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Response to Comment on “Unidirectional Freezing of Waste-Activated Sludge: The Presence of Sodium Chloride” SIR: We appreciate the comments made by Parker and Collins to our paper (1) and respond as follows. Freezing Temperature. Vol’khin and Zolotavin (2) demonstrated that, at a freezing temperature lower than the eutectic temperature of the water/electrolyte system, the presence of electrolyte will not affect the settleability of the freeze/thaw conditioned sludge. Above the eutectic temperature of NaCl solution, in principle, one cannot completely freeze the whole sludge body. There might still exist unfrozen, concentrated NaCl solution patches among the sludge flocs. Below the eutectic temperature (-21.12 °C), cyrohydrate (NaCl‚2H2O/ice mixture) forms that could weaken the interactions between the electrolyte and the sludge flocs. A lower freezing temperature would not affect the eutectic composition, thereby putting whether “curing” is an essential parameter into question. This merits some further investigations, nevertheless. Critical Freezing Speed. During a sludge freezing test, the temperature of certain surfaces of sample has to be lowered, from which ice front grows and migrates. The ice front (planar or dendritic in shape) may trap or reject the floc (3). If most particles are rejected by the ice front, as Parker and Collins addressed, the sludge ahead of the interface has been only thickened. At the end of a test (or a freeze/thaw conditioning process) where flocs gross migration occurs, all solids (flocs) have been pushed to the far-end of the frozen sample [for example, the top section in bottom freezing test or the central regime in a radial freezing test (4)] that cannot migrate any more. At this stage, the critical freezing speed loses its significance since any further freezing, no matter what the rate is, can induce no floc gross migration. Hung et al. (5) observed that the trapped flocs of original sludge (of lower concentration) in the growing ice has changed little as compared with the unconditioned sludge flocs. However, in the final phase of a freezing test, the ice front would still penetrate into the concentrated sludge body, transfer flocs from type I to type II, and induce subsequent dewaterability changes. This reflects the important role of solid content. The occurrence of gross migration can effectively raise the local solid contents before complete freezing, which is hardly
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achieved by purely mechanical means, especially for biological sludges. We had previously employed liquid nitrogen quenching (-198 °C) to treat sludges. However, no matter what the solid content is, there reveals no detectable sludge characteristic changes (6). Curing has no effects either. To recommend a fast freezing of thickened sludge in practice may be met with some skepticism. (Note: All sludge samples treated in Hung et al. or Chu et al. had been cured for 24 h under subfreezing temperatures.) Floc Shape. Notably, the fractal dimensions for type I and II flocs are similar, although their densities and appearance are different. In certain cases, a denser floc may exhibit a less fractal dimension (7). There exists no universal index to correlate the floc’s morphology and other characteristics (8). Interface Morphology. The occurrence of interfacial instability of solid/liquid interface may induce various morphologies, including dendrite, which is governed by supercooling, existence of impurity, boundary conditions, and others. As revealed in Parker et al. (4), the formation of dendrite is an important mechanism that entraps flocs, which we believe as well as an essential phenomena corresponding to the general conclusions drawn in ref 1.
Literature Cited (1) Chu, C. P.; Feng, W. H.; Tsai, Y. H.; Lee, D. J. Environ. Sci. Technol. 1997, 31, 1512. (2) Vol’khin, V. V.; Zolotavin, V. L. Colloid J. (USSR) 1961, 23, 113. (3) Parker, P. J.; Collins, A. G.; Dempsey, J. P. Proceedings of the ASCE 8th International Cold Regions Engineering Conference, Fairbanks, AK; 1996; p 757. (4) Hung, W. T.; Feng, W. H.; Tsai, I. H.; Lee, D. J. Water Res. 1997, 31, 2219. (5) Hung, W. T.; Chang, I. L.; Lin, W. W.; Lee, D. J. Environ. Sci. Technol. 1996, 30, 2391. (6) Chu, C. P.; Feng, W. H.; Chang, B. V.; Lee, D. J. J. Environ. Eng. In review. (7) Wu, C. C.; Huang, C.; Lee D. J. Colloids Surf. A In press. (8) Wu, R. M.; Lee, D. J. Water Res. Accepted for publication.
C. P. Chu, W. H. Feng, Y. H. Tsai, and D. J. Lee* Department of Chemical Engineering National Taiwan University Taipei, Taiwan 10617 ES972008N
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