Zone settling velocity versus freezing speed plot.

ent ice section below it. With an intermediate freezing speed, RM changes gradually between these two extremes. Sludge Characteristics. Figure 3 presents the CST data under various freezing speeds. Clearly, the CSTs for all conditioned sludges are all much less than that for the original sludge (140-170 s) and are close to that for the supernatant (31-40 s). That is, if the sludge has been completely frozen, the sludge filterability is markedly improved regardless of the freezing speed. A further reduction in freezing speed only slightly reduced the CST. From the CST data, the coefficient, χ, originally proposed by Vesilind (31), could be calculated and is plotted in Figure 4. (Note: The corresponding instrument constant, φ, as suggested in ref 31 is approximately 0.06 in this work.) If only the sample has completely frozen, the coefficient can be largely improved, as evidenced by the increase in χ from approximately 5 × 10-6 for the original sludges to 1.2-2 × 10-5 kg2 s-2 m-4 for conditioned sludges, a 2.5-4 times dewaterability improvement. Also, a decrease in freezing speed from above 40 to less than 2 µm/s would lead to an increase in χ for less than 30%. The above conclusions correspond to those drawn from CST tests.

Figures 5 and 6 illustrate the corresponding ZSV and final sediment height data, respectively. Notably, a strong correlation arises between the sludge settleability and the freezing speed. A conditioned sludge with a high freezing speed, unlike the sludge filterability, will exhibit a very low ZSV and an almost negligible reduction in final sediment height. When the freezing speed is low, the ZSV increased significantly. The final sediment height is also markedly reduced, reflecting a decrease in bound-water content (24). (Note: After 27 h of further settling, a slight settling of the sediment of sludge s still occurs, denoted as the horizontally dashed line in Figure 4. However, this effect on ZSV and final sediment height was only slight.) In the intermediate speed region, the ZSV and equilibrium sediment height change gradually. Figure 7 depicts the floc density versus size relationships for the original and conditioned sludges under various freezing speeds. Two points are worth mentioning. First, when the freezing speed was high (larger than 40 µm/s), the floc density was close to that of the original sludge. When the freezing was lower, when compared with the original sludge flocs, the floc density increased by 1.6 (v )

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FIGURE 6. hf/h0 versus freezing speed plot. Horizontal dashed lines are for original sludges.

FIGURE 8. Microphotographs of sludge sample 200×: (a) original sludge flocs; (b) the same flocs with Indian ink.

FIGURE 7. Effective floc density versus floc diameter plot.

5.6 µm/s) to 2.6 (v ) 2.25 µm/s) times if based on the same floc size. Second, the fractal dimensions for original and conditioned sludges were basically the same, which closely corresponds to the results of a previous study (24). The above results reveal that the basic packing feature of flocs remained unchanged after freeze/thaw treatment. The density, however, was a strong function of freezing speed. The term “dewaterability” had been used to describe various sludge characteristics, e.g., the filterability (for instance, see refs 3, 19, and 20), the settleability (13), or the residual moisture after a dewatering operation (10, 15, 38). Investigators normally expect that a conditioned sludge with good filterability would also exhibit a satisfactory settleability and a low bound-water content. Based on our observations, we can conclude that the effects of freezing are not the same for the sludge filterability and settleability. The sludge characteristics should therefore not only be controlled by different factors but also be discussed separately. Photographic Observations. Figure 8 shows the microscopic photos for the original and the Indian ink stained

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sludge sample (200×). Clearly loose, fractal-like shapes as reported before (24) were observed. A certain amount of filaments were found in the sludge sample (Figure 7a), indicating that the sludge settleability would be poor. The stained sludge photo (Figure 7b) shows that there were regions of all sizes that could not be penetrated by the Indian ink, indexing the possible positions where the ECPs (extracellular polymers) were crowded (39). Figures 9 and 10 show the microscopic photos for the typical floc types observed in the conditioned sludge. Two types of flocs in these sludge samples were found, referred to here as type I flocs (Figure 8a,b) and type II flocs (Figure 9a,b), respectively. The appearance of type I flocs resembles the original sludge flocs (Figures 7a and 8a); in addition, their effective floc densities were similar (Figure 6). The interior ECP distributions were similar as well (Figures 7b and 8b). The above results demonstrate that the formation of ice crystals did not significantly change the basic architectural features of the flocs. However, closer observation revealed that the solid phase’s local structure had become more compact. Such compactness may have occurred due to the squeezing action of the neighboring ice fronts that penetrated into the floc. On the other hand, type II flocs, as demonstrated in Figure 9a, show a very different character. Instead of the

FIGURE 9. Microphotographs of conditioned sludge sample 200×; type I: (a) conditioned sludge flocs; (b) the same flocs with Indian ink.

loose, fractal-like shape, the type II flocs possess a nearly round shape, more compact structure, and a typically larger floc size than type I flocs. The stained picture (Figure 9b) also reveals an extremely crowded ECPs distribution in the floc interior. Previous works noted the compact flocs (22, 24). At a high freezing speed, at which no gross migration occurred, the flocs constituting the conditioned sludge were almost all of type I. A few type II flocs and flocs that exhibited a shape between types I and II exist as well. The number was insignificant, and therefore discussion can be neglected here. Gross migration was obvious for sludge samples treated with a low freezing speed. The flocs in the concentrated top section were almost all of type II. The small amount of flocs trapped in the bottom ice section were almost all of type I. However, since the solids concentration in the top section was significantly higher than that in the bottom section, the mixed thawed sample consisted almost entirely of type II flocs. In the intermediate speed region, the ratio of the amount of flocs being pushed up to the upper section to those being trapped was a function of freezing speed. Unsurprisingly, the flocs remaining in the bottom section were still of type

FIGURE 10. Microphotographs of conditioned sludge sample 200×, type II: (a) conditioned sludge flocs; (b) the same flocs with Indian ink.

I, and the flocs in the upper concentrated section were of type II. Therefore, we can conclude that the ratio of type I to type II flocs was a continuous function of freezing speed. Furthermore, we can speculate that the change in sludge characteristics after freeze/thaw treatment is caused primarily by the change in floc structure. Due to the almost unchanged floc density (free-settling test) and the basic architectural features of the type I flocs, the subsequent hindered settling characteristics and final settling height would not be improved markedly. However, since the local structure has become more compact due to the squeezing action of ice crystal growth, the bound water would decrease and the subsequent cake would exhibit a lower compressibility, thereby resulting in much better filterability (i.e., a much larger χ). On the other hand, the type II flocs exhibit a round shape and a more compact interior full of ECPs. The floc density is therefore greatly increased. The bound-water content should also be significantly reduced (i.e., a much lower final sediment height; these results shown in ref 24). The formation of type II flocs would improve not only the sludge filterability but also the sludge settleability. In the intermediate speed region, since the ratio of the type I to II flocs changes continuously with the freezing

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speed, the sludge characteristics should also be of an average nature. This observation can account for the gradual change in sludge characteristics observed in Figures 3-6. Critical Freezing Speped. Available literature usually suggest that a sufficiently low freezing speed would achieve an improvement in sufficient sludge. Information regarding the critical freezing speed that can serve for engineering design is highly desired. Considering the present study, however, no strict definition of such a critical freezing speed is available. If the purpose is to improve the sludge filterability, a rather high freezing speed can be employed only if the sludge is completely frozen (as high as 70 µm/s in this work). (Note: Partially unfrozen samples can easily appear in fast freezing tests. Nevertheless they are usually rather difficult to identify from the appearance of the frozen sample.) If one wants to improve the sludge settleability and reduce the bound-water content in addition to improving filterability, the freezing speed should be less than a critical value, i.e., approximately 3 µm/s in this study. These values are definitely strong functions of particle size, surface properties, and the other dissolved impurities (8). Results in this study also indicate that a sludge with more filaments tends to be more easily trapped by the freezing front. For activated sludges, the freezing speed investigated in available literature ranges from 0.6 to 8 µm/s (6, 9, 13-16), or 11 µm/s (24). Most of these experimental results are thus typical of type II floc characteristics.

Acknowledgments We thank the National Science Council, ROC, for financial support of this work. Neili Bread plant, Presidental Enterprise Co., Taoyuan, Taiwan, is appreciated for providing activated sludge samples.

Literature Cited (1) (2) (3) (4)

Vol’khin, V. V.; Ponomarev, E. I. Colloid J. (USSR) 1965, 27, 10. Doe, P. W.; Benn, D.; Bays, L. R. J. Inst. Water Eng. 1965, 19, 251. Katz, W. J.; Mason, D. G. Water Sewage Works 1970, Apr, 110. Farrell, J. B.; Smith, J. E.; Dean, R. B.; Grossmann, E., III; Grant, O. L. J. Am. Water Works Assoc. 1970, 62, 787. (5) Baskerville, R. C. Filte Sep. 1971, Mar/Apr, 141. (6) Logsdon, G. S.; Edgerley, E., Jr. J. Am. Water Works Assoc. 1971, 63, 734. (7) Ezekwo, G.; Tong, H. M.; Gryte, C. C. Water Res. 1980, 14, 1079.

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(8) Halde, R. Water Res. 1980, 14, 575. (9) Ide, T.; Kawasaki, K.; Yamashiro, H.; Matsuda, A. Gesuido Kyokaishi 1986, 23, 21. (10) Martel, C. J. J. Water Pollut. Control Fed. 1989, 61, 237. (11) Martel, C. J. J. Environ. Eng. 1989, 115, 799. (12) Vesilind, P. A.; Martel, C. J. J. Environ. Eng. 1990, 116, 854. (13) Kawasaki, K.; Matsuda, A.; Ide, T.; Murase, T. Kagaku Kogaku Ronbunsyu 1990, 16, 16. (14) Kawasaki, K.; Matsuda, A.; Ide, T.; Murase, T. Kagaku Kogaku Ronbunsyu 1990, 16, 401. (15) Kawasaki, K.; Matsuda, A.; Murase, T. Kagaku Kogaku Ronbunsyu 1990, 16, 1241. (16) Kawasaki, K.; Matsuda, A.; Mizukawa, Y. J. Chem. Eng. Jpn. 1991, 24, 743. (17) Martel, C. J.; Diener, C. J. J. Am. Water Works Assoc. 1991, 83, 51. (18) Vesilind, P. A. J. Environ. Eng. 1990, 116, 646. (19) Vesilind, P. A.; Wallinmaa, S.; Martel, C. J. Can. J. Civ. Eng. 1991, 18, 1078. (20) Vesilind, P. A.; Hung, W.; Martel, C. J. J. Cold Region Eng. ASCE 1991, 5, 77. (21) Matsuda, A.; Kawasaki, K.; Mizukawa, Y. J. Chem. Eng. Jpn. 1992, 25, 100. (22) Lee, D. J. J. Chin. Inst. Chem. Eng. 1994, 25, 201. (23) Lee, D. J. J. Chem. Technol. Biotechnol. 1994, 61, 139. (24) Lee, D. J.; Hsu, Y. H. Environ. Sci. Technol. 1994, 28, 1444. (25) Hong, S. G.; Young, J. D.; Chen, G. W.; Chang, I. L.; Hung, W. T.; Lee, D. J. J. Environ. Sci. Health 1995, A30, 1717. (26) Lee, D. J.; Lee, S. F. J. Chem. Technol. Biotechnol. 1995, 62, 359. (27) Randall, C. W.; Khan, M. Z.; Stephens, N. T. Water Res. 1975, 9, 917. (28) Vesilind, P. A. Personal communication, 1995. (29) Lee, D. J.; Hsu, Y. H. Ind. Eng. Chem. Res. 1992, 31, 2379. (30) Lee, D. J.; Hsu, Y. H. Ind. Eng. Chem. Res. 1993, 32, 1180. (31) Vesilind, P. A. J. Water Pollut. Control Fed. 1988, 60, 215. (32) Ramalho, R. S. Introduction of wastewater treatment process; Academic Press: New York, 1977. (33) Chen, G. W.; Chang, I. L.; Hung, W. T.; Lee, D. J. Water Res. Accepted for publication. (34) Lee, D. J.; Hsu, Y. H. Water Environ. Res. 1995, 67, 310. (35) Lee, D. J.; Chen, G. W.; Liao, Y. C.; Hsieh, C. C. Water Res. 1996, 30, 541. (36) Tambo, N.; Watanabe, Y. Water Res. 1979, 13, 409. (37) Li, D. H.; Ganczarczyk, J. J. Environ. Sci. Technol. 1989, 23, 1385. (38) Lee, D. J. Water Sci. Technol. In press. (39) Jenkins, D.; Richard, R. G.; Daigger, G. T. Manual on the causes and control of activated sludge bulking and foaming 2nd ed.; Lewis Publishers: London, 1993.

Received for review November 28, 1995. Revised manuscript received March 4, 1996. Accepted March 6, 1996.X ES950889X X

Abstract published in Advance ACS Abstracts, May 1, 1996.