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Microbial activity in biological activated carbon bed by pulse responses
~
Pergamon
. Wat Sci. Tech.Vol. 34. No. ~-6. pp, 213-222.1996. Copynght ~ 1996IAWQ. Published by Elsevier ScienceLtd Printed In Great Britain . All rights reserved.
PH:S0273-1223(96)00648-8
0273·1223196 $1soo+ 0'00
MICROBIAL ACTIVITY IN BIOLOGICAL ACTIVATED CARBON BED BY PULSE RESPONSES Akiyoshi Sakoda, JianzhongWang andMotoyuki Suzuki Institute ofIndustrial Science. UniversityofTokyo. 7·22-1 Roppongi, Minato.ku, Tokyo 106. Japan
ABSTRACT The moment analysis of pulse responses was applied to the biological activated carbon (BAC) in order to elucidate its microbial activity and adsorption capacity separately. The microbial activity derived from this approach was focussed on and the following was found in this work. First. the activity of micro-organisms attached on activated carbon was higher than those on other carriers. Second. the microbial activities of bench-scale BACs treating pond water varied withthe pretreatments by ozone and chlorine.bUI did not change considerably during the operation for about one year. Also. an the empirical relationship was found It was between the microbial degradation rates of pulse-injected glucose and background dissolved organics. concluded that this approach is useful for evaluating the microbial activity in BAC in a relatively easy manner. Copyright e 1996 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Biological activated carbon:adsorption: microbial degradation; drinking water treatment; pulse response method: momentanalysis. INTRODUCTION Where drinking water sources are pollutedby various kinds of trace dissolvedsubstances, it is often difficult for the conventional treatment proces~es consisting of c?agulation,sedi~en~tion and filtration to purify such polluted raw water enough to satisfy the water quality standard of drinking water. Granular activated carbon (GAC) is already employedwidely in the waterworksto meet this demand. Recently,GAC with thin biofilms on its external surface has been studied as a cost efficient and high performance unit operation for drinking water treatments especially in Japan (Magara, 1989: Magara et al., 1993: Taniguchiet al, 1993: Kajino et al, 1993: Umiga et al, 1991). The GAC with biofilms is often called biological activated carbon (BAC) (Rollingerand Doot,1987: Zhanget al, 1991). A lot of experimental BAC processes have already shown the long-term and apparent steady-state removal of trihalomethaneprecursors.odoro~s compoun~s and so o~. In those cases, it i~ usu~lIy said that organics are removed by cooperation of physical adsorptiononto activated carbon and microbial degradation. Also the microbial degradation of adsorbedorgani~s is.called b ~oregeneration (Chudyk and Snoeylnk, 1984; Speitel et al, 1987). which can extend the service hfe of activatedCarbon. However, the understandingof 213
A. SAKODA et al.
214
BAC is still vague in regard to the roleof activated carbon, the interaction between activated carbon and micro-organisms,etc. The first step for elucidating the mechanisms of BAC is to evaluate its adsorption characteristics and microbial activity separately and simultaneously. In order to establish such a simple methodology that is suitable for this purpose, we have been interested in applying the moment analysis of pulse responses (Suzuki and Sohn, 1989; Wang et al, 1996a,b,c), which is commonly utilized for elucidating the physicochemical phenomena taking place, for example, in a packed bed of solids (Suzuki, 1990). This method can simultaneously give us the informationregardingadsorption equilibrium and kinetics, microbial degradation and other mass transfer kinetics involving axial dispersion and diffusion in biofilms (Wang et al, 1996a). In this work, the microbial activity derived from this approachwas studied and reorganized. THEORY Moment Analysis of Pulse Responses The conceptional sketch of the pulse responses from BAC is shown in Fig. 1. When the adsorption of a traceronto activated carbon takes place in a GAC bed, the response time is delayed as compared with that of the inert bed. The delay of the response time corresponds to the adsorption capacity. On the other hand, when biodegradation of a tracer occurs, the amount of a tracer that flows out of a bed is smaller than the input. Both phenomena take place in BAC. These pulse responses can give us quantitative information regardingadsorption and biodegradationwhen the moment analysis is applied.
(a) Inert bed
(b) GAC bed
c:
o ~
..::..: Reduction of adsorption capacity ........ Microbial degradation
.=c:
B
c:
8
o
Time after pulse injection FigureI. Pulseresponses from(a) inert.(b) GACand (c) BACbeds.
Assumptions. Simplifications and Basic EQuations Basic equations were derived on the basis of the following assumptions and simplifications. (1) When activated carbon fiber (ACF) (Kutics, 1992) is employed as a model activated carbon, the adsorption rate is extremely rapid and equilibrium adsorption can be assumed (Sakoda et al, 1987). On the other hand, when granularactivated carbon (GAC) is employed, the adsorption rate controlled by surface diffusion should be taken into account. However, the resultant zeroth and first moments are the same regardless of the consideration of the adsorption kinetics (Suzuki, 1973). Since the mathematical model developed in this work was used for the zeroth and first moment analysis only, the adsorption kinetics was not included in the model.
Microbial activity in biological activated carbon bed
21S
(2) The adsorption equilibrium relation between the tracer and activated carbon is written in a linear expression. (3) Microbial degradation takes place in a biofilm attached to activated carbon and its rate is described as the first-order reaction in regard to the tracer and micro-organism concentrations. (4) The tracer concentration in the biofilm is the same as that in the bulk solution. (5) The adsorption onto the biofilm is negligible. The basic equations describing the mass balance of a tracer in a BAC bed are written as follows. a~ Ez-
~
~
ai - uaz - K YC - k,ao-v(C-C) =Eat1"'''
I
(1)
(2) Where Ez is the axial dispersion coefficient, C is the tracer concentration in the bulk solution.j C is the tracer concentration at the interface between the biofilm and activated carbon, z is the axial distance, u is the k, is the firstvelocity of the solution. X is the amount .of micro:>rganisrns attached to the activated carbon, order rate constantof microbial degradation, kbav IS the overall mass transfer coefficient in the biofilm, e is the bed void fraction, g is the packing density of activated carbon. and K a is the linear adsorption equilibriumconstant. Theoretical Moments The zeroth moment. JDo. and the first absolute moment.mi' are defined by Eqs. (3) and (4). respectively .
~o= foOl> Cdt/M
(3)
(4)
Where t is the time elapsed from the injection of a tracer into the entrance of the BAC bed. C is the effluent concentrationof tracer and M is the amounto f tracer injected. By solving Eqs. (I) and (2) in the Laplace domain.th~ theoretic.aI moments. mo and mJ' given above were derived as the middle term of Eqs, (5) and (6) respectively. Basically, the zeroth moment, mOo and the first absolute moment, mr. derived by pulse response experiments can be analyzed by Eqs, (5) and (6) respectively and give the parameters expressing microbial degradation~d adsorption such as Kr and Ka. However, the forms of the middle terms of Eqs. (5) and (6) are toocomplicated to be used for this purpose. By limiting the application of these theoreti~al ~oments to B~C used for. drinking water treatment only. theoretical moments of and mJ were simplified to the nght hand SIde terms of Eqs. (5) and (6) respectively (Wang et al, 1996a).
roo
(5)
A. SAKODA et al.
216
£+'YKa J11 = I+4tr'PeB (l/u) ='YKa(l/u)
J
(6)
PeB is the bedlength-basisPeelet numberdefinedby Eq.(7), and tr is definedby Eq. (8). PeB =uz/Ez
(7) (8)
EXPERIMENTAL Materials Two series of pulse response experiments, the lab- andbench-scale experiments, were carried out. The differences between the two were found in the materials used, the preparation of BAC, the manner of measuring the amount of micro-organisms attached, and the scale of BAC.However, the procedures of obtaining and analyzing pulse responses were the same and the parameters derived from the two are compared anddiscussed. The bench-scaleexperiments were carried out at the water reservoir for Kure City Waterworks in Hiroshimaprefecture,Japan. Activated carbon fiber (ACF, A-15, Osaka Gas Co., JAPAN) was used as a model activated carbon in the lab-scale experiments, where carbon fiber (CF) and glass wool (OW) were also used as biofilm carriers for comparison. On the other hand, granular activated carbon (OAC, F-400, Calgon, USA) was used in the bench-scaleexperiments. In both cases,glucose was chosen as a tracer andpulse-injected into the entrance of the BAC, since its biodegradability and adsorbability are suitable for this purpose(Suzuki and Sohn,
1989). Preparationof BACs For the lab-scaleexperiments, BAC was prepared in a batch mixing culture using theglucose-peptone medium, the composition of which is shownin Table 1. 2000 mI of the medium and 3 g of a biofilmcarrier, ACF, CF or OW, were put in a glass beaker of 3000 ml involume, and then 2 ml of activated sludge was added.The mixturewas aerated at 25 1 C for several hours, and thinbiofilms were formed on the external surface of the carrier. The BAC thusprepared was gently packed in a glass column of 1.6em in diameter and 15 em long. The packing density of ACF was about0.1 g/ml. The nutrientsolutioncontaining50 mg/l (NH4hS04' 3.5 mg/lKH2P04, 7 mg/l K2HP04 flowed at the rate of 0.1ml/s, Table 1.Composition of the glucose-peptone medium Component glucose peptone KH2P04 (NH4)2S0 4 MgS04·7H20 NaHC03 NaCI
Amount [m~/I) 500 200 50 150 150 150 50
Microbial activity in biological activated carbon bed
2\7
For the bench-scale experiments, GAC was packed in a plastic column of 7.9 em in diameter and 80 em long with a packing density of 0.47 g/ml, and the water in the water reservoir for Kure City Waterworks was (SV) of 4 hr-I . Three filtered through the membrane and introduced into the column at a space velocity series of experimental set-ups were prepared; without pretreatment, with ozone pretreatment and with chlorine pretreatment. The ozone and chlorine concentrations were 2 mg-ozone I mg-OOC and 0.2 mgll of respectively. In all cases, GACs naturally changed to BACs without any operations such as the addition nutrients.An experimental apparatus with ozone pretreatment is illustrated in Fig. 2.
Ozone generator Treatedwaste
Ozonationreactor
Membranefilter
Pump
Pump
Fractioncollector
Figure2.Experimentalapparatusforbench-scaleexperiments at the waterreservoir
Measurements of the amount ofmjcro-or~anisms
attached
of mic~rganisms a~hed on the carrier was adjusted by In the lab-scale experiments, the am.oun~ choosing the initial glucose concentranon1D the medium properly; tt was measured by athermogravimetric analysis (TGA). The BAC sample was put on a micro-balance placed in a nitrogen stream of 20 mVmin, and the temperature was increased gradually at the rateof 5 Clmin. The decrease in weight during the temperature rise from110 C to SOO C, OW, was regarded as the wei~ht of micro-organisms attached, X, by SUbstituting the weight of organics adsorbed, q., and the blank weight loss of ACF, OW()o measured in advance. This derivation is expressed by Eq. (9) and the details will be described elsewhere (Wanget al., 1996a).
(9) In the bench-scale experiments at the water reservoir, the amount of micro-organisms attached on GAC was not controlled and it was measured by counting the numberof micro-organisms under a microscope. About 0.5 g of BAC was taken out of three locations in each BAC column, placed in a sterilized test tube, rinsed with 2 ml of sterilized saline twice and shaken in 2ml of sterilized saline with ultrasonic vibration. Then, 0.3 rnlof 5 mg/l DAPI was added. The mixture was filtrated through a 0.2~ filter an~ the number of microorganisms on the filter was countered. The number was converted to the weight by using the unit cell weight of2 x 10-\0mglcell (Gaudy, 1980).
A. SAKODA et al,
218
PulseResponses The 1 mJ or 100 mJ of 8,000 mg/l glucosesolutionwasinjected into the entranceof the BAC by a syringein the lab- andbench-scaleexperiments respectively. The effluent wasfractionated by a fraction collector and the glucoseconcentrationin eachfraction was assayed by the glucose-oxidase method.
RESULTS AND DISCUSSION Chan~es
in GlucosePulseResponses
Figure3 shows typical glucosepulseresponses from thelab-scale BACs. The Y axis of Fig.3 representsthe glucose concentration expressed as a carbonconcentration. Both the area and theresidence time of the response were smaller when the amount ofmicro-organisms attached was larger as shown in Fig. 1. As far as themicrobial activity in BAC is concerned, the peak area only needs to beanalyzed as follows. Equations(5) and (8) give the following linearizedexpression. - In~
= krX(z/u)
(10)
The parameter. kr was determined by plotting (-lnJ.1o}/(zlu) experimentally obtained against the amount of micro-organismsattached,X. accordingto Eq. (10).
80
-
Biomass [g/I] 0
--0--
~
0
*• •
60
I
0)
E ...... U
c:
0
1.1 2.1 3.0
40
0
Q)
UJ
0
0
::J
o
20
......~ . .~~F=O-I
o~IIIIIt~D-L--.....--::=:t
o
1800
3600
5400
7200
Time [5] Figure 3. Typicalglucosepulseresponses fromlab-scaleBACs.
Effects of BiofilmCarrier The above-mentioned (- Inrna )I(zlu) vs. X plots of the data obtained in lab-scaleexperiments are shown in Fig. 4. where the biofilm carriers were compared foractivated carbon fiber, carbon fiber and glass wool. Although data were a littlescattered. it can be said that the rate constant. rk of the biofilmattached on the
Microbialactivity inbiological activated carbon bed
219
activated carbon was about two times larger then those on other carriers. Similar experimental results were found in the literature (Bouwer and McCarty, 1982; Li and DiGiano, 1983; Nishijima et al, 1992). The reason for this phenomenon was not understood there either. The oxygen adsorbed on the activated carbon may enhance the microbial activity. The discussion on this subject is beyond the objective of this work at this stage.
4
/
-. 3
o Activated carbon fiber o Carbon fiber 6. Glasswool 0
..-.#
.
......'
.'
~ (')
o
0
,.-
--x
::J
N
.....'.'
0
0
o .'.....
.. .·····0
'EJ 1 l-
....
••••••••••••••
A.·····
ft/€ .-.>:
.-
.......'...... r>: .:::....
o o
.........6.. ~
6. ....
.., .'e-:
'
.'
'
.'.' .' .' .'
...
.... .' .
0
2
....'
0
6..
....
. 1
2 Biomass [9 / I]
3
4
Figure4.Degradationof glucose by micro-organismson differentbiofilm carriers
Effects of Pretreatmentsby Ozone and Cblorine The rate constants of microbial degradation of glucose were compared in Fig. S for various bench-scale BACs with different pretreatments. The microbial .activity in te.nns of ~e k, value did not change considerably after the operation for about 100 days m all casesWith and Without pretreatment. As often reported in the literature, it was found that the pre-ozonation of raw water was effective for enhancing the microbial activity in BAC. probably due to the degradation ofmacromolecules to smaller molecules with higher biodegradability. When chlorine was added before BAC under the conditions mentioned before. the microbial activity in BAC was not reduced as significantly as expected. The procedure developed in this acti~ ity in the bench-scale practical work successfullygives quantitative informationregarding the microbial BACs. The reason for the difference is another story and may need to be studied in further investigations. The comparison of the microbial activityin terms of kr .values derived from the lab- and bench-scale experiments is also shown in Fig.S. The k, values of the blofllms na~allY formed on activated carbon by larger than.those of the biofilms contact with pond water for a long time were~bout one order of ma~ltude ma:m reason 15 considered to be that the prepared in the laboratory in a manner mentioned before. The dOminant species of micro-organisms attachedw e r ~ completely different. The microbial magic of wild biofilms should be studied in future works. In practice, the constant value ofk, for each pretreatment is Useful for estimating the amount ofmicro-organisms active in BACs by the glucose pulse responses.
A. SAKODA et al.
220
Bench-scale BACs
"".\~\. .
50
•:..
'
..
·. ··... ··,. ... ... '\.~
40
"
...... en
-C>
30
~
E ~
.::t:.
... With ozone pretreatment
•
20
•
Without pretreatment
,
('l')
0 ......
...
..<,.:.a....
•
• •
•
With chlorine pretreatment
10 ....c::
Lab-scale BACs .
200
300
Time [day] FigureS. Comparisonof glucose blodegradatlonrates invarious BACs.
Microbial Activities from PracticalQr~anics
Removal and Glucose Pulse Responses
The relation between the resultant k, and the practical organics removal is shown in Fig. 6. K, in Fig. 6 represents the microbial degradation rate constant derived from the difference in organics concentration between the influent and effluent of the bench-scale BACs in the apparent steady-state after 100 days of operation. The details of the derivation of K, will be described elsewhere (Wang et al.; 1996c). This empirical relation is useful for evaluating the potential microbial activity in BAC by simply introducing glucose pulses. The moment analysis of pulse responses is useful for evaluating the microbial activity in BAC in a relatively o f this approach to the pilot- or full-scale easy manner. Further investigations may involve the application BACs treating the raw waterof drinking water supplies.
Microbial activity in biological activated carbon bed
221
60 50
...... 40 ~
wI chlorine
cfJ
wI ozone
E 30 0 ......
w/o pretreatment
~
~
20 10 0
0
10
20
30
40
50
60
kr [cm3/g/s1 Figure6.Relation between biodegradationratesofpulse-injected glucose. kr anddissolvedorganicsubstances. K r in steady-stateBACoperations.
CONCLUSIONS (I) The microbial activity in BAC was simply andquantitatively evaluated by the moment analysis of the glucosepulseresponses in termsof themicrobial degradationrateofglucose. (2) The activity of micro-organisms attached on activated carbon was higher thanthose on other carriers. Activated carbonmayenhancethe activityof micro-organismsattached. (3) The rateconstantsof microbial degradationof glucose in bench-scale BACs treating pond water varied with the pretreatmentand did not changeconsiderably during about one year ofoperation. The procedure developed in this worksuccessfully gave quantitative informationregardingmicrobial activity in the benchscalepracticalBACs. (4) The empirical relationbetween the microbial degradationratesofpulse-injected glucose andbackground dissolved organics is useful for evaluating the potential microbial activity of BAC by glucose pulse responses.
ACKNOWLEDGEMENTS The authors expressappreciation to the Kure City Waterworks and Prof.Mitsumasa Okada's group at the HiroshimaUniversity for theirkindcooperation.
A. SAKODA et al.
222
REFERENCES Bouwer, E. J. andMcCarty. P. L. (\982) Environ. Sci. Tech.16.836. Chudyk. W. A. and Snoeylnk, V. L. (1984) Environ. Sci. Tech. 18(1). Gaudy. A. F. and Gaudy.E. T. (\980)"Microbiology forEnvironmentalScientists andEngineers".Mc-Graw-Hill, New York. Kajino, K. et aI. (1993)Suido KyokaiZasshi62(1). 14. Kutics, K. and Suzuki.M. (\992) Wat. Sci. Tech. 16.655. u. A. Y. and DiGiano. F. A. (1983)J. WPCF55. 392. Magan, Y. (1989)Suishlts«OdakuKenkyu12. 140. Magara,Y et al.(1993) Yousu; To Haisui35. 671. Nishijima, H. et al. (1992)Mizu KankyoGakkaishi 15. 683. Rollinger. V. and Doot, W. (1987) Appl. Environ. Microbiol. 53. 777. Sakoda,A.•Kawazoe, K. andSuzuki. M. (\987) WaterRes. 21.717. Speitel, G. E. Jr et al. (1987)J. Environ. Eng.•ASCE113.32. Suzuki. M. (1973)J. Chern. Eng. Japan6. 540. Suzuki. M. and Sohn,J. E. (\989) "NewDirections in Sorption Technology".227. Suzuki. M. (1990) "AdsorptionEngineering".ElsevierlKodansya, Tokyo. Taniguchi.G. et al. (\991) Suido KyokaiZasshi62(1). 10. Umiga, N. et aI. (1991)Suido KyokaiZassh!60(6).2. Wang. 1.. Sakoda,A. andSuzuki. M. (1996a) Kagaku KogakuRonbunsyu,in press. Wang. J•• Sakoda,A. andSuzuki. M. (1996b) Kagaku KogakuRonbunsyu,in press. Wang.1.. Sakoda,A. andSuzuki. M. et al. (\996c) Suido KyokaiZasshi,in press. Zhang.X.1.. Wang.Z. S. andou. X. S. (\991) WaterRes. 25. 165.
Pergamon
. Wat Sci. Tech.Vol. 34. No. ~-6. pp, 213-222.1996. Copynght ~ 1996IAWQ. Published by Elsevier ScienceLtd Printed In Great Britain . All rights reserved.
PH:S0273-1223(96)00648-8
0273·1223196 $1soo+ 0'00
MICROBIAL ACTIVITY IN BIOLOGICAL ACTIVATED CARBON BED BY PULSE RESPONSES Akiyoshi Sakoda, JianzhongWang andMotoyuki Suzuki Institute ofIndustrial Science. UniversityofTokyo. 7·22-1 Roppongi, Minato.ku, Tokyo 106. Japan
ABSTRACT The moment analysis of pulse responses was applied to the biological activated carbon (BAC) in order to elucidate its microbial activity and adsorption capacity separately. The microbial activity derived from this approach was focussed on and the following was found in this work. First. the activity of micro-organisms attached on activated carbon was higher than those on other carriers. Second. the microbial activities of bench-scale BACs treating pond water varied withthe pretreatments by ozone and chlorine.bUI did not change considerably during the operation for about one year. Also. an the empirical relationship was found It was between the microbial degradation rates of pulse-injected glucose and background dissolved organics. concluded that this approach is useful for evaluating the microbial activity in BAC in a relatively easy manner. Copyright e 1996 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Biological activated carbon:adsorption: microbial degradation; drinking water treatment; pulse response method: momentanalysis. INTRODUCTION Where drinking water sources are pollutedby various kinds of trace dissolvedsubstances, it is often difficult for the conventional treatment proces~es consisting of c?agulation,sedi~en~tion and filtration to purify such polluted raw water enough to satisfy the water quality standard of drinking water. Granular activated carbon (GAC) is already employedwidely in the waterworksto meet this demand. Recently,GAC with thin biofilms on its external surface has been studied as a cost efficient and high performance unit operation for drinking water treatments especially in Japan (Magara, 1989: Magara et al., 1993: Taniguchiet al, 1993: Kajino et al, 1993: Umiga et al, 1991). The GAC with biofilms is often called biological activated carbon (BAC) (Rollingerand Doot,1987: Zhanget al, 1991). A lot of experimental BAC processes have already shown the long-term and apparent steady-state removal of trihalomethaneprecursors.odoro~s compoun~s and so o~. In those cases, it i~ usu~lIy said that organics are removed by cooperation of physical adsorptiononto activated carbon and microbial degradation. Also the microbial degradation of adsorbedorgani~s is.called b ~oregeneration (Chudyk and Snoeylnk, 1984; Speitel et al, 1987). which can extend the service hfe of activatedCarbon. However, the understandingof 213
A. SAKODA et al.
214
BAC is still vague in regard to the roleof activated carbon, the interaction between activated carbon and micro-organisms,etc. The first step for elucidating the mechanisms of BAC is to evaluate its adsorption characteristics and microbial activity separately and simultaneously. In order to establish such a simple methodology that is suitable for this purpose, we have been interested in applying the moment analysis of pulse responses (Suzuki and Sohn, 1989; Wang et al, 1996a,b,c), which is commonly utilized for elucidating the physicochemical phenomena taking place, for example, in a packed bed of solids (Suzuki, 1990). This method can simultaneously give us the informationregardingadsorption equilibrium and kinetics, microbial degradation and other mass transfer kinetics involving axial dispersion and diffusion in biofilms (Wang et al, 1996a). In this work, the microbial activity derived from this approachwas studied and reorganized. THEORY Moment Analysis of Pulse Responses The conceptional sketch of the pulse responses from BAC is shown in Fig. 1. When the adsorption of a traceronto activated carbon takes place in a GAC bed, the response time is delayed as compared with that of the inert bed. The delay of the response time corresponds to the adsorption capacity. On the other hand, when biodegradation of a tracer occurs, the amount of a tracer that flows out of a bed is smaller than the input. Both phenomena take place in BAC. These pulse responses can give us quantitative information regardingadsorption and biodegradationwhen the moment analysis is applied.
(a) Inert bed
(b) GAC bed
c:
o ~
..::..: Reduction of adsorption capacity ........ Microbial degradation
.=c:
B
c:
8
o
Time after pulse injection FigureI. Pulseresponses from(a) inert.(b) GACand (c) BACbeds.
Assumptions. Simplifications and Basic EQuations Basic equations were derived on the basis of the following assumptions and simplifications. (1) When activated carbon fiber (ACF) (Kutics, 1992) is employed as a model activated carbon, the adsorption rate is extremely rapid and equilibrium adsorption can be assumed (Sakoda et al, 1987). On the other hand, when granularactivated carbon (GAC) is employed, the adsorption rate controlled by surface diffusion should be taken into account. However, the resultant zeroth and first moments are the same regardless of the consideration of the adsorption kinetics (Suzuki, 1973). Since the mathematical model developed in this work was used for the zeroth and first moment analysis only, the adsorption kinetics was not included in the model.
Microbial activity in biological activated carbon bed
21S
(2) The adsorption equilibrium relation between the tracer and activated carbon is written in a linear expression. (3) Microbial degradation takes place in a biofilm attached to activated carbon and its rate is described as the first-order reaction in regard to the tracer and micro-organism concentrations. (4) The tracer concentration in the biofilm is the same as that in the bulk solution. (5) The adsorption onto the biofilm is negligible. The basic equations describing the mass balance of a tracer in a BAC bed are written as follows. a~ Ez-
~
~
ai - uaz - K YC - k,ao-v(C-C) =Eat1"'''
I
(1)
(2) Where Ez is the axial dispersion coefficient, C is the tracer concentration in the bulk solution.j C is the tracer concentration at the interface between the biofilm and activated carbon, z is the axial distance, u is the k, is the firstvelocity of the solution. X is the amount .of micro:>rganisrns attached to the activated carbon, order rate constantof microbial degradation, kbav IS the overall mass transfer coefficient in the biofilm, e is the bed void fraction, g is the packing density of activated carbon. and K a is the linear adsorption equilibriumconstant. Theoretical Moments The zeroth moment. JDo. and the first absolute moment.mi' are defined by Eqs. (3) and (4). respectively .
~o= foOl> Cdt/M
(3)
(4)
Where t is the time elapsed from the injection of a tracer into the entrance of the BAC bed. C is the effluent concentrationof tracer and M is the amounto f tracer injected. By solving Eqs. (I) and (2) in the Laplace domain.th~ theoretic.aI moments. mo and mJ' given above were derived as the middle term of Eqs, (5) and (6) respectively. Basically, the zeroth moment, mOo and the first absolute moment, mr. derived by pulse response experiments can be analyzed by Eqs, (5) and (6) respectively and give the parameters expressing microbial degradation~d adsorption such as Kr and Ka. However, the forms of the middle terms of Eqs. (5) and (6) are toocomplicated to be used for this purpose. By limiting the application of these theoreti~al ~oments to B~C used for. drinking water treatment only. theoretical moments of and mJ were simplified to the nght hand SIde terms of Eqs. (5) and (6) respectively (Wang et al, 1996a).
roo
(5)
A. SAKODA et al.
216
£+'YKa J11 = I+4tr'PeB (l/u) ='YKa(l/u)
J
(6)
PeB is the bedlength-basisPeelet numberdefinedby Eq.(7), and tr is definedby Eq. (8). PeB =uz/Ez
(7) (8)
EXPERIMENTAL Materials Two series of pulse response experiments, the lab- andbench-scale experiments, were carried out. The differences between the two were found in the materials used, the preparation of BAC, the manner of measuring the amount of micro-organisms attached, and the scale of BAC.However, the procedures of obtaining and analyzing pulse responses were the same and the parameters derived from the two are compared anddiscussed. The bench-scaleexperiments were carried out at the water reservoir for Kure City Waterworks in Hiroshimaprefecture,Japan. Activated carbon fiber (ACF, A-15, Osaka Gas Co., JAPAN) was used as a model activated carbon in the lab-scale experiments, where carbon fiber (CF) and glass wool (OW) were also used as biofilm carriers for comparison. On the other hand, granular activated carbon (OAC, F-400, Calgon, USA) was used in the bench-scaleexperiments. In both cases,glucose was chosen as a tracer andpulse-injected into the entrance of the BAC, since its biodegradability and adsorbability are suitable for this purpose(Suzuki and Sohn,
1989). Preparationof BACs For the lab-scaleexperiments, BAC was prepared in a batch mixing culture using theglucose-peptone medium, the composition of which is shownin Table 1. 2000 mI of the medium and 3 g of a biofilmcarrier, ACF, CF or OW, were put in a glass beaker of 3000 ml involume, and then 2 ml of activated sludge was added.The mixturewas aerated at 25 1 C for several hours, and thinbiofilms were formed on the external surface of the carrier. The BAC thusprepared was gently packed in a glass column of 1.6em in diameter and 15 em long. The packing density of ACF was about0.1 g/ml. The nutrientsolutioncontaining50 mg/l (NH4hS04' 3.5 mg/lKH2P04, 7 mg/l K2HP04 flowed at the rate of 0.1ml/s, Table 1.Composition of the glucose-peptone medium Component glucose peptone KH2P04 (NH4)2S0 4 MgS04·7H20 NaHC03 NaCI
Amount [m~/I) 500 200 50 150 150 150 50
Microbial activity in biological activated carbon bed
2\7
For the bench-scale experiments, GAC was packed in a plastic column of 7.9 em in diameter and 80 em long with a packing density of 0.47 g/ml, and the water in the water reservoir for Kure City Waterworks was (SV) of 4 hr-I . Three filtered through the membrane and introduced into the column at a space velocity series of experimental set-ups were prepared; without pretreatment, with ozone pretreatment and with chlorine pretreatment. The ozone and chlorine concentrations were 2 mg-ozone I mg-OOC and 0.2 mgll of respectively. In all cases, GACs naturally changed to BACs without any operations such as the addition nutrients.An experimental apparatus with ozone pretreatment is illustrated in Fig. 2.
Ozone generator Treatedwaste
Ozonationreactor
Membranefilter
Pump
Pump
Fractioncollector
Figure2.Experimentalapparatusforbench-scaleexperiments at the waterreservoir
Measurements of the amount ofmjcro-or~anisms
attached
of mic~rganisms a~hed on the carrier was adjusted by In the lab-scale experiments, the am.oun~ choosing the initial glucose concentranon1D the medium properly; tt was measured by athermogravimetric analysis (TGA). The BAC sample was put on a micro-balance placed in a nitrogen stream of 20 mVmin, and the temperature was increased gradually at the rateof 5 Clmin. The decrease in weight during the temperature rise from110 C to SOO C, OW, was regarded as the wei~ht of micro-organisms attached, X, by SUbstituting the weight of organics adsorbed, q., and the blank weight loss of ACF, OW()o measured in advance. This derivation is expressed by Eq. (9) and the details will be described elsewhere (Wanget al., 1996a).
(9) In the bench-scale experiments at the water reservoir, the amount of micro-organisms attached on GAC was not controlled and it was measured by counting the numberof micro-organisms under a microscope. About 0.5 g of BAC was taken out of three locations in each BAC column, placed in a sterilized test tube, rinsed with 2 ml of sterilized saline twice and shaken in 2ml of sterilized saline with ultrasonic vibration. Then, 0.3 rnlof 5 mg/l DAPI was added. The mixture was filtrated through a 0.2~ filter an~ the number of microorganisms on the filter was countered. The number was converted to the weight by using the unit cell weight of2 x 10-\0mglcell (Gaudy, 1980).
A. SAKODA et al,
218
PulseResponses The 1 mJ or 100 mJ of 8,000 mg/l glucosesolutionwasinjected into the entranceof the BAC by a syringein the lab- andbench-scaleexperiments respectively. The effluent wasfractionated by a fraction collector and the glucoseconcentrationin eachfraction was assayed by the glucose-oxidase method.
RESULTS AND DISCUSSION Chan~es
in GlucosePulseResponses
Figure3 shows typical glucosepulseresponses from thelab-scale BACs. The Y axis of Fig.3 representsthe glucose concentration expressed as a carbonconcentration. Both the area and theresidence time of the response were smaller when the amount ofmicro-organisms attached was larger as shown in Fig. 1. As far as themicrobial activity in BAC is concerned, the peak area only needs to beanalyzed as follows. Equations(5) and (8) give the following linearizedexpression. - In~
= krX(z/u)
(10)
The parameter. kr was determined by plotting (-lnJ.1o}/(zlu) experimentally obtained against the amount of micro-organismsattached,X. accordingto Eq. (10).
80
-
Biomass [g/I] 0
--0--
~
0
*• •
60
I
0)
E ...... U
c:
0
1.1 2.1 3.0
40
0
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0
0
::J
o
20
......~ . .~~F=O-I
o~IIIIIt~D-L--.....--::=:t
o
1800
3600
5400
7200
Time [5] Figure 3. Typicalglucosepulseresponses fromlab-scaleBACs.
Effects of BiofilmCarrier The above-mentioned (- Inrna )I(zlu) vs. X plots of the data obtained in lab-scaleexperiments are shown in Fig. 4. where the biofilm carriers were compared foractivated carbon fiber, carbon fiber and glass wool. Although data were a littlescattered. it can be said that the rate constant. rk of the biofilmattached on the
Microbialactivity inbiological activated carbon bed
219
activated carbon was about two times larger then those on other carriers. Similar experimental results were found in the literature (Bouwer and McCarty, 1982; Li and DiGiano, 1983; Nishijima et al, 1992). The reason for this phenomenon was not understood there either. The oxygen adsorbed on the activated carbon may enhance the microbial activity. The discussion on this subject is beyond the objective of this work at this stage.
4
/
-. 3
o Activated carbon fiber o Carbon fiber 6. Glasswool 0
..-.#
.
......'
.'
~ (')
o
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....
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ft/€ .-.>:
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.......'...... r>: .:::....
o o
.........6.. ~
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2
....'
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. 1
2 Biomass [9 / I]
3
4
Figure4.Degradationof glucose by micro-organismson differentbiofilm carriers
Effects of Pretreatmentsby Ozone and Cblorine The rate constants of microbial degradation of glucose were compared in Fig. S for various bench-scale BACs with different pretreatments. The microbial .activity in te.nns of ~e k, value did not change considerably after the operation for about 100 days m all casesWith and Without pretreatment. As often reported in the literature, it was found that the pre-ozonation of raw water was effective for enhancing the microbial activity in BAC. probably due to the degradation ofmacromolecules to smaller molecules with higher biodegradability. When chlorine was added before BAC under the conditions mentioned before. the microbial activity in BAC was not reduced as significantly as expected. The procedure developed in this acti~ ity in the bench-scale practical work successfullygives quantitative informationregarding the microbial BACs. The reason for the difference is another story and may need to be studied in further investigations. The comparison of the microbial activityin terms of kr .values derived from the lab- and bench-scale experiments is also shown in Fig.S. The k, values of the blofllms na~allY formed on activated carbon by larger than.those of the biofilms contact with pond water for a long time were~bout one order of ma~ltude ma:m reason 15 considered to be that the prepared in the laboratory in a manner mentioned before. The dOminant species of micro-organisms attachedw e r ~ completely different. The microbial magic of wild biofilms should be studied in future works. In practice, the constant value ofk, for each pretreatment is Useful for estimating the amount ofmicro-organisms active in BACs by the glucose pulse responses.
A. SAKODA et al.
220
Bench-scale BACs
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50
•:..
'
..
·. ··... ··,. ... ... '\.~
40
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...... en
-C>
30
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... With ozone pretreatment
•
20
•
Without pretreatment
,
('l')
0 ......
...
..<,.:.a....
•
• •
•
With chlorine pretreatment
10 ....c::
Lab-scale BACs .
200
300
Time [day] FigureS. Comparisonof glucose blodegradatlonrates invarious BACs.
Microbial Activities from PracticalQr~anics
Removal and Glucose Pulse Responses
The relation between the resultant k, and the practical organics removal is shown in Fig. 6. K, in Fig. 6 represents the microbial degradation rate constant derived from the difference in organics concentration between the influent and effluent of the bench-scale BACs in the apparent steady-state after 100 days of operation. The details of the derivation of K, will be described elsewhere (Wang et al.; 1996c). This empirical relation is useful for evaluating the potential microbial activity in BAC by simply introducing glucose pulses. The moment analysis of pulse responses is useful for evaluating the microbial activity in BAC in a relatively o f this approach to the pilot- or full-scale easy manner. Further investigations may involve the application BACs treating the raw waterof drinking water supplies.
Microbial activity in biological activated carbon bed
221
60 50
...... 40 ~
wI chlorine
cfJ
wI ozone
E 30 0 ......
w/o pretreatment
~
~
20 10 0
0
10
20
30
40
50
60
kr [cm3/g/s1 Figure6.Relation between biodegradationratesofpulse-injected glucose. kr anddissolvedorganicsubstances. K r in steady-stateBACoperations.
CONCLUSIONS (I) The microbial activity in BAC was simply andquantitatively evaluated by the moment analysis of the glucosepulseresponses in termsof themicrobial degradationrateofglucose. (2) The activity of micro-organisms attached on activated carbon was higher thanthose on other carriers. Activated carbonmayenhancethe activityof micro-organismsattached. (3) The rateconstantsof microbial degradationof glucose in bench-scale BACs treating pond water varied with the pretreatmentand did not changeconsiderably during about one year ofoperation. The procedure developed in this worksuccessfully gave quantitative informationregardingmicrobial activity in the benchscalepracticalBACs. (4) The empirical relationbetween the microbial degradationratesofpulse-injected glucose andbackground dissolved organics is useful for evaluating the potential microbial activity of BAC by glucose pulse responses.
ACKNOWLEDGEMENTS The authors expressappreciation to the Kure City Waterworks and Prof.Mitsumasa Okada's group at the HiroshimaUniversity for theirkindcooperation.
A. SAKODA et al.
222
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