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Flow through activated sludge flocs
War. Res. Vol. 22, No. 6, pp. 789-792, 1988 Printed in Great Britain. All rights reserved
0043-1354/88 $3.00+ 0.00 Copyright © 1988 Pergamon Press pie
RESEARCH NOTE FLOW THROUGH ACTIVATED SLUDGE FLOCS DA-HONG LI and J. GANCZARCZYK*• Department of Civil Engineering, University of Toronto, Toronto, Canada M5S IA4 (Received March 1987; accepted in revised form December 1987)
Abstract--Studies on the sedimentation of activated sludge floes formed in the presence of impermeable solid particles provided experimental evidence to the theoretical considerations by Logan and Hunt regarding the possibility of fluid flow through the floes. These studies indicated that the flow through the floes containing biomass carriers was seriously impeded as compared to the flow through floes without carriers. Key words--activated sludge floes, floe physical charactristics, permeability, porosity
INTRODUCTION Activated sludge floes are irregularly shaped, fragile, almost transparent aggregates which have a high water content and spread over a wide size range (Li and Ganczarczyk, 1986). Coinciding with the high water content, high porosity of activated sludge floes has been reported by many investigators (Mueller et al., 1966; Smith and Coakley, 1984; Li and Ganczarczyk, 1987). However, diffusion models have been predominantly applied in the analysis of mass transfer within the activated sludge floc (e.g. Baillod and Boyle, 1970; Smith and Coakley, 1984), despite the fact that the floes are highly porous and the outcomes from measurements of substrate uptake rates of the floes are sometimes controversial. These diffusion models led to the conclusion that cells within a bacterial floc could never have a greater substrate uptake rate than dispersed cells (Aris, 1975; Matson and Characklis, 1976). Based on the diffusion theory, many researchers hypothesized that an anoxic core existed in a bacterial floc of certain size as a result of oxygen transfer limitations (Wuhrmann, 1964; Matson and Characklis, 1976; Benefield and Molz, 1984). For non-biological systems, some theoretical analyses described possible fluid flow through highly porous aggregates (Ooms et al., 1970; Neale et al., 1973). The analyses showed that the hydrodynamic resistance experienced by an aggregate permeable to the liquid flow would be less than that by an impermeable aggregate. Correspondingly, the terminal settling velocity of a permeable aggregate would be higher than that of an impermeable aggregate of the same size and density. Settling experiments of porous particles made of steel wool (Matsumoto and Suganuma, 1977) and semi-rigid plastic (Masliyah *To whom correspondence should be addressed. 789
and Polikar, 1980) substantiated the theoretical hypothesis at small Reynolds number regions. Recently, Hunt and Logan (1984) and Logan and Hunt (1987, 1988) indicated that the traditional diffusion models for substrate transport into microbial aggregates such as activated sludge floes were difficult to justify. From the ecological and genetic point of view, bacteria should not expend energy to form floes under low-nutrient conditions, if floc formation only resulted in a reduced nutrient availability to the cells. They theorized that bioflocculation was an advantageous microbial response to substrate limitations. According to their prediction, the microbial aggregates were so porous that they might be permeable to fluid flow within certain shear rates or during gravitational settling. The intra-aggregate fluid velocity, which was assumed to be zero in the diffusion model, was considered significant. In contrast, mass transfer by diffusion might be negligible because the flow through the floc allowed cells within the floes to have access to the same substrate concentrations available to dispersed cells. By the advective mass transport, oxygen deficiency would not occur in the activated sludge floes and the maximum uptake rate of cells within the floes in sheared fluids could theoretically be increased over that of an identical amount of dispersed cells. EXPERIMENTAL CONCEPT In this paper, experimental evidence is presented to support the hypothesis of the fluid flow through activated sludge floe. This evidence was obtained from a sedimentation study of activated sludge floes which were formed with and without impermeable carriers. As Fig. 1 depicts, the floc permeability will be seriously limited by the presence within the floc of a barrier of a solid biomass carrier. Therefore, a
790
Research Note
possible flow through a porous floc
the carrier floc with partially blocked flow
conditions by the stroboscopic technique developed previously (Li and Ganczarczyk, 1987). Using this multiexposure technique, settling flocs were photographed for their velocity determinations. The longest and shortest dimensions of the flocs were measured from photographic negatives using an image analysis system BIOQUANT II (R&M Biometrics, U.S.A.) which consisted of an IBM-PC computer, a video monitor with microscope, a HIPAD digitizing tablet and a printer. When a negative was placed under the microscope, images of the flocs were seen on the monitor screen and measurements were made using the digitizing tablet.
Fig. 1. Experimental concept for testing the flow through activated sludge flocs. RESULTS AND DISCUSSION deviation of settling velocity by a floc with from t h a t o f the n o r m a l flocs m a y represent a density difference for flocs of c o m p a r a b l e also the effect of substantially reduced floc a n d permeability.
a carrier not only sizes but porosity
MATERIAL AND METHODS Three types of carrier/activated sludge flocs were investigated and compared with the control flocs which did not contain carriers. The carrier/activated sludge process is characterized by the presence of solid carriers as supporting media for biomass growth. This modified activated sludge process has been reported to be able to improve the performance of the conventional process (Lu and Ganczarczyk, 1983). The solid carriers used in this study were activated carbon, coke and a non-ionic synthetic ion-exchange resin, XAD-4 (Rohm and Haas, Canada). Their physical characteristics are given in Table 1. As shown in Table 1, number distribution measured by an image analysis system does not appear matched with the mass distribution, mainly because of the breakage of the particles. The value of the wet bulk density (wet mass per unit volume of the particles with internal pores and interparticle space filled with water after centrifuging) is an underestimate due to the inclusion in the measurement of interparticle water. The control and carrier/activated sludge systems under study were operated as laboratory fill-and-draw reactors. The same organic loadings, sludge retention time, turbulence intensity and air supply rate were maintained for the four reactors, while the mixed liquor suspended solids (MLSS) were allowed to reach the respective maximum levels. The detailed operating parameters of these experiments were reported by Senthilnathan and Ganczarczyk (1987). When the reactors reached a steady state of operation, the settling velocity of the activated sludge flocs formed with and without solid carriers was measured under quiescent Table 1. Physical characteristics of the solid carriers Activated carbon Coke Resin Intervals (~m) (%) (%) (%) Size 53 74 15 15 distribution* 74-106 35 35 106-150 35 35 150-212 15 15 Size 53-74 43 32 -distribution'~ 74-106 28 29 -106-150
Density gem 3
150-212 >212 True solid Wet bulk
13
28
13 I 1.83 1.20
10 3 1.84 1.37
*By sieve and based on mass.
try microscopic observation and based on number.
2
10 88 1.06 1.03
Three basic types of carrier/activated sludge flocs were observed u n d e r the optical microscope in these experiments. The first type comprised o f large activated c a r b o n and coke carrier particles on which biomass grew forming a biofilm with a thickness varying from 20 to 9 0 B m . The second type comprised of biomass flocs e n t r a p p i n g one or several small solid carrier particles, particularly the debris from the d a m a g e o f activated c a r b o n particles and, to a smaller extent, of coke carrier particles. In this case, the biomass of the floc could hardly be called biofilm. As to the third type, the biomass formed o n m o s t of the large synthetic resin particles a n d on a very small n u m b e r of c a r b o n a n d coke particles was so t e n u o u s t h a t the thickness o f the biofilm could not be measured by the m e t h o d applied. The size o f the flocs was expressed as the average floc diameter, defined as one half of the sum o f the longest a n d shortest dimensions of the floc. T o c o m p a r e the floc size with t h a t of the carriers, the m e a s u r e d floc diameters were divided into the same intervals by which the carrier particle size distributions were presented in Table 1 (Table 2). The s h o w n floc size distribution was s o m e w h a t distorted due to the limitation o f the p h o t o g r a p h i c technique applied which m a d e detection of flocs < 2 5 / l m difficult. The settling velocity data o b t a i n e d from measurements of 70-150 flocs for each reactor are presented in Fig. 2 as a function of average floc diameter. They are relatively scattered as expected, but the carrier flocs settled as a rule at lower velocities t h a n flocs in the control, a l t h o u g h some overlapping existed. A regression analysis showed t h a t linear functions h a d higher correlation coefficients t h a n non-linear functions for the data of control, c a r b o n and coke flocs, while no relationship could be f o u n d
Table 2. Floc size distributions* Carbon Coke Control carriers carriers Intervals (~m) (%) (%) (%) < 53 35 17 10 53 74 25 21 13 74-106 17 24 19 106-150 8 19 17 150-212 7 12 13 > 212 8 7 28 *By microscopic observation and based on number.
Resin carriers (%) 7 21 24 17 20 11
Research Note 4 3.5
Control
3
%
2.5
2
:
.• .
1.5
0.5 Activated Carbon Carrier • ee
1
•
• •:
...,jj.'c
E
Coke Carrier
•
2.5
•• ••
1.5
•
C 2
%•
ResinCarrier
15
~J~',t4~.~:
I
I,_-'qit ~ ' = "
0.s . ' . q - f ' r 01
/ 100
" • •
~ 200 Diameter
= = 300 400 (microns)
500
Fig, 2. Floc settling velocities. to represent the very scattered resin floc data. The linear functions resulting from the regression analysis, presented in Fig. 3, confirmed that in general the carrier floc settling velocity decreased as compared to the control flocs, or that the carrier flocs settled at least at the same velocity if the decrease was not significant. Stokes' law indicates that the settling velocity of a particle in the laminar hydraulic region is proportional to its buoyant density. The density of activated sludge floc reported in the literature is generally <1.1 g c m -3 compared to the wet bulk density of 1.20 or 1.37 g c m -3 of the activated sludge carbon and coke carriers studied. As viewed from the difference in density, the carbon and coke carrier flocs were expected to settle at higher velocities than did the control flocs. However, the difference in density was not the only factor influencing the settling velocity brought about when the carbon and coke
carriers were introduced into the activated sludge flocs. It appears that the impermeable carriers also blocked some or most floc internal pores and reduced drastically the overall floc porosity and permeability. Moreover, the porosity of the biological part of the carrier floc, which may be regarded as a biofilm, was likely reduced as well. Biofilm growing on solid surface was observed to be denser than activated sludge flocs (Hermanowicz and Ganczarczyk, 1983). If reduced floc porosity was the factor that offsets the effect of increased floc density on floc settling velocity, fluid flow through the studied flocs without carriers must have occurred and was substantially impeded by the presence of the carriers. It can be seen from Fig. 3 that the slopes of the lines for the solid carrier flocs are less steep than that for the control flocs, indicating that difference in settling velocity is less substantial for small flocs. This may be explained by the observation that most small solid particles were entrapped by large activated sludge flocs, leaving less chance for small flocs to contain a solid carrier. The presence of the solid carriers in the small flocs and therefore the effect of carriers on the settling of the small flocs were limited. Theoretical analysis by Logan and Hunt (1987) also showed that the difference between settling velocities of permeable and impermeable aggregates increased with the increase of aggregate size. The situation in the resin/activated sludge system is different. Resin particles are relatively large (Table 1) and do not break under experimental conditions. No small floes can entrap a resin particle and biomass growth on resin particles results in larger carrier flocs. Consequently, the complete overlapping of control and resin data in the range of < l S 0 / ~ m in floc diameter was observed. The difference in settling velocity takes place only when the flocs are > 150 p m where a considerable number of resin particles existed. It is expected that activated sludge flocs generated under different conditions from those applied in this study could have different permeabilities. Further research on this subject is under way. CONCLUSIONS
30i 2.5
........--""'
2.0
E
.~ 1.0 O.5
791
I
I
I
I
FIoc Diameter ~ m )
Fig. 3. Comparison o f floc settling velocities. (1) Control (=0.84), (2) coke carrier (=0.65), (3) activated sludge carrier (r = 0.50).
The settling velocities of carrier/activated sludge flocs formed in the presence of activated carbon, coke and resin were found to be at least no higher than the settling velocities of the comaparable size flocs formed without carriers. Considering that the carbon and coke have densities much higher than and the resin has density closer to that of floc biomass, differences in the settling velocities of activated sludge flocs and carrier/activated sludge flocs provide an evidence of fluid flow through activated sludge flocs.
Acknowledgements--The authors wish to thank S. Hermanowicz, University of California, Berkeley, for providing both comments and information on the work by Hunt and Logan. Information from PR. Senthilnathan,
792
Research Note
University of Toronto, on the physical characteristics of the solid carriers and his work on operation of the carrier/activated sludge systems are appreciated. REFERENCES
Aris R. (1975) The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts, Vol. L The Theory of the Steady State. Clarendon Press, Oxford. Baillod C. R. and Boyle W. C. (1970) Mass transfer limitations in substrate removal. J. sanit. Engng Div. Am. Soc. cir. Engrs 996, 525-545. Benefield L. and Molz F. (1984) A model for activated sludge process which considers wastewater characteristics, floc behavior and microbial population. Biotechnol. Bioengng 2,6, 352-361. Hermanowicz S. W. and Ganczarczyk J. J. (1983) Some fluidization characteristics of biological beds. Biotechnol. Bioengng 25, 1321-1330. Hunt J. R. and Logan B. E. (1984) Bioflocculation as a microbial response to substrate limitations. Presented at the division of Microbial and Biochemical Technology, American Chemical Society, 188th Meeting, Philadelphia, Pa. Li D. H. and Ganczarczyk J. J. (1986) Physical characteristics of activated sludge flocs. CRC Crit. Rev. envir. Control 17, 53-87. Li D. H. and Ganczarczyk J. J. (1987) Stroboscopic determination of settling velocity, size and porosity of activated sludge flocs. Wat. Res. 21, 257-262. Logan B. E. and Hunt J. R. (1987) Advantages to microbes of growth in permeable aggregates in marine systems. Limnol. Oceanogr. 32, 1034-1048.
Logan B. E. and Hunt J. R. (1988) Bioflocculation as microbial response to substrate limitation. Biotechnol. Bioengng 31, 92-101. Lu Y. S. and Ganczarczyk J. J. (1983) Application of carrier/activated sludge process for treatment of phenolic wastewater. Proc. 38th Ind. Waste Conf., pp. 643-657. Butterworth, Boston. Masliyah J. H. and Polikar M. (1980) Terminal velocity of porous spheres. Can. J. chem. Engng 58, 299-302. Matson J. V. and Characklis W. G. (1976) Diffusion into microbial aggregates. Wat. Res. 10, 877-885. Matsumoto K. and Suganuma A. (1977) Settling velocity of a permeable floc. Chem. Engng Sci. 32, 445-447. Mueller J. A., Voelkel K. G. and Boyle W. C. (1966) Nominal diameter of floc related to oxygen transfer. J. sanit. Engng Div. Am. Soc. civ. Engrs 92, 9-20. Neale G. N., Epstein N. and Nader W. (1973) Creeping flow relative to permeable spheres. Chem. Engng Sci. 28, 1865-1874. Ooms G., Mijnlieff P. F. and Beckers H. L. (1970) Frictional force exerted by a flowing fluid on a permeable particle, with particular reference to polymer coils. J. chem. Phys. 53, 4123-4230. Senthilnathan PR. and Ganczarczyk J. J. (1987) Determination of biomass in activated sludge containing large quantities of carbonaceous solid carriers. Paper presented at 22nd Canadian Symposium on Water Pollution Research, Toronto. Smith P. G. and Coakley P. (1984) Diffusivity, tortuosity and pore structure of activated sludge. Wat. Res. 18, 117 122. Wuhrmann K. (1964) Microbial aspects of water pollution control. Adv. appl. Microbiol. 6, 119-149.
0043-1354/88 $3.00+ 0.00 Copyright © 1988 Pergamon Press pie
RESEARCH NOTE FLOW THROUGH ACTIVATED SLUDGE FLOCS DA-HONG LI and J. GANCZARCZYK*• Department of Civil Engineering, University of Toronto, Toronto, Canada M5S IA4 (Received March 1987; accepted in revised form December 1987)
Abstract--Studies on the sedimentation of activated sludge floes formed in the presence of impermeable solid particles provided experimental evidence to the theoretical considerations by Logan and Hunt regarding the possibility of fluid flow through the floes. These studies indicated that the flow through the floes containing biomass carriers was seriously impeded as compared to the flow through floes without carriers. Key words--activated sludge floes, floe physical charactristics, permeability, porosity
INTRODUCTION Activated sludge floes are irregularly shaped, fragile, almost transparent aggregates which have a high water content and spread over a wide size range (Li and Ganczarczyk, 1986). Coinciding with the high water content, high porosity of activated sludge floes has been reported by many investigators (Mueller et al., 1966; Smith and Coakley, 1984; Li and Ganczarczyk, 1987). However, diffusion models have been predominantly applied in the analysis of mass transfer within the activated sludge floc (e.g. Baillod and Boyle, 1970; Smith and Coakley, 1984), despite the fact that the floes are highly porous and the outcomes from measurements of substrate uptake rates of the floes are sometimes controversial. These diffusion models led to the conclusion that cells within a bacterial floc could never have a greater substrate uptake rate than dispersed cells (Aris, 1975; Matson and Characklis, 1976). Based on the diffusion theory, many researchers hypothesized that an anoxic core existed in a bacterial floc of certain size as a result of oxygen transfer limitations (Wuhrmann, 1964; Matson and Characklis, 1976; Benefield and Molz, 1984). For non-biological systems, some theoretical analyses described possible fluid flow through highly porous aggregates (Ooms et al., 1970; Neale et al., 1973). The analyses showed that the hydrodynamic resistance experienced by an aggregate permeable to the liquid flow would be less than that by an impermeable aggregate. Correspondingly, the terminal settling velocity of a permeable aggregate would be higher than that of an impermeable aggregate of the same size and density. Settling experiments of porous particles made of steel wool (Matsumoto and Suganuma, 1977) and semi-rigid plastic (Masliyah *To whom correspondence should be addressed. 789
and Polikar, 1980) substantiated the theoretical hypothesis at small Reynolds number regions. Recently, Hunt and Logan (1984) and Logan and Hunt (1987, 1988) indicated that the traditional diffusion models for substrate transport into microbial aggregates such as activated sludge floes were difficult to justify. From the ecological and genetic point of view, bacteria should not expend energy to form floes under low-nutrient conditions, if floc formation only resulted in a reduced nutrient availability to the cells. They theorized that bioflocculation was an advantageous microbial response to substrate limitations. According to their prediction, the microbial aggregates were so porous that they might be permeable to fluid flow within certain shear rates or during gravitational settling. The intra-aggregate fluid velocity, which was assumed to be zero in the diffusion model, was considered significant. In contrast, mass transfer by diffusion might be negligible because the flow through the floc allowed cells within the floes to have access to the same substrate concentrations available to dispersed cells. By the advective mass transport, oxygen deficiency would not occur in the activated sludge floes and the maximum uptake rate of cells within the floes in sheared fluids could theoretically be increased over that of an identical amount of dispersed cells. EXPERIMENTAL CONCEPT In this paper, experimental evidence is presented to support the hypothesis of the fluid flow through activated sludge floe. This evidence was obtained from a sedimentation study of activated sludge floes which were formed with and without impermeable carriers. As Fig. 1 depicts, the floc permeability will be seriously limited by the presence within the floc of a barrier of a solid biomass carrier. Therefore, a
790
Research Note
possible flow through a porous floc
the carrier floc with partially blocked flow
conditions by the stroboscopic technique developed previously (Li and Ganczarczyk, 1987). Using this multiexposure technique, settling flocs were photographed for their velocity determinations. The longest and shortest dimensions of the flocs were measured from photographic negatives using an image analysis system BIOQUANT II (R&M Biometrics, U.S.A.) which consisted of an IBM-PC computer, a video monitor with microscope, a HIPAD digitizing tablet and a printer. When a negative was placed under the microscope, images of the flocs were seen on the monitor screen and measurements were made using the digitizing tablet.
Fig. 1. Experimental concept for testing the flow through activated sludge flocs. RESULTS AND DISCUSSION deviation of settling velocity by a floc with from t h a t o f the n o r m a l flocs m a y represent a density difference for flocs of c o m p a r a b l e also the effect of substantially reduced floc a n d permeability.
a carrier not only sizes but porosity
MATERIAL AND METHODS Three types of carrier/activated sludge flocs were investigated and compared with the control flocs which did not contain carriers. The carrier/activated sludge process is characterized by the presence of solid carriers as supporting media for biomass growth. This modified activated sludge process has been reported to be able to improve the performance of the conventional process (Lu and Ganczarczyk, 1983). The solid carriers used in this study were activated carbon, coke and a non-ionic synthetic ion-exchange resin, XAD-4 (Rohm and Haas, Canada). Their physical characteristics are given in Table 1. As shown in Table 1, number distribution measured by an image analysis system does not appear matched with the mass distribution, mainly because of the breakage of the particles. The value of the wet bulk density (wet mass per unit volume of the particles with internal pores and interparticle space filled with water after centrifuging) is an underestimate due to the inclusion in the measurement of interparticle water. The control and carrier/activated sludge systems under study were operated as laboratory fill-and-draw reactors. The same organic loadings, sludge retention time, turbulence intensity and air supply rate were maintained for the four reactors, while the mixed liquor suspended solids (MLSS) were allowed to reach the respective maximum levels. The detailed operating parameters of these experiments were reported by Senthilnathan and Ganczarczyk (1987). When the reactors reached a steady state of operation, the settling velocity of the activated sludge flocs formed with and without solid carriers was measured under quiescent Table 1. Physical characteristics of the solid carriers Activated carbon Coke Resin Intervals (~m) (%) (%) (%) Size 53 74 15 15 distribution* 74-106 35 35 106-150 35 35 150-212 15 15 Size 53-74 43 32 -distribution'~ 74-106 28 29 -106-150
Density gem 3
150-212 >212 True solid Wet bulk
13
28
13 I 1.83 1.20
10 3 1.84 1.37
*By sieve and based on mass.
try microscopic observation and based on number.
2
10 88 1.06 1.03
Three basic types of carrier/activated sludge flocs were observed u n d e r the optical microscope in these experiments. The first type comprised o f large activated c a r b o n and coke carrier particles on which biomass grew forming a biofilm with a thickness varying from 20 to 9 0 B m . The second type comprised of biomass flocs e n t r a p p i n g one or several small solid carrier particles, particularly the debris from the d a m a g e o f activated c a r b o n particles and, to a smaller extent, of coke carrier particles. In this case, the biomass of the floc could hardly be called biofilm. As to the third type, the biomass formed o n m o s t of the large synthetic resin particles a n d on a very small n u m b e r of c a r b o n a n d coke particles was so t e n u o u s t h a t the thickness o f the biofilm could not be measured by the m e t h o d applied. The size o f the flocs was expressed as the average floc diameter, defined as one half of the sum o f the longest a n d shortest dimensions of the floc. T o c o m p a r e the floc size with t h a t of the carriers, the m e a s u r e d floc diameters were divided into the same intervals by which the carrier particle size distributions were presented in Table 1 (Table 2). The s h o w n floc size distribution was s o m e w h a t distorted due to the limitation o f the p h o t o g r a p h i c technique applied which m a d e detection of flocs < 2 5 / l m difficult. The settling velocity data o b t a i n e d from measurements of 70-150 flocs for each reactor are presented in Fig. 2 as a function of average floc diameter. They are relatively scattered as expected, but the carrier flocs settled as a rule at lower velocities t h a n flocs in the control, a l t h o u g h some overlapping existed. A regression analysis showed t h a t linear functions h a d higher correlation coefficients t h a n non-linear functions for the data of control, c a r b o n and coke flocs, while no relationship could be f o u n d
Table 2. Floc size distributions* Carbon Coke Control carriers carriers Intervals (~m) (%) (%) (%) < 53 35 17 10 53 74 25 21 13 74-106 17 24 19 106-150 8 19 17 150-212 7 12 13 > 212 8 7 28 *By microscopic observation and based on number.
Resin carriers (%) 7 21 24 17 20 11
Research Note 4 3.5
Control
3
%
2.5
2
:
.• .
1.5
0.5 Activated Carbon Carrier • ee
1
•
• •:
...,jj.'c
E
Coke Carrier
•
2.5
•• ••
1.5
•
C 2
%•
ResinCarrier
15
~J~',t4~.~:
I
I,_-'qit ~ ' = "
0.s . ' . q - f ' r 01
/ 100
" • •
~ 200 Diameter
= = 300 400 (microns)
500
Fig, 2. Floc settling velocities. to represent the very scattered resin floc data. The linear functions resulting from the regression analysis, presented in Fig. 3, confirmed that in general the carrier floc settling velocity decreased as compared to the control flocs, or that the carrier flocs settled at least at the same velocity if the decrease was not significant. Stokes' law indicates that the settling velocity of a particle in the laminar hydraulic region is proportional to its buoyant density. The density of activated sludge floc reported in the literature is generally <1.1 g c m -3 compared to the wet bulk density of 1.20 or 1.37 g c m -3 of the activated sludge carbon and coke carriers studied. As viewed from the difference in density, the carbon and coke carrier flocs were expected to settle at higher velocities than did the control flocs. However, the difference in density was not the only factor influencing the settling velocity brought about when the carbon and coke
carriers were introduced into the activated sludge flocs. It appears that the impermeable carriers also blocked some or most floc internal pores and reduced drastically the overall floc porosity and permeability. Moreover, the porosity of the biological part of the carrier floc, which may be regarded as a biofilm, was likely reduced as well. Biofilm growing on solid surface was observed to be denser than activated sludge flocs (Hermanowicz and Ganczarczyk, 1983). If reduced floc porosity was the factor that offsets the effect of increased floc density on floc settling velocity, fluid flow through the studied flocs without carriers must have occurred and was substantially impeded by the presence of the carriers. It can be seen from Fig. 3 that the slopes of the lines for the solid carrier flocs are less steep than that for the control flocs, indicating that difference in settling velocity is less substantial for small flocs. This may be explained by the observation that most small solid particles were entrapped by large activated sludge flocs, leaving less chance for small flocs to contain a solid carrier. The presence of the solid carriers in the small flocs and therefore the effect of carriers on the settling of the small flocs were limited. Theoretical analysis by Logan and Hunt (1987) also showed that the difference between settling velocities of permeable and impermeable aggregates increased with the increase of aggregate size. The situation in the resin/activated sludge system is different. Resin particles are relatively large (Table 1) and do not break under experimental conditions. No small floes can entrap a resin particle and biomass growth on resin particles results in larger carrier flocs. Consequently, the complete overlapping of control and resin data in the range of < l S 0 / ~ m in floc diameter was observed. The difference in settling velocity takes place only when the flocs are > 150 p m where a considerable number of resin particles existed. It is expected that activated sludge flocs generated under different conditions from those applied in this study could have different permeabilities. Further research on this subject is under way. CONCLUSIONS
30i 2.5
........--""'
2.0
E
.~ 1.0 O.5
791
I
I
I
I
FIoc Diameter ~ m )
Fig. 3. Comparison o f floc settling velocities. (1) Control (=0.84), (2) coke carrier (=0.65), (3) activated sludge carrier (r = 0.50).
The settling velocities of carrier/activated sludge flocs formed in the presence of activated carbon, coke and resin were found to be at least no higher than the settling velocities of the comaparable size flocs formed without carriers. Considering that the carbon and coke have densities much higher than and the resin has density closer to that of floc biomass, differences in the settling velocities of activated sludge flocs and carrier/activated sludge flocs provide an evidence of fluid flow through activated sludge flocs.
Acknowledgements--The authors wish to thank S. Hermanowicz, University of California, Berkeley, for providing both comments and information on the work by Hunt and Logan. Information from PR. Senthilnathan,
792
Research Note
University of Toronto, on the physical characteristics of the solid carriers and his work on operation of the carrier/activated sludge systems are appreciated. REFERENCES
Aris R. (1975) The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts, Vol. L The Theory of the Steady State. Clarendon Press, Oxford. Baillod C. R. and Boyle W. C. (1970) Mass transfer limitations in substrate removal. J. sanit. Engng Div. Am. Soc. cir. Engrs 996, 525-545. Benefield L. and Molz F. (1984) A model for activated sludge process which considers wastewater characteristics, floc behavior and microbial population. Biotechnol. Bioengng 2,6, 352-361. Hermanowicz S. W. and Ganczarczyk J. J. (1983) Some fluidization characteristics of biological beds. Biotechnol. Bioengng 25, 1321-1330. Hunt J. R. and Logan B. E. (1984) Bioflocculation as a microbial response to substrate limitations. Presented at the division of Microbial and Biochemical Technology, American Chemical Society, 188th Meeting, Philadelphia, Pa. Li D. H. and Ganczarczyk J. J. (1986) Physical characteristics of activated sludge flocs. CRC Crit. Rev. envir. Control 17, 53-87. Li D. H. and Ganczarczyk J. J. (1987) Stroboscopic determination of settling velocity, size and porosity of activated sludge flocs. Wat. Res. 21, 257-262. Logan B. E. and Hunt J. R. (1987) Advantages to microbes of growth in permeable aggregates in marine systems. Limnol. Oceanogr. 32, 1034-1048.
Logan B. E. and Hunt J. R. (1988) Bioflocculation as microbial response to substrate limitation. Biotechnol. Bioengng 31, 92-101. Lu Y. S. and Ganczarczyk J. J. (1983) Application of carrier/activated sludge process for treatment of phenolic wastewater. Proc. 38th Ind. Waste Conf., pp. 643-657. Butterworth, Boston. Masliyah J. H. and Polikar M. (1980) Terminal velocity of porous spheres. Can. J. chem. Engng 58, 299-302. Matson J. V. and Characklis W. G. (1976) Diffusion into microbial aggregates. Wat. Res. 10, 877-885. Matsumoto K. and Suganuma A. (1977) Settling velocity of a permeable floc. Chem. Engng Sci. 32, 445-447. Mueller J. A., Voelkel K. G. and Boyle W. C. (1966) Nominal diameter of floc related to oxygen transfer. J. sanit. Engng Div. Am. Soc. civ. Engrs 92, 9-20. Neale G. N., Epstein N. and Nader W. (1973) Creeping flow relative to permeable spheres. Chem. Engng Sci. 28, 1865-1874. Ooms G., Mijnlieff P. F. and Beckers H. L. (1970) Frictional force exerted by a flowing fluid on a permeable particle, with particular reference to polymer coils. J. chem. Phys. 53, 4123-4230. Senthilnathan PR. and Ganczarczyk J. J. (1987) Determination of biomass in activated sludge containing large quantities of carbonaceous solid carriers. Paper presented at 22nd Canadian Symposium on Water Pollution Research, Toronto. Smith P. G. and Coakley P. (1984) Diffusivity, tortuosity and pore structure of activated sludge. Wat. Res. 18, 117 122. Wuhrmann K. (1964) Microbial aspects of water pollution control. Adv. appl. Microbiol. 6, 119-149.