two-phase anaerobic digestion of a mixture of fruit and vegetable wastes

Biological Wastes 29 (1989) 189-199

Two-Phase Anaerobic Digestion of a Mixture of Fruit and Vegetable Wastes

A. Mtz. Viturtia, J. Mata-Alvarez*

Departament d'Enginyeria Quimica i Metallflrgia, Facultat de Quimica, Universitat de Barcelona, C/Marti i Franqu6s I, pl. 6, E-08028 Barcelona, Spain

F. Cecchi & G. Fazzini

Dipartimento di Scienze Ambientali, Universita degli Studi di Venezia, Larga S. Marta 2137, 1-30123 Venezia, Italy

(Received 14 September 1988; revised version received 18 December 1988: accepted 30 December 1988)

A B S T R A C T

Two-phase anaerobic digestion of a mixture of fruit and vegetable solid wastes was studied at laboratory scale, using digesters operated in the mesophilic range. An optimal configuration for a hybrid (up-flow anaerobic sludge bed-anaerobic filter ) digester was determined, in order to improve the methanogenic step. Biodegradation achieved in two weeks was around 75%. The process was stable, even when the pH was at the lowest levels (around 5, in the hydrolyzer ). The distribution and concentration of volatile fatty acids produced was also studied.

INTRODUCTION

Fruit and vegetable wastes are produced in large quantities in markets, and constitute a source of nuisance in municipal landfills because of their high

* To whom correspondence should be addressed.

189 Biological Wastes 0269-7483/89/$03.50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

190 A. Mtz. Viturtia et al.

biodegradability. A possible way to dispose of these wastes is using the anaerobic digestion processes (Cecchi et al., 1987; Mata-Alvarez & Mtz. Viturtia, 1986a). Anaerobic digestion of vegetable and fruit wastes represents an alternative treatment to conventional methods of disposal and allows the production, on one hand, of biogas that can be used to generate electricity and, on the other hand, of a compost from the digested wastes (Cecchi et aL, 1988; Mata-Alvarez & Mtz. Viturtia, 1986b).

Given the very large biodegradable organic content of these wastes, a convenient way to perform the anaerobic fermentation process would seem to be the use of a two-phase system. Although conventional single-phase, stirred-tank systems are more simple and easier to operate, they may be too sensitive to overloading because of propionate accumulation (Verrier et al., 1987). In two-phase systems, hydrolysis of polymers and acidification of easily degradable compounds take place in the first phase, yielding volatile fatty acids (VFA, mg HAc/liter). In the second phase, what mainly takes place is acetogenesis and methanogenesis. Some kinetic considerations argue in favor of the two-phase approach, in particular when optimal growth conditions for hydrolytic and methanogenic bacteria are taken into account (Ghosh et al., 1985). Examples of the application of this technology are the digestion processes proposed for agricultural solid wastes by some authors (Colleran et aL, 1983; Rijkens et al., 1984). In this approach the solid organic matter, loaded discontinuously, is broken down in the first stage (first digester) by acid-forming anaerobic bacteria. The soluble organic matter and VFA are pumped into a specific methane reactor (second digester), containing a high concentration of methanogenic bacteria and producing biogas. The main operating variables of a system like that described here, were analyzed in a previous study by a computer simulation (Mata-Alvarez, 1987). The results obtained have been taken into account in the design of these experiments.

The aim of the present work was to assess the performance of a two-phase anaerobic digestion of a mixture of fruit and vegetable wastes and to obtain data on the degradation rate in the mesophilic range. Because of the relevance of the methanogenic step, this study first compares the behavior of different supports in the methanizer of a two-phase system. The reactor configuration used is a hybrid UASB-AF (up-flow anaerobic sludge bed- anaerobic filter), which can be regarded as having some of the characteristics of both types of digester. The sludge-bed volume was smaller than the packed-bed compartment volume. However, the ratio between both volumes was not decided in advance. As a consequence, it was decided to compare the operational performance at different values of this ratio together with the pore size of the support. In a second series of experiments several runs were made with the best configuration found.

Two-phase anaerobic digestion of fruit and vegetable wastes 191

METHODS

The experimental set-up (Fig. 1) consisted of four systems composed of a hydrolyzer and a methanizer with respective working volumes of 1.3 and 0.5 liters. The reactors were connected by a leachate recirculation line, with a variable flow rate. At the top, the hybrid methanizers contained a polyurethane support with different pore size and percentage of volume occupation. Two different polyurethane supports were tested, the first with a pore size of 80 ppi (pores per inch), and the second of 15 ppi. Two levels of the percentage of volume occupation were used: 20 and 40% (total methanizer working volume). Table 1 shows the different configurations tested. The size of the support pieces placed in the methanizers was 1 × i x 1 cm 3 for the 15ppi support, and 1 × 2 × 0 . 5 c m 3 for the 80ppi support. Prior to the experimentation, methanizers were fed with a mixture of acetate and methanol for more than four months.

The reactor was not continuously fed, but operated in a batch mode. The solid waste was loaded to the hydrolyzer, recirculation was started and, once

I A A

1F f C ] D

- - - " B F

- - _ _ - - _ "

~ - ~ _ . i - • _

B - - - - - - - - - B

= fi

- 2 B B

F i g . 1. Schematic of the two-phase experimental device used to digest fruit and vegetable wastes. A, Biogas outlet; B, sample port, C, hydrolyzer; D, methanizer; E, hydrolyzer filter; F,

methanizer anaerobic filter section; G, recirculation pump.


TABLE 1 Characteristics of the Polyurethane Supports in the

Methanizer (see Methods)

System Support pore size Volume occupation (ppi) (%)

1 15 40 2 15 20 3 80 40 4 80 20

the b iodegrada t ion was comple ted , it was then unloaded. Reci rcula t ion o f the leachate was p rov ided by a p u m p opera ted by a p r o g r a m m e d timer. A filter was placed at the hydro lyzer b o t t o m to prevent suspended solids f rom

gett ing into the pump. Each hydro lyzer was loaded with a s t andard mixture o f fruit and

vegetable wastes with the compos i t ion shown in Table 2. This compos i t ion represents an app rox ima te average o f the local fruit and vegetable wastes. To ta l weight was 1 kg per digester. To ta l initial mois ture was 93-6% measured at 105°C overnight , with a volati le solids (VS, %(w/w)) con ten t o f 5.7% (w/w) ob ta ined after 4 h at 650°C. Cow m a n u r e (50 g wet wt) was used as inoculum. Its character is t ics and those o f the substra te are also presented

TABLE 2 Characteristics of the Components of the Substrate Loaded to the Hydrolyzer

Total N Total P Moisture TS VS Substrate (mg/kg) (mg/kg) (%) (% (w/w)) (% (w/w)) composition ° (%)

Substrate Orange 1 573 208 85.70 14.30 13.78 7.5 Cauliflower 2697 379 91.56 8.44 7.75 10.0 Cucumber 685 293 96.22 3.78 3.37 25.0 Lettuce 1 945 319 94.33 5.67 4'22 25"0 Tomato 771 288 93-93 6.07 5.56 25.0 Water-melon 1 117 285 92.01 7.99 7.43 7.5

Mixture 1 321"7 300 93.60 6"40 5"65 100.0

Inoculum Cow manure 12355 891 84'68 15-32 13-23 Digested pig 6 348 1 860 97.66 2.34 1.38

manure

a Percentage values refer to total weight of the different components used as a substrate.


in Table 2. The C : N : P ratio was well balanced, being around 120:7:1, therefore no other nutrients were added to the hydrolyzer reactors. The hydrolyzers were finally filled with water until a total moisture of 95 % (w/w) was reached.

Each methanizer was filled with 400 ml inoculum (Table 2) from a pig manure digester operated at 35°C. The working temperature of hydrolyzers and methanizers was 35°C. Although evidence exists that liquefaction proceeds at an optimal rate when the hydrolyzer is operated in the thermophilic range (Verrier et al., 1987), the control of individual reactor temperatures was not possible because of the continuous recirculation between the reactors.

Monitored parameters were total solids (TS, % (w/w)), VS, pH and VFA on the leachate at the outlets of both hydrolyzer and methanizer. Initial and final solids analyses were also performed on the solid waste. All the analyses were performed in accordance with Standard Methods (1985). VFA were determined by gas chromatography after centrifugation of the samples at 12 000 g for 30 min. Samples were diluted (1"1) with formic acid in order to saturate the column. Biogas production was measured twice a day by means of an automatic displacement device. Biogas composition was analyzed once a week by gas chromatography.

RESULTS AND DISCUSSION

Support testing

The first part of the study was carried out to choose the best configuration among the four analyzed. The digestion was carried out during a period of time long enough to allow each configuration to yield a good performance. The results obtained are summarized in Table 3. As can be seen, similar values for cumulative biogas production, average methane content in gas and total volatile solids reduction were obtained, but the duration of the experiment to reach these yields varied from 25 to 55 days in accordance with the system used. Without any regulation, the average pH of the hydrolyzer remained around 5.7-6-0 during the whole experimentation period, while that of the methanizer oscillated between 7.2 and 7.7. Although perhaps a little lower than the pH reported as optimal for hydrolysis (6-5) (Verrier et al., 1987; Ishida et al., 1979), it yielded an optimal methanizer pH. The VFA values at the methanizer inlet were about 6000mg/liter for methanizers 1 and 2, 7000 mg/liter for methanizer 3 and 8000 mg/liter for methanizer 4. The VS values of the hydrolyzer leachate at the beginning of the digestion were about 20g/liter. The VS contents at the methanizer outlets had an average value of 10 g/liter.


TABLE 3 Summarized Results of the Two-Phase Digestion of Fruit and Vegetable Wastes Using

Different Support in the Methanizer ~

System Digestion Cumulative Overall Ultimate Biodegradation period methane methane methane as VS (days) production b in biogas yield, B o reduction

(m 3 C n J k g VS) (%) (m 3 CH,/kg VS) (%)

! 55 0.345 61 0.372 83 2 52 0-355 63 0.370 82 3 30 0.368 62 0"389 87 4 25 0-383 62 0.394 90

a Recirculation HRT in hydrolyzer and methanizer were 7-5 and 3 days respectively in all cases. b At the end of the digestion period.

It is clear from Table 3 that the best performance considering the fastest digestion, the highest VS reduction and the highest cumulative biogas production was obtained using system 4, that is, using the support with a pore s i zeo f 80ppi and a digester volume occupation of 20%. Using the 15 ppi support, no differences appear between the two levels of volume occupation. Table 3 also shows the ultimate methane yield (B o (infinite time), m 3 at STP C H J k g VS) reached in the four systems. In accordance with Chen & Hashimoto (1980), this parameter is estimated from a representation of B (cumulative methane production per kg VS added), versus the inverse of time. The plot approaches a straight line with 1/t ~ 0 as B ~ B 0.

As a consequence of the results obtained, three of the four systems were modified to adapt them to the best configuration found (system 4, Table 1). Moreover, due to the relatively high level of VFA observed in the hydrolyzer in all the experiments, the leachate recirculation flow rate was increased from 0.17 to 0.5 liters/day. As a result, the new H R T were 2.6 days for the hydrolyzer and 1 day for the methanizer.

Operation with the best configuration

The results obtained during a fermentation period of about 31 days are shown in Table 4 and Figs 2-4. As can be seen, the digestion of this kind of waste proceeds very rapidly. The waste biodegradability is high, as can be seen from the values of the ultimate methane yield Bo. These estimates (Table 4) are higher than those of the first experimental series (Table 3), and can be considered more representative for this kind of wastes because of the larger biodegradation achieved in the replicated experiment. Nearly all the


TABLE 4 Results Obtained in Four Replications When the Systems were Operated at Hydrolyzer and

Methanizer HRT of 2.6 and 1 Day, Respectively °

Digester Digestion Methane Calculated Calculated Ultimate Overall period production biodegradation b biodegradation methane methane

• (days) (m 3 CH4/kg VS) (%) at day 25 yield (Bo) in biogas (%) (m 3 CH4/kg VS) (%)

1 31 0"51 98'5 96"7 0'526 70 2 32 0"49 95'9 94-5 0'513 68 3 29 0"50 95'7 95" 1 0'530 70 4 31 0"51 97"2 96"7 0"526 69

a Support characteristics: porous support size 80 ppi, support volume occupation 20%. b Calculated from the ratio between cumulative methane production and ultimate methane yield.

gas is produced in the methanizer (Fig. 2). During the first two weeks, about 75% of the total biodegradation was achieved (Fig. 3).

The VFA reached a maximum of about 10000ppm in the hydrolyzer outlet and a maximum of about 5000 ppm at the methanizer outlet (Fig. 4). Both achieved maximum levels in the second half of the first week. The pH levels oscillated between 5 and 8, in accordance with the acid profiles. On average, VFA concentration was 60% lower in the methanizer outlet than in the hydrolyzer. Figure 5 shows the VFA removal profile achieved in the methanizer of system 1 (considered as representative of the four tested). The

350

0 4 8 12 16 20 24 28 32 Time (day)

Fig. 2. Biogas production profiles observed in digester 1 (Table 4) during the second set of experiments. I--1, Hydrolyzer; +, methanizer; O, overall. Methane content of biogas was

around 44% in hydrolyzers and around 79% in methanizers.


Fig. 3.

100

90- ,¢

8o- v

.~ 70-

~60-

~ 5o- g

40-

E 3o- ®

20- o...

10-

4 8 12 16 2 0 24 28 32 T i m e ( d a y )

Biodegradation achieved during the second set of experiments calculated as the ratio between cumulative methane production and ultimate methane yield.

organic load referred to the VFA (kg VFA/m 3 per day) and the percentage of acetic acid with respect to the other VFA are also indicated in this figure. As can be seen, the maximum load, about 9 kg VFA/m 3 per day, was achieved at the end of the first week and it was not related to the removal percentage but to the biogas production (Fig. 2). The relative amount of acetic acid in the VFA fed to the methanizer and directly utilized by methanogenic bacteria increased as the reaction progressed.

All the results seem to indicate that the methanizer could take a larger

c6~ .o

84

> 2

/

. . . . , . . . . j , , •

6 11 16 21 26 31 T i m e ( d a y )

9

.8

7

-6

-5

-4

Fig. 4. VFA and pH profiles observed during the second set of experiments in the hydrolyzer (H) and the methanizer (M). [-'1, VFA in H; +, VFA in M; A, pH in H;

<% pH in M.


lOO

9 0 -

8 0 -

-~ 70 -

E 6 0 -

• 5 0 -

4O-

~ 30_ ~ a .

20-

10-

\ / "

J / /

f , .~, i

/ / \ \ ,

\

\

\

\

t

I &

7 ~

E

5 > v

c ¢

3 6

o . . . . i . . . . i . . . . i . . . . r . . . . p - . I ,

6 11 16 21 26 31 Time (days)

Fig. 5. VFA removal profiles and methanizer organic loading rates observed during the second set of experiments in digester 1 (Table 4). F], VFA removal; + , organic loading rate,

OLR referred to VFA; A, % acetic acid in VFA.

load considering that gas production was not disturbed and even was at its maximum rate, when methanizer organic load referred to VFA concentration was at its maximum level. As a consequence, a smaller methanizer volume could be used without lowering the overall yield. No problems arose even when the pH was at the lowest levels (a value of 5 for the hydrolyzer and value of 7 for the methanizer, Fig. 4). Leachate ammoniacal nitrogen concentration was checked to ensure the right nutrient balance. This is more convenient than just verifying substrate composition, as has been pointed out by Glauser et al. (1987). The results showed that it was fairly constant during all the experiments, with values around 1200 mg/liter.

Influence of the recirculation rate

Recirculation rate was claimed to be a significant parameter in the operation of a two-phase digestion (Mata-Alvarez, 1987). Here, the increase of three times the recirculation rate made the biodegradation percentage achieved at day 25 increase from 90% (the best value observed operating system 4, in the first set of experiments, Table 3) to 94-97% (second set, Table 4). The percentage of methane was also higher during the second experimentation period, increasing from 61-63% to 68-70%.

Discontinuous versus continuous systems

The digestion system used in this study was a discontinuous one. Batch treatment of fruit and vegetable wastes may be appropriate when small


quantities are to be digested. This could be the case for small populations. The use of a two-phase discontinuous system presents an additional advantage: the hydrolyzer can be used as a methanizer after a certain period of time. As can be seen in Fig. 2, methane production starts around day 12, when the volatile acid concentration is low enough. At a given point, methane production could proceed in the hydrolyzer without recirculating the leachate. As a consequence, the methanizer could be disconnected and the digestion be carried out using just one phase. The methanizer could then be used to treat the leachate coming from a second hydrolyzer which would be beginning the cycle. This concept could even be used to treat the leachate coming from several sections of a landfill: when the VFA concentration is low enough, the methanizer could be switched to treat the leachate coming from another landfill section.

Continuous systems are more appropriate to treat large quantities of residues. The experimentation carried out in this study could also serve as a guide for the design of such a system.

According to the results obtained, it can be concluded that the digestion of fruit and vegetable wastes proceeds at a considerable rate using a two-phas e anaerobic digestion. The yields achieved (Fig. 3) are comparable with others reported in the literature. Lane (1979, 1984) obtained 0.429-0.568m a CH4/kg VS in a single-phase anaerobic digestion of several vegetable and fruit wastes. Hill & Roberts (1982), also in a single-phase digestion of tomato wastes, obtained 0.61 m 3 CH4/kg VS removed. Verrier et al. (1983) reported a yield of 2.4 m 3 CH4/m 3 digester per day in a two-phase digestion of carrot parings with a methanizer retention time of 2 days, and with a 95.9% COD removal.

ACK NOW LEDGMENTS

The authors thank the Instituto Trevigiano di Ricerca Scientifica del Comune di Treviso for its auspices and support. Financial support from the NATO, grant no. 0178/87, is also gratefully acknowledged.

REFERENCES

Cecchi, F., Traverso, P. G., Mata-Alvarez, J., Clancy, J. & Zaror, C. (1987). M.S.W.: Guidelines for research: From the harvesting procedure to the anaerobic process. In Biomass Energy: From Harvesting to Storage, ed. G. L. Ferrero, G. Grassi & H. E. Williams. Elsevier Applied Science, London, pp. 225-35.

Cecchi, F., Traverso, P. G., Mata-Alverez, J., Clancy, J. & Zaror, C. (1988). State of the art of R & D in the anaerobic digestion process of municipal solid waste in Europe. Biomass, 16, 257-84.


Chen, Y. & Hashimoto, A. (1980). Substrate utilization kinetic model for biological treatment processes. Biotechnol. and Bioeng., 22, 2081.

Colleran, E., Wilkie, A., Barry, M., Faherty, G., O'Kelly, N. & Reynolds, P. J. (1983). One- and two-stage anaerobic filter digestion of agricultural wastes. 3rd Int. Symp. on Anaerobic Digestion, 14-19 August, Boston, MA, pp. 285-312.

Ghosh, S., Ombregt, J. P. & Pipyn, P. (1985). Methane production from industrial wastes by two-phase digestion. Wat. Res., 19, 1083-8.

Glauser, M., Aragno, M. & Gandolla, M. (1987). Anaerobic digestion of urban wastes: Sewage sludge and organic fraction of garbage. Bioenvironmental Systems, Vol. III, ed. D. L. Wise. CRC Press, Florida, pp. 143-225.

Hill, D. J. & Roberts, D. W. (1982). Conversion of tomato, peach and honey dew solid waste into methane gas. ASAE, 25, No. 3, 820-6.

Ishida, M., Odaware, Y., Gejo, T. & Okumura, H. (1979). Biogasification of municipal waste. In Recycling Berlin, 1979, ed. K. J. Thome-Kozmiensky, East Berlin, GDR. pp. 797-802.

Lane, A. G. (1979). Methane from anaerobic digestion of fruit and vegetable processing wastes. Food Technology in Australia, 31, No. 5, May.

Lane, A. G. (1984). Laboratory scale anaerobic digestion of fruit and vegetable solid waste. Biomass, 5, 245-59.

Mata-Alvarez, J. (1987). A dynamic simulation of a two-phase anaerobic digestion system for solid wastes. Biotechnol. and Bioeng., 30, 844-51.

Mata-Alvarez, J. & Mtz. Viturtia, A. (1986a). Laboratory simulation of solid waste fermentation with leachate recycle. J. Chem. Tech. Biotechnol., 36, 547-56.

Mata-Alvarez, J. & Mtz. Viturtia, A. (1986b). An examination of different supports for a hybrid digester. EWPCA Conference on Anaerobic Waste Water Treatment, 15-19 September, Amsterdam, 740-3.

Rijkens, B. A., Voetberg, J. W., Hofenk, G. & Lips, S. J. J. (1984). Two phase anaerobic digestion of solid organic wastes yielding biogas and compost. Final Report, E.C. Contract No. ESE-E-R-O40-NL. IBVL Wageningen, The Netherlands.

Standard methods for the examination of water and wastewater (1985). 16th edn, American Public Health Association, American Water Works Association, Water Pollution Control Federation, 1193 pp.

Verrier, D., Roy, F. & Florentz, M. (1983). Two-stage anaerobic digestion of solid vegetable waste. Third Int. Syrup. on Anaer. Digest., August, Boston, MA.

Verrier, D., Roy, F. & Albagnac, G. (1987). Two-phase methanization of solid vegetable waste. Biological Wastes, 22, 163-77.

two-phase anaerobic digestion of a mixture of fruit and vegetable wastes

Documents