bioreactor performance in anaerobic digestion of fruit and vegetable wastes

Process Biochemistry 40 (2005) 989–995

Review

Bioreactor performance in anaerobic digestionof fruit and vegetable wastes

H. Bouallaguia,b,∗, Y. Touhamia, R. Ben Cheikhb, M. Hamdia

a UR-Procédés Microbiologiques et Alimentaires, Institut National des Sciences Appliquées et de Technologie (INSAT),B.P. 676, 1080 Tunis, Tunisia

b Ecole Nationale d’Ingénieurs de Tunis (ENIT), B.P. 37, 1002 Tunis, Tunisia

Received 2 December 2003; received in revised form 16 March 2004; accepted 28 March 2004

Abstract

This work reviews the potential of anaerobic digestion for material recovery and energy production from fruit and vegetable wastes (FVW).These wastes contain 8–18% total solids (TS), with a total volatile solids (VS) content of 86–92%. The organic fraction includes about 75%easy biodegradable matter (sugars and hemicellulose), 9% cellulose and 5% lignin. Anaerobic digestion of FVW was studied under differentoperating conditions using different types of bioreactors. It permits the conversion of 70–95% of organic matter to methane, with a volumetricorganic loading rate (OLR) o f 1–6.8 g versatile solids (VS)/l day. A major limitation of anaerobic digestion of FVW is a rapid acidificationof these wastes decreasing the pH in the reactor, and a larger volatile fatty acids production (VFA), which stress and inhibit the activity ofmethanogenic bacteria. Continuous two-phase systems appear as more highly efficient technologies for anaerobic digestion of FVW. Theirgreatest advantage lies in the buffering of the organic loading rate taking place in the first stage, allowing a more constant feeding rate of themethanogenic second stage. Using a two-stage system involving a thermophilic liquefaction reactor and a mesophilic anaerobic filter, over95% volatile solids were converted to methane at a volumetric loading rate of 5.65 g VS/l d. The average methane production yield was about420 l/kg added VS.© 2004 Elsevier Ltd. All rights reserved.

Keywords: Fruit and vegetable wastes; Anaerobic digestion; Limitation step; Bioreactors performance

1. Introduction

Fruit and vegetable wastes (FVW) are produced in largequantities in markets, and constitute a source of nuisancein municipal landfills because of their high biodegradabil-ity [1,2]. In the central distribution market for food (meat,fish, fruit, and vegetables) Mercabarna (Barcelona), the to-tal amount of wastes coming from fruit and vegetables isaround 90 tonnes per day during 250 days per year[3]. Thewhole production of FVW collected from the market of Tu-nis (Tunisia) has been measured and estimated to be 180tons per month[4]. In India, FVW constitute about 5.6 mil-lion tonnes annually and currently these wastes are disposedby dumping on the outskirts of cities[5].

The most promising alternative to incinerating and com-posting these wastes is to digest its organic matter using the

∗ Corresponding author. Tel.:+216 22 524 406; fax:+216 71 704 329.E-mail address: [email protected] (H. Bouallagui).

anaerobic digestion[4,6]. The main advantage of this pro-cess is the production of biogas, which can be used to pro-duce electricity[3,7,8]. A valuable effluent is also obtained,which eventually can be used as an excellent soil condi-tioner after minor treatments[9,10]. High organic loadingrates (OLR) and low sludge production are among the manyadvantages anaerobic process exhibit over other biologicalunit operations[11,12].

The successful application of anaerobic technology tothe treatment of solid wastes is critically dependent on thedevelopment and use of high rate anaerobic bioreactors[13,14]. The reactor design has a strong effect on digesterperformance[15]. In recent years, a number of novel re-actor designs have been adapted and developed allowing asignificantly higher rate of reaction per unit volume of re-actor[16,17]. Different anaerobic processes, such as batch,continuous one-stage, and continuous two-stage systems,with a variety of methanizers like, continuously stirredtank reactor (CSTR), tubular reactor, anaerobic sequencingbatch reactor (ASBR), upflow anaerobic sludge blanket

0032-9592/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.procbio.2004.03.007

990 H. Bouallagui et al. / Process Biochemistry 40 (2005) 989–995

(UASB) and anaerobic filters have been applied to FVWtreatment. These processes differ especially in the waythe microorganisms are retained in the bioreactor, and theseparation between the acidogenic and the methanogenicbacteria which reduce the anaerobic digestion limitations.Methanogenic bacteria may have long mass doubling timesin anaerobic reactors and this makes it very difficult toobtain fast acting reactors without retaining most of thebiomass normally washed out with the effluent[18,19].

The aim of this paper was to review the energetic potentialof FVW and to examine the performance of several groups ofanaerobic bioreactors used for anaerobic digestion of thesewastes.

2. Characteristics of FVW and anaerobicdigestion limitations

The putrescible FVW used in overall reported studieswere collected from food markets andTable 1shows themost important constituents of FVW in three works whereanaerobic digestion was operated[1,20,21]. The total initialsolid concentration of FVW is between 8 and 18%, witha total volatile solids (VS) content of about 87%. The or-ganic fraction includes about 75% sugars and hemicellulose,9% cellulose and 5% lignin[20]. The easy biodegradableorganic matter content of FVW (75%) with high moisturefacilitates their biological treatment and shows the trend ofthese wastes for anaerobic digestion[1,21]. However, com-plex vegetable processing effluent, such as olive mill wastescontaining large amounts of phenolic and non-biodegradablecompounds are resistant to biological degradation[22]. Aer-obic processes are not favoured for FVW treatment becausethey require preliminary treatment to minimise the organicloading rate[23]. The COD/N ratio of FVW is balanced,being around 100/4 and therefore, no nitrogen was added tothe reactors. In fact the optimum C:N ratio for microbial ac-tivity involved in bioconversion of vegetable biomasses tomethane is 100–128:4[24].

Before being loaded to the reactors, FVW must undergosome pre-treatments[5,9]. They were shredded to small

Table 1Composition of different fruit and vegetable wastes

Wastes (g/kg) Potato peelings Salad waste Green peas and carrots Mixture of FVW Mixture of FVW

Total solids 119.2 79.4 179.4 90.4 84.4Volatile solids 105.5 72.1 171 82.9 77.5Total COD 126 97.8 185 104.5 –Particulate COD 80.6 39.3 123.9 – –Total suspended solids 80 39 145 – 58.6Total Kjeldhal Nitrogen – – – 2 2.7Cellulose 12.9 13.5 16.1 9.2 –Sugars and hemicellulose – – – 62 –Lignin – – – 4.5 –

References [21] [21] [21] [20] [1]

particles and homogenized to facilate digestion. They werealso diluted to decrease the concentration of organic matterand then to operate the reactors with optimal organic load-ing rate[3,4]. Due to the lower pH of FVW, some authorsbuffered these waste by the addition of sodium hydroxidesolutions[5,6]. Without any regulation, the pH quickly de-creased and tended to inhibit the methanogenic bacteria[20].Converti et al. pre-treated organic matter of FVW at hightemperature to improve the efficiency of their anaerobic di-gestion[9], while Srilatha et al. pre-treated orange process-ing waste by solid state fermentation using selected strainsof Sporotrichum, Aspergillus, Fusarium, andPenicillium toimprove biogas and methane productivity at higher OLR[5].

The biomethanation of FVW is accomplished by a se-ries of biochemical transformations, which can be roughlyseparated into four metabolic stages[25,23] (Fig. 1). First,particulate organic materials of FVW like cellulose, hemi-cellulose, pectin, and lignin, must undergo liquefaction byextracellular enzymes before being taken up by acidogenicbacteria[26]. The rate of hydrolysis is a function of factors,such as pH, temperature, composition, and particle size ofthe substrate and high concentrations of intermediate prod-ucts[27,28]. After that, soluble organic components includ-ing the products of hydrolysis are converted into organicacids, alcohols, hydrogen, and carbon dioxide by acidogens.The products of the acidogenesis are then converted intoacetic acid, hydrogen, and carbon dioxide. Finally, methaneis produced by methanogenic bacteria from acetic acid, hy-drogen, and carbon dioxide as well as directly from othersubstrates of which formic acid and methanol are the mostimportant[28].

In general, hydrolysis is the rate limiting step if the sub-strate is in particulate form[29,30]. However, the anaerobicdegradation of cellulose-poor wastes like FVW is limitedby methanogenesis rather than by the hydrolysis[31,32].These wastes, are very rapidly acidified to volatile fattyacids (VFA) and tend to inhibit methanogenesis when thefeedstock is not adequately buffered[23]. In one-stage sys-tems, all these reactions take place simultaneously in a sin-gle reactor, while in two-or multistage systems, the reac-tions take place sequentially in at least two reactors. In a

H. Bouallagui et al. / Process Biochemistry 40 (2005) 989–995 991

Fig. 1. Reactions scheme for anaerobic digestion of particulate organic material of FVW[3,20,25,28,49].

well-balanced anaerobic digestion process, all products ofa previous metabolic stage are converted into the next onewithout significant build up of intermediate products[33].The overall result is a nearly complete conversion of theanaerobically biodegradable organic material into end prod-ucts like methane, carbon dioxide, hydrogen sulphide, andammonia.

3. Anaerobic bioreactors used for FVW biomethanation

3.1. Batch systems

In batch systems, digesters are filled once with fresh FVW,with or without addition of seed materials, and allowed to gothrough all degradation steps sequentially. The hallmark of

Table 2Performance data of different anaerobic processes applied for FVW treatment

Process Volume (l) Loading rate(gVS/(l day))

HRT (day) VS removal (%) Methane yield(litre/gVS)

References

Batch system 10 1.06 47 65 0.16 [35]Batch system 5 0.9 32 58 0.26 [36]Continuous one-stage CSTR 3 1.6 20 88 0.47 [3]Continuous one-stage CSTR 16 3.6 23 83 0.37 [20]Continuous tubular reactor 18 2.8 20 76 0.45 [4]Two-stage system: solid bed hydrolyser

and UASB methaniser100 + 25 6.8 2.5 94 0.35 [52]

Two-stage system: ASBR hydrolyser andanaerobic filter methaniser

2.5 + 10 4.4 7+ 10 87.5 0.34 [21]

Two-stage system: CSTRhydrolyser and anaerobicfilter methaniser

7 + 4 5.65 2+ 2.3 96 0.42 [20]

batch systems is the clear separation between a first phase,where acidification proceeds much faster than methanogen-esis, and a second phase, where acids are transformed intobiogas[34].

Converti et al., tested the anaerobic batch digestion ofFVW, under both mesophilic and thermophilic conditions[9]. The results showed that, under mesophilic and ther-mophilic conditions, the mixture of vegetable wastes wasquickly digestible, and the first-order kinetic constant around4.1 10−3 l/(h g) VSS was estimated for these materials.

Anaerobic batch digestion of mixed vegetable waste wasalso carried out successfully at 5% total solid concentration[35] (Table 2). Digestion of the waste after 47 days resultedin 0.16 m3biogas/kg TS added with a maximum gas produc-tion rate on day 26. Whereas, Bouallagui et al.,[36] andMarouani et al.,[37] showed that the anaerobic treatment of


FVW at 8% TS in a batch digester was inhibited by the VFAaccumulation and irreversible decreasing pH problems.

Batch systems have, up to now, not succeeded in takinga substantial market share. However, the specific featuresof batch processes, such as simple design and process con-trol, robustness towards coarse and heavy contaminants, andlower investment costs make them particularly attractive fordeveloping countries[34]. The dependence of the biometha-nation yield on the starting level of digestible organic sub-stances observed in batch digestion tests suggested the op-erating conditions for the fed-batch or continuous digestionof the materials under consideration.

Application of sequencing batch reactor (SBR) technol-ogy in anaerobic treatment of FVW is of interest because ofits inherent operational flexibility, characterised by a highdegree of process flexibility in terms of cycle time and se-quence, no requirement for separate clarifiers, and retentionof a higher concentration of slow-growing anaerobic bac-teria within the reactor[38]. Research into the ASBR pro-cess has been carried out by several investigators[39,40].Satisfactory high solid content waste degradation and sus-pended solid removal (90–93%) using the ASBR were re-ported[41,42].

3.2. Continuous one-stage systems

About 90% of the full scale plants, currently in use in Eu-rope for the anaerobic digestion of organic fraction of munic-ipal solid wastes and biowastes, rely on continuous one-stagesystems[14]. However, a considerable amount of literaturehas appeared concerning wastes treatment in two phases;first an acid forming phase followed by a methanogenicphase[43–45]. A likely reason for this discrepancy is thattwo-and multistage systems afford more possibilities to theresearcher to control and investigate the intermediate stepsof the digestion process. Industrialists, on the other hand,prefer one-stage systems because of their simpler designsand lower investment costs.

Different experiments on vegetable wastes anaerobic di-gestion were carried out using different one-stage systems(Table 2). Mata-Alvarez et al. examined the performance ofthe mesophilic one-stage completely stirred reactor (Fig. 2a)for the treatment of the organic fraction of the wastes comingfrom a large food market[3]. The maximum organic load-ing rate (OLR) tested was below 3 kg TVS/(m3 day). TheOLR of 6 kg TVS/(m3 day) was found to be a limit conditionfor a similar waste digestion[31]. Moreover, as mentionedby Mata-Alvarez et al., this waste was presumably morebiodegradable, which meant a larger and faster VFA pro-duction which stressed the validity of this OLR limit[32].Overloading of digesters with FVW above 4 kg TVS/m3 daywas also reported by Lane to result in a fall in pH and gasyield and an increase in the CO2 content of gas producedusing a continuously stirred tank reactor (CSTR)[46].

A semi-continuously mixed tubular digester was tested(Fig. 2b) [4,47]. The best results were obtained by applying

an HRT of 20 days with an OLR of 2.8 kg TVS/(m3 day).The pH may fall in the hydrolysis shortly to 6.1, but itremains most of the time at 7.2. When reducing the HRT to10 days, the pH fell to 5 and inhibition was observed. Themost significant factor of the tubular reactor is its abilityto separate acidogenesis and methanogenesis longitudinallydown the reactor, allowing the reactor to behave as a systemof two phases.

In one-step anaerobic digestion of solid wastes, problemsmay occur if the substrate is easily degradable because insolid waste digestion, there is no possibility for the accu-mulation/retention of biomass within the reactor, the slowergrowing methanogens are overfed at higher loading rates[6].

In a one-stage system, combining acidogens andmethanogens in one vessel, hydrogen formed by acidogenicmetabolism is assimilated by the methanogens to reducecarbon dioxide to methane and water[48]. On increasing thefeeding rate of the substrate, acidogenic activity, includingmainly acetate, carbon dioxide, and hydrogen production,is increased, whereas the methanogenic population cannotincrease its activity to the same extent. At a loading rate,were the hydrogen consuming reactions become saturated,accumulation of hydrogen partially inhibits its further for-mation and consequently more organic electron sink willbe formed, causing imbalances and cessation of methaneproduction[49,50].

3.3. Continuous two-stage systems

Both groups of acidogenic and methanogenic organismsare different with respect to their nutritional requirements,physiology, pH optima, growth, and nutrient uptake kinet-ics, and their ability to withstand environmental stress fac-tors [13]. With conventional digestion processes, by com-bining acidogens and methanogens in one reactor, uniformconditions are imposed on both groups. However, two-phaseanaerobic digestion implies a process configuration employ-ing separate reactors for acidification and methanogenesisconnected in series, allowing optimisation of both processes[51].

The two-phase anaerobic digestion of a mixture offruit and vegetable wastes was studied in different works(Table 2). The two-step technology applied by Rajeshwariet al., allowed the conversion of over 94% of vegetablemarket waste into biogas (Fig. 2c) [52]. The raw wastewas acidified in a solid bed reactor. The leachate obtainedafter completion of acidification phase was further treatedin an UASB reactor for biogas production. A different kindof FVW have been subjected to two-phase anaerobic di-gestion[21]. The hydrolysis–acidification step was carriedout in ASBR and methane fermentation was performed ina fixed film reactor operated in the upflow mode (Fig. 2e).The global degradation yield remained above 87% and thebiogas production yield was about 0.29 l per g of input totalCOD. Using a two-stage system involving a thermophilic


Fig. 2. Processes used for FVW anaerobic treatment: (a) continuously stirred tank reactor (CSTR)[3,6]; (b) tubular reactor[4,47]; (c) two-phase integratedanaerobic solid bed hydrolyser (SBH) and upflow anaerobic sludge blanket (UASB)[52]; (d) two-phase integrated anaerobic continuously stirred tankreactor and fixed film reactor (FFR)[20]; and (e) two-phase integrated anaerobic sequencing batch reactor and fixed film reactor (FFR)[21].

liquefaction CSTR reactor and a mesophilic anaerobic filter,more than 95% volatile solids were converted to methaneat a volumetric loading rate of 5.65 g VS/l d (Fig. 2d). Themethane production yield was about 420 l/kg added VS[20].

These authors generally found that phase-separated di-gesters may offer the best choice for high efficiency, con-cerning both depuration rates and energy recovery.

4. Post-treatment of anaerobic digestion effluent

Post-treatments are necessary if anaerobic effluents needto be discharged into surface waters, because anaerobic di-gestion alone is not able to produce effluents that can meetthe discharge standards applied in most industrialized coun-tries, particularly for COD and nitrogen[53]. Up to now,there is a definite lack of practical experience and know-howin the treatment of those effluents[54]. In view of the in-

creasingly wide acceptance awarded to the fermentation pro-cess, it appears necessary to include the treatment of the gen-erated wastewater in the overall process and to grant it thesame priority as the fermentation step. The SBR technologycan successfully be used for carbon and nitrogen removalswith anoxic/aerobic processes. Garrido et al. reported that98 and 99% removals were achieved with the conventionalSBR reactor for COD and nitrogen, respectively[55].

5. Conclusion

Anaerobic digestion represents a commercially viableprocess to convert FVW to methane gas, a useful energysource. The overall results of anaerobic digestion of FVWsuggest that the two-stage system is a promising processto treat these wastes with high efficiency in term of degra-dation yield and biogas productivity. This efficiency is


possible by the adaptation of each ecosystem to its ownsubstrate. The biochemical reactions involved in anaerobicdigestion of FVW are taken subsequently under conditionssimilar to those of the rumen. It is appropriate to view thegastrointestinal tract as an ecological system and that byapplying ecological principles, a better understanding ofdistribution and interaction of organisms can be achieved,and then it could help to design and construct a suitablebioreactor for FVW anaerobic treatment.

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bioreactor performance in anaerobic digestion of fruit and vegetable wastes

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