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Comment on “Unidirectional Freezing of Waste-Activated Sludge: The Presence of Sodium Chloride” SIR: We have greatly enjoyed the paper by Chu et al. (1) and welcome this opportunity to comment on their work. This discussion is not meant as a criticism of their work. Rather, the majority of our comments are meant to enhance their work by comparing it with relevant results in the literature. Freezing Temperature. In addition to the works cited by Chu et al., Vol’khin and Zolotavin (2) also discussed the impact of electrolytes on freezing and thawing. Their results indicate that the conclusions of Chu et al. may only be applicable over a limited freezing temperature range. Vol’khin and Zolotavin examined the individual effects of 13 electrolytes on the settled volume of iron hydroxide precipitate following freezing and thawing. For each electrolyte studied, the settled sludge volume varied depending on the freezing temperature. Two results were reported for experiments in which the freezing temperature was lower than the temperature of formation of the electrolyte’s “cryohydrate”: (1) settleability was better compared with precipitates frozen at higher (warmer) temperatures; and (2) settleability appeared to be independent of electrolyte concentration. Since the improvement in settleability occurred at lower freezing temperatures, it could conceivably be attributed to an increase in freezing rate or a decrease in the “curing” temperature. (Curing is the storage of frozen sludge at subfreezing temperatures.) Although Vol’khin and Zolotavin’s experimental procedure did not isolate freezing speed from curing temperature, the improvements were likely due to the colder curing temperatures, since higher freezing rates generally lead to poorer dewaterability. Chu et al. reported that dewaterability was not as good for NaCl-amended sludge as it was for sludge without NaCl. Moreover, dewaterability continued to worsen as electrolyte concentration increased. According to Vol’khin and Zolotavin, electrolytes would not have had such a deleterious effect and settleability would not have been influenced by electrolyte concentration if the freezing temperature had been lower. Chu et al. froze their samples in the presence of sodium chloride at -17 °C. The temperature of formation of NaCl cryohydrate is -21.3 °C. Critical Freezing Speed. The use of the term “critical freezing speed” by Chu et al. may confuse some readers. Of the referenced works mentioned in Chu et al.’s discussion on critical freezing speed, only Uhlmann et al. (3) actually use a similar term, “critical velocity”. Uhlmann et al. refer to critical velocity as the maximum rate at which an interface can move such that particles are continuously rejected. The other references noted by Chu et al. do investigate freezing rate and comment on the “best” freezing speeds in terms of performance of the process. We feel that the term “critical freezing speed” should only be used to describe the freezing rate that allows migration of flocs in front of the ice/water interface. This is consistent with the usage in the crystal growth literature. Perhaps “optimum freezing speed” would be a better term to describe the freezing rate that yields the best dewaterability. Furthermore, the above is not just an argument concerning terminology. The freezing rate that allows flocs to be completely rejected does not necessarily correspond to the rate that yields the best dewaterability. Despite the emphasis in the freeze/thaw literature on the benefits of migration and the importance of mechanisms occurring in the unfrozen
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sludge, migration does not need to occur for dewaterability to improve. In fact, if sludge is continuously rejected ahead of the interface, the result is only a thickening of the sludge. Sludge flocs must be entrapped within the ice matrix in order to be transformed into granular particles. And, for flocs to be entrapped, the ice must grow at a rate greater than the critical velocity. Therefore, mechanical freezing processes must operate at a freezing speed greater than the critical velocity, not, as Chu et al. state, less than the critical velocity. We would suggest that possibly the benefits attributed to migration can be achieved by thickening the sludge before freezing. Hung et al. (4) noted that entrapped flocs which had migrated were more compact (type II) than flocs entrapped without migrating. Possibly type II flocs can be obtained by freezing a thickened sludge, even at rates that prevent migration. Rather than entrapping single flocs as would occur for a dilute sludge, larger portions of thickened sludge would most likely be entrapped. These larger entrapped portions then might become compact particles (type II flocs) following curing. Floc Shape. The observations made by Chu et al. concerning floc shape are very interesting. However, using the ratio of type I to type II flocs as a quantitative measure is perplexing since distinguishing between the two types of flocs appears to depend on the analyst’s discretion. An approach that quantifies the shapes of individual flocs (e.g., by fractal dimension) and correlates these shape descriptions to type I or type II flocs might be more useful. Interface Morphology. Chu et al. note that an increasingly “wavy” ice front is expected as the electrolyte concentration increases; the wavy front then inhibits gross floc migration. We have confirmed this hypothesis in an investigation of the ice/water interface during sludge freezing (5). A dendritic interface was formed by freezing the ice at high rates; very likely a similar interface could be formed by increasing the electrolyte concentration rather than the rate of freezing. At high freezing rates (>200 mm/h), a dendritic interface was noted, physically similar to the “wavy front” discussed by Chu et al. As hypothesized by Chu et al., the dendrites affected migration; dendrites grew past and through flocs, effectively confining them and preventing migration. As freezing rates decreased, the interface became less dendritic and smoother; the smooth interface pushed the flocs rather than fragmenting or entrapping them. Possibly a similar interface was present in the experiments Chu et al. performed at high electrolyte concentrations and may account for the relatively poor dewaterability noted at these concentrations.
Literature Cited (1) Chu, C. P.; Feng, W. H.; Tsai, Y. H.; Lee, D. J. Environ. Sci. Technol. 1997, 31, 1512-1517. (2) Vol’khin, V. V.; Zolotavin, V. L. Colloid J. (USSR) 1961, 23, 113117. (3) Uhlmann, D. R.; Chalmers, B.; Jackson, K. A. J. Appl. Phys. 1964, 35, 2986-2993. (4) Hung, W. T.; Chang, I. L.; Lin, W. W.; Lee, D. J. Environ. Sci. Technol. 1996, 30, 2391-2396. (5) Parker, P. J.; Collins, A. G.; Dempsey, J. P. Proceedings, ASCE 8th International Cold Regions Engineering Conference, Fairbanks, AK; 1996; pp 757-767.
Philip J. Parker* and Anthony G. Collins Department of Civil and Environmental Engineering Clarkson University Potsdam, New York 13699-5710 ES970582T
S0013-936X(97)00582-8 CCC: $14.00
1997 American Chemical Society
