Anaerobic digestion by a ceramic membrane enclosed reactor

JOURNAL OF FERMENTATION AND BIOENGINEERING

Vol. 71, No. 2, 122-125. 1991

Anaerobic Digestion by a Ceramic Membrane Enclosed Reactor E I S U K E K A Y A W A K E , * Y O S H I H I S A N A R U K A M I , AND M A S A T S U G U Y A M A G A T A

Ceramic Membrane Project Team, Water and Sewage Plant Div., Kubota Corporation, 1-2-47, Shikitsuhigashi, Naniwa-ku, Osaka 556-91, Japan Received 23 May 1990/Accepted 25 October 1990 A reactor with an external pressure type ceramic membrane module was designed to complete a compact system which brings about a reduction of permeability power requirements. The applicability of the reactor to methane fermentation was examined using heat-treated liquor obtained from sewage sludge. The experiments were carried out at 3 steps of CODer-loading (Run 1; 4.53, Run 2; 7.98, Run 3; 15.4 kg CODcr/m 3 • d). The permeation flux stabilized at 0.06 in Run 1 and 0.11 m3/m2.d in Run 2, where the membrane surface was backwashed with N2 gas. In Run 3, the membrane surface was additionally washed by circulating the gas yielded by the fermentation to increase the permeation flux, which stabilized at 0.2 m3/m2.d. The CODer removal was 79.3 in Run 1, 83.2 in Run 2, and 80.2% in Run 3, respectively. In all cases, the BOD removal exceeded 95%, and there was no significant difference in the gas yield, which was 0.41 to 0.43 (//g CODer supplied) and 0.50 to 0.54 (l/g CODer decomposed). Since the BOD could be greatly reduced even at 0.6 d of HRT and the microbiol cell concentration could be easily controlled, it seems possible to that the effciency of this reactor can be further improved.

matized for about 4 months by a semi-continuous operation feeding the heat-treated liquor as a substrate. The temperature in the digestor was kept at 35°C. Methane fermentation apparatus and membrane module Figure 1 shows the scheme o f the methane fermentation device. The working volume of the fermentor is 200/. Inside the fermentor an external pressure type ceramic m e m b r a n e module was installed, and filtration o f the fermentation b r o t h was done by suction. Stirring o f the fermentation b r o t h was accomplished by using a circulating p u m p (Figs. 1, 4). Back washing o f the membrane m o d u l e was regularly p e r f o r m e d with nitrogen gas. The permeate through m e m b r a n e module was suctioned by a p u m p (Figs. 1,6). A part o f the permeate was returned into the reactor for water level adjustment (Figs.l, 8) and the rest was discharged. The reactor temperature was adjusted to 35-38°C. The p r o d u c e d gas was stored in a gas holder after desulphurization. It was then burned with a gas burner. Figure 2 is a drawing o f the test m e m b r a n e module

With the advancement o f m e m b r a n e separation technology, reactors c o m b i n e d with membranes have come to be widely used in biological reactions (1-3). By introducing m e m b r a n e technology into a bioreactor, various operational merits can be added to the reactor. These include the retention o f a high concentration o f microbial cells or enzymes (4-6), removal o f inhibitors (7), and enhancement o f the decomposition of n o n d e c o m p o s a b l e suspended substances (8). In general, most bioreactors combined with a m e m b r a n e separator consist o f a bio-reaction section and a m e m b r a n e separation section, where cross-flow filtration is p e r f o r m e d primarily by a tubular-type module (9-11). However, these bioreactors still have some problems to be addressed; (i) a large permeability power requirement, (ii) a large space requirement, and (iii) the complexity o f the system. As a consequence, we have designed a m e m b r a n e m o d u l e enclosed in a bioreactor to complete a compact system which brings a b o u t a reduction o f the permeability power required. Generally, ceramic membranes have high mechanical strength, and resistance to chemicals like hydrochloric acid or active chlorine as well as to high temperatures (12, 13). In this work, the applicability of a reactor with an external pressure type ceramic m e m b r a n e module to methane fermentation was examined and the results are discussed from the viewpoint o f process efficiency.

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MATERIALS AND METHODS Influent Heat-treated liquor (14), which was obtained from heat-treated sewage sludge, was used as the test liquor (influent) for methane fermentation. Microorganisms and acclimation Anaerobically digested sludge from a sewage treatment plant was used after screening through a 2 m m mesh net (approximate size) to remove coarse solid admixtures. The sludge was accli-

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FIG. 1. Schematic diagram of the anaerobic fermentation system with a membrane module. 1, Fermentor; 2, membrane module; 3, desulfurization trap; 4, fermentation broth circulation pump; 5, gas circulation pump; 6, effluent pump; 7, backwashing with N2; 8, permeate return line; 9, bio-gas outlet; 10, thermostat; 11, backwashing pressure gauge; 12, suction pressure gauge.

* Corresponding author. 122

VOL. 71, 1991

ANAEROBIC DIGESTION

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Reactor operation CODcF loading (kg/m3.d) (kg/kg MLVSS.d) BOD loading (kg/m3.d) HRT (d) MLVSS (g//) Temperature (°C)

4 0.4 2 2 10 37

8 0.8 4 1 10 37

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Membrane operation Flow velocitya (m/s) Suction pressure (mmHg) Back washing pressure (kg/cm 2) Filtration time (min) Back washing time (s) Gas circulation (l/rain)

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a Velocity of fermentation broth on membrane surface. (size: 500 L X 100w x 800 H, m e m b r a n e area: 1.06 m 2, m e m b r a n e area/reactor capacity ratio: 5.45 m2/m3). The membrane is a tubular external pressure type ceramic m e m b r a n e with 0.1/~m poresize (outside diameter 1 0 m m ¢ ; inside diameter 7 m m ¢ ; length 450 mm) m a n u f a c t u r e d by K u b o t a Co. (Osaka). The operating conditions of the reactor are shown in Table 1. Analysis TOC was determined with a TOC analyzer (Shimadzu type 500, Kyoto). CODer, B O D , MLVSS, KrN and T-P were measured according to the Japanese Industrial Standards 0 5 ) . Total organic acid was measured by rapid colorimetric determination according to Montgomery et al. (16).

FIG. 3. Time course of permeation flux in each run. be affected by the growth or microbiol compaction on the m e m b r a n e surface (17). In this test, the flow velocity, 0.2 or 0.3 m / s , was low; it is, therefore, assumed that the permeation flux declined due to the growth of a deposit layer, and then stabilized when the growth reached a steady state. The balanced flux of R u n 2, which was higher than that of R u n 1, seemed to depend on the higher flow. While in Runs 1 and 2 the m e m b r a n e surface was washed only by the circulation of the fermentation broth, in R u n 3 it was additionally washed by the circulation of the gas yielded. As a result, the permeation flux stabilized at 0.2 m3/m 2. d after 10 d of operation. Since the other operating conditions in R u n 3 were the same as those in Run 2, the two-fold increase in the permeation flux was considered to be due to the effect of gas cleaning. In this method, it is difficult to increase the flow velocity for economical reasons. It is also unavoidable that the permeation flux gradually reduces. Nevertheless, it was found that the permeation flux was increased by circulating the gas yielded by the fermentation.

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(d) FIG. 4. Methane fermentation of heat-treated liquor of sewage sludge in the membrane-coupled bioreactor. Symbols: • , influent pH; Q, effluent pH; $, influent CODer; ©, effluent CODer; A, MLVSS; A, gas yield. Operating conditions: see Runs 1 to 3 in Table i.

124

K A Y A W A K E ET AL. T A B L E 2.

J. FERMENT. BIOENG.,

Qualities of heat-treated liquor (influent) of sewage sludge and o f effluent after methane fermentation by membrane-coupled reactor Run 1

pH O R P (mV) Alkalinity (mg//) SS (mg//) CODcr (rag//) BOD (mg//) T O C (mg//) Kr-N (mg//) NH3-N (mg//) NO2 -N (mg//) NO3 -N (mg/l) T-P (rag//) PO 3 -P (mg//) VFA (mg//) MLVSS a (mg//)

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5.5 100 350 180 10630 4370 2800 950 287 < 0.1 <.0.1 53.2 50.0 1190

7.7 400 2270 ND b 2200 179 684 642 264 <~0.1 <~0.01 27.3 23.6 191 11000

5.9 220 920 520 9230 4910 3210 892 160 ~" 0.1 <~0.1 20.5 18.6 2680

8.1 400 2680 ND b 1550 157 751 740 245 ~ 0.1 <~.0.01 13.2 12.5 229 10200

5.8 150 760 310 10250 5000 3000 699 310 ~ 0.1
8.0 400 2530 ND b 2030 235 759 562 256 • 0.1 <0.1 25.5 20.7 189 21400

Includes excess sludge withdrawn. b Not detected.

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Treatment efficiency of membrane-coupled methane fermentation Figure 4 indicates the performance o f p H , CODcr, MLVSS and gas yield during the methane fermentation o f the heat-treated liquor from sewage sludge in the m e m b r a n e coupled reactor. The H R T was fixed at 2, 1 and 0.5 h. The influent p H was 5.0 to 6.0, and the effluent p H was 7.5 to 8.0. The C O D c r value o f the effluent finally stabilized at 1,000 to 1,500 mg/l. The MLVSS concentration could be maintained at 8.3 to 12.5 g/l in Runs 1 and 2, and at 17.4 to 21.7 g/l in Run 3, as desired. Control o f MLVSS was p e r f o r m e d by draining the fermentation broth, according to the MLVSS in the reactor. In all runs there were no significant differences in the gas yields, 0.41 to 0.43 (m3/kg CODcr supplied) and 0.50 to 0.54 (m3/kg C O D c r decomposed). Methane content was 63 to 66% (data not shown). Since both the CODcr value o f the effluent and the gas yield were constant after a b o u t 20-d operation in all cases, steady-states might have been established. Table 2 indicates the quality o f effluent under steadystates. The CODcr, BOD and T O C concentrations of T A B L E 3.

treated water were 1,550 to 2,200 m g / l , 157 to 235 m g / l , and 684 to 759mg/1, respectively. The increase o f NH 3 concentration seems to have been caused by deamination of protein in the influent. P h o s p h o r u s contained as p h o s p h a t e in the feed water remained virtually unchanged. SS was not detected in the effluent. The effluent showed high transparency, which is an advantageous point in a m e m b r a n e separation reactor. Table 3 summarizes the steady-state data obtained in each operation shown in Fig.4. In all cases, removal of CODcr, BOD and T O C was about 80, 95 and 75o/00, respectively. A l t h o u g h Kr-N and T-P removal was high in Run 1, it d r o p e d somewhat in both cases in Runs 2 and 3. F r o m the quality o f the treated water, further treatment is seen to be necessary before recycling or discharging into the environment. However, as there is no SS in the treated water, m e m b r a n e separation appears to have an advantage both as a treatment m e t h o d and also as a pretreatment method. Figure 5 indicates the relationship between CODcr loading and the sludge yield. The sludge yield per feed

S u m m a r y of steady-state data in membrane-coupled m e t h a n e fermentation ~

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CODcr loading (kg/m3.d) (kg/kgMLVSS. d) BOD loading (kg/m3.d) T O C loading (kg/m3.d) H R T (d) CODcr removal ( ~ ) BOD removal (%) T O C removal (~00) Kr-N removal (%) T-P removal (%) Sludge produced b ( k g M L V S S / m 3) Sludge yield ( k g M L V S S / k g C O D fed) Gas production rate (m3/m3-d) Gas yield (m3/kgCOD fed) (m3/kgCOD used) Methane content (%)

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Run 2

Run 3

4.53 0.41 2.30 1.48 2.02

7.98 0.66 4.07 2.80 1.10

15.4 0.51 7.81 5.38 0.58

79.3 95.9 75.6 32.5 52.3 0.381 0.040 2.13 0.43 0.54 66

83.2 96.8 76.6 17.0 35.5 0.407 0.046 3.73 0.41 0.50 65

80.2 95.3 74.7 19.7 36.2 0.400 0.042 7.10 0.42 0.54 65

a These were obtained after 20-d operation in each run (see Fig. 4). b Excess sludge withdrawn was also counted,

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Relationship between CODcr loading and sludge produc-

VOL. 71, 1991

ANAEROBIC DIGESTION

C O D c r was 0 . 0 4 6 g M L V S S / g C O D c r u n d e r a C O D c r l o a d i n g o f 0.66 k g / k g , d ( R u n 2), b u t this v a l u e decreased as the C O D c r l o a d i n g was l o w e r e d : 0.042 ( R u n 3), and 0.040 ( R u n 1). T h e s a m e t e n d e n c y was o b s e r v e d in the sludge c o n c e n t r a t i o n , i n c l u d i n g excess sludge, and in the sludge yield per B O D feed. C o n s e q u e n t l y , if the a m o u n t o f sludge p r o d u c t i o n decreases, it is better to o p e r a t e the system u n d e r r e d u c e d C O D c r l o a d i n g a n d / o r increased M L V S S c o n c e n t r a t i o n in the reactor. T h u s , c o n t i n u o u s m e t h a n e f e r m e n t a t i o n c o u l d be o p e r a t e d satisfactorily by the c e r a m i c m e m b r a n e enclosed b i o r e a c t o r . C o m p a r i n g the t r e a t m e n t efficiency with the that o f o t h e r i m m o b i l i z e d m e t h o d s (18), the B O D c o u l d be largely r e m o v e d even at 0.6 d o f H R T in this r e a c t o r , while o t h e r i m m o b i l i z e d m e t h o d s r e q u i r e d 2 d for similar treatm e n t . A l s o , since the bacterial c o n c e n t r a t i o n can be easily c o n t r o l l e d in this reactor, it seems possible that the efficiency can be i m p r o v e d f u r t h e r . ACKNOWLEDGMENT As a member of the Aqua Renaisance Research Association (ARRA), we study this investigation which is consigned by the New Energy and Industrial Technology Development Organization to the ARRA. REFERENCES 1. Higasa, M.: Wastewater treatment by high-density activated sludge method combined with ultrafiltration membranes. J. Water and Waste, 27, 1015-1023 (1985). 2. Kanayama, H.: Application of ultrafiltration for waste water reuse system of buildings. Japan J. Water Pollut. Res., 13, 13-16 (1990). 3. Magara, Y.: Membrane application to human excreta treatment system. Japan J. Water Pollut. Res., 13, 17-20 (1990). 4. Ferras, E., Mineir, M., and Goma, G.: Acetonobutyric fermentation: improvement of performance by coupling continuous fermentation and ultrafiltration. Biotechnol. Bioeng., 28, 523533 (1986). 5. Kyung, H. K. and Philipp, G.: Continuous production of ethanol by yeast "immobilized" in a membrane-contained fermentor,

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Biotechnol. Bioeng., 26, 252-256 (1984). 6. Taniguchi, M., Kotani, N., and Kobayashi, T.: High concentration cultivation of lactic acid bacteria in fermentor with crossflow filtration. J. Ferment. Technol., 65, 179-184 (1987). 7. Solomon, B.A., Colton, C.K., Friedman, L.I., Castino, F., Wiltbank, T.B., and Martin, D.M.: Microporous membrane fltration for continuous-flow plasmapheresis, p. 489-601. In Cooper, A. R. (ed.), Ultrafiltration membranes and applications. Plenum Press, New York (1980). 8. Kayawake, E., Toya, S., Rokudai, M., Shimizu, Y., Honda, S., Tanaka, R., and Eguchi, K.: Anaerobic digestion of artificial wastewater containing cellulose by a membrane reactor. Hakkokogaku, 67, 255-261 (1989). 9. Rautenhach, R. and Albrecht, R.: Module design and module characterization, p. 131-171. In Rautenbach, R. and Albrecht, R. (ed.), Membrane processes. John Wiley & Sons, New York (1980). 10. Murkes, J. and Carsson, C. G.: Low shear crossflow filtration, p. 33-68. In Murkes, J. and Carsson, C. G. (ed.), Crossflow filtration. John Wiley & Sons, New York (1988). l 1. Hashimoto, K.: Ultrafiltration method, p. 60-66. In Ogiwara, B. and Hashimoto, K. (ed.), Separation methods using membrane. Kodansha, Tokyo (1980). 12. Matsumoto, Y., Nakao, S., and Kimura, S.: Cross-flow filtration of polymer solutions by ceramic microfiltration membranes. Kagakukogaku Ronbunshu, 13, 100-106 (1987). 13. Matsumoto, K., Kawahara, M., and Ohya, H.: Cross-flow filtration of yeast by microporous ceramic membrane with backwashing. J. Ferment. Technol., 66, 199-205 (1988). 14. Shimizu, K., Toda, I., Uede, K., Uchimura, T., Hiraoka, M., and Murakami, T.: Heat treatment of sewage sludge by lower temperature system. J. Water and Waste, 17, 163-173 (1975). 15. Japanese Industrial Standards Committee: Testing Methods for Industrial Wastewater JIS K0102-1986 (in Japanese), Japanese Standard Assoc., Tokyo (1986). 16. Montgomery, H. A. C., Dymock, J. F., and Thorn, N.S.: The rapid colorimetric determination of organic acids and their salts in sewage-sludge liquor. Analyst, 37, 949-955 (1962). 17. Shimizu, Y., Rokudai, M., Toya, S., Kayawake, E., Yazawa, T., Tanaka, H., and Egnchi, K.: Effect of operating time on the permeation flux of alumina membrane. Kagakukogaku Ronbunshu, 15, 788-794 (1989). 18. Kawasugi, T., Toriyama, A., Kayawake, E., and Shioyama, A. : Recent progress in methane fermentation. Hakkokogaku, 64, 205-207 (1986).