Two-phase continuous anaerobic digestion of fruit and vegetable wastes

reso~¥ces, ELSEVIER

Resources, Conservation and Recycling 13 (1995) 257-267

conservation and recycling

Two-phase continuous anaerobic digestion of fruit and vegetable wastes A. Mtz.-Viturtia a, j. Mata-Alvarez "'*, F. Cecchi b a University of Barcelona, Department of Chemical Engineering, Martfi Franquds 1/6pit, E-08028 Barcelona, Spain b University of Venice, Department of Environmental Sciences, Calle Larga S. Marta 2137, 1-30123 Venezia, Italy

Received 1 July 1994; accepted 1 October 1994

Abstract Results of a two-phase mesophilic (35°(2) anaerobic digestion treatment of fruit and vegetable wastes, carried out at laboratory scale, are presented and discussed. They are contrasted with other results obtained with a similar waste, but digested in a one-phase system. The yields are lower in this simple two-phase system, because of the higher organic loading rate. It is concluded that this twophase system does not seem appropriate to treat these wastes unless it is equipped with some type of control of the hydrolytic step. A one-phase system is simpler and can yield at least the same yields. Keywords: Anaerobic digestion; Fruit waste; Vegetable waste

1. Introduction Recycling municipal solid waste ( M S W ) becomes more obvious every day and, accordingly, many towns are starting new selective collection programs to make this possible. Public awareness o f the environmental hazards of raw incineration and landfilling are increasing the complexity of these operations and, consequently, their costs. The trend is then to minimize the amount o f waste to be treated with these procedures. Recycling of the organic fraction of municipal solid wastes ( O F M S W ) is widely accomplished through the composting process. Nowadays, a new and interesting way of composting O F M S W , alternative to the classic one, is the combination of anaerobic and aerobic techniques. As recently pointed out, the inclusion o f an anaerobic digestion step, previous to the composting process, is becoming more acceptable, especially for wet organic material * Corresponding author. 0921-3449/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI092 ! -3449 ( 94 ) 00048 -4

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(easily degradable fractions of refuse) in contrast with the direct application of a single composting step [ 1,2]. Furthermore, single aerobic composting requires a net energy input of 30 k w h / t MSW [3], but combining an anaerobic step, organic matter is decomposed releasing methane gas, so that the energy balance becomes positive. Given these trends, the aim of this paper is to offer more data on the anaerobic digestion of a wet waste such as that coming from a fruit and vegetable market. The behaviour of a simple two-phase system, without any added control, under different operating parameters is considered, and the results are compared with those obtained with a one-phase system.

2. Experimental procedures Experiments were carried out at laboratory scale. Two parallel systems were placed in a thermostatic chamber maintained at 35 +__0.5°C. Each system consisted of a hydrolyser and a methanizer reactor. The hydrolysers were reactors of 1.3 dm 3 working volume. The bottom was filled with Raschig rings (5 mm) to assure perfect leachate percolation. At the top of the hydrolyser there was a feeding port. The bottom of the hydrolyser was connected to the leachate recirculation line, which couples the hydrolyser to the methanizer by means of a peristaltic pump (average flow rate 500 cm3/day). The methanizers were hybrid-type reactors of 0.5 dm 3 working volume. Their lower section consisted in an upflow anaerobic sludge blanket reactor. At the top, there was an anaerobic filter using polyurethane foam as a support, with a pore diameter of 1.5 mm (more details of this reactor can be found in [4] ). Fig, 1 shows a basic scheme of the systems used. SUBSTRATE FEEDER BIOGAS $

l,

OUTLET

BIOGAS OUTLET $

I I LEACHATE SAMPLE

HYDROL¥SER

Illllllll I

Mrrrm-I NIZER

llill i ~*'-',,htg /111111/ IIIIIIII

iiin~lllll IIIIIIIII

LEACHATE

SAMPLE

Fig. 1. Schemeof the two-phasesystemused.

I

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259

Table 1 Basic characteristics of the substrate fed to the hydrolyser

Tomato Lettuce Cucumber Cauliflower Orange Melon Total

% weight

TS (g/kg)

VS (g/kg)

N (mg/l)

P (rag/l)

25.0 25.0 25.0 10.0 7.5 7.5 100.0

60.7 56.7 37.8 84.4 143.0 79.9 64.0

55.6 42.2 33.7 77.5 137.8 74.3 56.5

771 1945 685 2697 1573 1117 1322

288 319 293 379 208 285 300

TS: total solids; VS: volatile solids; N: total N; P: total P. Density of shredded substrate: 0.995 kg/1.

2.1. Substrate The substrate consisted of a mixture of fruits and vegetables, simulating the sorted wastes produced by the central market in Barcelona [5]. Substrate composition is presented in Table 1. Before feeding it into the hydrolyser, the substrate was shredded. Density of the substrate, after shredding was about 0.995 g/cm 3. Assuming a carbon content around 45% of the total solid (TS) content, which is roughly the value for these type of waste [6] and considering the data shown in Table 1, a C/N ratio of around 24 results. This value assures the absence of ammonia toxicity and, at the same time, a suitable nutrient balance [ 7]. 2.2. Procedure Initially, hydrolysers were fed with 1 kg of substrate and 250 cm 3 of water. After adding 50 g of inoculum (cow manure, TS --- 15.4%; VS = 13.2%) to the hydrolysers and filling the methanizers with 100 cm 3 of water and 400 cm 3 of methanogenic inoculum (digested pig manure, TS = 19.2 mg/dm3; VS = 11.3 mg/dm3), the recirculation pump was switched on. Hydrolysers were left unfed for 33 days. After this period, the feeding operation for the first stationary run was started (run R1 ), adding only the substrate of fruit and vegetable wastes. A feed flow rate of 0.1 kg/day was used, until achieving steady-state performance. The reactors were operated for a period of two to three retention times before the steady state were considered achieved. Then, after 1 month of steady operation, the feed flow rate was changed and set to their successive values. Table 2 shows the four feed flow rates used, together with the resulting hydraulic retention times (HRT) and resulting organic loading rates (OLR). 2.3. Analytical methods Total solids (TS), volatile solids (VS), total nitrogen and total phosphorous analysis were carried out in accordance with the methodology reported by Standard Methods [ 8 ] and Bond and Straub [9]. Biogas production was measured by a device designed by the authors [ 10]. Biogas composition analysis was carried out by means of a Gas Chromato-

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Table 2 Substrate fed and resulting organicloading rate (OLR) for the four runs tested Run

R1 R2 R3 R4

Flow rate (g/day)

100.0 200.0 300.0 400.0

OLR (g VS/dm3. d)

Retention time (days)

3.14 6.28 9.42 12.56

hydrolyser

methanizer

overall

12.93 6.47 4.31 3.23

4.97 2.49 1.66 1.24

17.91 8.95 5.97 4.48

graph Shimadzu GC-9A. Column and conditions were: thermal conductivity detector; Porapack Q,80/100 column of 1/8" and 3 m length. Working temperature: 37°C (constant); injector temperature: 45°C; detector temperature: 100°C. Time: 5.5 min. Volatile fatty acids (VFA) were determined by the same GC. Column and conditions were: flame ionization detector; column: GP 15% SP-1220/1% H3PO4 in 100/120 Chromosorb W A W 6, 1/8" and 3 m length. Samples were injected after centrifugation and dilution with formic acid 1:1. Carrier gas was N 2. Injector temperature: 240°C; initial working temperature: 110°C. First ramp: 10°C/min until 130°C; second ramp: 3°C/rain until 155°C; third ramp: 20°C/rnin until 195°C; time at 195°C: 7 rain.

3. Results and discussion Results are summartzed in Table 3. They correspond t o t h e mean value of the responses of the two systems fed with the same organic flow rate, and operated at steady state during 1 month. As can be seen, overall gas production rate (GPR) ranged from 1,96 to 2.91 dm 3 of biogas/dm 3 of digesters/d, The increase with the load is only apparent, because biodegradation of the volatile solids (VS) fed decreases, with the increasing load. GPR in the hydrolyser also becomes more significant with the increased load. Thus, at the lower OLR, Table 3 Some yields of the two-pase anaerobic digestion of fruit and vegetablewastes Run

R1

R2

R3

R4

Organic loading rate (g VS/dm3. d) Biogas production (dm3/dm3/d)

H M O

3.1 0.84 1.11 1.96

6.3 1.22 1.65 2.88

9.4 1.38 1.53 2.91

12.6 1.43 1.09 2.52

Biogas composition (% of CH4)

H M O

43.1 79.9 64.1

44.0 79.5 64.4

40.0 78.5 60.3

29.0 75.0 48.8

72.4

53.1

37.5

27.2

VS removal (%) H, hydrolyser;M, methanizer;O, overall.

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261

60-

tu o9

-i 50

°

I

-1- 45 z

I

~

J

~

-

i.u 400 0

o_

30

I METHANE ]

., ~ - - -

~~~.__------~+

25 OLR (g VS/L.d) Fig. 2. Profile of percentage of biogas ( B ) and methane ( + ) produced in the hydrolyzer as a function of the organic loading rate applied to the two-phase system.

more than 50% of the biogas is produced in the methanizer, whereas at the higher OLR, the situation is reversed. Fig. 2 shows the percentages of the biogas and methane produced in the hydrolyser. Both profiles show a similar trend, which is discussed below together with the volatile fatty acids profile. Biogas composition (Table 3) for the three first runs ranges from approx. 60 to 64% CI-L, but at O L R = 12, it decreases substantially (48%), which seems to indicate some problems of the methanogenic step. Percentages of methane in the hydrolyser are much lower than in the methanizer, indicating the degree of phase separation. It is interesting to observe that biogas composition is much more affected by the highest OLR in the hydrolyser than in the methanizer. This seems to indicate some difficulties of the hydrolyser. Regarding the VS removal (Table 3) its maximum value (around 72%) is less than what can be expected of such biodegradable waste. As discussed later, experiments conducted on similar waste, in a one-phase system, showed that removal of approx. 90% can be achieved by the anaerobic digestion [5]. Percentage removal values can be compared with the specific biogas production (SGP) or with the specific methane production (SMP), computed in accordance with the following Eqs. [ 11 ] : SGP -- GPR × OLR

( 1)

SMP = MPR × OLR

(2)

where SGP and SMP are the specific gas and methane production, respectively, and GPR and MPR are the gas and methane production rates, respectively. Fig. 3 presents the SGP and SMP as a function of the OLR. With the decrease of the OLR there is a constant increase in SGP or SMP. This trend suggests that higher specific yields can be achieved using a more reduced OLR. This agrees with the fact mentioned above that higher biodegradations can be expected of such waste. The maximum value achieved for the SGP (0.623 m3/kg VS) is very close to that estimated from the same wastes, when they were converted to volatile fatty acids (VFA)

262

A. Mtz,-Viturtia et al. /Resources, Conservation and Recycling 13 (1995) 257-267 0.7

(/) >

==

o.6 ~ 0.5

~E W 0.4 ' a W 0.3

0

~

0.2 0.1

3

~,

~

~

-~

8

~

lb

1'1

1'2

13

OLR (g VS/L.d)

Fig. 3. Profile of the overall specific biogas (11) and methane ( + ) production as a function of the organic loading rate applied to the two-phase system.

in a hydrolytic reactor [ 12]. In that study, assuming that all the acids were converted to biogas, a value of 0.627 Nm3/kg VS was estimated for SGP. On the other hand, SGP lies within the range of values reported in the literature. Thus, Gamboni and Pacciaroni [13] obtained a yield of 0.38 m3/kg VS digesting vegetable wastes; Lane [ 14-16] reported higher yields (0.50-0.57 m3/kg VS) digesting different fruit and vegetable wastes. These latter yields are very similar to the values described by Cohen et al. [ 17] digesting vegetable wastes (0.50 m3/kg VS) or by Terai et al. [ 18] (0.5 ma/kg VS) digesting fruit wastes from industry. Differences can be explained because of the various OLRs and different substrates used in the experiments. It is interesting to point out that, although the HRT used here may seem too high if compared with others in literature (for instance 2-3 days reported by Ghosh and Klass [ 19]; Verrier et al. [20] ; Witty and M ~ k [21] and Wolfgang [22] ), those authors worked with lower substrate concentrations and also with lower digester loads, than these used in the present study.

3.1. Volatile fatty acids Concerning the overall VFA production, there is a maximum at OLR = 6.3 kg VS/m 3. d. At higher loads, the overall VFA concentration decreases, and the same happens at lower loads. It seems that, by increasing the organic loading rate in this two-phase system, hydrolytic step efficiency declines. At OLR below the optimal, that is, at which VFA concentration is the highest, VFA concentration decreases as a consequence of their metabolization by the methanogenic bacteria. Table 4 shows that the percentage of total VFA in methanizer increases with the OLR. This, together with the decrease in the overall VFA observed in run R4 (at the highest OLR) agrees with the trend observed in Fig. 2: The higher the OLR, the higher the percentage of the biogas produced in the hydrolyser (because of the decrease of VFA). Figs. 4 and 5 present the individual distribution of VFA observed in the hydrolyser and methanizer, respectively. No acids with a longer chain than valeric were detected in either

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263

Table 4 Volatile fatty acids productionand pH in the two-pase anaerobicdigestion of fruit and vegetable wastes Run

R1

R2

R3

R4

Organic loading rate (kg VS/m3. d)

3.1

6.3

9.4

12.6

pH

H M

6.70 7.37

6.77 7.26

6.84 7.31

6.98 7.36

VFA in leachate ( mg/din 3)

H M O

3718 379 2791

4631 530 3492

4171 514 3155

2539 393 1943

3.8

4.2

4.5

5.6

% of VFA in methanizer H, hydrolyser;M, methanizer; O, overall.

case. As can be seen in the hydrolyser at the highest and, especially, at the lowest OLR, when the SGP is the highest, distribution is in decreasing order, acetic, propionic, butyric and valeric. In contrast, at the two middle OLRs, when the VFA concentration achieves its maximum values, distribution is very flat, and concentration is similar among all the acids. In any case, these profiles agree with a high hydrolytic activity, which produces all these acids. In the methanizer, where methanogenic bacteria are present in much larger amounts, both attached and suspended, distribution is quite similar for all the OLRs tested. In this case, propionic acid accounts for nearly 50% of the total acids present, acetic around 30% and the rest in approximately the same amounts for butyric and valeric. This agrees with the observations of Mosey [23], that propionic is the last acid to disappear. Its relatively high concentration does not inhibit the biodegradation, as happens when one-phase reactors are involved [24]. The relatively low concentration of valeric and butyric acids is an indication of a high acetogenic activity, which drastically reduces the VFA acids coming

LU

¢n ._1

o tr-

-1-

z Z

o rn

~3 >

3.14

6.28 9.42 OLR (g VS/L.d)

12.56

Fig. 4. Percentageof the distribution of individual volatile fatty acids produced in the hydrolyzer (A, acetic acid; P, propionicacid; B, butyric acid; V, valeric acid).

264

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# iii 1"4 < "r"

,r,

z Z

__o I--eta tr t~ >

3.14

6.28

9.42

12.56

oLlq (g V$/l_.d) Fig. 5. Percentageof the distributionof individual volatilefatty acidsproducedin the methanizer(A, aceticacid; P, propionicacid; B, butyric acid; V, valericacid). from the hydrolyser. The lower acetic acid concentration compared with that of the propionic acid, indicates high methanogenic activity. Values of pH for the four OLRs tested are also presented in Table 4. As can be seen, there is no correlation between pH and VFA concentration, and values are similar for hydrolysers and methanizers, respectively. The pH values reported here for the hydrolytic step are slightly higher than those reported as optimal in the literature. For instance, the lowest optimal values for the hydrolytic step (between 4.2 and 4.6) are reported by Witty and MRrk [21], Roy et al. [25] and Verstraete et al. [26]. However, most of the authors recommend pH comprised between 5 and 6 [17,19,26-32]. Finally, some authors recommend higher pH values for hydrolysis: 6.5 [33] or from 6 to 7 [22]. Thus, it seems that if an optimized hydrolytic step is desired, some actions should be taken to decrease the pH. A further indication of a lower hydrolysis efficiency at higher pHs is the fact that the maximum pH in the hydrolyser (6.98) corresponds to the higher OLR (12.6 kg VS/m3d), when the hydrolytic reactor showed the lowest yields. Methanizers present similar VFA concentrations and the pH values are all of the same order (7.3-7.4) which is within the optimal values pointed out by other authors for methanogenesis (6.5-8.0, [22]; 7.2-7.7, [28];7.3-7.5, [34] or 7.5 by [33] ).

3.2. One-phase vs. two-phase system There is controversy on the use of one- or two-phase systems for the anaerobic digestion of wastes. The results obtained in this paper can serve as a basis for a comparison of these two configurations. In a recent paper, the biomethanization of a very similar waste was Teported [ 5 ]. The substrate was mainly a mixture of fruit and vegetable wastes. In that case, it was diluted to a TS content of around 4%., with a VS content of around 85% of TS, close to the 88% of the wastes of the present study. Due to the presence of some fish wastes (around 10% TS), C / N ratio was 16.4 compared with 22.3 of this study. However, both

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265

values are within the limits that can be considered non-problematic either from the nutrient or from the toxicity point of view [ 7 ]. The digestion was carded out using OLRs ranging from 1.7 to 2.8 kg VS/m 3. d (much lower than in this study). Four steady states were obtained. The results showed higher yields: SGP was close to 0.762 m3/kg VS in all the OLRs tested, whereas the maximum value achieved for the SGP in the present study was 0.623 m3/kg VS. Similarly, VS removal efficiencies were around 89% in comparison with the maximum value obtained here (72%). Apart from the fact that the wastes were very similar but not identical, the reason for the lower yields obtained with a two-phase system should be attributed to the higher OLR applied. In fact, as Fig. 3 suggests, higher yields can be achieved using a lower OLR in the two-phase approach. Thus, it appears that two-phase systems are also sensitive to the increase in OLR. As a consequence, it seems that no definite advantages arise in using the two-phase approach as far as OLR is concerned, unless a more sophisticated system is used (for instance, with a pH control to maintain it a the optimum values in the hydrolyser). Thus, unless toxicity can arise, it seems that the one-phase system should be used if a simpler system is preferred. For some specific wastes rich in proteins, as for example food wastes or for some agro-industrial wastes, ammonia is formed during the anaerobic degradation chain, leading to bacteria-inhibition problems. In accordance with Weiland [ 35 ], residues which show C/N ratios lower than 10 cannot be treated in one-phase systems because of the instability problems at COD loading rates over 3 kg/m 3 day and process complete failure at COD loading rates above 5 kg. This author recommends the one-phase systems for residues with a C/N ratio above 15. Ammonia toxicity can increase if the digester is operated at thermophilic conditions, due to the higher ammonia solubility and to the displacement of the ammonium-ammonia equilibrium towards the unionized form.

4. Conclusions Anaerobic digestion of a fruit and vegetable waste has been carded out using a simple two-phase system. From the values of pH and VFA concentration in both hydrolyser and methanizer, it appears that phase separation has been achieved without difficulty. Yields ranged from 0.2 to 0.63 m 3 CHa/kg VS with OLRs ranging from 3 to 12.5 kg VS/m3-d. At the same time, at an overall HRT of 18 days, the VS removal was around 72%, but at 4-5 days, it was only 27%. At high organic loads hydrolysis does not seem to be very effective. OLR affects the hydrolytic step. Furthermore, at higher loads, more methanization takes place in the hydrolyser than in methanizer, because VFA are in a lower concentration and, thus, phase separation is not so effective. At low organic loads, VFA concentrations are poor, because they are immediately metabolized and the yields per kg VS are high. To upgrade yields in a two-phase system, more investigations would be needed concerning the system set-up, the control ofpH in both reactor, etc., in order to optimize conditions in both hydrolyser and methanizer. However, if the two-phase system is operated in a straightforward manner, like the one-phase system, this latter would be the best choice: It is simpler and can be applied successfully to the treatment of this type of waste without any type of control action.

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Acknowledgements Authors gratefully a c k n o w l e d g e the support o f C I C Y T spanish project N A T 9 1 - 0 0 1 8 and N A T 0 Grant C R G 9 1 0 6 6 3 .

References [ 1] Cecchi, F., Mata-Alvarez, J. and Verstraete, W., 1992. Memorandum of the international symposium on anaerobic digestion of solid waste held in Venice, April 1992.7th International Recycling Congress, October 28-30, Berlin, Germany. [ 2] Edelmann, W.E., 1992. Proceedings of the International Symposium on Anaerobic Digestion of Solid Waste, April 14-17, 1992. Venice, Italy. In: Cecchi, F., Mata-Alvarez, J. and Pohland, F.G. (Eds.), pp. 225-238. [3] O'Keefe, D.M., Chynoweth, D.P., BarkdoU, A.W., Nordstedt, R.A., Owens, J.M. and Sifontes, J., 1992. International Symposium on Anaerobic Digestion of Solid Waste, April 14-17, 1992. Venice, Italy. In: Cecchi, F., Mata-Alvarez, J. and Pohland, F.G. (Eds.), pp. 117-125. [4] Mtz-Viturtia, A. and Mata-Alvarez, J., 1987. Proc. 4th Mediterranean Congress on Chemical Engineering, Vol. I/, 11-13 Nov, Barcelona. pp. 790-791. [5] Mata-Alvarez, J., Llabr6s, P., Cecchi, F. and Pavan, P., 1992. Bioresources Technol., 39: 39--48. [6] Ceechi, F., Traverso, P.G., Mata-Alvarez, J., Clancy, J. and Zaror, C., 1988. Biomass, 16: 257-284. [7] Kayhanian, M., Lindenauer, K., Hardy, S. and Tchobanoglous, G., 1991. Biocycle, 32: 48-53. [8] Standard methods for the examination of water and waste-water, 1985.16th edn., American Public Health Association, American Water Works Association and Water Pollution Control Federation. [9] Bond, R.G. and Stranb, C.P., 1973. Handbook of Environmental Control, Vol. II, Solid Waste, CRC Press, OH. [ 10] Mata-Alvarez, J., Mtz-Viturtia, A. and Torres, R., 1986. Bioteehnol. Lea., 8(10): 719-720. [ 11] Mata-Alvarez, J., Cecchi, F., Pavan, P. nad Fazzini, G., 1990. Biological Wastes, 33:181-199. [12] Mtz.-Viturtia, A., Mata-Alvarez, J., Sans, C., Costa, J. and Cecchi, F., 1992. Chemicals production from waste. Environ. Technol., 13: 1033-1041. [ 13] Gamboni, M. and Pacciaroni, F., 1982. Com. Naz. Ric. Sviluppo Energ. Nucl. Energ. Altem. (Papp, Tee.) ENEA-RT/BIO (Italy), ENEA-TR/BIO (82) 24. [14] Lane, A.G., 1983. Biomass, 3(4): 247-268. [15] Lane, A.G., 1984. Biomass, 5(4): 245-259. [16] Lane, A.G., 1984a. Food Technol. Aust., 36(3): 125-127. [ 17] Cohen, A., Koevoets, W.A.A. and Zoetemeyer, R.J., 1983. Proc. European Syrup. Anaerobic Waste Water Treatment, 171, 23-25 Nov,, Noordwijkerhout, Netherlands. p. 171. [18] Terai, T., Takahashi, M. and Kamaya, A., 1984. Suishitsu Odaku ni Kansum Kenkyu Shusho, 13: 97-104. [ 19] Ghosh, S. and Klass, D.L., 1978. Process Biochem., 13(4): 15-25. [20] Verrier, D., Roy, F. and Florentz, M., 1983. Proc. 3th Int. Syrup. on Anaerobic Digestion, 14-19 Aug., Boston, USA. [21] Witty, W. and Mark, H., 1983. Proc. European Syrup. Anaerobic Waste Water Treatment, 23-25 Nov., Noordwijkerhout, Netherlands, pp. 139-154. [22] Wolfgang, O.U., 1983. Proc. European Syrup. Anaerobic Waste Water Treatment, 23-25 Nov., Noordwijkerhout, Netherlands, pp. 130-138. [23] Mosey, F.E., 1981. Water Pollution Control, 80(2): 273-291. [24] Verrier, D., Roy, F. and Albagnac, G., 1987. Biological Wastes, 22: 163~177. [25] Roy, F., Verrier, D. and Florentz, M., 1983. Proc. European Syrup. Anaerobic Waste Water Treatment, 2325 Nov., Noordwijkerhout, Netherlands, pp. 175-183. [26] Verstraete, W., De Baere, L. and Rozzi, A., 1981. Trib. Cebedean, 453--454(34): 367-375. [ 27 ] Eastman, J.A. and Ferguson, J.F., 1981. J. Water Pollut. Control Fed., 53 (3): 352-366. [28] Mata-Alvarez, J. and Mtz-Viturfia, A., 1986. EWPCA Conference on Anaerobic Waste Water Treatment, 15-19 Sept., Amsterdam, pp. 740-743.

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[29] Joergensen, M.H., 1978. Eur. J. Appl. Microbiol. Biotechnol., 6(2): 181-187. [30] Arntz, H.J., Stopper, E. and Buchholz, K., 1985. Biotechnol. Lett., 7(2): 113-118. [31] Zoetemeyer, R.J., Matthijsen, A., Van den Heuvel, J.C., Cohen, A. and Boelhouwer, C., 1982. Biomass, 2(3): 187-199. [32] Zoetemeyer, R.J., Van den Heuvel, J.C. and Cohen, A., 1982a. Water Res., 16(3): 303-311. [33] Ishida, M., Odaware, Y., Gejo, T. and Okumura, H., 1979. Recycling Berlin, In: Thome-Kozmiensky, K.J. (Ed.), E. Berlin. [34] Tortes, R. and Mata-Alvarez, J., 1987. Proc. 4th Mediterranean Congress on Chemical Engineering, 11-13 Nov., Barcelona, pp. 788-789. [35] Weiland, P., 1992. International Symposium on Anaerobic Digestion of Solid Waste, April 14-17, 1992. Venice, Italy. In: Cecchi, F., Mata-Alvarez, J. and Pohland, F.G. (Eds.), pp. 193-199.