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Cross-flow microfiltration of activated sludge using submerged membrane with air bubbling
JOURNAL OF
Cross-Flow Microfiltration of Activated Sludge Using Submerged Membrane with Air Bubbling YASUTOSHI Biotechnology
SHIMIZU,*
KATSUSHI
URYU, YU-ICHI
OKUNO,
AND
& Water Treatment Laboratory, R & D Div., TOT0 Ltd., 2-l-l Kitakyushu 802, Japan Received 30 March 1995/Accepted
ATSUO
WATANABE
Nakashima,
Kokurakita-ku,
13 October 1995
For the purpose of developing a wastewater treatment membrane bioreactor system, cross-flow microfiltration of intermittently aerated activated sludge was carried out, as a part of the solid-liquid separation process in the bioreactor. The tubular alumina microfiltration membrane, having a pore size of 0.5 pm, was submerged in the activated sludge. A cross-flow stream over the membrane surface was produced by air bubbling, generated by a diffuser situated underneath the membrane. An exponential relationship between the steady-state flux, J,,, and the causative factors such as operating parameters and fluid characteristics was found and expressed as J,= VL=K’u*1~oMLSS-o~5, where V,, ZC’,U* and MLSS are the lift velocity, filtration constanf, air-liquid two-phase flow velocity and MLSS, respectively. The relationship, as expressed by the equation, is consistent with that for conventional cross-flow filtration using a circulation pump. [Key words: cross-flow filtration, microfiltration,
bioseparation,
Cross-flow microfiltration is a valuable solid-liquid separation process, in which a cross-flow stream induces the back-transport of filtered colloidal particles from the membrane surface and provides a high filtration flux. Several types of operation methods can be used for the microfiltration. The most common method which is already being used in a wide range of bioindustries involves the use of circulation pumps which generate cross-flow feed streams over membrane surfaces, as shown in Fig. 1. This method will be referred to as the conventional filtration method. Though the conventional filtration method is simple, easy to use and allows control of filtration flux by controlling pumping conditions such as feed velocity and transmembrane pressure (l-3), it unfortunately requires input of tremendous amounts of energy and may possibly reduce the activity of biocatalysts, i.e., bacterial cells and enzymes in the circulating feed. This is because this method requires about 10 to 20 times as much circulation feed as filtrate by volume to maintain a high filtration flux, and the high discharge rate circulation pump used in the method generates excessive shear stress which can damage the biocatalysts (4). A second method has been proposed to eliminate the disadvantages mentioned above. In this method a crossflow stream is generated by rotating membranes or rotating impellors set in the vicinity of the membrane surfaces (5, 6). Since this method does not require a high discharge rate circulation pump, it does not incur the above-mentioned disadvantages. Moreover, it can be used to filter highly viscous fluids which cannot be processed using the conventional method (6). The third and newest method has been applied to solid-liquid separation in wastewater treatment membrane bioreactors (7, 8). In this method, a cross-flow stream over the membrane surface is produced by air bubbling, which is generated by a diffuser situated underneath the membrane in a feed. The air bubbles flow upward with a fluid along the membrane surface and
submerged membrane,
activated
sludge]
induce a moderate shear stress which generates the backtransport of filtered colloidal particles from the membrane surface. This method not only produces cross-flow filtration without a high discharge rate circulation pump and membrane casing, but also can be set up using simple equipment, such as a low rate suction pump, air blower and a vessel. Therefore, this method is regarded as an energy efficient and low shear stress filtration method, being applicable to membrane bioreactor systems. In order to design a membrane filtration system, flux prediction equations, which describe the relationships of the operating parameters, fluid characteristics and membrane properties to the flux, are required. For the conventional cross-flow filtration method, we have already proposed three equations below (9). Figure 2 shows a cross-flow microfiltration model for a suspension with particles of different sizes, dPl and dP2.
(1) -.0.5dp,0.67Tf1.0 V,, = KU’ .OC Pn {
4 > I/L(min)+Jss=v~/~(rnin) Ji 5 VL(min)AJss= Ji
1
(2) (3)
The steady-state filtration flux, J,,, is expressed in terms of the balance between the initial filtration flux, Ji, and the lift velocities for each particle, VL, (VL1 for dpl and VL, for dp2 in the case of Fig. 2). Ji is determined by the hydraulic resistances of plugging, R,, which is attributable to membrane properties such as pore size and surface charge, and of the membrane itself, R,. Therefore, at the initial stage of filtration, the flux Ji, that is, the flux after the initial rapid decline, is dependent only on the membrane properties, as expressed in Eq. 1. The V,, for the particles of size dpn is expressed as a function of the operating parameters of feed velocity u and fluid characteristics of particle concentration C,, of noted particles n and viscosity of filtrate vf (Eq. 2). J,, is expressed with the hydraulic resistances as in
* Corresponding author. 55
56
J. FERMENT.WOENO.,
SHIMIZU ET AL.
I Generation of cross-flow stream
(
disE$nRte)
(Filtrate
flow
IO-
membranes or Rotating impellers
Air bubble upflow stream
20
l-3
1
rate) :QF
FIG. 1.
Cross-flow microfiltration methods proposed in bioindustry.
Eq. 4. R, represents the resistance of the particle-packed layer on the membrane. 1 J=hP SS (4) ?r &+R,+R, When Ji > VLcti,,), R, is formed onto (R,+R& until J,,= VLctinj, as shown in Fig. 2. In this case, the particles causing the minimum lift velocity, VL(mi,,j,are deposited selectively in the vicinity of the steady-state period, and J,, is controlled only by VrIL(minj. When JiS VLfmin), R, does not develop during the filtration and a further decline in flux does not occur after the initial stage. Jss can be expressed as Js,=Ji(b) as shown in Fig. 2. This occurs when a high-resistance membrane or a membrane with pores larger than the particles, which cause plugging, is used. The abovementioned models could consistently predict the fluxes for mono-dispersed colloidal particles such as latexes and microbial cells, as well as fluxes for particles of various sizes (9, 10). The filtration for the second method will be modeled as a function of rotating speed of membranes or impellors in the near future (6). For the third type of filtration method, the cross-flow
(Initial
stage)
II
Operating FIG. 2.
III
Rotating
Circulation pumping
:QP
II
microfiltration of a submerged membrane with air bubbling, the effect of operating parameters and fluid characteristics on the flux has not been discussed quantitatively. Consequently, we study the effect and construct the flux-prediction equations for the third type of filtration method, referring to the conventional filtration method model using a circulation pump. In an effort to adapt the method for use with wastewater treatment membrane bioreactors, activated sludge was used as experimental colloidal particles. MATERIALS
AND METHODS
Experimental colloidal particles were obtained from domestic wastewater-derived activated sludge fermented in a 0.63 m3 membrane bioreactor. Four hollow-fiber microfiltration membrane modules (Mitsubishi Rayon Co. Ltd, Tokyo), having a pore size D, of 0.1 pm and membrane area of 6 m* for each module, and air diffusers were placed in the bioreactor vessel. Domestic wastewater, with a BOD of 0.2 kg.m-3 and total nitrogen content (T-N) of 0.05 kg.m-3, was fed into the bioreactor at the hydraulic retention time (HRT) of 12 h. The bioreactor was intermittently aerated Activated
IIRm(Steady
sludge
state)
II
time
Cross-flow microfiltration model for suspension with particles of different sizes.
CROSS-FLOW MICROFILTRATION
VOL. 81, 1996
OF SUBMERGED MEMBRANE
57
1 Air
FIG. 3. Schematic diagram of filtration apparatus. 1, 10 I filtration vessel; 2, tubular alumina membrane; 3, filtrate reservoir; 4, electric balance; 5, suction pump; 6, diffuser; 7, air flow meter.
'I1
10
1
Particle size distribution
by an air blower on an aeration and non-aeration cycle of 25 and 35 min, respectively, at a flow rate of 18.0m3. h-i. The membrane was suction filtered using a fixed delivery pump at a rate of 0.14 m3. hh’. The filtration was conducted intermittently to synchronize the aeration time for the bioreactor and with air bubbling. This procedure was described in detail elsewhere (11). Particle size Characterization of activated sludge distribution of the activated sludge was measured with a laser diffraction particle size analyzer, SALD-1100 (Shimadzu Co., Kyoto), with a measurable size range from 0.1 pm to 500 pm. The viscosity of the activated sludge was measured using a Brookfield viscometer. MLSS was measured as 110°C dry sludge weight per volume, which was collected on a 0.22 pm pore membrane filter. The molecular weights of organic materials in the activated sludges and f&rates were measured by HPLC using KB-803 and KB-806 GPC columns (Showa Denko Co., Tokyo) with a measurable molecular weight range of under 107Da. For the measurements, particles were removed by centrifugation at 3,000 x g for 10 min. Cross-flow microfiltration Cross-flow microfiltration was carried out using the filtration apparatus shown in Fig. 3. A TOT0 tubular alumina membrane, ASOOSU (12), with nominal pore size D, of 0.5 pm, an outer diameter of 10 mm, an inner diameter of 7 mm and a length of 200 mm, was submerged in the activated sludge with a MLSS of 3-20 kg+mp3. A cross-flow stream was produced by air bubbling generated by a diffuser situated underneath the submerged membrane. The bubbling strengths, V,, i.e., the bubbling air flow rates per unit projection membrane area for the base, were set between 0 and 300 m3 - rne2. h- l. The cross-flow steam velocity u* for air-liquid two-phase flow was measured using a submersible electromagnetic current meter (Kenek Ltd., Tokyo). Filtration was conducted for 4 h at a transmembrane pressure AP of 10-80 kPa and temperature of 20°C. The filtrate was weighed on an electric balance every 30s. For each filtration experiment, the activated sludge was newly prepared from the 0.45 m3 membrane bioreactor. After the filtration experiment, the membrane surface was cleaned with a sponge to remove the particle-packed layer which had accumulated during filtration. Pure water flux through the membrane was then measured to calculate the hydraulic resistances of adsorption on and
1000
100
@[pm1 of intermittently
aerated acti-
vated sludge.
plugging of membrane pores, R,, and of the particlepacked layer which formed on the membrane, R,. Details of the measurements and calculations are given elsewhere (9). RESULTS AND DISCUSSION Physical properties of intermittently aerated activated sludge The particle size distribution of the intermittently aerated activated sludge is shown in Fig. 4. The particles were found to be larger than 0.8 pm. It is known that filtration with membranes whose pore size is larger than the filtered particle size causes plugging and reduces flux (13). Therefore, a membrane with a pore size smaller than 0.8 pm was selected for the filtration experiment. Dissolved organics in the reactor had molecular weight within a range of 102 to lo3 Da, as did the filtrate with the same concentrations. Since the organics in the sludge were allowed to pass through the membrane or membrane surface particle-packed layer during filtration (14), the effect of the concentration polarization of organics on the flux was judged to be negligible. Flux-prediction equation The influence of operat1.o
I
I
I
I
20
40
60
80
6 ?JE “i 0.5 s 7
0.c
'0
100
AP [kPa] FIG. 5. Effect of transmembrane pressure, AP, on steady-state flux, J, of intermittently aerated activated sludge: D,=O.S pm, MLSS=8kg.m-3, T=20”C. Symbols: 0, V,=7.2m3.m-2.h-L; 0, V~=210m3.m-2.h-1.
58
SHIMIZU ET AL.
J.
FERMENT. BIOENG..
ing parameters and fluid characteristics on steady-state flux of a submerged membrane was examined using intermittently aerated activated sludge. The filtration flux declined rapidly within 1 h after the start of filtration, with further declines being quite small. The flux value after 4-h operation was regarded as the steady-state value,
Jw
Figures 5 and 6 show J,, and filtration resistances R plotted against the transmembrane pressure AP. J,, was almost consistent for AP over 50 kPa. This relationship can be explained as the increment of R, (Fig. 6). R, can be expressed as a function of the particle-packed layer thickness, a,, layer density, p, and voidage, E, as expressed in Eq. 5. R, =
aAd
- &)P&
(5)
The 8, was measured as being less than 1 mm by microscopic observation. The air-liquid two-phase flow profile on the membrane surface would not be changed by the formation of the layers, as the layers were sufficiently thin compared with the width of the airliquid two-phase flow region, which was estimated to be several centimeters by eye measurement. The influence of AP on J,, of the submerged membrane was consistent with that for the conventional filtration method mentioned above. Consequently, the fluxprediction equations for conventional filtration method using a circulation pump were assumed to be extensible to filtration by submerged membranes with air bubbling. The influence of the bubbling strength V, on J,, is shown in Fig. 7. Here, the relationship Jssm Vao.3 was observed. This relationship was expressed as Jssmu*l.O using the air-liquid two-phase flow velocity U* measurement results (Fig. 8). It was also consistent with the previously reported relationships for conventional filtration method expressed in Eq. 2 (9). It was thought that the effect of operating parameter of fluidity on the lift velocity or on the flux was not expressed only by V,, as the fluidity varies not only according to V, but also according to the fluid characteristics of viscosity, particle concentration and so on of the suspension. To construct universal flux-prediction equations, the influence of the fluidity was expressed in terms of a*. It was shown that a 0.3-power exponential relationship exists between U* and V,. This relationship is consis-
40 60 A P [kPa]
80
1
IO
100
1000
Va [r&ri?b’] FIG. 7. Effect of bubbling strength, V,, on steady-state flux, J,,, of intermittently aerated activated sludge: D,=O.S pm, AP==30 kPa, ML&S=8 kg.mm3, T=2O”C.
tent with the results of Taylor’s study concerning the upflow centerline velocity of a plume and air flow rate (15, 16). The effect of particle concentration on flux was discussed in terms of wet particle concentration, C,, in previous papers (9, 10). However, particle concentration is generally used as the dry weight of MLSS in the field of wastewater treatment. Therefore, the particle concentration was expressed as MLSS in this study. The influence of MLSS on J,,, which was examined under the same V, values, was found to be divided into two regions as shown in Fig. 9. In the first region, i.e., low MLSS, the influence was consistent with the previously reported relationship for the conventional filtration method, Js,=MLSS-o.s (8). However, the experimental flux value in the region of high MLSS (the second region) was smaller than the predicted value for the conventional filtration. The discrepancy in the second region was explained as the effect of a deceleration of air-liquid two-phase flow velocity M* resulting from a decline of the fluidity in the region of high MLSS. Figure 10 shows
100
FIG. 6. Filtration resistances, R of intermittently aerated activated sludge: D,=O.S pm, V,=7.2mr.m-z.h-1, A4LSS=g kg.mm3, T=2O”C.
u*[m.s’] FIG. 8. Effect of air-liquid two-phase flow velocity, a*, on steady-state flux, J,,, of intermittently aerated activated sludge: D, =OS ,um, AP=30 kPa, MLSS=I kg.me3, T=2O”C.
CROSS-FLOW MICROFILTRATION
VOL. 81, 1996 0
F
I
I
OF SUBMERGED MEMBRANE
1
I 4
(1 st region) +
i -+
59
(2nd region)
d
‘1
10
100
MLSS [kg.n?]
100 MLSS [kg.rti3]
FIG. 10.
FIG. 9. Effect of MLSS on steady-state flux, J,,, of intermittently aerated activated sludge: D,=O.S pm, bp=30 kPa, V,=210m3.m-2. h-’ MLSS=8k . ), T=2O”C. Symbols: 0, measured values; 0, )corrected valie?
the viscosity of the suspension in relation to the MLSS. The corrected J,, values under the same u* condition, based on the measurement values of u* for the fluids and the relationship Jss~u*l.o mentioned above, were found to correspond with the previously reported relationship Js,~A4LSS-o~5 in all experimental regions, as shown in Fig. 9. In all experiments, R, and R, were quite small and J,, was governed by R,. In this situation, J,, values were equal to the lift velocities Vr (9). An exponential relationship between I’, and the parameters was obtained and is expressed as follows. q+=K’u*l.o~Lss-o.5
10
sion TV. Moreover, the velocity profiles in the vicinity of the membrane surface for the conventional filtration model and for the filtration with submerged membrane model may be different. Detailed studies on the velocity profiles and the meaning of K values for these two types of filtration models are therefore required. NOMENCLATURE aAV
C, DP 4 J Ji
JSS
K (6)
The filtration constant K’ of the intermittently aerated activated sludge was calculated as 2.6 x lo-’ kg”.‘. m-1.5. This value was of the same order of magnitude as the conventional filtration values reported for bacterial cells, for example, 3.1 x lop5 kgO.5.m-1.5 for methanogenic waste (9). The flux prediction model helps us to understand the filtration mechanism for submerged membranes with air bubbling. The back-transport, or lift velocity Vr of the filtered colloidal particles from the membrane surface, is thought to be generated by the velocity gradient (=shear stress) on the membrane surface during the conventional filtration method (17, 18). From the above discussion, the back-transport of the filtered colloidal particles from the submerged membrane surface was found to be controlled by the air-liquid two-phase flow, being induced by air bubbling, as well as the case for the conventional filtration method in which cross-flow pumping generates the fluid flow over the membrane surface. The difference between the conventional filtration model and the filtration with submerged membrane model resulted from the effects of operating parameters on the flux. The operating parameter of feed velocity u is an independent variable in the conventional filtration model, while the air-bubbling-induced feed velocity for the submerged membrane is assumed to be not only controlled by the operating parameter of bubbling strength I’, but also to be affected by the fluid characteristics of MLSS, particle size, temperature and viscosity of suspen-
Viscosity of intermittently aerated activated sludge.
K MLSS AP R, R, Rp T U U*
V, VL
6, E ‘lf
7s
P
mean specific filtration resistance, me kg -I : particle concentration, kg. m ~ 3 : membrane pore size, m : particle size, m : filtration flux, m3.m-*.d~-l or m-s--l : initial filtration flux, m3 -m--2.d-1 or m-s-’ : steady-state filtration flux, m3 .m~2.d~1 or m.ss’ : filtration constant, defined by Eq. 2, ke5+m-2.17. Pa-s : filtration constant, defined by Eq. 7, kg”,’ .m-1.5 : dry sludge concentration, (Mixed Liquor Suspended Solid), kg-mm3 : transmembrane pressure, Pa : hydraulic resistance of particle-packed layer on membrane (cake resistance), rn-’ : membrane resistance, rn-’ : plugging resistance, ml : temperature, “C : feed velocity, rn.sl : air-liquid two-phase flow velocity, m-s-’ : bubbling strength, bubbling air flow rate per unit projection membrane area for base, m3mp2.hp1 : lift velocity, rn. s-l : thickness of particle-packed layer on membrane, m : voidage, : viscosity of filtrate, Pa. s : viscosity of suspension, Pa.s : density, kg. mp3 :
REFERENCES 1. Porter, M. C.: Concentration polarization with membrane ultrafiltration. Ind. Eng. Chem. Prod. Res. Develop., 11,234248 (1972). 2. Fane, A. G., Fell, C. J. D., and Walters, A. G.: The relationship between membrane surface pore characteristics and flux for ultrafiltration membranes. J. Membrane Sci., 9, 245-262
60
J. FERMENT.BIOENG..
SHIMIZU ET AL. (1981).
3.
K.:
13.
14.
15. 16. 17.
18.
Surface modification of alumina membranes for membrane bioreactor. J. Ceram. Sot. Jpn., Inter. Ed., 95, 1012-1018 (1987). Sbimizu, Y., Rokudai, M., Kayawake, E., Yazawa, T., Tanaka, H., and Egucbi, K.: Effect of membrane resistance on filtration characteristics for methanogenic waste. Kagakukogaku Ronbunshu, 16, 145-151 (1990). (in Japanese) Sbimlzu, Y., Rokudal, M., Yazawa, T., and Tanaka, H.: Application of membrane bioreactor for sewage sludge treatment. Filtration characteristics of anaerobic digestion for artificial sewage containing cellulose. J. Jpn. Sewage Works Associ. Res., 29, 74-81 (1992). (in Japanese) Taylor, Sir G.: Proc. R. Sot., Ser. A, 231-466 (1995). Kobus, H. E.: Analysis of the flow induced by air-bubble systems. Proc. 11th Conf. Coastal Eng., 2, 1016-1031 (1968). Fane, A. G., Fell, C. J. D., and Nor, M. T.: Ultrafiltration in the presence of suspended matter. Inst. Chem. Eng. Symp. Ser., 73, Cl-Cl2 (1982). Green, G. and Belfort, G.: Fouling of ultrafiltration membranes: lateral migration and the particle trajectory model. Desalination, 35, 129-147 (1980).
Cross-Flow Microfiltration of Activated Sludge Using Submerged Membrane with Air Bubbling YASUTOSHI Biotechnology
SHIMIZU,*
KATSUSHI
URYU, YU-ICHI
OKUNO,
AND
& Water Treatment Laboratory, R & D Div., TOT0 Ltd., 2-l-l Kitakyushu 802, Japan Received 30 March 1995/Accepted
ATSUO
WATANABE
Nakashima,
Kokurakita-ku,
13 October 1995
For the purpose of developing a wastewater treatment membrane bioreactor system, cross-flow microfiltration of intermittently aerated activated sludge was carried out, as a part of the solid-liquid separation process in the bioreactor. The tubular alumina microfiltration membrane, having a pore size of 0.5 pm, was submerged in the activated sludge. A cross-flow stream over the membrane surface was produced by air bubbling, generated by a diffuser situated underneath the membrane. An exponential relationship between the steady-state flux, J,,, and the causative factors such as operating parameters and fluid characteristics was found and expressed as J,= VL=K’u*1~oMLSS-o~5, where V,, ZC’,U* and MLSS are the lift velocity, filtration constanf, air-liquid two-phase flow velocity and MLSS, respectively. The relationship, as expressed by the equation, is consistent with that for conventional cross-flow filtration using a circulation pump. [Key words: cross-flow filtration, microfiltration,
bioseparation,
Cross-flow microfiltration is a valuable solid-liquid separation process, in which a cross-flow stream induces the back-transport of filtered colloidal particles from the membrane surface and provides a high filtration flux. Several types of operation methods can be used for the microfiltration. The most common method which is already being used in a wide range of bioindustries involves the use of circulation pumps which generate cross-flow feed streams over membrane surfaces, as shown in Fig. 1. This method will be referred to as the conventional filtration method. Though the conventional filtration method is simple, easy to use and allows control of filtration flux by controlling pumping conditions such as feed velocity and transmembrane pressure (l-3), it unfortunately requires input of tremendous amounts of energy and may possibly reduce the activity of biocatalysts, i.e., bacterial cells and enzymes in the circulating feed. This is because this method requires about 10 to 20 times as much circulation feed as filtrate by volume to maintain a high filtration flux, and the high discharge rate circulation pump used in the method generates excessive shear stress which can damage the biocatalysts (4). A second method has been proposed to eliminate the disadvantages mentioned above. In this method a crossflow stream is generated by rotating membranes or rotating impellors set in the vicinity of the membrane surfaces (5, 6). Since this method does not require a high discharge rate circulation pump, it does not incur the above-mentioned disadvantages. Moreover, it can be used to filter highly viscous fluids which cannot be processed using the conventional method (6). The third and newest method has been applied to solid-liquid separation in wastewater treatment membrane bioreactors (7, 8). In this method, a cross-flow stream over the membrane surface is produced by air bubbling, which is generated by a diffuser situated underneath the membrane in a feed. The air bubbles flow upward with a fluid along the membrane surface and
submerged membrane,
activated
sludge]
induce a moderate shear stress which generates the backtransport of filtered colloidal particles from the membrane surface. This method not only produces cross-flow filtration without a high discharge rate circulation pump and membrane casing, but also can be set up using simple equipment, such as a low rate suction pump, air blower and a vessel. Therefore, this method is regarded as an energy efficient and low shear stress filtration method, being applicable to membrane bioreactor systems. In order to design a membrane filtration system, flux prediction equations, which describe the relationships of the operating parameters, fluid characteristics and membrane properties to the flux, are required. For the conventional cross-flow filtration method, we have already proposed three equations below (9). Figure 2 shows a cross-flow microfiltration model for a suspension with particles of different sizes, dPl and dP2.
(1) -.0.5dp,0.67Tf1.0 V,, = KU’ .OC Pn {
4 > I/L(min)+Jss=v~/~(rnin) Ji 5 VL(min)AJss= Ji
1
(2) (3)
The steady-state filtration flux, J,,, is expressed in terms of the balance between the initial filtration flux, Ji, and the lift velocities for each particle, VL, (VL1 for dpl and VL, for dp2 in the case of Fig. 2). Ji is determined by the hydraulic resistances of plugging, R,, which is attributable to membrane properties such as pore size and surface charge, and of the membrane itself, R,. Therefore, at the initial stage of filtration, the flux Ji, that is, the flux after the initial rapid decline, is dependent only on the membrane properties, as expressed in Eq. 1. The V,, for the particles of size dpn is expressed as a function of the operating parameters of feed velocity u and fluid characteristics of particle concentration C,, of noted particles n and viscosity of filtrate vf (Eq. 2). J,, is expressed with the hydraulic resistances as in
* Corresponding author. 55
56
J. FERMENT.WOENO.,
SHIMIZU ET AL.
I Generation of cross-flow stream
(
disE$nRte)
(Filtrate
flow
IO-
membranes or Rotating impellers
Air bubble upflow stream
20
l-3
1
rate) :QF
FIG. 1.
Cross-flow microfiltration methods proposed in bioindustry.
Eq. 4. R, represents the resistance of the particle-packed layer on the membrane. 1 J=hP SS (4) ?r &+R,+R, When Ji > VLcti,,), R, is formed onto (R,+R& until J,,= VLctinj, as shown in Fig. 2. In this case, the particles causing the minimum lift velocity, VL(mi,,j,are deposited selectively in the vicinity of the steady-state period, and J,, is controlled only by VrIL(minj. When JiS VLfmin), R, does not develop during the filtration and a further decline in flux does not occur after the initial stage. Jss can be expressed as Js,=Ji(b) as shown in Fig. 2. This occurs when a high-resistance membrane or a membrane with pores larger than the particles, which cause plugging, is used. The abovementioned models could consistently predict the fluxes for mono-dispersed colloidal particles such as latexes and microbial cells, as well as fluxes for particles of various sizes (9, 10). The filtration for the second method will be modeled as a function of rotating speed of membranes or impellors in the near future (6). For the third type of filtration method, the cross-flow
(Initial
stage)
II
Operating FIG. 2.
III
Rotating
Circulation pumping
:QP
II
microfiltration of a submerged membrane with air bubbling, the effect of operating parameters and fluid characteristics on the flux has not been discussed quantitatively. Consequently, we study the effect and construct the flux-prediction equations for the third type of filtration method, referring to the conventional filtration method model using a circulation pump. In an effort to adapt the method for use with wastewater treatment membrane bioreactors, activated sludge was used as experimental colloidal particles. MATERIALS
AND METHODS
Experimental colloidal particles were obtained from domestic wastewater-derived activated sludge fermented in a 0.63 m3 membrane bioreactor. Four hollow-fiber microfiltration membrane modules (Mitsubishi Rayon Co. Ltd, Tokyo), having a pore size D, of 0.1 pm and membrane area of 6 m* for each module, and air diffusers were placed in the bioreactor vessel. Domestic wastewater, with a BOD of 0.2 kg.m-3 and total nitrogen content (T-N) of 0.05 kg.m-3, was fed into the bioreactor at the hydraulic retention time (HRT) of 12 h. The bioreactor was intermittently aerated Activated
IIRm(Steady
sludge
state)
II
time
Cross-flow microfiltration model for suspension with particles of different sizes.
CROSS-FLOW MICROFILTRATION
VOL. 81, 1996
OF SUBMERGED MEMBRANE
57
1 Air
FIG. 3. Schematic diagram of filtration apparatus. 1, 10 I filtration vessel; 2, tubular alumina membrane; 3, filtrate reservoir; 4, electric balance; 5, suction pump; 6, diffuser; 7, air flow meter.
'I1
10
1
Particle size distribution
by an air blower on an aeration and non-aeration cycle of 25 and 35 min, respectively, at a flow rate of 18.0m3. h-i. The membrane was suction filtered using a fixed delivery pump at a rate of 0.14 m3. hh’. The filtration was conducted intermittently to synchronize the aeration time for the bioreactor and with air bubbling. This procedure was described in detail elsewhere (11). Particle size Characterization of activated sludge distribution of the activated sludge was measured with a laser diffraction particle size analyzer, SALD-1100 (Shimadzu Co., Kyoto), with a measurable size range from 0.1 pm to 500 pm. The viscosity of the activated sludge was measured using a Brookfield viscometer. MLSS was measured as 110°C dry sludge weight per volume, which was collected on a 0.22 pm pore membrane filter. The molecular weights of organic materials in the activated sludges and f&rates were measured by HPLC using KB-803 and KB-806 GPC columns (Showa Denko Co., Tokyo) with a measurable molecular weight range of under 107Da. For the measurements, particles were removed by centrifugation at 3,000 x g for 10 min. Cross-flow microfiltration Cross-flow microfiltration was carried out using the filtration apparatus shown in Fig. 3. A TOT0 tubular alumina membrane, ASOOSU (12), with nominal pore size D, of 0.5 pm, an outer diameter of 10 mm, an inner diameter of 7 mm and a length of 200 mm, was submerged in the activated sludge with a MLSS of 3-20 kg+mp3. A cross-flow stream was produced by air bubbling generated by a diffuser situated underneath the submerged membrane. The bubbling strengths, V,, i.e., the bubbling air flow rates per unit projection membrane area for the base, were set between 0 and 300 m3 - rne2. h- l. The cross-flow steam velocity u* for air-liquid two-phase flow was measured using a submersible electromagnetic current meter (Kenek Ltd., Tokyo). Filtration was conducted for 4 h at a transmembrane pressure AP of 10-80 kPa and temperature of 20°C. The filtrate was weighed on an electric balance every 30s. For each filtration experiment, the activated sludge was newly prepared from the 0.45 m3 membrane bioreactor. After the filtration experiment, the membrane surface was cleaned with a sponge to remove the particle-packed layer which had accumulated during filtration. Pure water flux through the membrane was then measured to calculate the hydraulic resistances of adsorption on and
1000
100
@[pm1 of intermittently
aerated acti-
vated sludge.
plugging of membrane pores, R,, and of the particlepacked layer which formed on the membrane, R,. Details of the measurements and calculations are given elsewhere (9). RESULTS AND DISCUSSION Physical properties of intermittently aerated activated sludge The particle size distribution of the intermittently aerated activated sludge is shown in Fig. 4. The particles were found to be larger than 0.8 pm. It is known that filtration with membranes whose pore size is larger than the filtered particle size causes plugging and reduces flux (13). Therefore, a membrane with a pore size smaller than 0.8 pm was selected for the filtration experiment. Dissolved organics in the reactor had molecular weight within a range of 102 to lo3 Da, as did the filtrate with the same concentrations. Since the organics in the sludge were allowed to pass through the membrane or membrane surface particle-packed layer during filtration (14), the effect of the concentration polarization of organics on the flux was judged to be negligible. Flux-prediction equation The influence of operat1.o
I
I
I
I
20
40
60
80
6 ?JE “i 0.5 s 7
0.c
'0
100
AP [kPa] FIG. 5. Effect of transmembrane pressure, AP, on steady-state flux, J, of intermittently aerated activated sludge: D,=O.S pm, MLSS=8kg.m-3, T=20”C. Symbols: 0, V,=7.2m3.m-2.h-L; 0, V~=210m3.m-2.h-1.
58
SHIMIZU ET AL.
J.
FERMENT. BIOENG..
ing parameters and fluid characteristics on steady-state flux of a submerged membrane was examined using intermittently aerated activated sludge. The filtration flux declined rapidly within 1 h after the start of filtration, with further declines being quite small. The flux value after 4-h operation was regarded as the steady-state value,
Jw
Figures 5 and 6 show J,, and filtration resistances R plotted against the transmembrane pressure AP. J,, was almost consistent for AP over 50 kPa. This relationship can be explained as the increment of R, (Fig. 6). R, can be expressed as a function of the particle-packed layer thickness, a,, layer density, p, and voidage, E, as expressed in Eq. 5. R, =
aAd
- &)P&
(5)
The 8, was measured as being less than 1 mm by microscopic observation. The air-liquid two-phase flow profile on the membrane surface would not be changed by the formation of the layers, as the layers were sufficiently thin compared with the width of the airliquid two-phase flow region, which was estimated to be several centimeters by eye measurement. The influence of AP on J,, of the submerged membrane was consistent with that for the conventional filtration method mentioned above. Consequently, the fluxprediction equations for conventional filtration method using a circulation pump were assumed to be extensible to filtration by submerged membranes with air bubbling. The influence of the bubbling strength V, on J,, is shown in Fig. 7. Here, the relationship Jssm Vao.3 was observed. This relationship was expressed as Jssmu*l.O using the air-liquid two-phase flow velocity U* measurement results (Fig. 8). It was also consistent with the previously reported relationships for conventional filtration method expressed in Eq. 2 (9). It was thought that the effect of operating parameter of fluidity on the lift velocity or on the flux was not expressed only by V,, as the fluidity varies not only according to V, but also according to the fluid characteristics of viscosity, particle concentration and so on of the suspension. To construct universal flux-prediction equations, the influence of the fluidity was expressed in terms of a*. It was shown that a 0.3-power exponential relationship exists between U* and V,. This relationship is consis-
40 60 A P [kPa]
80
1
IO
100
1000
Va [r&ri?b’] FIG. 7. Effect of bubbling strength, V,, on steady-state flux, J,,, of intermittently aerated activated sludge: D,=O.S pm, AP==30 kPa, ML&S=8 kg.mm3, T=2O”C.
tent with the results of Taylor’s study concerning the upflow centerline velocity of a plume and air flow rate (15, 16). The effect of particle concentration on flux was discussed in terms of wet particle concentration, C,, in previous papers (9, 10). However, particle concentration is generally used as the dry weight of MLSS in the field of wastewater treatment. Therefore, the particle concentration was expressed as MLSS in this study. The influence of MLSS on J,,, which was examined under the same V, values, was found to be divided into two regions as shown in Fig. 9. In the first region, i.e., low MLSS, the influence was consistent with the previously reported relationship for the conventional filtration method, Js,=MLSS-o.s (8). However, the experimental flux value in the region of high MLSS (the second region) was smaller than the predicted value for the conventional filtration. The discrepancy in the second region was explained as the effect of a deceleration of air-liquid two-phase flow velocity M* resulting from a decline of the fluidity in the region of high MLSS. Figure 10 shows
100
FIG. 6. Filtration resistances, R of intermittently aerated activated sludge: D,=O.S pm, V,=7.2mr.m-z.h-1, A4LSS=g kg.mm3, T=2O”C.
u*[m.s’] FIG. 8. Effect of air-liquid two-phase flow velocity, a*, on steady-state flux, J,,, of intermittently aerated activated sludge: D, =OS ,um, AP=30 kPa, MLSS=I kg.me3, T=2O”C.
CROSS-FLOW MICROFILTRATION
VOL. 81, 1996 0
F
I
I
OF SUBMERGED MEMBRANE
1
I 4
(1 st region) +
i -+
59
(2nd region)
d
‘1
10
100
MLSS [kg.n?]
100 MLSS [kg.rti3]
FIG. 10.
FIG. 9. Effect of MLSS on steady-state flux, J,,, of intermittently aerated activated sludge: D,=O.S pm, bp=30 kPa, V,=210m3.m-2. h-’ MLSS=8k . ), T=2O”C. Symbols: 0, measured values; 0, )corrected valie?
the viscosity of the suspension in relation to the MLSS. The corrected J,, values under the same u* condition, based on the measurement values of u* for the fluids and the relationship Jss~u*l.o mentioned above, were found to correspond with the previously reported relationship Js,~A4LSS-o~5 in all experimental regions, as shown in Fig. 9. In all experiments, R, and R, were quite small and J,, was governed by R,. In this situation, J,, values were equal to the lift velocities Vr (9). An exponential relationship between I’, and the parameters was obtained and is expressed as follows. q+=K’u*l.o~Lss-o.5
10
sion TV. Moreover, the velocity profiles in the vicinity of the membrane surface for the conventional filtration model and for the filtration with submerged membrane model may be different. Detailed studies on the velocity profiles and the meaning of K values for these two types of filtration models are therefore required. NOMENCLATURE aAV
C, DP 4 J Ji
JSS
K (6)
The filtration constant K’ of the intermittently aerated activated sludge was calculated as 2.6 x lo-’ kg”.‘. m-1.5. This value was of the same order of magnitude as the conventional filtration values reported for bacterial cells, for example, 3.1 x lop5 kgO.5.m-1.5 for methanogenic waste (9). The flux prediction model helps us to understand the filtration mechanism for submerged membranes with air bubbling. The back-transport, or lift velocity Vr of the filtered colloidal particles from the membrane surface, is thought to be generated by the velocity gradient (=shear stress) on the membrane surface during the conventional filtration method (17, 18). From the above discussion, the back-transport of the filtered colloidal particles from the submerged membrane surface was found to be controlled by the air-liquid two-phase flow, being induced by air bubbling, as well as the case for the conventional filtration method in which cross-flow pumping generates the fluid flow over the membrane surface. The difference between the conventional filtration model and the filtration with submerged membrane model resulted from the effects of operating parameters on the flux. The operating parameter of feed velocity u is an independent variable in the conventional filtration model, while the air-bubbling-induced feed velocity for the submerged membrane is assumed to be not only controlled by the operating parameter of bubbling strength I’, but also to be affected by the fluid characteristics of MLSS, particle size, temperature and viscosity of suspen-
Viscosity of intermittently aerated activated sludge.
K MLSS AP R, R, Rp T U U*
V, VL
6, E ‘lf
7s
P
mean specific filtration resistance, me kg -I : particle concentration, kg. m ~ 3 : membrane pore size, m : particle size, m : filtration flux, m3.m-*.d~-l or m-s--l : initial filtration flux, m3 -m--2.d-1 or m-s-’ : steady-state filtration flux, m3 .m~2.d~1 or m.ss’ : filtration constant, defined by Eq. 2, ke5+m-2.17. Pa-s : filtration constant, defined by Eq. 7, kg”,’ .m-1.5 : dry sludge concentration, (Mixed Liquor Suspended Solid), kg-mm3 : transmembrane pressure, Pa : hydraulic resistance of particle-packed layer on membrane (cake resistance), rn-’ : membrane resistance, rn-’ : plugging resistance, ml : temperature, “C : feed velocity, rn.sl : air-liquid two-phase flow velocity, m-s-’ : bubbling strength, bubbling air flow rate per unit projection membrane area for base, m3mp2.hp1 : lift velocity, rn. s-l : thickness of particle-packed layer on membrane, m : voidage, : viscosity of filtrate, Pa. s : viscosity of suspension, Pa.s : density, kg. mp3 :
REFERENCES 1. Porter, M. C.: Concentration polarization with membrane ultrafiltration. Ind. Eng. Chem. Prod. Res. Develop., 11,234248 (1972). 2. Fane, A. G., Fell, C. J. D., and Walters, A. G.: The relationship between membrane surface pore characteristics and flux for ultrafiltration membranes. J. Membrane Sci., 9, 245-262
60
J. FERMENT.BIOENG..
SHIMIZU ET AL. (1981).
3.
K.:
13.
14.
15. 16. 17.
18.
Surface modification of alumina membranes for membrane bioreactor. J. Ceram. Sot. Jpn., Inter. Ed., 95, 1012-1018 (1987). Sbimizu, Y., Rokudai, M., Kayawake, E., Yazawa, T., Tanaka, H., and Egucbi, K.: Effect of membrane resistance on filtration characteristics for methanogenic waste. Kagakukogaku Ronbunshu, 16, 145-151 (1990). (in Japanese) Sbimlzu, Y., Rokudal, M., Yazawa, T., and Tanaka, H.: Application of membrane bioreactor for sewage sludge treatment. Filtration characteristics of anaerobic digestion for artificial sewage containing cellulose. J. Jpn. Sewage Works Associ. Res., 29, 74-81 (1992). (in Japanese) Taylor, Sir G.: Proc. R. Sot., Ser. A, 231-466 (1995). Kobus, H. E.: Analysis of the flow induced by air-bubble systems. Proc. 11th Conf. Coastal Eng., 2, 1016-1031 (1968). Fane, A. G., Fell, C. J. D., and Nor, M. T.: Ultrafiltration in the presence of suspended matter. Inst. Chem. Eng. Symp. Ser., 73, Cl-Cl2 (1982). Green, G. and Belfort, G.: Fouling of ultrafiltration membranes: lateral migration and the particle trajectory model. Desalination, 35, 129-147 (1980).