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Floc strength and dewatering efficiency of alum sludge
Advances in Environmental Research 7 (2003) 617–621
Floc strength and dewatering efficiency of alum sludge Chih Chao Wu*, Jerry J. Wu, Ruey Yi Huang Department of Environmental Engineering and Science Feng Chia University, Taichung, 407 Taiwan, ROC
Abstract The effect of floc strength on the dewatering efficiency of alum sludge is studied in this work. Addition of polymer strengthens the floc structure of the sludge, which reduces floc breakage and sludge dewaterability. The result of vacuum filtration suggests that a confined floc strength growth benefits the filterability, although the strongest floc structure does not guarantee maximal dewatering efficiency. For centrifugal dewatering, increasing floc strength can not reduce the moisture content of the sludge cake when the centrifugal G value is under 7000. At high G, a strong floc structure is necessary to reduce cake blinding from floc deformation, and the lowest moisture content is found in sludge cake with the highest floc strength. Therefor, floc strength should be included with other traditional dewaterability indices in determining sludge dewatering. Centrifugation is suggested for the dewatering of sludge with high floc strength, and vacuum filtration for middle floc strength. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polymer conditioning; Sludge dewatering; Floc strength; Fractal dimension; Floc size; Vacuum filtration; Centrifugation
1. Introduction Polymer conditioning has been widely employed to improve the dewatering characteristics of sludge and the mechanical dewatering efficiency. It has been reported that over-intensive shear and insufficient polymer results in floc breakage and deterioration of dewaterability (Werle and Novak, 1984; Novak et al., 1988; Wu et al., 1997). After chemical conditioning, sludges are dewatered with mechanical dewatering devices of various shear stresses. The degree of floc breakage and deformation in cake filtration strongly affects the performance of the dewatering equipment. Capillary suction time (CST) and specific resistance of filtration (SRF) are the most commonly used indices for determining the optimal dose of polymer conditioning. Although for many years floc strength has been adopted to assess the floc breakage caused by the surrounding shear force, most researchers have focused their studies on the effect of floc strength on particles aggregation (or flocculation) of the dilute solution with very little *Corresponding author. Tel:q886-4-2451-7250 ext 5213; fax: q886-4-2451-2405. E-mail address: [email protected] (C.C. Wu).
attention on the sludge system. This paper presents insight on floc strength and the dewatering efficiency of the polymer-conditioned sludge. 2. Experimental 2.1. Sludge conditioning and dewatering Sludge samples were collected from the sedimentation tank of the Feng-yuan water treatment facility (Taichung, Taiwan). The cationic polyelectrolyte (polymer PC-320) was obtained from the Taiwan Polymer Company. Polymer PC-320 is a copolymer of acrylamide and diallyldimethyl-amonium chloride, with an average molecular weight of 1.1;1.2=107, and a 20% charge density. Polymer solution (0.1% wyw) was prepared as suggested by the polymer manufacturer. A Couette flocculator was adopted as the mixing apparatus to generate a Taylor vortex, in which a better velocity-gradient distribution than the traditional mixing was provided. The Couette flocculator consisted of two concentric cylinders (inner: Øs170 mm, Hs200 mm; outer: Øs180 mm, Hs250 mm) with a space of 5 mm in which the test sludge and polymer were added. The
1093-0191/03/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 2 . 0 0 0 4 8 - 5
C.C. Wu et al. / Advances in Environmental Research 7 (2003) 617–621
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Table 1 The CSTs and floc strengths of alum sludge with cationic polymer conditioning
CST (per second) Floc strength
0 mgyl
10 mgyl
20 mgyl
30 mgyl
127.2 2.41
27.6 4.56
19.2 5.48
21.9 5.88
Data are the average of three trials.
outer cylinder was stationary. The device was coupled with and powered by a variable speed motor connected to a torquemeter. The rotational speed was set to achieve the desired velocity gradient (G). Sludge dewaterability was evaluated by the capillary suction time (CST) and the specific resistance of filtration (SRF). The Triton CST apparatus model 200 was used to measure the CST value. A standard Buchner funnel apparatus with a 9-cm funnel was used for vacuum filtration and SRF determination. To reduce the sedimentation effect on SRF determination, stirring was provided inside the Buchner funnel during filtration (Wu et al., 2000). The data shown in Table 1 are the CSTs of test samples and measured just after the end of the conditioning period. For the convenience of data comparison, we define here the optimal dose as 20 mgy l based on the dose of minimum CST. Doses below 20 mgyl are referred to as under-dosing, and doses over 20 as overdosing, in this study. In centrifugation, settled sludge samples were centrifuged at five different rev.ymin for 1 min. The supernatant was decanted and the dry solids content of the sludge cake was determined by drying at 105 8C for 24 h. All the data are reported as the average of three trials.
polymer-conditioning (first mixing), a portion of the sludge was kept in the Couette flocculator and mixed for an extra 60 s; this is referred to as the breaking test hereafter. Four Gs were chosen for the mixing of the breaking test. At the end of the breaking mixing, approximately 400 flocs were removed for image analysis to determine the perimeter and diameter of the flocs. A log–log plot of the calculated average perimeter vs. velocity gradient and their linear regressions were established. The intercept of the regressed lines was taken as the ‘floc strength coefficient’ for each sludge. 3. Results and discussion 3.1. Floc breakage and floc strength Fig. 1 shows the influence of G on floc breakage, as illustrated by the mean floc size of the sludge sample. The mean floc size of the unconditioned alum sludge remained approximately 75 mm over four various velocity gradients (Gs), suggesting that the flocs were resistant to the selected shear with no detectable breakage. When 10 mgyl cationic polymer was added, the mean floc size increased to 375 mm before the breaking test was initiated. This enlargement in floc size was significant as shown by the decrease of CST, as seen in Table 1. When 280ys was applied, breakage of the flocs occurred and the mean floc size decreased to the size of the original alum floc. With the optimal dosage of 20 mgyl, as determined from the unconditional sludge, the mean floc size grew up to 530 mm, the largest size produced in this test, and then remained at approximately 450 mm during the subsequent breakage test. The floc size decreased dramatically down to 150 mm, when the velocity gradient of 480ys was employed.
2.2. Fractal dimension analysis Fractal dimension, D, of the floc was determined from the log–log plot of the size vs. density. The slope was calculated via linear regression analysis, from which the D was evaluated as Ds3qS (Lee and Hsu, 1994). A free settling test was employed to measure the wet density of the sludge floc. The settling travel of the sludge aggregate in a quiescent column was recorded by a video camera equipped with a close-up lens. The floc diameter and terminal velocity were determined from the replayed tape. Floc densities were calculated from terminal settling velocity and the diameter of the floc using a modified Stoke’s equation and presented as wet density (Tambo and Watanabe, 1979). The details of the experiment can be found our previous work (Wu et al., 1997). 2.3. Floc strength measurement Floc strength was determined following the method given by Leentvaar and Rebhun (1983). After each
Fig. 1. Variation of mean floc size floc of polymer-conditioned sludge with increasing shear stress in breaking test.
C.C. Wu et al. / Advances in Environmental Research 7 (2003) 617–621 Table 2 Fractal dimensions of conditioned and unconditioned sludges under various degree of breaking test Dose
After conditioning After breaking test G (per second) 170 280 360 480
0 mgyl
10 mgyl
20 mgyl
30 mgyl
1.54
1.80
1.83
1.83
1.54 1.53 1.50 1.52
1.68 1.63 1.59 1.56
1.82 1.83 1.76 1.70
1.85 1.83 1.84 1.81
Such a change suggested that even under the optimal dose, the aggregated floc could not withstand the shear at 480ys. Under over-dosing conditioning, 30 mgyl, the size of the floc remained fairly stable at approximately 450 mm and then grew to 530 mm when Gs exceeded 280ys. Floc breakage started to occur when the Gs was greater than 280ys. However, the mean floc size stayed at approximately 375 mm. The increase in floc size before the breaking process was due to over-dosing. As mentioned earlier, the optimal dose, 20 mgyl, was quantitatively determined by the CST response at the selected mixing intensity in the conditioning stage. Such shear stress was unable to provide sufficient mixing intensity for a complete flocculation. Consequently, the shear stress applied in the subsequent breaking test compensated for the insufficient mixing of the previous conditioning stage and produced the observed growth in floc size. However, when more intensity was provided, the same trend in floc breakage as occurred in other doses was observed. The much higher threshold Gs of the sludge conditioned by 30 mgyl polymer suggested that the addition of polymer enhanced the floc strength of the sludge. Fractal dimension analysis is effective in investigating intrafloc structures, which reflects the manner in which the primary particles are packed within the floc (Wen and Lee, 1998). The theoretical values of D vary from 1 to 3, which provide an useful index for the degree of floc compactness and how the particles are packed (Lee, 1994; Wu et al., 1997). Table 2 lists fractal dimensions of sludge flocs under various conditioning and breaking tests. Without the excess mixing, as seen in the 1st row data, the D increased dramatically from 1.54 to 1.80 with the addition of 10 mgyl polymer, and remained unchanged when more polymers were added. This result indicated that polymer addition strongly influenced floc structures, whether compactness or configuration. Two critical findings were discovered from the variations in D values after the breakage test. First, the D at 10 and 20 mgyl polymer conditioning decreased gradually with the
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increasing shear, but those at 0 and 30 mgyl remained unchanged. Second, it was apparent that the effect of Gs on the floc structure of the sludge conditioned with 10 mgyl polymer was more significant on D, dropping from 1.80 to 1.56. A more moderate decrease, from 1.83 to 1.70, was observed at 20 mgyl polymer addition. This implies that polymer addition enhances floc strength to resist the breaking process. These two observations suggest that the architecture of the flocs of the conditioned sludge had been rearranged according to the floc strength and the intensity of the following shear. Recalling the discussion of floc size variation in Fig. 1, less floc breakage and larger average floc diameter were associated with the dose of 30 mgyl. A relationship between sludge conditioning and floc strength was thus deduced. When a low dose of polymer, 10 mgyl, is added, the sludge flocs formed are easily broken and lose their floc compactness. At optimal-dosing, floc strength increases, causing less floc breakage and floc compactness. At over-dosing, very little breakage occurs. Besides the changes in floc size, a direct measurement of floc strength also provides evidence for this theory. The extent of floc strength can be quantified by the floc strength coefficient with the measurement proposed by Leentvaar and Rebhun (1983). Table 1 shows that floc strengths increased with polymer doses, indicating that the addition of polymer does enforce the strength of aggregated flocs, even at over-dosing at 30 mgyl. To investigate more about the over-dosing conditioning, 40 mgyl polymer was applied in a similar test. Unfortunately, since the excess amount of polymer caused the fast sedimentation fast in the Couette Flocculator, the floc did not travel long in the Taylor vortex during the conditioning period, which made the following breaking test meaningless. Based on the experimental results, it is reasonable to believe that the more polymer is applied, the stronger the floc is. The conclusion that polymer addition enforces floc strength is consistent with previous reports about suspension systems (Hannah et al., 1967; Leentvaar and Rebhun, 1983). 3.2. Floc strength and dewatering performance In the following section, the effect of floc strength on dewatering performance is studied. Vacuum filtration was used to evaluate the filterability. Fig. 2 is a typical filtration result where the volume of the filtrate is plotted as a function of filtration time for conditioned and unconditioned sludges. To evaluate the dewatering performance, an index T85, defined as the cumulative filtration time to remove 85% of expressible water from the sludge cake, was adopted. The addition of polymer improved filtration efficiency no matter how little of the polymer was used. However, 10 mgyl has been found to be the most efficient dose. This phenomenon
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C.C. Wu et al. / Advances in Environmental Research 7 (2003) 617–621
Fig. 2. Filtration of sludges under various polymer-conditioning. (Floc strength of each sludge sample is given behind each polymer dose.)
could not be explained by the enhancement in floc strength only. When the SRF was plotted against the corresponding floc strength, as shown in Fig. 3, the R square of the linear regression was only 0.2061. Such poor correlation indicates that floc strength has very little influence on the dewatering by vacuum-filtration. However, it should be noted that a confined strength growth is favorable for improving the filtration rate of the sludge and the over-growth is associated with an increase in undesired resistance. In other words, the floc strength must be kept in the range of 2.41;5.48 to optimize the polymer conditioning and the dewatering. During filtration, flocs of weak structure tend to break when they are transported through the filter medium to form filter cakes. Not only are the pores between the filter medium and the cake blinded by the fragments from floc breakage, but the filtration resistance also rises with ease. Therefore, a minimum floc strength is necessary to avoid floc breakage during filtration. In view of the above discussion, the upper limit of floc strengths seem meaningless for filtration. Actually, the growth of floc strength is caused by the addition of polymer which also increases the viscosity of the sludge system. As we know, a major improvement in cake filtration results from the enlargement of floc size that greatly reduces the blind effect. That is why the addition of 10 mgyl polymer can produce the best filterability (lowest T85 in Fig. 2 and SRF in Fig. 3). Beyond that dose, the residual polymer in the suspension increased and the viscosity rose. Christensen et al. (1993) had concluded that the increasing viscosity in the liquid phase is the primary source for the overdosing of polymer conditioning. The increase in floc strength by adding polymers at 20 and 30 mgyl is additional
evidence of the change in rheological behavior. Under these conditions, the deteriorated filtration masked the filtration improvement by the enlarged floc. Therefore, floc strength is definitely a valuable index in characterizing the performance of polymer conditioning and dewatering. Among conventional mechanical dewatering devices, vacuum filtration is typically classified as a low-pressure system, in which the major dewatering mechanism is filtration rather than consolidation. In high-pressure systems such as belt press, plate press and centrifugation, the consolidation stage generally follows filtration. In the beginning of the consolidation stage, the filter cake has already formed and begun to endure the pressure. Upon using a high-pressure filtration, a minimum floc strength within the sludge cake must be maintained to achieve satisfactory dewatering performance. The following test with a high-speed centrifuge was then conducted to study the influence of G on dewaterability. The residual moisture content of the dewatered cakes was measured. An interesting result was found in Fig. 4. The moisture contents of the cakes from conditioned sludges decreased with the increasing G values continuously, while those from the unconditioned sludge decreased with G initially and then remained within a finite range when G values were larger than 6244. This means that water can still be expressed out of the cake of conditioned sludge when the G is increased over 6244. Thus, although conditioning does not produce more removable water, it forms strong flocs to withstand the high centrifugation force. However, weak flocs tend to retain more moisture due to the increased resistance of cake filtration caused by the broken and deformed flocs. Therefore, the dewatering systems involving high pressure, increasing floc
Fig. 3. Linear regression of SRF vs. floc strength of sludges under various conditioning.
C.C. Wu et al. / Advances in Environmental Research 7 (2003) 617–621
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Acknowledgments This work was financially supported by the grant of National Science Council, ROC. Sincere thanks are expressed to Dr Jill Ruhsing Pan for her study advise and linguistic help. References
Fig. 4. Dewaterability of conditioned sludges under various centrifugation forces.
strength should improve the dewaterability and result in a cake of low-moisture content. 4. Conclusions Strong flocs can not guarantee optimal dewatering efficiency, although a confined floc strength growth is advantageous for vacuum filtration. For centrifugal dewatering, increasing floc strength can not improve moisture removal when the centrifugation G value is low. At high G, a strong floc structure is required to inhibit cake blinding from floc deformation. To optimize the dewatering process, the floc strength of the sludge should be considered beside the traditional dewaterability indices. Centrifugation dewatering is suggested for sludge with strong flocs, while vacuum filtration is recommended for sludge with medium strength flocs.
Christensen, J.R., Sorensen, P.B., Christensen, G.L., 1993. Mechanisms for overdosing in sludge conditioning. J. Environ. Eng. ASCE 119, 159–171. Hannah, S.A., Cohen, J.M., Robeck, G.G., 1967. Measurement of floc strength by particle counting. J. AWWA 59, 843–858. Lee, D.J., 1994. Floc structure and bound water content in excess activated sludge. J. Chin. Inst. Chem. Eng. 25, 201–207. Lee, D.J., Hsu, Y.H., 1994. Fast freezeythaw treatment on excess activated sludges: floc structure and sludge dewaterability. Environ. Sci. Technol. 28, 1444–1449. Leentvaar, J., Rebhun, M., 1983. Strength of ferric hydroxide flocs. Water Res. 17, 895–902. Novak, J.T., Prencteville, J.F., Sherrard, J.H., 1988. Mixing intensity and polymer performance on sludge dewatering. J. Environ. Eng. ASCE 114, 190–198. Tambo, N., Watanabe, Y., 1979. Physical characteristics of flocs—I. The floc density function and aluminum floc. Water Res. 13, 409–419. Wen, H.J., Lee, D.J., 1998. Strength of cationic polymerflocculated clay flocs. Adv. Environ. Res. 2, 390–397. Werle, C.P., Novak, J.T., 1984. Mixing intensity and polymer conditioning. J. Environ. Eng. ASCE 110, 919–934. Wu, C.C., Huang, C., Lee, D.J., 1997. Effects of polymer dosage on alum sludge dewatering characteristics and physical properties. Colloids Surf. 122, 89–96. Wu, C.C., Lee, D.J., Huang, C., 2000. Determination of the optimal dose of polyelectrolyte sludge conditioner considering particle sedimentation effects. Adv. Environ. Res. 4, 245–249.
Floc strength and dewatering efficiency of alum sludge Chih Chao Wu*, Jerry J. Wu, Ruey Yi Huang Department of Environmental Engineering and Science Feng Chia University, Taichung, 407 Taiwan, ROC
Abstract The effect of floc strength on the dewatering efficiency of alum sludge is studied in this work. Addition of polymer strengthens the floc structure of the sludge, which reduces floc breakage and sludge dewaterability. The result of vacuum filtration suggests that a confined floc strength growth benefits the filterability, although the strongest floc structure does not guarantee maximal dewatering efficiency. For centrifugal dewatering, increasing floc strength can not reduce the moisture content of the sludge cake when the centrifugal G value is under 7000. At high G, a strong floc structure is necessary to reduce cake blinding from floc deformation, and the lowest moisture content is found in sludge cake with the highest floc strength. Therefor, floc strength should be included with other traditional dewaterability indices in determining sludge dewatering. Centrifugation is suggested for the dewatering of sludge with high floc strength, and vacuum filtration for middle floc strength. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polymer conditioning; Sludge dewatering; Floc strength; Fractal dimension; Floc size; Vacuum filtration; Centrifugation
1. Introduction Polymer conditioning has been widely employed to improve the dewatering characteristics of sludge and the mechanical dewatering efficiency. It has been reported that over-intensive shear and insufficient polymer results in floc breakage and deterioration of dewaterability (Werle and Novak, 1984; Novak et al., 1988; Wu et al., 1997). After chemical conditioning, sludges are dewatered with mechanical dewatering devices of various shear stresses. The degree of floc breakage and deformation in cake filtration strongly affects the performance of the dewatering equipment. Capillary suction time (CST) and specific resistance of filtration (SRF) are the most commonly used indices for determining the optimal dose of polymer conditioning. Although for many years floc strength has been adopted to assess the floc breakage caused by the surrounding shear force, most researchers have focused their studies on the effect of floc strength on particles aggregation (or flocculation) of the dilute solution with very little *Corresponding author. Tel:q886-4-2451-7250 ext 5213; fax: q886-4-2451-2405. E-mail address: [email protected] (C.C. Wu).
attention on the sludge system. This paper presents insight on floc strength and the dewatering efficiency of the polymer-conditioned sludge. 2. Experimental 2.1. Sludge conditioning and dewatering Sludge samples were collected from the sedimentation tank of the Feng-yuan water treatment facility (Taichung, Taiwan). The cationic polyelectrolyte (polymer PC-320) was obtained from the Taiwan Polymer Company. Polymer PC-320 is a copolymer of acrylamide and diallyldimethyl-amonium chloride, with an average molecular weight of 1.1;1.2=107, and a 20% charge density. Polymer solution (0.1% wyw) was prepared as suggested by the polymer manufacturer. A Couette flocculator was adopted as the mixing apparatus to generate a Taylor vortex, in which a better velocity-gradient distribution than the traditional mixing was provided. The Couette flocculator consisted of two concentric cylinders (inner: Øs170 mm, Hs200 mm; outer: Øs180 mm, Hs250 mm) with a space of 5 mm in which the test sludge and polymer were added. The
1093-0191/03/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 2 . 0 0 0 4 8 - 5
C.C. Wu et al. / Advances in Environmental Research 7 (2003) 617–621
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Table 1 The CSTs and floc strengths of alum sludge with cationic polymer conditioning
CST (per second) Floc strength
0 mgyl
10 mgyl
20 mgyl
30 mgyl
127.2 2.41
27.6 4.56
19.2 5.48
21.9 5.88
Data are the average of three trials.
outer cylinder was stationary. The device was coupled with and powered by a variable speed motor connected to a torquemeter. The rotational speed was set to achieve the desired velocity gradient (G). Sludge dewaterability was evaluated by the capillary suction time (CST) and the specific resistance of filtration (SRF). The Triton CST apparatus model 200 was used to measure the CST value. A standard Buchner funnel apparatus with a 9-cm funnel was used for vacuum filtration and SRF determination. To reduce the sedimentation effect on SRF determination, stirring was provided inside the Buchner funnel during filtration (Wu et al., 2000). The data shown in Table 1 are the CSTs of test samples and measured just after the end of the conditioning period. For the convenience of data comparison, we define here the optimal dose as 20 mgy l based on the dose of minimum CST. Doses below 20 mgyl are referred to as under-dosing, and doses over 20 as overdosing, in this study. In centrifugation, settled sludge samples were centrifuged at five different rev.ymin for 1 min. The supernatant was decanted and the dry solids content of the sludge cake was determined by drying at 105 8C for 24 h. All the data are reported as the average of three trials.
polymer-conditioning (first mixing), a portion of the sludge was kept in the Couette flocculator and mixed for an extra 60 s; this is referred to as the breaking test hereafter. Four Gs were chosen for the mixing of the breaking test. At the end of the breaking mixing, approximately 400 flocs were removed for image analysis to determine the perimeter and diameter of the flocs. A log–log plot of the calculated average perimeter vs. velocity gradient and their linear regressions were established. The intercept of the regressed lines was taken as the ‘floc strength coefficient’ for each sludge. 3. Results and discussion 3.1. Floc breakage and floc strength Fig. 1 shows the influence of G on floc breakage, as illustrated by the mean floc size of the sludge sample. The mean floc size of the unconditioned alum sludge remained approximately 75 mm over four various velocity gradients (Gs), suggesting that the flocs were resistant to the selected shear with no detectable breakage. When 10 mgyl cationic polymer was added, the mean floc size increased to 375 mm before the breaking test was initiated. This enlargement in floc size was significant as shown by the decrease of CST, as seen in Table 1. When 280ys was applied, breakage of the flocs occurred and the mean floc size decreased to the size of the original alum floc. With the optimal dosage of 20 mgyl, as determined from the unconditional sludge, the mean floc size grew up to 530 mm, the largest size produced in this test, and then remained at approximately 450 mm during the subsequent breakage test. The floc size decreased dramatically down to 150 mm, when the velocity gradient of 480ys was employed.
2.2. Fractal dimension analysis Fractal dimension, D, of the floc was determined from the log–log plot of the size vs. density. The slope was calculated via linear regression analysis, from which the D was evaluated as Ds3qS (Lee and Hsu, 1994). A free settling test was employed to measure the wet density of the sludge floc. The settling travel of the sludge aggregate in a quiescent column was recorded by a video camera equipped with a close-up lens. The floc diameter and terminal velocity were determined from the replayed tape. Floc densities were calculated from terminal settling velocity and the diameter of the floc using a modified Stoke’s equation and presented as wet density (Tambo and Watanabe, 1979). The details of the experiment can be found our previous work (Wu et al., 1997). 2.3. Floc strength measurement Floc strength was determined following the method given by Leentvaar and Rebhun (1983). After each
Fig. 1. Variation of mean floc size floc of polymer-conditioned sludge with increasing shear stress in breaking test.
C.C. Wu et al. / Advances in Environmental Research 7 (2003) 617–621 Table 2 Fractal dimensions of conditioned and unconditioned sludges under various degree of breaking test Dose
After conditioning After breaking test G (per second) 170 280 360 480
0 mgyl
10 mgyl
20 mgyl
30 mgyl
1.54
1.80
1.83
1.83
1.54 1.53 1.50 1.52
1.68 1.63 1.59 1.56
1.82 1.83 1.76 1.70
1.85 1.83 1.84 1.81
Such a change suggested that even under the optimal dose, the aggregated floc could not withstand the shear at 480ys. Under over-dosing conditioning, 30 mgyl, the size of the floc remained fairly stable at approximately 450 mm and then grew to 530 mm when Gs exceeded 280ys. Floc breakage started to occur when the Gs was greater than 280ys. However, the mean floc size stayed at approximately 375 mm. The increase in floc size before the breaking process was due to over-dosing. As mentioned earlier, the optimal dose, 20 mgyl, was quantitatively determined by the CST response at the selected mixing intensity in the conditioning stage. Such shear stress was unable to provide sufficient mixing intensity for a complete flocculation. Consequently, the shear stress applied in the subsequent breaking test compensated for the insufficient mixing of the previous conditioning stage and produced the observed growth in floc size. However, when more intensity was provided, the same trend in floc breakage as occurred in other doses was observed. The much higher threshold Gs of the sludge conditioned by 30 mgyl polymer suggested that the addition of polymer enhanced the floc strength of the sludge. Fractal dimension analysis is effective in investigating intrafloc structures, which reflects the manner in which the primary particles are packed within the floc (Wen and Lee, 1998). The theoretical values of D vary from 1 to 3, which provide an useful index for the degree of floc compactness and how the particles are packed (Lee, 1994; Wu et al., 1997). Table 2 lists fractal dimensions of sludge flocs under various conditioning and breaking tests. Without the excess mixing, as seen in the 1st row data, the D increased dramatically from 1.54 to 1.80 with the addition of 10 mgyl polymer, and remained unchanged when more polymers were added. This result indicated that polymer addition strongly influenced floc structures, whether compactness or configuration. Two critical findings were discovered from the variations in D values after the breakage test. First, the D at 10 and 20 mgyl polymer conditioning decreased gradually with the
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increasing shear, but those at 0 and 30 mgyl remained unchanged. Second, it was apparent that the effect of Gs on the floc structure of the sludge conditioned with 10 mgyl polymer was more significant on D, dropping from 1.80 to 1.56. A more moderate decrease, from 1.83 to 1.70, was observed at 20 mgyl polymer addition. This implies that polymer addition enhances floc strength to resist the breaking process. These two observations suggest that the architecture of the flocs of the conditioned sludge had been rearranged according to the floc strength and the intensity of the following shear. Recalling the discussion of floc size variation in Fig. 1, less floc breakage and larger average floc diameter were associated with the dose of 30 mgyl. A relationship between sludge conditioning and floc strength was thus deduced. When a low dose of polymer, 10 mgyl, is added, the sludge flocs formed are easily broken and lose their floc compactness. At optimal-dosing, floc strength increases, causing less floc breakage and floc compactness. At over-dosing, very little breakage occurs. Besides the changes in floc size, a direct measurement of floc strength also provides evidence for this theory. The extent of floc strength can be quantified by the floc strength coefficient with the measurement proposed by Leentvaar and Rebhun (1983). Table 1 shows that floc strengths increased with polymer doses, indicating that the addition of polymer does enforce the strength of aggregated flocs, even at over-dosing at 30 mgyl. To investigate more about the over-dosing conditioning, 40 mgyl polymer was applied in a similar test. Unfortunately, since the excess amount of polymer caused the fast sedimentation fast in the Couette Flocculator, the floc did not travel long in the Taylor vortex during the conditioning period, which made the following breaking test meaningless. Based on the experimental results, it is reasonable to believe that the more polymer is applied, the stronger the floc is. The conclusion that polymer addition enforces floc strength is consistent with previous reports about suspension systems (Hannah et al., 1967; Leentvaar and Rebhun, 1983). 3.2. Floc strength and dewatering performance In the following section, the effect of floc strength on dewatering performance is studied. Vacuum filtration was used to evaluate the filterability. Fig. 2 is a typical filtration result where the volume of the filtrate is plotted as a function of filtration time for conditioned and unconditioned sludges. To evaluate the dewatering performance, an index T85, defined as the cumulative filtration time to remove 85% of expressible water from the sludge cake, was adopted. The addition of polymer improved filtration efficiency no matter how little of the polymer was used. However, 10 mgyl has been found to be the most efficient dose. This phenomenon
620
C.C. Wu et al. / Advances in Environmental Research 7 (2003) 617–621
Fig. 2. Filtration of sludges under various polymer-conditioning. (Floc strength of each sludge sample is given behind each polymer dose.)
could not be explained by the enhancement in floc strength only. When the SRF was plotted against the corresponding floc strength, as shown in Fig. 3, the R square of the linear regression was only 0.2061. Such poor correlation indicates that floc strength has very little influence on the dewatering by vacuum-filtration. However, it should be noted that a confined strength growth is favorable for improving the filtration rate of the sludge and the over-growth is associated with an increase in undesired resistance. In other words, the floc strength must be kept in the range of 2.41;5.48 to optimize the polymer conditioning and the dewatering. During filtration, flocs of weak structure tend to break when they are transported through the filter medium to form filter cakes. Not only are the pores between the filter medium and the cake blinded by the fragments from floc breakage, but the filtration resistance also rises with ease. Therefore, a minimum floc strength is necessary to avoid floc breakage during filtration. In view of the above discussion, the upper limit of floc strengths seem meaningless for filtration. Actually, the growth of floc strength is caused by the addition of polymer which also increases the viscosity of the sludge system. As we know, a major improvement in cake filtration results from the enlargement of floc size that greatly reduces the blind effect. That is why the addition of 10 mgyl polymer can produce the best filterability (lowest T85 in Fig. 2 and SRF in Fig. 3). Beyond that dose, the residual polymer in the suspension increased and the viscosity rose. Christensen et al. (1993) had concluded that the increasing viscosity in the liquid phase is the primary source for the overdosing of polymer conditioning. The increase in floc strength by adding polymers at 20 and 30 mgyl is additional
evidence of the change in rheological behavior. Under these conditions, the deteriorated filtration masked the filtration improvement by the enlarged floc. Therefore, floc strength is definitely a valuable index in characterizing the performance of polymer conditioning and dewatering. Among conventional mechanical dewatering devices, vacuum filtration is typically classified as a low-pressure system, in which the major dewatering mechanism is filtration rather than consolidation. In high-pressure systems such as belt press, plate press and centrifugation, the consolidation stage generally follows filtration. In the beginning of the consolidation stage, the filter cake has already formed and begun to endure the pressure. Upon using a high-pressure filtration, a minimum floc strength within the sludge cake must be maintained to achieve satisfactory dewatering performance. The following test with a high-speed centrifuge was then conducted to study the influence of G on dewaterability. The residual moisture content of the dewatered cakes was measured. An interesting result was found in Fig. 4. The moisture contents of the cakes from conditioned sludges decreased with the increasing G values continuously, while those from the unconditioned sludge decreased with G initially and then remained within a finite range when G values were larger than 6244. This means that water can still be expressed out of the cake of conditioned sludge when the G is increased over 6244. Thus, although conditioning does not produce more removable water, it forms strong flocs to withstand the high centrifugation force. However, weak flocs tend to retain more moisture due to the increased resistance of cake filtration caused by the broken and deformed flocs. Therefore, the dewatering systems involving high pressure, increasing floc
Fig. 3. Linear regression of SRF vs. floc strength of sludges under various conditioning.
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Acknowledgments This work was financially supported by the grant of National Science Council, ROC. Sincere thanks are expressed to Dr Jill Ruhsing Pan for her study advise and linguistic help. References
Fig. 4. Dewaterability of conditioned sludges under various centrifugation forces.
strength should improve the dewaterability and result in a cake of low-moisture content. 4. Conclusions Strong flocs can not guarantee optimal dewatering efficiency, although a confined floc strength growth is advantageous for vacuum filtration. For centrifugal dewatering, increasing floc strength can not improve moisture removal when the centrifugation G value is low. At high G, a strong floc structure is required to inhibit cake blinding from floc deformation. To optimize the dewatering process, the floc strength of the sludge should be considered beside the traditional dewaterability indices. Centrifugation dewatering is suggested for sludge with strong flocs, while vacuum filtration is recommended for sludge with medium strength flocs.
Christensen, J.R., Sorensen, P.B., Christensen, G.L., 1993. Mechanisms for overdosing in sludge conditioning. J. Environ. Eng. ASCE 119, 159–171. Hannah, S.A., Cohen, J.M., Robeck, G.G., 1967. Measurement of floc strength by particle counting. J. AWWA 59, 843–858. Lee, D.J., 1994. Floc structure and bound water content in excess activated sludge. J. Chin. Inst. Chem. Eng. 25, 201–207. Lee, D.J., Hsu, Y.H., 1994. Fast freezeythaw treatment on excess activated sludges: floc structure and sludge dewaterability. Environ. Sci. Technol. 28, 1444–1449. Leentvaar, J., Rebhun, M., 1983. Strength of ferric hydroxide flocs. Water Res. 17, 895–902. Novak, J.T., Prencteville, J.F., Sherrard, J.H., 1988. Mixing intensity and polymer performance on sludge dewatering. J. Environ. Eng. ASCE 114, 190–198. Tambo, N., Watanabe, Y., 1979. Physical characteristics of flocs—I. The floc density function and aluminum floc. Water Res. 13, 409–419. Wen, H.J., Lee, D.J., 1998. Strength of cationic polymerflocculated clay flocs. Adv. Environ. Res. 2, 390–397. Werle, C.P., Novak, J.T., 1984. Mixing intensity and polymer conditioning. J. Environ. Eng. ASCE 110, 919–934. Wu, C.C., Huang, C., Lee, D.J., 1997. Effects of polymer dosage on alum sludge dewatering characteristics and physical properties. Colloids Surf. 122, 89–96. Wu, C.C., Lee, D.J., Huang, C., 2000. Determination of the optimal dose of polyelectrolyte sludge conditioner considering particle sedimentation effects. Adv. Environ. Res. 4, 245–249.