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Effect of dissolved organic material and cations on freeze-thaw conditioning of activated and alum sludges
PII: S0043-1354(01)00174-9
Wat. Res. Vol. 35, No. 18, pp. 4299–4306, 2001 r 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
EFFECT OF DISSOLVED ORGANIC MATERIAL AND CATIONS ON FREEZE-THAW CONDITIONING OF ACTIVATED AND ALUM SLUDGES BANU O¨RMECI1* and P. AARNE VESILIND2 1
Department of Civil and Environmental Engineering, Duke University, P.O. Box 90287, Durham, NC 27706-0287, USA and 2 Department of Civil and Environmental Engineering, Bucknell University, Lewisburg, PA 17837, USA (First received 1 August 2000; accepted in revised form 1 March 2001)
AbstractFFreeze-thaw conditioning effectively dewaters alum and activated sludges, but it works better on alum sludge than it does on activated sludge. The main difference between alum sludge and activated sludge is that activated sludge has high concentrations of both dissolved organic material and ions. Dissolved organic material and ions may possibly alter the freezing process and decrease the effectiveness of freeze-thaw conditioning on activated sludge. The objective of this study is to investigate the effect of dissolved organic material and cations on freeze-thaw conditioning of sludges, and to improve the effectiveness of freeze-thaw conditioning on activated sludge. The results of this study show that although protein, carbohydrate and cation concentrations in activated sludge supernatant are initially high, they dramatically increase after freeze-thaw conditioning. The increase is likely to come from the release of extracellular and intracellular material to sludge supernatant. The observed increase in the DNA concentration in activated sludge supernatant after freeze-thaw conditioning indicates that freeze-thaw causes cell disruption. Alum sludge supernatant, on the other hand, initially contains low concentrations of proteins, carbohydrates and cations which do not noticeably change after freeze-thaw conditioning. When ECPs (extracellular polymers) and cations are extracted from activated sludge before freeze-thaw conditioning, the sludge settles and dewaters better after the freeze-thaw. The resulting aggregates are smaller and denser resembling the ‘‘coffee ground’’ aggregates of alum sludge. r 2001 Elsevier Science Ltd. All rights reserved Key wordsFsludge, dewatering, freeze-thaw, organic material, extracellular polymers, cations
INTRODUCTION
Freeze-thaw conditioning prior to sludge dewatering is becoming increasingly popular due to the improvements in the design and efficiency of the facilities (Hellstrom and Kvarnstrom, 1997; Martel, 1998). Some of the advantages of freeze-thaw conditioning are its simplicity, low-cost at moderate to cold climates, and effectiveness with certain types of sludges. Freeze-thaw conditioning works amazingly well on alum sludge. It dramatically converts alum sludge from a fine particle suspension to a mixture of clear water and granular particles. The granular particles resemble coffee grounds in both size and appearance, and they do not break apart upon vigorous mixing. On the other hand, freeze-thaw conditioning is not as effective for activated sludge. After freeze-thaw conditioning, sludge flocs turn into
*Author to whom all correspondence should be addressed. Tel.: +1-919-6605-200; fax: +1-919-6605-219; e-mail: [email protected]
fluffy and watery aggregates which disintegrate upon agitation. The improvements achieved through freeze-thaw conditioning may be temporary and reversible. The main difference between alum sludge and activated sludge is the presence of various organic material, dissolved ions, and microorganisms in the latter one. It is possible that the high organic material and ion concentrations in activated sludge, which are not present in alum sludge, interfere with the freezing process and decrease the freeze-thaw effectiveness. If these components are withdrawn from activated sludge using extraction techniques, it might be possible to improve the effectiveness of freeze-thaw conditioning on activated sludge, and bring it closer to that of alum sludge. The objective of this study is to gain a better understanding of the effects of dissolved organic material and cations on freeze-thaw conditioning of activated and alum sludges, and improve the effectiveness of freeze-thaw conditioning on activated sludge by extracting extracellular polymers and cations together with other dissolved material from sludge matrix.
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Banu O¨rmeci and P. Aarne Vesilind
4300 LITERATURE REVIEW
Freeze-thaw technology works on the principle that ice crystals grow by incorporating water molecules only. Because the structure of ice crystal is highly organized and symmetrical, it cannot accommodate any other atoms or molecules. Each ice crystal continues to grow as long as water molecules are available. All other impurities and solid particles are forced to the boundaries of the ice crystal where they become compressed or dehydrated (Chalmers, 1959). Clements et al. (1950) reported that to obtain any conditioning action by freeze-thaw, the sludge must be completely frozen at a relatively slow rate. Flash or fast freezing produces little or no conditioning action since the slow diffusion of water molecules through the interface results in the capture of solids in the ice matrix. Halde (1980) reported that when freezing rate is slow enough, particles of nearly all materials are rejected by the moving ice–water interface, and dewaterability of freeze-thawed sludge is greatly improved. Nevertheless, when the freezing rate is high, particles in the solution eventually are trapped in the developing ice layer resulting in little improvement in sludge dewaterability. Particle size is another factor that affects the freezing process. Corte (1962) showed that fine particles migrate under a wide range of freezing rates and coarse particles migrate only at low freezing rates. This means large particles are more likely to be trapped in the advancing ice front compared to the smaller particles. Corte (1962) argued that a layer of water must be continuously present between a particle and the ice front. Hoekstra and Miller (1967) named this layer of water surrounding the particle a ‘‘transition layer’’. Transition layer is more easily replenished when particles are small (Vesilind et al., 1991). Hoekstra and Miller (1967) suggested that transition layer freezes later than free water because of the accumulation of dissolved ions, which are rejected by the ice front, in the transition layer. The high ion concentration in the transition layer depresses the freezing point, and delays the freezing of the transition layer. Kawasaki and Matsuda (1995) examined the effect of sodium chloride on sludge dewaterability, settleability, and residual moisture, and concluded that sodium chloride addition retarded the rejection of sludge particles by the ice–water interface. Similar results were also reported by Chu et al. (1997). The difference between the morphology of the ice/ water interfaces of alum sludge and activated sludge may be one of the factors that cause the difference in their freeze-thaw conditioning. Martel (2000) stated that alum sludge ice crystals grow in columns and activated sludge ice crystals grow in branching treelike structures called dendrites. When dendrites are formed at the ice/water interface, solid particles are trapped in the advancing ice front, and sludge does
not freeze well. Martel attributed the formation of dendrities to the presence of dissolved solids in activated sludge. He also showed that addition of dissolved solids, such as NaCl, to alum sludge can change the ice crystal growth from columnar to dendritic. Freeze-thaw conditioning has probably found its best application on alum sludge. Baskerville (1971) stated that upon freeze-thaw conditioning, alum sludge is remarkably stable to stirring, with capillary suction time (CST) being almost constant irrespective of the stirring period. Martel and Diener (1991) observed that freeze-thaw conditioning dramatically converts alum sludge from a fine particle suspension to a mixture of clear water and granular particles. The granular particles resemble coffee grounds in both size and appearance, and they do not break apart even after vigorous agitation. Martel and Diener achieved a 96% reduction in volume simply by placing the mixture on a porous medium and draining away the water. The dewaterability of the activated sludge is also significantly improved by freeze-thaw conditioning. After a freeze-thaw treatment, the bound water content of the sludge cake, the floc volume and the sludge compressibility decreases, and the sludge filterability increases (Lee and Hsu, 1994, 1995; Chen et al., 1996; Parker et al., 1998). However, unlike the case in alum sludge the aggregates are not as small, dense and strong, and they are not very resistant to agitation. Baskerville (1971) observed an increase in capillary suction time (CST) of the freeze-thawed activated sludge which indicated a deterioration in sludge filterability with stirring. Some researchers observed a change in the quality of activated sludge supernatant after freeze-thaw conditioning. Randall et al. (1975) and Hong et al. (1995) reported that both BOD and COD values of an activated sludge supernatant increased significantly after freeze-thaw conditioning. Hung et al. (1996b) suggested that the action of ice front may release some of the ECPs from sludge body to sludge supernatant. Activated sludge flocs have a complex structure compared to alum sludge flocs. ECPs and divalent cations are two important components of the activated sludge flocs. ECPs produced by sludge bacteria are involved in the formation of a threedimensional matrix where divalent cations, particularly Ca+2 and Mg+2, act as bridging agents with probably specific affinities for each kind of extracellular polymer (Urbain et al., 1993). Virtually all possible sugars, organic acids, nucleic acids, proteins, and lipids have been found as components of ECPs from various pure and mixed cultures (Gehr and Henry, 1983). Extraction of ECPs results in deflocculation and a decrease in average particle size (Eriksson and Alm, 1991; Bruus et al., 1992). Although the effects of divalent cations on activated sludge characteristics have been known and studied for some time (Eriksson and Alm, 1991; Bruus et al.,
Effect of dissolved organic material and cations on freeze-thaw conditioning
1992; Keiding and Nielsen, 1997), the importance of monovalent (Bruus et al., 1992; Chu et al., 1997; Higgins and Novak, 1997) and trivalent cations (Nielsen and Keiding, 1998) on floc structure is just beginning to surface. It is hypothesized in this study that the presence of dissolved organic material and ions in activated sludge adversely affects its freeze-thaw conditioning. It may be possible to improve the freeze-thaw conditioning of activated sludge by extracting ECPs and cations from floc structure, and removing other impurities from sludge supernatant. Removal of these components is likely to increase the effectiveness of freeze-thaw conditioning by reducing particle entrapment during freezing.
MATERIALS AND METHODS
The activated sludge used in this research was collected from North Durham Wastewater Treatment Facility, Durham, NC. The sludge was taken to the laboratory within 30 min, and was kept aerated throughout the experiments. In order to avoid differences between samples that may come from ongoing physical, biological, and chemical changes in sludge, all experiments in a particular group were finished on the same day. The alum sludge was
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collected from Brown Water Treatment Facility, Durham, NC. A freezer (ScienTemp Corp., MI) with a digital setting was used to freeze the sludge samples. The volume of all samples was 100 ml. Sludge samples were frozen in 130 ml transparent glass bottles which enabled visual examination of aggregates after freeze-thaw conditioning. The preliminary experiments showed that activated sludge froze completely at @81C, and therefore @81C was used to freeze the samples. All samples were kept frozen for 36 h in the freezer, and thawed for 10 h at room temperature. Preliminary experiments showed that thawing times longer than 12 h started undesired anaerobic reactions in sludge and changed the sludge characteristics. ECPs and ions were removed from activated sludge by using a physical extraction method suggested by Gehr and Henry (1983). This method is a combination of centrifugation and blending, and it was repeated 5 times to extract as much ECPs and ions as possible from the samples. The ECP contents of the samples were quantified through determining the carbohydrate and protein concentrations by colorimetric methods. Dye-binding method (Bradford, 1976) was used to determine the protein concentration, and Anthrone method (Dreywood, 1946) was used to determine the carbohydrate concentration in samples. Cysteine method (Ashwell, 1957) was used to estimate the DNA content of sludge bulk water. Specific resistance to filtration (SRF) test with the omission of vacuum was used to measure the dewaterability of freeze-thaw conditioned sludges. Although many researchers have been using capillary suction time (CST) test
Fig. 1. The difference between the protein and carbohydrate concentrations (a) and cation concentrations (b) in activated sludge before and after freeze-thaw conditioning.
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Fig. 2. The difference between the protein and carbohydrate concentrations (a) and cation concentrations (b) in alum sludge before and after freeze-thaw conditioning.
to measure the dewaterability of freeze-thawed sludges, preliminary experiments showed that CST should not be used for freeze-thawed sludges. Freeze-thaw conditioned sludges contain high amounts of free water, therefore filtration and penetration of sludge water into CST paper are very fast, and result in underestimation of CST values (Vesilind and O¨rmeci, 2000). Dewaterability of the samples is reported in terms of specific resistance to filtration constant (b). Settling tests were conducted in 100 ml graduated cylinders, and the height of the interface was recorded after 4 min of settling. A Hach 2100 turbidimeter was used to measure the turbidity of the sludge supernatant, following 4 min of settling.
RESULTS AND DISCUSSION
In the first part of this study, naturally occuring ECP and cation concentrations in activated and alum sludge supernatants were measured before and after freeze-thaw conditioning, and the results were compared. Protein and carbohydrate concentrations in the sludge supernatant increased remarkably after freeze-thaw conditioning of activated sludge (Fig. 1(a)). In addition, a similar increase in monovalent, divalent, and trivalent cation concentrations was also
observed after freeze-thaw conditioning (Fig. 1(b)). The increase in proteins, carbohydrates, and cations may be due to the release of ECPs from sludge flocs, which would result in the collapse of cation–polymer bridges, and this in turn would cause the release of divalent and possibly other cations to the sludge supernatant. However, ECPs may not be the only source of this increase. Another possibility is that the increase in cations, carbohydrates, and proteins may be due to a release of intracellular material to sludge supernatant caused by cell disruption. The water in sludge cells expands during the freezing process, and cells that cannot resist the pressure of the expanding ice may burst. It is also possible that the compression and suction on cells applied by the advancing ice front may cause the cells to rupture. Intracellular material is mainly composed of proteins, carbohydrates, various ions, RNA and DNA. Presence of DNA in a medium directly indicates the presence of intracellular material, and thus presence of cell disruption. Therefore, if DNA concentration in sludge supernatant increases after freeze-thaw conditioning, then it is possible to conclude that freezethaw conditioning causes cell disruption. The DNA
Effect of dissolved organic material and cations on freeze-thaw conditioning
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Fig. 3. The protein and carbohydrate concentrations (a) and cation concentrations (b) in the treated and untreated samples before freeze-thaw conditioning.
concentration in the activated sludge supernatant before freeze-thaw conditioning was 4.5 mg/l and it increased to 26.7 mg/l after the freeze-thaw which indicated that intracellular material was released to sludge supernatant during freeze-thaw conditioning. In contrast to the activated sludge results, the initial concentrations of proteins, carbohydrates, and cations were low in alum sludge, and they did not change noticeably after the freeze-thaw (Figs 2(a) and (b)). Alum sludge is composed of mostly inorganic particles and some organic material that comes from the decay of plants and organisms in water. Unlike activated sludge, alum sludge is not biologically very active and contains low levels of intracellular and extracellular material. Thus, it is unlikely that there will be a significant increase in protein, carbohydrate, and cation concentrations in sludge supernatant. It is hypothesized in this study that the presence of dissolved organic material and ions in activated sludge may adversely affect the freezing process, and their removal is likely to improve the effectiveness of freeze-thaw conditioning on activated sludge. In the second part of this study, ECPs and ions were extracted from activated sludge samples by using a
combination of centrifugation and blending as suggested by (Gehr and Henry, 1983). The extraction procedure was repeated 5 times to remove as much ECPs and ions as possible. After the removal of these components from sludge matrix, dewaterability, settling, supernatant, and aggregate characteristics of the treated and untreated samples were compared. Figures 3(a) and (b) show the protein, carbohydrate, and cation concentrations of the treated and untreated samples before freeze-thaw conditioning. The extraction method clearly reduced the concentrations of carbohydrates and cations in the treated sample. A small increase in the protein concentration was observed which was possibly due to a small-scale cell disruption that might have occurred during blending and centrifugation. After freeze-thaw conditioning, protein, carbohydrate, and cation concentrations in both treated and untreated samples were higher than their initial concentrations as expected, but the treated sample contained much lower concentrations of these components compared to the untreated one (Figs 4(a) and (b)). In addition, dewaterability, settling, and supernatant characteristics of the treated sample showed a
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Fig. 4. The protein and carbohydrate concentrations (a) and cation concentrations (b) in the treated and untreated samples after freeze-thaw conditioning.
remarkable improvement after freeze-thaw conditioning. The filtration constant dropped from 0.21 to 0.18 s/cm6, the height of the interface and the turbidity of the supernatant dropped to one-third of their initial values (Table 1). The treated sample dewatered within seconds, leaving a clear supernatant. The flocs of the treated sample also showed a notable improvement by turning into smaller and denser aggregates after freeze-thaw conditioning. These aggregates looked more like alum sludge aggregates rather than activated sludge aggregates (Figs 5(a)–(c)). The results of this study indicate that presence of dissolved ions and organic material in activated sludge adversely affects the freezing process and decrease the effectiveness of freeze-thaw conditioning on activated sludge. When these components are removed from activated sludge, freeze-thaw effectiveness increases remarkably. This can be explained as follows: During freezing, dissolved ions and organic material are pushed by the ice front and accumulate at the ice/water interface depressing the freezing temperature at the interface. The water surrounding the interface contains fewer of these components and it freezes first due to having a higher freezing point
Table 1. The filtration constants (a) heights of the interfaces (b) and supernatant turbidities (c) of the treated and untreated samples after freeze-thaw conditioning Sludge
Untreated Treated
Filtration constant (s/cm6)
Height of the interface (ml)
Turbidity (NTU)
0.21 0.18
23 7
34.7 13.6
(Martel, 2000). Therefore, in the presence of dissolved ions and organic material, particles are more easily trapped in the growing ice layer before they are fully compressed and dehydrated. Presence of these components may affect the morphology of the ice/ water interface as well. Martel (2000) suggested that in the presence of dissolved solids, ice crystals grow in branching tree-like structure called dendrities. The formation of dendritics causes the solids and dissolved material to be trapped between the dendrities, rather than being pushed by the growing ice layer. This action decreases the effectiveness of freeze-thaw conditioning. The observed increase in the protein, carbohydrate, and cation concentrations in activated sludge
Effect of dissolved organic material and cations on freeze-thaw conditioning
Fig. 5. Photomicrograph of sludge aggregates after freezethaw conditioning (a) activated sludge (b) alum sludge (c) treated activated sludge ( 100 magnification).
supernatant after freeze-thaw conditioning may partly be explained by the release of ECPs from activated sludge flocs to the sludge supernatant. The release of ECPs may improve freeze-thaw conditioning in two ways. First, the release of ECPs may decrease the average floc size. Since small particles are more easily pushed by the ice/water interface and can migrate under a wide range of freezing rates (Corte, 1962), the decrease in the floc size is likely to improve the freeze-thaw conditioning. Second, it may also be easier to replenish the transition layer between smaller particles and ice front which is crucial for particle rejection (Vesilind et al., 1991). However, the release of ECPs may adversely affect the freeze-thaw conditioning by increasing the dissolved ion and organic material concentrations in sludge supernatant and by promoting particle entrapment. This last effect is probably the strongest, and is likely to overcome the first two positive effects.
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This study also indicates that cell disruption occurs during freezing of activated sludge. Cell disruption releases intracellular material, and increases the concentrations of dissolved ions and organic material in sludge supernatant further. In addition, cell disruption produces smaller particles and increases the total solid surface area, which in turn may increase the surface water content in activated sludge. Surface water is tightly held to the solid surfaces by hydrogen bonds and is very difficult to freeze. Therefore, cell disruption is likely to decrease the effectiveness of freeze-thaw conditioning. Alum sludge, on the other hand, contains low concentrations of dissolved ions and organic material, and particles are more easily pushed by the growing ice/water interface without being trapped. In addition, ice crystals grow in columns and the ice/ water interface is planar (Martel, 2000) as long as the freezing rate is slow enough. As a result, alum sludge freeze-thaws better than activated sludge. This study has shown that removal of dissolved organic material and ions from activated sludge improves its freeze-thaw conditioning. This can be achieved in a reasonable and economic fashion by including elutriation in a treatment process. Elutriation is simply washing the sludge prior to further conditioning. It reduces the flocculant demand of sludge by improving the physical and chemical quality of its solid and liquid components. Elutriation used to be a popular conditioning method before the development of inorganic polyelectrolytes, and has been abandoned since then. However, it might find a new application in freeze-thaw conditioning by reducing the dissolved organic material and ion concentrations in activated sludge, and thereby increasing the effectiveness of freeze-thaw conditioning. This research has also shown that the lower is the concentration of impurities in sludge, the better it freeze-thaws. Therefore, if freeze-thaw conditioning is chosen to be the method of dewatering, simply avoiding the excessive use of coagulants and chemicals in a treatment plant could improve the effectiveness of freeze-thaw conditioning. CONCLUSIONS
1. In activated sludge, freeze-thaw conditioning dramatically increases the concentrations of proteins, carbohydrates, and cations in the sludge supernatant. 2. In alum sludge, freeze-thaw conditioning does not noticeably increase the concentrations of proteins, carbohydrates, and cations in the sludge supernatant. 3. Alum sludge is likely to freeze-thaw better than activated sludge due to its low dissolved ion and organic material contents. High concentrations of dissolved ions, and organic material found in activated sludge may promote particle
Banu O¨rmeci and P. Aarne Vesilind
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entrapment during freezing, and decrease the effectiveness of freeze-thaw conditioning. 4. All the commonly used measures of sludge dewaterability indicate that activated sludge dewaters better following the reduction of protein, carbohydrate, and cation concentrations. The resulting aggregates are smaller and denser, and they resemble the alum sludge aggregates. 5. Freeze-thaw conditioning causes cell disruption and releases intracellular material to the sludge supernatant. REFERENCES
Ashwell G. (1957) Colorimetric analysis of sugars. In Methods in Enzymology, Vol. III, eds S. P. Colowick, and N. O. Kaplan, pp. 99–105. Academic Press, New York. Baskerville R. C. (1971) Freezing and thawing as a technique for improving the dewaterability of aqueous suspensions. Filtr. Sep. 3, 141. Bradford M. M. (1976) A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. 72, 248. Bruus J. H., Nielsen P. H. and Keiding K. (1992) On the stability of activated sludge flocs with implications to dewatering. Water. Res. 26, 1597. Chalmers B. (1959) How water freezes. Sci. Am. 200, 114. Chen G. W., Chang I. L., Hung W. T. and Lee D. J. (1996) Freeze/thaw treatment of zone settling of waste activated sludges. J. Environ. Sci. Health A 31(3), 621. Chu C. P., Feng W. H., Tsai Y. H. and Lee D. J. (1997) Unidirectional freezing of waste activated sludge: the presence of sodium chloride. Environ. Sci. Technol. 31, 1512. Clements G. S., Stephenson R. J. and Regan C. J. (1950) Sludge dewatering by freezing with added chemicals. J. Proc. Inst. Sewage Purif. 4, 318. Corte A. (1962) Vertical migration of particles in front of a moving freezing plane. J. Geophys. Res. 67, 1085. Dreywood R. (1946) Qualitative test for carbohydrate material. Ind. Eng. Chem. 18, 499. Eriksson L. and Alm B. (1991) Study of flocculation mechanisms by observing effects of a complex agent on activated sludge properties. Water Sci. Technol. 24, 21. Gehr R. and Henry J. G. (1983) Removal of extracellular material. Techniques and pitfalls. Water Res. 17, 1243. Halde R. (1980) Concentration of impurities by progressive freezing. Water. Res. 14, 575.
Hellstrom D. and Kvarnstrom E. (1997) Natural sludge dewatering. I: Combination of freezing, thawing, and drying as dewatering methods. J. Cold Regions Eng. 11(1), 1–14. Higgins M. J. and Novak J. T. (1997) The effect of cations on the settling and dewatering of activated sludges: laboratory results. Water Environ. Res. 69(2), 215–224. Hoekstra P. and Miller R. D. (1967) On the mobility of water molecules in the transition layer between ice and a solid surface. J. Colloid Interface Sci. 25, 166. Hong S. G., Young J. D., Chen G. W., Chang I. L., Hung W. T. and Lee D. J. (1995) Freeze/thaw treatment on waste activated sludge: a FTIR spectroscopic study. J. Environ. Sci. Health A 30(8), 1717. Kawasaki K. and Matsuda A. (1995) Effect of dissolved solid concentration on freezing and thawing of excess activated sludge. Kogaku Kogaku Ronbunsyu 21, 859. Keiding K. and Nielsen P. H. (1997) Desorption of organic macromolecules from activated sludge: Effect of ionic composition. Water Res. 31, 1665. Lee D. J. and Hsu Y. H. (1994) Fast freeze/thaw treatment on excess activated sludges: floc structure and sludge dewaterability. Environ. Sci. Technol. 28, 1444. Lee D. J. and Hsu Y. H. (1995) Measurement of bound water in sludges: a comparative study. Water Environ. Res. 67, 310. Martel C. J. (1998) Cold-weather clean. How freezing temperatures can be used for wastewater treatment. Water Environ. Technol. 10(8), 50–53. Martel C. J. (2000) Influence of dissolved solids on the mechanisms of freeze-thaw conditioning. Water Res. 34, 657. Martel C. J. and Diener C. J. (1991) A pilot scale study of alum sludge dewaterability in a freezing bed. Journal of AWWA 83, 51. Nielsen P. H. and Keiding K. (1998) Disintegration of activated sludge flocs in presence of sulfide. Water Res. 32, 313. Parker P. J., Collins A. G. and Dempsey J. P. (1998) Alum residual floc interactions with an advancing ice/water interface. J. Environ. Eng. 124(3), 249–253. Randall C. W., Ali Khan M. Z. and Stephens N. T. (1975) Waste activated sludge conditioning by direct slurry freezing. Water Res. 9, 917. Urbain V., Block J. C. and Manem J. (1993) Bioflocculation in activated sludge: an analytic approach. Water Res. 27, 829. Vesilind P. A. and O¨rmeci B. (2000) A modified capillary suction time apparatus for measuring the filterability of super-flocculated sludges. Water Sci. Technol. 42(9), 135. Vesilind P. A., Wallinmaa S. and Martel J. (1991) Freezethaw sludge conditioning and double layer compression. Can. J. Civ. Eng. 18, 1078.
Wat. Res. Vol. 35, No. 18, pp. 4299–4306, 2001 r 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
EFFECT OF DISSOLVED ORGANIC MATERIAL AND CATIONS ON FREEZE-THAW CONDITIONING OF ACTIVATED AND ALUM SLUDGES BANU O¨RMECI1* and P. AARNE VESILIND2 1
Department of Civil and Environmental Engineering, Duke University, P.O. Box 90287, Durham, NC 27706-0287, USA and 2 Department of Civil and Environmental Engineering, Bucknell University, Lewisburg, PA 17837, USA (First received 1 August 2000; accepted in revised form 1 March 2001)
AbstractFFreeze-thaw conditioning effectively dewaters alum and activated sludges, but it works better on alum sludge than it does on activated sludge. The main difference between alum sludge and activated sludge is that activated sludge has high concentrations of both dissolved organic material and ions. Dissolved organic material and ions may possibly alter the freezing process and decrease the effectiveness of freeze-thaw conditioning on activated sludge. The objective of this study is to investigate the effect of dissolved organic material and cations on freeze-thaw conditioning of sludges, and to improve the effectiveness of freeze-thaw conditioning on activated sludge. The results of this study show that although protein, carbohydrate and cation concentrations in activated sludge supernatant are initially high, they dramatically increase after freeze-thaw conditioning. The increase is likely to come from the release of extracellular and intracellular material to sludge supernatant. The observed increase in the DNA concentration in activated sludge supernatant after freeze-thaw conditioning indicates that freeze-thaw causes cell disruption. Alum sludge supernatant, on the other hand, initially contains low concentrations of proteins, carbohydrates and cations which do not noticeably change after freeze-thaw conditioning. When ECPs (extracellular polymers) and cations are extracted from activated sludge before freeze-thaw conditioning, the sludge settles and dewaters better after the freeze-thaw. The resulting aggregates are smaller and denser resembling the ‘‘coffee ground’’ aggregates of alum sludge. r 2001 Elsevier Science Ltd. All rights reserved Key wordsFsludge, dewatering, freeze-thaw, organic material, extracellular polymers, cations
INTRODUCTION
Freeze-thaw conditioning prior to sludge dewatering is becoming increasingly popular due to the improvements in the design and efficiency of the facilities (Hellstrom and Kvarnstrom, 1997; Martel, 1998). Some of the advantages of freeze-thaw conditioning are its simplicity, low-cost at moderate to cold climates, and effectiveness with certain types of sludges. Freeze-thaw conditioning works amazingly well on alum sludge. It dramatically converts alum sludge from a fine particle suspension to a mixture of clear water and granular particles. The granular particles resemble coffee grounds in both size and appearance, and they do not break apart upon vigorous mixing. On the other hand, freeze-thaw conditioning is not as effective for activated sludge. After freeze-thaw conditioning, sludge flocs turn into
*Author to whom all correspondence should be addressed. Tel.: +1-919-6605-200; fax: +1-919-6605-219; e-mail: [email protected]
fluffy and watery aggregates which disintegrate upon agitation. The improvements achieved through freeze-thaw conditioning may be temporary and reversible. The main difference between alum sludge and activated sludge is the presence of various organic material, dissolved ions, and microorganisms in the latter one. It is possible that the high organic material and ion concentrations in activated sludge, which are not present in alum sludge, interfere with the freezing process and decrease the freeze-thaw effectiveness. If these components are withdrawn from activated sludge using extraction techniques, it might be possible to improve the effectiveness of freeze-thaw conditioning on activated sludge, and bring it closer to that of alum sludge. The objective of this study is to gain a better understanding of the effects of dissolved organic material and cations on freeze-thaw conditioning of activated and alum sludges, and improve the effectiveness of freeze-thaw conditioning on activated sludge by extracting extracellular polymers and cations together with other dissolved material from sludge matrix.
4299
Banu O¨rmeci and P. Aarne Vesilind
4300 LITERATURE REVIEW
Freeze-thaw technology works on the principle that ice crystals grow by incorporating water molecules only. Because the structure of ice crystal is highly organized and symmetrical, it cannot accommodate any other atoms or molecules. Each ice crystal continues to grow as long as water molecules are available. All other impurities and solid particles are forced to the boundaries of the ice crystal where they become compressed or dehydrated (Chalmers, 1959). Clements et al. (1950) reported that to obtain any conditioning action by freeze-thaw, the sludge must be completely frozen at a relatively slow rate. Flash or fast freezing produces little or no conditioning action since the slow diffusion of water molecules through the interface results in the capture of solids in the ice matrix. Halde (1980) reported that when freezing rate is slow enough, particles of nearly all materials are rejected by the moving ice–water interface, and dewaterability of freeze-thawed sludge is greatly improved. Nevertheless, when the freezing rate is high, particles in the solution eventually are trapped in the developing ice layer resulting in little improvement in sludge dewaterability. Particle size is another factor that affects the freezing process. Corte (1962) showed that fine particles migrate under a wide range of freezing rates and coarse particles migrate only at low freezing rates. This means large particles are more likely to be trapped in the advancing ice front compared to the smaller particles. Corte (1962) argued that a layer of water must be continuously present between a particle and the ice front. Hoekstra and Miller (1967) named this layer of water surrounding the particle a ‘‘transition layer’’. Transition layer is more easily replenished when particles are small (Vesilind et al., 1991). Hoekstra and Miller (1967) suggested that transition layer freezes later than free water because of the accumulation of dissolved ions, which are rejected by the ice front, in the transition layer. The high ion concentration in the transition layer depresses the freezing point, and delays the freezing of the transition layer. Kawasaki and Matsuda (1995) examined the effect of sodium chloride on sludge dewaterability, settleability, and residual moisture, and concluded that sodium chloride addition retarded the rejection of sludge particles by the ice–water interface. Similar results were also reported by Chu et al. (1997). The difference between the morphology of the ice/ water interfaces of alum sludge and activated sludge may be one of the factors that cause the difference in their freeze-thaw conditioning. Martel (2000) stated that alum sludge ice crystals grow in columns and activated sludge ice crystals grow in branching treelike structures called dendrites. When dendrites are formed at the ice/water interface, solid particles are trapped in the advancing ice front, and sludge does
not freeze well. Martel attributed the formation of dendrities to the presence of dissolved solids in activated sludge. He also showed that addition of dissolved solids, such as NaCl, to alum sludge can change the ice crystal growth from columnar to dendritic. Freeze-thaw conditioning has probably found its best application on alum sludge. Baskerville (1971) stated that upon freeze-thaw conditioning, alum sludge is remarkably stable to stirring, with capillary suction time (CST) being almost constant irrespective of the stirring period. Martel and Diener (1991) observed that freeze-thaw conditioning dramatically converts alum sludge from a fine particle suspension to a mixture of clear water and granular particles. The granular particles resemble coffee grounds in both size and appearance, and they do not break apart even after vigorous agitation. Martel and Diener achieved a 96% reduction in volume simply by placing the mixture on a porous medium and draining away the water. The dewaterability of the activated sludge is also significantly improved by freeze-thaw conditioning. After a freeze-thaw treatment, the bound water content of the sludge cake, the floc volume and the sludge compressibility decreases, and the sludge filterability increases (Lee and Hsu, 1994, 1995; Chen et al., 1996; Parker et al., 1998). However, unlike the case in alum sludge the aggregates are not as small, dense and strong, and they are not very resistant to agitation. Baskerville (1971) observed an increase in capillary suction time (CST) of the freeze-thawed activated sludge which indicated a deterioration in sludge filterability with stirring. Some researchers observed a change in the quality of activated sludge supernatant after freeze-thaw conditioning. Randall et al. (1975) and Hong et al. (1995) reported that both BOD and COD values of an activated sludge supernatant increased significantly after freeze-thaw conditioning. Hung et al. (1996b) suggested that the action of ice front may release some of the ECPs from sludge body to sludge supernatant. Activated sludge flocs have a complex structure compared to alum sludge flocs. ECPs and divalent cations are two important components of the activated sludge flocs. ECPs produced by sludge bacteria are involved in the formation of a threedimensional matrix where divalent cations, particularly Ca+2 and Mg+2, act as bridging agents with probably specific affinities for each kind of extracellular polymer (Urbain et al., 1993). Virtually all possible sugars, organic acids, nucleic acids, proteins, and lipids have been found as components of ECPs from various pure and mixed cultures (Gehr and Henry, 1983). Extraction of ECPs results in deflocculation and a decrease in average particle size (Eriksson and Alm, 1991; Bruus et al., 1992). Although the effects of divalent cations on activated sludge characteristics have been known and studied for some time (Eriksson and Alm, 1991; Bruus et al.,
Effect of dissolved organic material and cations on freeze-thaw conditioning
1992; Keiding and Nielsen, 1997), the importance of monovalent (Bruus et al., 1992; Chu et al., 1997; Higgins and Novak, 1997) and trivalent cations (Nielsen and Keiding, 1998) on floc structure is just beginning to surface. It is hypothesized in this study that the presence of dissolved organic material and ions in activated sludge adversely affects its freeze-thaw conditioning. It may be possible to improve the freeze-thaw conditioning of activated sludge by extracting ECPs and cations from floc structure, and removing other impurities from sludge supernatant. Removal of these components is likely to increase the effectiveness of freeze-thaw conditioning by reducing particle entrapment during freezing.
MATERIALS AND METHODS
The activated sludge used in this research was collected from North Durham Wastewater Treatment Facility, Durham, NC. The sludge was taken to the laboratory within 30 min, and was kept aerated throughout the experiments. In order to avoid differences between samples that may come from ongoing physical, biological, and chemical changes in sludge, all experiments in a particular group were finished on the same day. The alum sludge was
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collected from Brown Water Treatment Facility, Durham, NC. A freezer (ScienTemp Corp., MI) with a digital setting was used to freeze the sludge samples. The volume of all samples was 100 ml. Sludge samples were frozen in 130 ml transparent glass bottles which enabled visual examination of aggregates after freeze-thaw conditioning. The preliminary experiments showed that activated sludge froze completely at @81C, and therefore @81C was used to freeze the samples. All samples were kept frozen for 36 h in the freezer, and thawed for 10 h at room temperature. Preliminary experiments showed that thawing times longer than 12 h started undesired anaerobic reactions in sludge and changed the sludge characteristics. ECPs and ions were removed from activated sludge by using a physical extraction method suggested by Gehr and Henry (1983). This method is a combination of centrifugation and blending, and it was repeated 5 times to extract as much ECPs and ions as possible from the samples. The ECP contents of the samples were quantified through determining the carbohydrate and protein concentrations by colorimetric methods. Dye-binding method (Bradford, 1976) was used to determine the protein concentration, and Anthrone method (Dreywood, 1946) was used to determine the carbohydrate concentration in samples. Cysteine method (Ashwell, 1957) was used to estimate the DNA content of sludge bulk water. Specific resistance to filtration (SRF) test with the omission of vacuum was used to measure the dewaterability of freeze-thaw conditioned sludges. Although many researchers have been using capillary suction time (CST) test
Fig. 1. The difference between the protein and carbohydrate concentrations (a) and cation concentrations (b) in activated sludge before and after freeze-thaw conditioning.
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Fig. 2. The difference between the protein and carbohydrate concentrations (a) and cation concentrations (b) in alum sludge before and after freeze-thaw conditioning.
to measure the dewaterability of freeze-thawed sludges, preliminary experiments showed that CST should not be used for freeze-thawed sludges. Freeze-thaw conditioned sludges contain high amounts of free water, therefore filtration and penetration of sludge water into CST paper are very fast, and result in underestimation of CST values (Vesilind and O¨rmeci, 2000). Dewaterability of the samples is reported in terms of specific resistance to filtration constant (b). Settling tests were conducted in 100 ml graduated cylinders, and the height of the interface was recorded after 4 min of settling. A Hach 2100 turbidimeter was used to measure the turbidity of the sludge supernatant, following 4 min of settling.
RESULTS AND DISCUSSION
In the first part of this study, naturally occuring ECP and cation concentrations in activated and alum sludge supernatants were measured before and after freeze-thaw conditioning, and the results were compared. Protein and carbohydrate concentrations in the sludge supernatant increased remarkably after freeze-thaw conditioning of activated sludge (Fig. 1(a)). In addition, a similar increase in monovalent, divalent, and trivalent cation concentrations was also
observed after freeze-thaw conditioning (Fig. 1(b)). The increase in proteins, carbohydrates, and cations may be due to the release of ECPs from sludge flocs, which would result in the collapse of cation–polymer bridges, and this in turn would cause the release of divalent and possibly other cations to the sludge supernatant. However, ECPs may not be the only source of this increase. Another possibility is that the increase in cations, carbohydrates, and proteins may be due to a release of intracellular material to sludge supernatant caused by cell disruption. The water in sludge cells expands during the freezing process, and cells that cannot resist the pressure of the expanding ice may burst. It is also possible that the compression and suction on cells applied by the advancing ice front may cause the cells to rupture. Intracellular material is mainly composed of proteins, carbohydrates, various ions, RNA and DNA. Presence of DNA in a medium directly indicates the presence of intracellular material, and thus presence of cell disruption. Therefore, if DNA concentration in sludge supernatant increases after freeze-thaw conditioning, then it is possible to conclude that freezethaw conditioning causes cell disruption. The DNA
Effect of dissolved organic material and cations on freeze-thaw conditioning
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Fig. 3. The protein and carbohydrate concentrations (a) and cation concentrations (b) in the treated and untreated samples before freeze-thaw conditioning.
concentration in the activated sludge supernatant before freeze-thaw conditioning was 4.5 mg/l and it increased to 26.7 mg/l after the freeze-thaw which indicated that intracellular material was released to sludge supernatant during freeze-thaw conditioning. In contrast to the activated sludge results, the initial concentrations of proteins, carbohydrates, and cations were low in alum sludge, and they did not change noticeably after the freeze-thaw (Figs 2(a) and (b)). Alum sludge is composed of mostly inorganic particles and some organic material that comes from the decay of plants and organisms in water. Unlike activated sludge, alum sludge is not biologically very active and contains low levels of intracellular and extracellular material. Thus, it is unlikely that there will be a significant increase in protein, carbohydrate, and cation concentrations in sludge supernatant. It is hypothesized in this study that the presence of dissolved organic material and ions in activated sludge may adversely affect the freezing process, and their removal is likely to improve the effectiveness of freeze-thaw conditioning on activated sludge. In the second part of this study, ECPs and ions were extracted from activated sludge samples by using a
combination of centrifugation and blending as suggested by (Gehr and Henry, 1983). The extraction procedure was repeated 5 times to remove as much ECPs and ions as possible. After the removal of these components from sludge matrix, dewaterability, settling, supernatant, and aggregate characteristics of the treated and untreated samples were compared. Figures 3(a) and (b) show the protein, carbohydrate, and cation concentrations of the treated and untreated samples before freeze-thaw conditioning. The extraction method clearly reduced the concentrations of carbohydrates and cations in the treated sample. A small increase in the protein concentration was observed which was possibly due to a small-scale cell disruption that might have occurred during blending and centrifugation. After freeze-thaw conditioning, protein, carbohydrate, and cation concentrations in both treated and untreated samples were higher than their initial concentrations as expected, but the treated sample contained much lower concentrations of these components compared to the untreated one (Figs 4(a) and (b)). In addition, dewaterability, settling, and supernatant characteristics of the treated sample showed a
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Fig. 4. The protein and carbohydrate concentrations (a) and cation concentrations (b) in the treated and untreated samples after freeze-thaw conditioning.
remarkable improvement after freeze-thaw conditioning. The filtration constant dropped from 0.21 to 0.18 s/cm6, the height of the interface and the turbidity of the supernatant dropped to one-third of their initial values (Table 1). The treated sample dewatered within seconds, leaving a clear supernatant. The flocs of the treated sample also showed a notable improvement by turning into smaller and denser aggregates after freeze-thaw conditioning. These aggregates looked more like alum sludge aggregates rather than activated sludge aggregates (Figs 5(a)–(c)). The results of this study indicate that presence of dissolved ions and organic material in activated sludge adversely affects the freezing process and decrease the effectiveness of freeze-thaw conditioning on activated sludge. When these components are removed from activated sludge, freeze-thaw effectiveness increases remarkably. This can be explained as follows: During freezing, dissolved ions and organic material are pushed by the ice front and accumulate at the ice/water interface depressing the freezing temperature at the interface. The water surrounding the interface contains fewer of these components and it freezes first due to having a higher freezing point
Table 1. The filtration constants (a) heights of the interfaces (b) and supernatant turbidities (c) of the treated and untreated samples after freeze-thaw conditioning Sludge
Untreated Treated
Filtration constant (s/cm6)
Height of the interface (ml)
Turbidity (NTU)
0.21 0.18
23 7
34.7 13.6
(Martel, 2000). Therefore, in the presence of dissolved ions and organic material, particles are more easily trapped in the growing ice layer before they are fully compressed and dehydrated. Presence of these components may affect the morphology of the ice/ water interface as well. Martel (2000) suggested that in the presence of dissolved solids, ice crystals grow in branching tree-like structure called dendrities. The formation of dendritics causes the solids and dissolved material to be trapped between the dendrities, rather than being pushed by the growing ice layer. This action decreases the effectiveness of freeze-thaw conditioning. The observed increase in the protein, carbohydrate, and cation concentrations in activated sludge
Effect of dissolved organic material and cations on freeze-thaw conditioning
Fig. 5. Photomicrograph of sludge aggregates after freezethaw conditioning (a) activated sludge (b) alum sludge (c) treated activated sludge ( 100 magnification).
supernatant after freeze-thaw conditioning may partly be explained by the release of ECPs from activated sludge flocs to the sludge supernatant. The release of ECPs may improve freeze-thaw conditioning in two ways. First, the release of ECPs may decrease the average floc size. Since small particles are more easily pushed by the ice/water interface and can migrate under a wide range of freezing rates (Corte, 1962), the decrease in the floc size is likely to improve the freeze-thaw conditioning. Second, it may also be easier to replenish the transition layer between smaller particles and ice front which is crucial for particle rejection (Vesilind et al., 1991). However, the release of ECPs may adversely affect the freeze-thaw conditioning by increasing the dissolved ion and organic material concentrations in sludge supernatant and by promoting particle entrapment. This last effect is probably the strongest, and is likely to overcome the first two positive effects.
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This study also indicates that cell disruption occurs during freezing of activated sludge. Cell disruption releases intracellular material, and increases the concentrations of dissolved ions and organic material in sludge supernatant further. In addition, cell disruption produces smaller particles and increases the total solid surface area, which in turn may increase the surface water content in activated sludge. Surface water is tightly held to the solid surfaces by hydrogen bonds and is very difficult to freeze. Therefore, cell disruption is likely to decrease the effectiveness of freeze-thaw conditioning. Alum sludge, on the other hand, contains low concentrations of dissolved ions and organic material, and particles are more easily pushed by the growing ice/water interface without being trapped. In addition, ice crystals grow in columns and the ice/ water interface is planar (Martel, 2000) as long as the freezing rate is slow enough. As a result, alum sludge freeze-thaws better than activated sludge. This study has shown that removal of dissolved organic material and ions from activated sludge improves its freeze-thaw conditioning. This can be achieved in a reasonable and economic fashion by including elutriation in a treatment process. Elutriation is simply washing the sludge prior to further conditioning. It reduces the flocculant demand of sludge by improving the physical and chemical quality of its solid and liquid components. Elutriation used to be a popular conditioning method before the development of inorganic polyelectrolytes, and has been abandoned since then. However, it might find a new application in freeze-thaw conditioning by reducing the dissolved organic material and ion concentrations in activated sludge, and thereby increasing the effectiveness of freeze-thaw conditioning. This research has also shown that the lower is the concentration of impurities in sludge, the better it freeze-thaws. Therefore, if freeze-thaw conditioning is chosen to be the method of dewatering, simply avoiding the excessive use of coagulants and chemicals in a treatment plant could improve the effectiveness of freeze-thaw conditioning. CONCLUSIONS
1. In activated sludge, freeze-thaw conditioning dramatically increases the concentrations of proteins, carbohydrates, and cations in the sludge supernatant. 2. In alum sludge, freeze-thaw conditioning does not noticeably increase the concentrations of proteins, carbohydrates, and cations in the sludge supernatant. 3. Alum sludge is likely to freeze-thaw better than activated sludge due to its low dissolved ion and organic material contents. High concentrations of dissolved ions, and organic material found in activated sludge may promote particle
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entrapment during freezing, and decrease the effectiveness of freeze-thaw conditioning. 4. All the commonly used measures of sludge dewaterability indicate that activated sludge dewaters better following the reduction of protein, carbohydrate, and cation concentrations. The resulting aggregates are smaller and denser, and they resemble the alum sludge aggregates. 5. Freeze-thaw conditioning causes cell disruption and releases intracellular material to the sludge supernatant. REFERENCES
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