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Ultra-rapid freezing of water treatment residuals
PII: S0043-1354(98)00449-7
Wat. Res. Vol. 33, No. 10, pp. 2239±2246, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
ULTRA-RAPID FREEZING OF WATER TREATMENT RESIDUALS M PHILIP J. PARKER1* and ANTHONY G. COLLINS2*
Department of Civil and Environmental Engineering, University of Wisconsin-Platteville, 137 Ottensman Hall, 1 University Plaza, Platteville, WI 53818, U.S.A. and 2Clarkson University, P.O. Box 5700, Potsdam, NY 13699-5700, U.S.A. 1
(First received July 1998; accepted in revised form October 1998) AbstractÐAn alum water treatment residual (``sludge'') was conditioned by freezing at ultra-rapid rates. Ultra-rapid rates were obtained by freezing on liquid nitrogen. The purpose of the experiments was to determine the conditions necessary for such rapid freezing to improve dewaterability. Contrary to results reported in the literature, ultra-rapid freezing worked extremely well. Although the conditioned ¯ocs (``zots'') were not as dense or granular as obtained at slower freezing rates, dewaterability was still greatly improved. Dewaterability, as measured by cake solids content (CSC), capillary suction time (CST), ®lterability coecient (w) and aggregate volume index (AVI), was best for samples with an initially high solids content and for samples that had been cured or stored at sub-freezing temperatures, for long times. Zot size analyses showed that zots decreased in size following ultra-rapid freezing. The results are explained using a proposed conceptual model of the residuals freezing process. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐfreeze/thaw conditioning, water treatment residuals, sludge, dewatering, liquid nitrogen, ultra-rapid freezing rates
INTRODUCTION
Water treatment sludge, or ``residuals,'' is composed of ¯ocs and water. The goal of sludge dewatering is the removal of water from residuals in order to minimize disposal costs. Water in residuals may be categorized as free water and bound water: free water is located between ¯ocs and may be removed mechanically; bound water is associated with the ¯ocs and may be trapped in ¯oc interstices, adsorbed to inter-particle surfaces, or exist as water of hydration. Most dewatering methods are limited in the sense that they only remove free water. However, bound water contents are signi®cant: 2 to 12 g/g-DS (dry solids) have been reported for a variety of water and wastewater sludges measured by several dierent methods (Lee, 1994; Lee and Hsu, 1995; Lee and Lee, 1995; Robinson and Knocke, 1992; Smith, 1992). Freeze/thaw conditioning transforms bound water into free water that can be removed mechanically. Frozen and thawed ¯ocs (termed ``zots'') are often larger than unfrozen ¯ocs and have a ``gritty'' consistency. The term ``zot'' was ®rst used by Russian researchers (Zolotavin et al., 1960), who recognized that the properties of *Author to whom all correspondence should be addressed. [Tel.: +1-608-3421235; fax: +1-608-3421566; e-mail: [email protected]].
chemical precipitates were drastically altered by freezing and thawing, thus warranting a separate term. The freezing rate reportedly has important consequences on the performance of the freeze/thaw process. Slow freezing has generally been reported to yield better dewaterability than fast freezing. For instance, Vesilind and Martel (1990) noted that the freezing rate had a dramatic eect on various biological sludges, with the best dewaterability noted for the most slowly frozen samples. Kawasaki et al. (1991) stated that ®lterability of waste activated sludge (WAS) was better for samples frozen at 1.9 mm/h than those frozen at 9.1 mm/h. Fast freezing, which refers to freezing rates of approximately 40 mm/h, led to a slight improvement in WAS dewaterability according to Lee and Hsu (1994). However, ¯ash freezing, on dry ice (ÿ788C) or liquid nitrogen (ÿ1968C), reportedly does not lead to any improvements and researchers have noted that ¯ocs retain their ``colloidal'' nature following ¯ash freezing (Clements et al., 1950; Vol'khin and Zolotavin, 1961; Doe et al., 1965; Benn and Doe, 1969; Katz and Mason, 1970; Logsdon and Edgerley, 1971; Martel, 1988). In the remainder of this paper, the term ``ultra-rapid freezing'' will be used instead of ``¯ash freezing''. The term is borrowed from the frozen food literature, where it has been used to describe food that is frozen on liquid nitrogen (Luyet, 1968a).
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Katz and Mason (1970) and Logsdon and Edgerley (1971) speculated on the causes of the failure of ultra-rapid freezing. They theorized that freeze/thaw conditioning at low rates is successful because water is released by the ¯ocs when they are entrapped in the ice. Moreover, the rate of dehydration depends on the temperature gradient between the ¯oc and the surrounding ice. Katz and Mason speculated that ultra-rapid freezing inhibits dehydration because the freezing rate is faster than the rate at which water can diuse from the inner portions of the ¯ocs to the surface. At ultra-rapid rates, very small crystals form on the surface and within the ¯oc; at lower rates, the ice crystals are much larger and grow slowly such that water can diuse from the interstices of the ¯oc. The same reasoning applies to freezing food; fast rates are desired so that water remains incorporated in the tissue. According to Luyet (1968b), ice formed at lower cooling rates exhibited well-formed crystals; as the cooling rate increased to ultra-rapid rates, ice needles formed with increasingly small radius (on the order of angstroms). Although experimental results and plausible theories have established the failure of ultra-rapid freezing, some reported results suggest that ultrarapid freezing may be successful under certain freezing conditions. Parker et al. (1998b) froze samples of alum residuals at four dierent rates. The fastest freezing, 37.5 mm/h, was achieved by freezing the residuals directionally on a cold surface at ÿ308C for 15 min. After being immediately thawed, the dewaterability was only slightly better than the unfrozen residuals and was poorer than the dewaterability of the more slowly frozen samples. This suggested that dewaterability would become worse as freezing rates increased further, in agreement with the consensus in the literature. However, Parker et al. also noted that when the residuals frozen at 37.5 mm/h were cured (stored at subfreezing temperatures), dewaterability improved drastically. In fact, for 6 h of curing or more, dewaterability of the most rapidly frozen residuals did not vary signi®cantly from the dewaterability of more slowly frozen residuals. Moreover, the reports of the failure of ultrarapid freezing have been based on results from biological sludges. However, freezing and thawing is generally less eective for biological sludges than for chemical sludges at non-ultra-rapid rates: the zots formed from biological sludges are less granular and tend to revert more readily to a ¯occulant nature. Therefore, it seems plausible that ultra-rapid freezing might be more eective for chemical sludges. This paper's objectives are to report the curing time and initial solids content for which ultra-rapid freezing is successful.
CONCEPTUAL MODEL OF RESIDUALS FREEZING
The conceptual model builds upon other models, most notably that of Vesilind and Martel (1990). The model presented here dierentiates between mechanisms occurring in the melt (unfrozen residuals), in the frozen residuals and at the interface between the frozen and unfrozen residuals. The accuracy of the model will be demonstrated using results from the literature. Melt Migration is the phenomenon by which ¯ocs in the melt travel ahead of the ice/water interface. If the ¯ocs do not migrate, they become incorporated in the ice and are said to be entrapped. Migration causes changes in the physical and chemical properties of the melt, such as increases in solids content and ionic strength. Theoretical and experimental work on particles or ¯ocs interacting with liquid/ solid interfaces has suggested that higher freezing rates decrease the amount of migration (Bolling and Cisse, 1971; Logsdon and Edgerley, 1971; Halde, 1980; Korber et al., 1985; Azouni et al., 1990; Lipp and Korber, 1993; Hung et al., 1996). Migration, and therefore entrapment, has at least two important implications in the design of residuals freezing processes. First, it is commonly accepted that freezing must be complete to obtain any improvement in dewaterability; therefore, only those ¯ocs which have been entrapped by the ice/ water interface are transformed into zots. Second, if ¯ocs are continuously rejected, the solids content in the melt increases; for a mechanical process where pumping/recycling of the melt might occur, such thickening may be problematic. Preliminary experiments in our lab showed that the rejected solids were not uniformly distributed in the unfrozen residuals. Instead, solids accumulated at the interface, creating a gel layer. This thickened gel layer had a consistency similar to centrifuged residuals. We hypothesize that as the ice/water interface advances into the gel layer, the layer interacts with the interface as a ``continuum'', not as individual ¯ocs. When entrapment does occur, ¯ocs are not entrapped singly, but as a group of several ¯ocs. We term this group an agglomerate. Interface Parker et al. (1996, 1998a) showed that under certain conditions, ¯ocs were fragmented by the ice/ water interface during entrapment. This behavior was most noticeable for the fastest freezing rate employed (200±600 mm/h). At this high rate, dendrites were present at the interface. Dendrites are branched, pointed ``needles'' found in any solid/ liquid interface when growth becomes unstable. We observed the dendrites to shoot through the unfrozen residuals, piercing ¯ocs and preventing migration. Dendrites did not form at lower freezing
Ultra-rapid freezing of residuals
rates; rather, the interface was smooth and ¯ocs were able to migrate. Similar to the conceptual model proposed by Vesilind and Martel (1990), we propose that dendrites may exist in full scale residuals freezing processes and break the ¯ocs into smaller pieces. The conceptual model then helps to explain the rate eects noted in the literature. Any deleterious eects of fast freezing can be attributed in part to dendrites fragmenting the ¯ocs as well as single ¯ocs being entrapped rather than agglomerates. Frozen residuals Once ¯ocs or agglomerates are entrapped, they are transformed into zots. Mechanisms occurring in the frozen residuals have been discussed in the literature (Clements et al., 1950; Katz and Mason, 1970; Logsdon and Edgerley, 1971; Vesilind and Martel, 1990). Below we summarize and expand upon these theories. Entrapped ¯ocs or agglomerates, along with some amount of associated unfrozen water, are physically con®ned in ``pockets'' within the ice. The term ``pocket'' is borrowed from the sea ice literature; in sea ice, brine cannot be incorporated in the ice crystal and is consequently con®ned in brine pockets located on inter- and intra-granular boundaries (Weeks and Ackley, 1982). The pockets of entrapped ¯ocs or agglomerates have very high solute concentrations, and therefore contain unfrozen water. Flocs within the pockets are dehydrated as unfrozen water is withdrawn and added to the surrounding ice. The mechanism may be similar to ``cryosuction'', whereby ice in frozen soils exerts a suction on the unfrozen water below the frozen layer (Williams and Smith, 1989). This dehydration mechanism has been used by some authors to explain their results (Clements et al., 1950; Katz and Mason, 1970; Logsdon and Edgerley, 1971; Vesilind and Martel, 1990).
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A complete discussion of the analyses performed is given in Parker and Collins (1997) and Parker et al. (1998b). Entrapment was quanti®ed by comparing the total solids (TS) content of the melted ice disc to the TS of the original residuals. Floc size was measured using a Mastersizer/E laser particle size analyzer. Zot size was measured by sieve analysis after drying, for ultra-rapidly and slowly frozen (ÿ108C) samples. Dewaterability was quanti®ed before and after freezing by capillary suction time (CST), ®lterability coecient (w), cake solids content (CSC), settleability and aggregate volume index (AVI). CST and w measure the rate of water removal from the sludge. For the CST test, the time needed for water to move from the residuals through a special ®lter paper is measured (see Vesilind (1988) for a thorough discussion of the test). w was developed as a more general form of CST and, by eliminating the eects of various CST apparatuses, can be used to compare the dewaterability of residuals as measured in dierent laboratories (Vesilind, 1988). Low CST's and high values of w indicate a residual from which water readily drains. Vesilind (1988) noted that w could be used as a general descriptor of ®lterability and encouraged the use of w as a method of comparing ®lterabilities of dierent residuals. CSC is the total solids content of the ®lter cake remaining after a vacuum of 38.1 cm Hg is exerted to residuals placed on a Whatman #42 ®lter paper. Settled residual height was measured after 24 h of settling in a 10-ml graduated cylinder. The volume of residuals after drying was also measured and used to compute the AVI. AVI is the ratio between the settled volume before and after drying and is thus a measure of ¯oc water content (Knocke et al., 1987). RESULTS AND DISCUSSION
After freezing at ultra-rapid rates, the conditioned residuals consisted of smaller zots that settled slower than noted in a previous study performed at a lower freezing rate. The zots did not have a granular texture. However, quantitative results demonstrate that ultra-rapid freezing still leads to signi®cant improvements in dewaterability. Figure 1 shows results from the size distribution analyses for thickened (10%) and unthickened (1%) samples frozen at ÿ196 and ÿ108C. The horizontal dashed line represents the mean ¯oc size in the
MATERIALS AND METHODS
Alum residuals at approximately 1% total solids (w/w) from the Potsdam (NY) Water Treatment Facility were frozen directionally in a machined aluminum pan, 15 cm in diameter. A sample volume of 250 ml was used, corresponding to a depth of 1.4 cm. The sides of the pan were insulated with rigid insulation and ¯oated for 2 min on liquid nitrogen (ÿ1968C). The entire sample did not freeze in 2 min; rather, a 3 mm-thick disc of frozen residuals formed on the bottom of the pan. By freezing a thin layer of ice, results could be correlated to a speci®c freezing rate and solids content; the rate of freezing was nearly constant across the ice layer and the variation in solids content was minimized. When freezing was completed, the unfrozen residuals were discarded and the ice disc was cured, melted and analyzed. Curing lasted for either 0, 1, 6 or 24 h at ÿ108C. The initial total solids (TS) contents employed were 1, 3, 5 and 10%. The 3, 5 and 10% solid content samples were obtained by centrifugation. Experiments were performed in triplicate.
Fig. 1. Eect of freezing rate and initial solids content on zot size. The eect of initial solids content and freezing rate on zot size is compared to the size of the unconditioned ¯ocs (dashed line).
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Fig. 2. Entrapment at ultra-rapid freezing rates. Even at ultra-rapid freezing rates, a signi®cant fraction of the ¯ocs are not entrapped in the ice; only at high solids content (10%) are the majority of ¯ocs entrapped in the ice.
unthickened residuals prior to freezing. Each bar on the graph is the average value of two samples analyzed in triplicate, with error bars representing the standard error. (In the following graphs, all error bars represent standard error. For some graphs, error bars were omitted if they cluttered the graph; however, for those graphs, repeatability was similar to the graphs containing error bars).
Three conclusions were drawn from Fig. 1 that were consistent with the conceptual model. First, thickening before freezing at slow or ultra-rapid rates formed zots that were larger than the original ¯ocs. In other words, agglomerates were entrapped rather than single ¯ocs, consistent with the prediction that the gel layer is entrapped as agglomerates. Second, slow freezing produced larger zots than
Fig. 3. CST and w results. The unconditioned residuals (®lled symbols) exhibit poorer ®lterability than the frozen residuals. Moreover, ®lterability improves as curing time increases.
Ultra-rapid freezing of residuals
ultra-rapid freezing, given the same initial solids content. This agrees with the prediction that slow freezing enhances migration, which thickens the residuals, and leads to larger zots. Finally, ultra-rapid freezing of the unthickened residuals created zots that were smaller than the original ¯ocs. This also is in agreement with the conceptual model, which suggests three mechanisms that will make zots formed by ultra-rapid freezing smaller than the original ¯ocs: ¯ocs will be incorporated singly, and not as agglomerates; ¯ocs may be fragmented by the dendrites; and ¯ocs will shrink once entrapped due to dehydration. Figure 2 shows the eect of initial solids content on entrapment. For a solids content of 5% or less, nearly half of the ¯ocs were entrapped, while half of the ¯ocs migrated. At these low initial solids concentrations, the amount of entrapment appeared to be independent of solids content. However, at an initial solids content of 10%, approximately 86% of the solids were entrapped. The large amount of rejection is surprising, since our conceptual model suggests that dendrites will be formed at ultra-rapid rates. The variation in dewaterability as measured by CST is shown in Fig. 3(a). Each symbol represents the mean of three analyses. The ®lled symbols represent samples that have not been frozen. CST could not be measured for the 10% TS sample, since an inadequate amount of water was released by the sample. Figure 3(a) shows that freezing without curing decreased CST by an order of magnitude when compared with the unfrozen samples and that curing caused CST to decrease further. 24 h of curing did not appear to yield appreciably better
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results than those obtained for 6 h of curing. With respect to solids content, the more dilute residuals generally yielded the best dewaterability as measured by CST. The ®lterability coecient (w) results, calculated from the CST values, are shown in Fig. 3(b). Note that high w-values indicate better dewaterability (Vesilind, 1988). As curing time increases, the ®lterability coecient increased to a ``plateau'' at about 6 h of curing. The highest values of w were obtained for the 10% TS samples. A comparison between Fig. 3(a) and (b) shows that w may be more useful than CST in the present study since it emphasizes the eect of solids content. CSC results are plotted in Fig. 4. In contrast to CST and w, CSC measures the fraction of water which can be removed, not the rate of water removal. Again, the ®lled symbols represent the unfrozen samples and the unfrozen 3, 5 and 10% TS samples were closely grouped at 14% CSC. Following ultra-rapid freeze/thaw conditioning, CSC results showed a marked increase for the higher initial TS samples. Except for the 1% TS sample, increasing curing from 1 to 6 h increased CSC. Additional curing to 24 h did not noticeably increase the CSC. The results in Figs 3 and 4 demonstrate an extraordinary improvement in dewaterability as a result of freezing and thawing. The CST values of the frozen and thawed residuals are similar to values for distilled water, which illustrates the ease at which water drains from the residuals. Also, the CSC values are greater than the total solids contents produced by traditional dewatering methods. Moreover, the results are not signi®cantly dierent
Fig. 4. CSC results. Similar to Fig. 3, the bene®cial eects of curing are noted. In addition, it appears that the residuals at the highest solids content initially produce the driest cake.
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from results obtained by Parker et al. (1998b) for a similar residual frozen at much lower rates. Figure 5 shows settleability results. No data are shown for the unfrozen samples since negligible settling occurred. According to Fig. 5(a), a slight decrease in settled volume was noted as curing time increased, given a constant solids content. These results appear to further con®rm the model prediction that ¯oc water is withdrawn during curing, creating denser, smaller zots. The AVI results are shown in Fig. 5(b). According to Knocke et al. (1987), a higher AVI indicates higher ¯oc water content. They reported AVI values of 200±1800 for unfrozen residuals. Figure 5(b) shows AVI's ranging from 2 to 15, indicating a dramatic decrease in ¯oc water content. As curing time increases, AVI decreases as the ¯ocs lose bound water and become smaller. The conceptual model suggests why ultra-rapid freezing is successful at high solids content and long curing times, as shown in Figs 3±5. Although ultrarapid freezing creates smaller zots than are formed at slower freezing rates, dehydration of the entrapped ¯ocs still occurs. Moreover, although
ultra-rapid freezing forms small ice crystals that do not promote dehydration, perhaps while curing, even the smallest ice crystals can exert enough suction over time to draw out inner bound water. Moreover, since the ¯ocs are incorporated as very small pieces during ultra-rapid freezing, the unfrozen bound water can travel on a relatively short path to the surface. However, the improvement noted for un-cured samples of low initial solid content residuals was not expected according to the conceptual model. For such samples, the model states that ¯ocs will have had no opportunity to increase in size prior to entrapment nor to be dehydrated following entrapment. However, this discrepancy may be explained by considering that the time the samples are in the frozen state (2 min on liquid nitrogen plus approximately 15 min of thawing) also acts as curing time. Moreover, this curing is very eective due to liquid nitrogen's extremely low temperature; a very large temperature gradient exists between the ice and entrapped ¯ocs, creating a relatively large suction which removes bound water from the ¯ocs. The eects of curing temperature have previously been
Fig. 5. Eect of ultra-rapid freezing on ®nal settled volume. The volume of solids following 24 h settling is shown in (a). AVI, a measure of water within the ¯ocs, is shown in (b).
Ultra-rapid freezing of residuals
investigated by Vesilind and Martel (1990). They froze two biological sludges at ÿ68C and cured them for 24 h at temperatures ranging from ÿ6 to ÿ308C. The highest w-values were obtained for the samples cured at the coldest temperatures. The results demonstrate that ultra-rapid freezing rates can be successful for alum residuals and should have important implications on the design of mechanical freezing processes. In the past, such processes have been designed without fully understanding the relationship between freezing rate and dewaterability and freezing temperatures have been chosen without understanding the implications on dewaterability (Honda et al., 1981; Khan, 1986; Sutphen, 1988). CONCLUSIONS
. Ultra-rapid freezing on liquid nitrogen successfully conditioned an alum water treatment residual. . The best dewaterability was obtained for samples that were cured for 6 or more hours and that had an initially high solids content. . The results are satisfactorily explained by a conceptual model. At ultra-rapid rates, ¯ocs are either entrapped singly or broken into smaller pieces, rather than being entrapped as agglomerates. Nevertheless, excellent results are obtained because the ¯ocs are still dehydrated while entrapped within the ice. . The results show that in practice, ultra-rapid freezing rates may be employed if they can be achieved economically. AcknowledgementsÐThis research was funded by the American Water Works Association Research Foundation (AWWARF) Project #386. The support of the project manager, Albert Ilges, is gratefully acknowledged. Assistance in performing much of the experimental work by two undergraduate research associates is also acknowledged: Kate Ashley (AWWARF support) and Christopher Burl (NSF support).
REFERENCES
Azouni M. A., Kalita W. and Yemmou M. (1990) On the particle behaviour in front of advancing liquid-ice interface. Journal of Crystal Growth 99, 201±205. Benn D. and Doe P. W. (1969) The disposal of sludge by the freezing and thawing process. Filtration and Separation 6(4), 383±389. Bolling G. F. and Cisse J. (1971) A theory for the interaction of particles with a solidifying front. Journal of Crystal Growth 10, 56±66. Clements G. S., Stephenson R. J. and Regan C. J. (1950) Sludge dewatering by freezing with added chemicals. The Public Works and Municipal Services Congress, London, pp. 318±337.
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Doe P. W., Benn D. and Bays L. R. (1965) The disposal of washwater sludge by freezing. Journal of the Institute of Water Engineers 4(19), 251±291. Halde R. (1980) Concentration of impurities by progressive freezing. Water Research 14(6), 575±580. Honda T., Hiro S. and Yamanishi K. (1981) Automatic operation of a freeze±thawing and dewatering plant for sewage sludges. Water Science and Technology 13, 599± 604. Hung W. T., Chang I. L., Lin W. W. and Lee D. J. (1996) Unidirectional freezing of waste-activated sludges: Eects of freezing speed. Environmental Science and Technology 30(7), 2391±2396. Katz W. J. and Mason D. G. (1970) Freezing methods used to condition activated sludge. Water and Sewage Works 117, 110±114. Kawasaki K., Matsuda A. and Mizukawa Y. (1991) Compression characteristics of excess activated sludges treated by freezing-and-thawing process. Journal of Chemical Engineering of Japan 24(6), 743±748. Khan M. Z. A. (1986) Conditioning of pulp and paper sludge using direct slurry freezing. 41st Industrial Waste Conference, West Lafayette, IN, pp. 429±435. Knocke W. R., Hamon J. R. and Dulin B. E. (1987) Eects of coagulation on sludge thickening and dewatering. Journal of the American Water Works Association 79(6), 89±98. Korber C., Rau G., Cosman M. D. and Cravalho E. G. (1985) Interaction of particles and a moving ice±liquid interface. Journal of Crystal Growth 72, 649±662. Lee D.-J. (1994) Measurement of bound water in waste activated sludge: Use of the centrifugal settling method. Journal of Chemical Technology and Biotechnology 61, 139±144. Lee D. J. and Hsu Y. H. (1994) Fast freeze/thaw treatment on excess activated sludges: Floc structure and sludge dewaterability. Environmental Science and Technology 28(8), 1444±1449. Lee D. J. and Hsu Y. H. (1995) Measurement of bound water in sludges. Water Environment Research 67(3), 310±317. Lee D.-J. and Lee S. F. (1995) Measurement of bound water content in sludge: The use of dierential scanning calorimetry (DSC). Journal of Chemical Technology and Biotechnology 62, 359±365. Lipp G. and Korber C. (1993) On the engulfment of spherical particles by a moving ice±liquid interface. Journal of Crystal Growth 130, 475±489. Logsdon G. S. and Edgerley E. (1971) Sludge dewatering by freezing. Journal American Water Works Association 63(11), 734±740. Luyet B. (1968a) Basic physical phenomena in the freezing and thawing of animal and plant tissues. In The Freezing Preservation of Foods, eds. D. K. Tressler, W. B. V. Arsdel and M. J. Copley, pp. 1±25. The AVI Publishing Company, Westport, CT. Luyet B. J. (1968b) The formation of ice and the physical behaviour of the ice phase in aqueous solutions and in biological systems. In Low Temperature Biology of Foodstus, eds. J. Hawthorn and E. J. Rolfe, p. 458. Pergamon Press, Oxford. Martel C. J. (1988) Development and Design of Sludge Freezing Beds. 88-20, US Army Cold Regions Research and Engineering Laboratory, Hanover, NH. Parker P. J. and Collins A. G. (1997) Feasibility study on freeze/thaw conditioning of a pulp mill waste activated sludge. Journal of Cold Regions Engineering (ASCE) 11(3), 245±250. Parker P. J., Collins A. G. and Dempsey J. P. (1996) Incorporation and rejection of alum sludge ¯ocs by an advancing freezing front. 8th International Cold Regions Conference, Fairbanks, AK, pp. 757±767.
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Parker P. J., Collins A. G. and Dempsey J. P. (1998a) Alum residual ¯oc interactions with an advancing ice/ water interface. Journal of Environmental Engineering 124(3), 249±253. Parker P. J., Collins A. G. and Dempsey J. P. (1998b) Signi®cance of freezing rate, solids content, and curing time in freeze/thaw conditioning of water treatment residuals. Environmental Science and Technology 32(3), 383±387. Robinson J. and Knocke W. R. (1992) Use of dilatometric and drying techniques for assessing sludge dewatering characteristics. Water Environment Research 64(1), 60± 68. Smith J. K. (1992). The use of dilatometry to measure the characteristics of water in sludge. M.Sc., Duke University, Durham. Sutphen R. L. (1988) Process and Systems for Dewatering Metal Oxide Sludge. Specialized Process Equipment, U.S.A. Vesilind P. A. (1988) Capillary suction time as a fundamental measure of sludge dewaterability. Journal of WPCF 60(2), 215±220.
Vesilind P. A. and Martel C. J. (1990) Freezing of water and wastewater sludges. Journal of Environmental Engineering 116(5), 854±862. Vol'khin V. V. and Zolotavin V. L. (1961) The eect of freezing on the properties of metal hydroxide coagulates. Part 2: Eect of electrolytes on changes of the volume of ferric hydroxide coagulate as a result of freezing. Colloid Journal of the USSR 23(2), 113±117. Weeks W. F. and Ackley S. F. (1982) The Growth, Structure and Properties of Sea Ice. 82-1, US Army Cold Regions Research and Engineering Laboratory. Williams P. J. and Smith M. W. (1989) The Frozen Earth. Cambridge University Press, Cambridge. Zolotavin V. L., Vol'khin V. V. and Rezvushkin V. V. (1960) Eect of freezing on the properties of coagulated metal hydroxides of coagulated ferric hydroxide. Part 1: In¯uence of freezing and thawing conditions on the properties of coagulated ferric hydroxide. Colloid Journal of the USSR 22(3), 317±324.
Wat. Res. Vol. 33, No. 10, pp. 2239±2246, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
ULTRA-RAPID FREEZING OF WATER TREATMENT RESIDUALS M PHILIP J. PARKER1* and ANTHONY G. COLLINS2*
Department of Civil and Environmental Engineering, University of Wisconsin-Platteville, 137 Ottensman Hall, 1 University Plaza, Platteville, WI 53818, U.S.A. and 2Clarkson University, P.O. Box 5700, Potsdam, NY 13699-5700, U.S.A. 1
(First received July 1998; accepted in revised form October 1998) AbstractÐAn alum water treatment residual (``sludge'') was conditioned by freezing at ultra-rapid rates. Ultra-rapid rates were obtained by freezing on liquid nitrogen. The purpose of the experiments was to determine the conditions necessary for such rapid freezing to improve dewaterability. Contrary to results reported in the literature, ultra-rapid freezing worked extremely well. Although the conditioned ¯ocs (``zots'') were not as dense or granular as obtained at slower freezing rates, dewaterability was still greatly improved. Dewaterability, as measured by cake solids content (CSC), capillary suction time (CST), ®lterability coecient (w) and aggregate volume index (AVI), was best for samples with an initially high solids content and for samples that had been cured or stored at sub-freezing temperatures, for long times. Zot size analyses showed that zots decreased in size following ultra-rapid freezing. The results are explained using a proposed conceptual model of the residuals freezing process. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐfreeze/thaw conditioning, water treatment residuals, sludge, dewatering, liquid nitrogen, ultra-rapid freezing rates
INTRODUCTION
Water treatment sludge, or ``residuals,'' is composed of ¯ocs and water. The goal of sludge dewatering is the removal of water from residuals in order to minimize disposal costs. Water in residuals may be categorized as free water and bound water: free water is located between ¯ocs and may be removed mechanically; bound water is associated with the ¯ocs and may be trapped in ¯oc interstices, adsorbed to inter-particle surfaces, or exist as water of hydration. Most dewatering methods are limited in the sense that they only remove free water. However, bound water contents are signi®cant: 2 to 12 g/g-DS (dry solids) have been reported for a variety of water and wastewater sludges measured by several dierent methods (Lee, 1994; Lee and Hsu, 1995; Lee and Lee, 1995; Robinson and Knocke, 1992; Smith, 1992). Freeze/thaw conditioning transforms bound water into free water that can be removed mechanically. Frozen and thawed ¯ocs (termed ``zots'') are often larger than unfrozen ¯ocs and have a ``gritty'' consistency. The term ``zot'' was ®rst used by Russian researchers (Zolotavin et al., 1960), who recognized that the properties of *Author to whom all correspondence should be addressed. [Tel.: +1-608-3421235; fax: +1-608-3421566; e-mail: [email protected]].
chemical precipitates were drastically altered by freezing and thawing, thus warranting a separate term. The freezing rate reportedly has important consequences on the performance of the freeze/thaw process. Slow freezing has generally been reported to yield better dewaterability than fast freezing. For instance, Vesilind and Martel (1990) noted that the freezing rate had a dramatic eect on various biological sludges, with the best dewaterability noted for the most slowly frozen samples. Kawasaki et al. (1991) stated that ®lterability of waste activated sludge (WAS) was better for samples frozen at 1.9 mm/h than those frozen at 9.1 mm/h. Fast freezing, which refers to freezing rates of approximately 40 mm/h, led to a slight improvement in WAS dewaterability according to Lee and Hsu (1994). However, ¯ash freezing, on dry ice (ÿ788C) or liquid nitrogen (ÿ1968C), reportedly does not lead to any improvements and researchers have noted that ¯ocs retain their ``colloidal'' nature following ¯ash freezing (Clements et al., 1950; Vol'khin and Zolotavin, 1961; Doe et al., 1965; Benn and Doe, 1969; Katz and Mason, 1970; Logsdon and Edgerley, 1971; Martel, 1988). In the remainder of this paper, the term ``ultra-rapid freezing'' will be used instead of ``¯ash freezing''. The term is borrowed from the frozen food literature, where it has been used to describe food that is frozen on liquid nitrogen (Luyet, 1968a).
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Katz and Mason (1970) and Logsdon and Edgerley (1971) speculated on the causes of the failure of ultra-rapid freezing. They theorized that freeze/thaw conditioning at low rates is successful because water is released by the ¯ocs when they are entrapped in the ice. Moreover, the rate of dehydration depends on the temperature gradient between the ¯oc and the surrounding ice. Katz and Mason speculated that ultra-rapid freezing inhibits dehydration because the freezing rate is faster than the rate at which water can diuse from the inner portions of the ¯ocs to the surface. At ultra-rapid rates, very small crystals form on the surface and within the ¯oc; at lower rates, the ice crystals are much larger and grow slowly such that water can diuse from the interstices of the ¯oc. The same reasoning applies to freezing food; fast rates are desired so that water remains incorporated in the tissue. According to Luyet (1968b), ice formed at lower cooling rates exhibited well-formed crystals; as the cooling rate increased to ultra-rapid rates, ice needles formed with increasingly small radius (on the order of angstroms). Although experimental results and plausible theories have established the failure of ultra-rapid freezing, some reported results suggest that ultrarapid freezing may be successful under certain freezing conditions. Parker et al. (1998b) froze samples of alum residuals at four dierent rates. The fastest freezing, 37.5 mm/h, was achieved by freezing the residuals directionally on a cold surface at ÿ308C for 15 min. After being immediately thawed, the dewaterability was only slightly better than the unfrozen residuals and was poorer than the dewaterability of the more slowly frozen samples. This suggested that dewaterability would become worse as freezing rates increased further, in agreement with the consensus in the literature. However, Parker et al. also noted that when the residuals frozen at 37.5 mm/h were cured (stored at subfreezing temperatures), dewaterability improved drastically. In fact, for 6 h of curing or more, dewaterability of the most rapidly frozen residuals did not vary signi®cantly from the dewaterability of more slowly frozen residuals. Moreover, the reports of the failure of ultrarapid freezing have been based on results from biological sludges. However, freezing and thawing is generally less eective for biological sludges than for chemical sludges at non-ultra-rapid rates: the zots formed from biological sludges are less granular and tend to revert more readily to a ¯occulant nature. Therefore, it seems plausible that ultra-rapid freezing might be more eective for chemical sludges. This paper's objectives are to report the curing time and initial solids content for which ultra-rapid freezing is successful.
CONCEPTUAL MODEL OF RESIDUALS FREEZING
The conceptual model builds upon other models, most notably that of Vesilind and Martel (1990). The model presented here dierentiates between mechanisms occurring in the melt (unfrozen residuals), in the frozen residuals and at the interface between the frozen and unfrozen residuals. The accuracy of the model will be demonstrated using results from the literature. Melt Migration is the phenomenon by which ¯ocs in the melt travel ahead of the ice/water interface. If the ¯ocs do not migrate, they become incorporated in the ice and are said to be entrapped. Migration causes changes in the physical and chemical properties of the melt, such as increases in solids content and ionic strength. Theoretical and experimental work on particles or ¯ocs interacting with liquid/ solid interfaces has suggested that higher freezing rates decrease the amount of migration (Bolling and Cisse, 1971; Logsdon and Edgerley, 1971; Halde, 1980; Korber et al., 1985; Azouni et al., 1990; Lipp and Korber, 1993; Hung et al., 1996). Migration, and therefore entrapment, has at least two important implications in the design of residuals freezing processes. First, it is commonly accepted that freezing must be complete to obtain any improvement in dewaterability; therefore, only those ¯ocs which have been entrapped by the ice/ water interface are transformed into zots. Second, if ¯ocs are continuously rejected, the solids content in the melt increases; for a mechanical process where pumping/recycling of the melt might occur, such thickening may be problematic. Preliminary experiments in our lab showed that the rejected solids were not uniformly distributed in the unfrozen residuals. Instead, solids accumulated at the interface, creating a gel layer. This thickened gel layer had a consistency similar to centrifuged residuals. We hypothesize that as the ice/water interface advances into the gel layer, the layer interacts with the interface as a ``continuum'', not as individual ¯ocs. When entrapment does occur, ¯ocs are not entrapped singly, but as a group of several ¯ocs. We term this group an agglomerate. Interface Parker et al. (1996, 1998a) showed that under certain conditions, ¯ocs were fragmented by the ice/ water interface during entrapment. This behavior was most noticeable for the fastest freezing rate employed (200±600 mm/h). At this high rate, dendrites were present at the interface. Dendrites are branched, pointed ``needles'' found in any solid/ liquid interface when growth becomes unstable. We observed the dendrites to shoot through the unfrozen residuals, piercing ¯ocs and preventing migration. Dendrites did not form at lower freezing
Ultra-rapid freezing of residuals
rates; rather, the interface was smooth and ¯ocs were able to migrate. Similar to the conceptual model proposed by Vesilind and Martel (1990), we propose that dendrites may exist in full scale residuals freezing processes and break the ¯ocs into smaller pieces. The conceptual model then helps to explain the rate eects noted in the literature. Any deleterious eects of fast freezing can be attributed in part to dendrites fragmenting the ¯ocs as well as single ¯ocs being entrapped rather than agglomerates. Frozen residuals Once ¯ocs or agglomerates are entrapped, they are transformed into zots. Mechanisms occurring in the frozen residuals have been discussed in the literature (Clements et al., 1950; Katz and Mason, 1970; Logsdon and Edgerley, 1971; Vesilind and Martel, 1990). Below we summarize and expand upon these theories. Entrapped ¯ocs or agglomerates, along with some amount of associated unfrozen water, are physically con®ned in ``pockets'' within the ice. The term ``pocket'' is borrowed from the sea ice literature; in sea ice, brine cannot be incorporated in the ice crystal and is consequently con®ned in brine pockets located on inter- and intra-granular boundaries (Weeks and Ackley, 1982). The pockets of entrapped ¯ocs or agglomerates have very high solute concentrations, and therefore contain unfrozen water. Flocs within the pockets are dehydrated as unfrozen water is withdrawn and added to the surrounding ice. The mechanism may be similar to ``cryosuction'', whereby ice in frozen soils exerts a suction on the unfrozen water below the frozen layer (Williams and Smith, 1989). This dehydration mechanism has been used by some authors to explain their results (Clements et al., 1950; Katz and Mason, 1970; Logsdon and Edgerley, 1971; Vesilind and Martel, 1990).
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A complete discussion of the analyses performed is given in Parker and Collins (1997) and Parker et al. (1998b). Entrapment was quanti®ed by comparing the total solids (TS) content of the melted ice disc to the TS of the original residuals. Floc size was measured using a Mastersizer/E laser particle size analyzer. Zot size was measured by sieve analysis after drying, for ultra-rapidly and slowly frozen (ÿ108C) samples. Dewaterability was quanti®ed before and after freezing by capillary suction time (CST), ®lterability coecient (w), cake solids content (CSC), settleability and aggregate volume index (AVI). CST and w measure the rate of water removal from the sludge. For the CST test, the time needed for water to move from the residuals through a special ®lter paper is measured (see Vesilind (1988) for a thorough discussion of the test). w was developed as a more general form of CST and, by eliminating the eects of various CST apparatuses, can be used to compare the dewaterability of residuals as measured in dierent laboratories (Vesilind, 1988). Low CST's and high values of w indicate a residual from which water readily drains. Vesilind (1988) noted that w could be used as a general descriptor of ®lterability and encouraged the use of w as a method of comparing ®lterabilities of dierent residuals. CSC is the total solids content of the ®lter cake remaining after a vacuum of 38.1 cm Hg is exerted to residuals placed on a Whatman #42 ®lter paper. Settled residual height was measured after 24 h of settling in a 10-ml graduated cylinder. The volume of residuals after drying was also measured and used to compute the AVI. AVI is the ratio between the settled volume before and after drying and is thus a measure of ¯oc water content (Knocke et al., 1987). RESULTS AND DISCUSSION
After freezing at ultra-rapid rates, the conditioned residuals consisted of smaller zots that settled slower than noted in a previous study performed at a lower freezing rate. The zots did not have a granular texture. However, quantitative results demonstrate that ultra-rapid freezing still leads to signi®cant improvements in dewaterability. Figure 1 shows results from the size distribution analyses for thickened (10%) and unthickened (1%) samples frozen at ÿ196 and ÿ108C. The horizontal dashed line represents the mean ¯oc size in the
MATERIALS AND METHODS
Alum residuals at approximately 1% total solids (w/w) from the Potsdam (NY) Water Treatment Facility were frozen directionally in a machined aluminum pan, 15 cm in diameter. A sample volume of 250 ml was used, corresponding to a depth of 1.4 cm. The sides of the pan were insulated with rigid insulation and ¯oated for 2 min on liquid nitrogen (ÿ1968C). The entire sample did not freeze in 2 min; rather, a 3 mm-thick disc of frozen residuals formed on the bottom of the pan. By freezing a thin layer of ice, results could be correlated to a speci®c freezing rate and solids content; the rate of freezing was nearly constant across the ice layer and the variation in solids content was minimized. When freezing was completed, the unfrozen residuals were discarded and the ice disc was cured, melted and analyzed. Curing lasted for either 0, 1, 6 or 24 h at ÿ108C. The initial total solids (TS) contents employed were 1, 3, 5 and 10%. The 3, 5 and 10% solid content samples were obtained by centrifugation. Experiments were performed in triplicate.
Fig. 1. Eect of freezing rate and initial solids content on zot size. The eect of initial solids content and freezing rate on zot size is compared to the size of the unconditioned ¯ocs (dashed line).
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Fig. 2. Entrapment at ultra-rapid freezing rates. Even at ultra-rapid freezing rates, a signi®cant fraction of the ¯ocs are not entrapped in the ice; only at high solids content (10%) are the majority of ¯ocs entrapped in the ice.
unthickened residuals prior to freezing. Each bar on the graph is the average value of two samples analyzed in triplicate, with error bars representing the standard error. (In the following graphs, all error bars represent standard error. For some graphs, error bars were omitted if they cluttered the graph; however, for those graphs, repeatability was similar to the graphs containing error bars).
Three conclusions were drawn from Fig. 1 that were consistent with the conceptual model. First, thickening before freezing at slow or ultra-rapid rates formed zots that were larger than the original ¯ocs. In other words, agglomerates were entrapped rather than single ¯ocs, consistent with the prediction that the gel layer is entrapped as agglomerates. Second, slow freezing produced larger zots than
Fig. 3. CST and w results. The unconditioned residuals (®lled symbols) exhibit poorer ®lterability than the frozen residuals. Moreover, ®lterability improves as curing time increases.
Ultra-rapid freezing of residuals
ultra-rapid freezing, given the same initial solids content. This agrees with the prediction that slow freezing enhances migration, which thickens the residuals, and leads to larger zots. Finally, ultra-rapid freezing of the unthickened residuals created zots that were smaller than the original ¯ocs. This also is in agreement with the conceptual model, which suggests three mechanisms that will make zots formed by ultra-rapid freezing smaller than the original ¯ocs: ¯ocs will be incorporated singly, and not as agglomerates; ¯ocs may be fragmented by the dendrites; and ¯ocs will shrink once entrapped due to dehydration. Figure 2 shows the eect of initial solids content on entrapment. For a solids content of 5% or less, nearly half of the ¯ocs were entrapped, while half of the ¯ocs migrated. At these low initial solids concentrations, the amount of entrapment appeared to be independent of solids content. However, at an initial solids content of 10%, approximately 86% of the solids were entrapped. The large amount of rejection is surprising, since our conceptual model suggests that dendrites will be formed at ultra-rapid rates. The variation in dewaterability as measured by CST is shown in Fig. 3(a). Each symbol represents the mean of three analyses. The ®lled symbols represent samples that have not been frozen. CST could not be measured for the 10% TS sample, since an inadequate amount of water was released by the sample. Figure 3(a) shows that freezing without curing decreased CST by an order of magnitude when compared with the unfrozen samples and that curing caused CST to decrease further. 24 h of curing did not appear to yield appreciably better
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results than those obtained for 6 h of curing. With respect to solids content, the more dilute residuals generally yielded the best dewaterability as measured by CST. The ®lterability coecient (w) results, calculated from the CST values, are shown in Fig. 3(b). Note that high w-values indicate better dewaterability (Vesilind, 1988). As curing time increases, the ®lterability coecient increased to a ``plateau'' at about 6 h of curing. The highest values of w were obtained for the 10% TS samples. A comparison between Fig. 3(a) and (b) shows that w may be more useful than CST in the present study since it emphasizes the eect of solids content. CSC results are plotted in Fig. 4. In contrast to CST and w, CSC measures the fraction of water which can be removed, not the rate of water removal. Again, the ®lled symbols represent the unfrozen samples and the unfrozen 3, 5 and 10% TS samples were closely grouped at 14% CSC. Following ultra-rapid freeze/thaw conditioning, CSC results showed a marked increase for the higher initial TS samples. Except for the 1% TS sample, increasing curing from 1 to 6 h increased CSC. Additional curing to 24 h did not noticeably increase the CSC. The results in Figs 3 and 4 demonstrate an extraordinary improvement in dewaterability as a result of freezing and thawing. The CST values of the frozen and thawed residuals are similar to values for distilled water, which illustrates the ease at which water drains from the residuals. Also, the CSC values are greater than the total solids contents produced by traditional dewatering methods. Moreover, the results are not signi®cantly dierent
Fig. 4. CSC results. Similar to Fig. 3, the bene®cial eects of curing are noted. In addition, it appears that the residuals at the highest solids content initially produce the driest cake.
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Philip J. Parker and Anthony G. Collins
from results obtained by Parker et al. (1998b) for a similar residual frozen at much lower rates. Figure 5 shows settleability results. No data are shown for the unfrozen samples since negligible settling occurred. According to Fig. 5(a), a slight decrease in settled volume was noted as curing time increased, given a constant solids content. These results appear to further con®rm the model prediction that ¯oc water is withdrawn during curing, creating denser, smaller zots. The AVI results are shown in Fig. 5(b). According to Knocke et al. (1987), a higher AVI indicates higher ¯oc water content. They reported AVI values of 200±1800 for unfrozen residuals. Figure 5(b) shows AVI's ranging from 2 to 15, indicating a dramatic decrease in ¯oc water content. As curing time increases, AVI decreases as the ¯ocs lose bound water and become smaller. The conceptual model suggests why ultra-rapid freezing is successful at high solids content and long curing times, as shown in Figs 3±5. Although ultrarapid freezing creates smaller zots than are formed at slower freezing rates, dehydration of the entrapped ¯ocs still occurs. Moreover, although
ultra-rapid freezing forms small ice crystals that do not promote dehydration, perhaps while curing, even the smallest ice crystals can exert enough suction over time to draw out inner bound water. Moreover, since the ¯ocs are incorporated as very small pieces during ultra-rapid freezing, the unfrozen bound water can travel on a relatively short path to the surface. However, the improvement noted for un-cured samples of low initial solid content residuals was not expected according to the conceptual model. For such samples, the model states that ¯ocs will have had no opportunity to increase in size prior to entrapment nor to be dehydrated following entrapment. However, this discrepancy may be explained by considering that the time the samples are in the frozen state (2 min on liquid nitrogen plus approximately 15 min of thawing) also acts as curing time. Moreover, this curing is very eective due to liquid nitrogen's extremely low temperature; a very large temperature gradient exists between the ice and entrapped ¯ocs, creating a relatively large suction which removes bound water from the ¯ocs. The eects of curing temperature have previously been
Fig. 5. Eect of ultra-rapid freezing on ®nal settled volume. The volume of solids following 24 h settling is shown in (a). AVI, a measure of water within the ¯ocs, is shown in (b).
Ultra-rapid freezing of residuals
investigated by Vesilind and Martel (1990). They froze two biological sludges at ÿ68C and cured them for 24 h at temperatures ranging from ÿ6 to ÿ308C. The highest w-values were obtained for the samples cured at the coldest temperatures. The results demonstrate that ultra-rapid freezing rates can be successful for alum residuals and should have important implications on the design of mechanical freezing processes. In the past, such processes have been designed without fully understanding the relationship between freezing rate and dewaterability and freezing temperatures have been chosen without understanding the implications on dewaterability (Honda et al., 1981; Khan, 1986; Sutphen, 1988). CONCLUSIONS
. Ultra-rapid freezing on liquid nitrogen successfully conditioned an alum water treatment residual. . The best dewaterability was obtained for samples that were cured for 6 or more hours and that had an initially high solids content. . The results are satisfactorily explained by a conceptual model. At ultra-rapid rates, ¯ocs are either entrapped singly or broken into smaller pieces, rather than being entrapped as agglomerates. Nevertheless, excellent results are obtained because the ¯ocs are still dehydrated while entrapped within the ice. . The results show that in practice, ultra-rapid freezing rates may be employed if they can be achieved economically. AcknowledgementsÐThis research was funded by the American Water Works Association Research Foundation (AWWARF) Project #386. The support of the project manager, Albert Ilges, is gratefully acknowledged. Assistance in performing much of the experimental work by two undergraduate research associates is also acknowledged: Kate Ashley (AWWARF support) and Christopher Burl (NSF support).
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