Biochemical Engineering Journal 21 (2004) 193–197

Short communication

Two-phases anaerobic digestion of fruit andvegetable wastes: bioreactors performance

H. Bouallaguia,∗, M. Torrijosc, J.J. Godonc, R. Molettac,R. Ben Cheikhb, Y. Touhamia, J.P. Delgenesc, 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, Tunisiac Laboratoire de Biotechnologie de l’Environnement, INRA, Avenue des Etangs, 11100 Narbonne, France

Received 8 December 2003; received in revised form 6 May 2004; accepted 14 May 2004

Abstract

The two-phase anaerobic digestion of a mixture of fruit and vegetable wastes (FVW) was studied, using two coupled anaerobic sequencingbatch reactors (ASBR) operated at mesophilic temperature. The effect of increasing loading rates on the acidification step was investigated.Results indicated that the hydrolysis yield (81%) stabilized at an OLR of 7.5 g COD/L.d. The volatile fatty acids concentration increasedwhen the loading rate was increased and reached its maximum value (13.3 g/L) at higher loading rate tested (10.1 g COD/L.d). Methanogenicfermentation of the liquefaction acidification products was efficiently performed in the ASBR reactor and high methane productivity wasobtained (320 L CH4 per kg of input COD). Total COD in the final effluent from the methanizer was usually below 1500 mg/L, and solubleCOD below 400 mg/L. Overall COD removal in the treatment system was 96%. Phase separation with conventional ASBR reactors resultedin high process stability, significant biogas productivity and better effluent quality from fruit and vegetable wastes anaerobic digestion.© 2004 Elsevier B.V. All rights reserved.

Keywords:Waste treatment; Anaerobic processes; Acidification; Two-phases; Anaerobic sequencing batch reactor; Biogas

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]. A possible way to dispose of these wastes is usingthe anaerobic digestion process[3,4]. The successful ap-plication of anaerobic technology to the treatment of solidwastes is critically dependent on the development and theuse of high rate anaerobic bioreactors[5,6]. In recent yearsa number of novel reactor designs have been adapted anddeveloped. These processes differ especially in the way mi-croorganisms are retained in the bioreactor and the separa-tion between the acidogenic and the methanogenic bacte-ria and then to reduce the anaerobic digestion limitations[7–9].

Given the very large biodegradable organic content ofFVW, a major limitation of anaerobic digestion of thesewastes in one stage system is a rapidly acidification de-

∗ Corresponding author. Tel.:+216 22 524 406; fax:+216 71 704 329.E-mail address:hassibbouallagui@yahoo.fr (H. Bouallagui).

creasing the pH in the reactor, and a larger volatile fattyacids production, which stressed and inhibited the activityof methanogenic bacteria. The two-phase systems appearas higher efficient technologies for anaerobic digestion ofFVW. Their greatest advantage lies in the buffering of theorganic loading rate taking place in the first stage, allowinga more constant feeding rate of the methanogenic secondstage[10–12].

Application of sequencing batch reactor (SBR) technol-ogy to 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 can retaina higher concentration of slow-growing anaerobic bacteriawithin the reactor. Research into the anaerobic sequencingbatch reactors (ASBR) process has been carried out by sev-eral investigators[13–15]. Satisfactory high-solid-contentwaste degradation and suspended solid removal (90–93.5%)using the ASBR were reported[16,17].

The aim of the present work was to assess the performanceof a two-phases anaerobic digestion of a mixture of fruit andvegetable wastes in an ASBR.

1369-703X/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.bej.2004.05.001

194 H. Bouallagui et al. / Biochemical Engineering Journal 21 (2004) 193–197

Nomenclature

FVW fruit and vegetable wasteASBR anaerobic sequencing batch reactorCOD chemical oxygen demand (g L−1)HRT hydraulic retention time (day)SRT solid retention time (day)TS total solid (%)TVS total volatile solid (%)TSS total supended solid (g L−1)VSS volatile suspended solid (g L−1)TNK total nitrogen Kjeldahl (g L−1)OLR organic loading rate (g L-1 d−1)VFA volatile fatty acid (g L−1)

2. Material and methods

2.1. Reactors design and operational conditions

Two laboratory-scale reactors of different volumes wereused. The hydrolysis–acidification step was carried out ina glass reactor of 1.5 L effective volume. The reactor wasstirred by an inox stirrer and operated with 1 cycle per dayand 21 h of reaction, 3 h of settling, draw off and fill duringthe last hour of the settling period. The pH was controlledby automatic addition of 2 N NaOH.

The methane fermentation was performed in a double-walled glass reactor of 5 L effective volume. Mixing in thereactor was done by a system of magnetic stirring. Themethanogenic ASBR was operated with cycles includingthe following four discrete steps: (i) fill (30 min), (ii) react(21 h), (iii) settle (2 h), (iv) draw off (30 min). The reactorswere maintained at 35◦C and inoculated with an anaerobicecosystem obtained from the settled output of an anaerobicfixed bed reactor treating winery effluent.

2.2. Analysis and fermentation parameters

Total solids (TS), total volatile solids (TVS), total sus-pended solids (TSS), pH, total nitrogen Kjeldahl (TNK)and chemical oxygen demand (COD) were determined ac-cording to the standard methods[18]. Volatile fatty acids(VFA) concentration was determined by semi-capillarygas chromatography (Chrompack CP 9000) with a Econo-fap FFAP (Altech) column. The maximum temperatureof injection and the regeneration temperature are 250 and200◦C, respectively. The volume of biogas produced wasmeasured by an Aalborg mass flow meter 0–20 mL/minand analysed by gas chromatography (Shimadzu GC 8A)[14].

Hydrolysis yield (HY)(%):it was calculated as following:

HY =100∗ [(total input COD− soluble input COD)

− (total output COD− soluble output COD)]

(total input COD− soluble input COD)

Acidification yield:total VFA in the reactor expressed inmg COD over total COD of crude input.

Pollutant removal yield (%):difference between the totalCOD at inlet and total COD at outlet× 100 over total CODat inlet.

3. Results and discussion

3.1. Substrate characteristics

The putrescible FVW used in this study was collectedfrom the group market of Narbonne (South France). Aftershredding to small particles and homogenizing, it was storedin 1 litre tins at 4◦C. The composition of the raw shreddedFVW is shown inTable 1. Total initial COD was about120 g/kg (humid weight), with soluble COD and particulateCOD of 79 g/kg and 41 g/kg, respectively. The COD/N ratiowas balanced, being around 120/3.8; therefore, no nitrogenwas added to the acidification reactor.

3.2. Hydrolysis and acidification stage

The acidification reactor was operated at a constant hy-draulic retention time (HRT) of 3 days and fed with differentdilutions of FVW to change the organic loading rate (OLR).The whole experiment was carried out over three runs (run1: OLR= 3.7 g COD/L.d; run 2: OLR= 7.5 g COD/L.d andrun 3: OLR= 10.1 g COD/L.d).

3.2.1. Start-up and pH adjusmentThe first part of this study was carried out at a low

OLR (run 1) in order to choose the best pH for theliquefaction-acidification step. When not controlled, pHdropped rapidly to 4, especially just at the end of the feedperiod, and an inhibition of hydrolysis was observed. Koster[19] mentioned that butyric acid-producing bacteria are notviable in a medium with a pH lower than 4.2. Several au-thors suggested that optimal pH for better hydrolytic andacidogenic bacteria activity is comprised between 5 and 6[2,11,20,21]. After that, pH was maintained at 6. In theseconditions, the rate of volatile fatty acids production waslow (1.2 g COD/L.d). Then acidification was activated andthe rate of volatile fatty production was improved to 4.5 g

Table 1Raw shredded FVW characteristics

Analysis Average values

pH 4.2Total solids (g/kg) 100Volatile solids (g/kg) 88Total COD (g/kg) 120Particulate COD (g/kg) 78.9Total suspended matter (g/kg) 74.4Total nitrogen Kjeldahl (g/kg) 3.8

H. Bouallagui et al. / Biochemical Engineering Journal 21 (2004) 193–197 195

Fig. 1. Cumulative total VFA production (�), C2 (�), C3 (�), C4 (�)and C5 (�) during a typical cycle of acidification reactor, operated atdifferent OLR: run 1 (a), run 2 (b) and run 3 (c).

COD/L.d by lowering pH to 5.5 by the addition of a 2 Nchlorhydric acid solution. The pH was maintained at thislevel during the whole experiments.

3.2.2. Particulate organic carbon solubilization and VFAproduction at different OLR

The results of the hydrolysis acidogenesis stage are shownin Table 2. Variation in OLR indicated that the hydrolysisyield (81%) stabilized at an OLR of 7.5 g COD/L.d. VFAconcentration increased dramatically with the increase ofthe OLR. The highest VFA concentration (13.3 g/L) wasobtained in run 3, with the highest OLR.Fig. 1 shows theVFA production and composition during a typical cycles ofreactor operation at different OLR. Butyric acid and valericacid were the major VFA produced at a lower OLR (run 1)(Fig. 1a). When increasing the OLR, acetic acid and butyric

acid were the main VFA produced, with an average value of40% for each at an OLR of 10.1 g COD/L.d. In agreementwith Sans et al.[22] and Raynal et al.[11], the VFA profileshowed that butyric acid concentrations were quite high forFVW acidification. The percent of soluble COD in the formof VFA in the outlet of the reactor increased as the OLRrose, with 97% of soluble COD as VFA in the last run(Table 2).

Total COD reduction results based on a COD balanceat steady-state conditions was about 45 for the highestOLR (run 3) (Table 2). During the liquefaction phase,the production of carbon dioxide and small quantities ofmethane and hydrogen resulted from COD degradation.The gas production in the acidogenic reactor (at 1 atmand 0◦C) was about 0.3 L/L.d (run 1), 0.41 L/L.d (run 2)and 0.52 L/L.d (run 3) (Table 2). Several analyses of gascomposition revealed that CO2 was the predominant gasgenerated (over than 70%), which is in agreement with theresults reported in the literature for the acedogenic step[1,11].

3.3. Methanogenic ASBR digestor performance

This step in the experiments was carried out to evalu-ate the degradation efficiency and biogas productivity ofan ASBR digester treating the supernatant of the acido-genic reactor effluent. The results are given inTable 3.The hydraulic retention time was constant (10 days) sothe loading rate (from 0.72 to 1.65 g/L.d) depended on theCOD at the outlet of the acidogenic reactor. In these oper-ating conditions, the maximum g COD total inlet/g volatilesuspended solid (VSS) was 0.18 and the SRT was about50 days.

3.3.1. Biogas production and organic matter degradationThe rate of biogas production as well as the pH were

monitored continuously. An example of the evolution of thebiogas production rate and of the pH obtained at the highestOLR, is presented inFig. 2. The biogas production ratewas greatest at the start of the cycle, and then decreasedwith time, reaching very low levels after about 8 h. At thistime, the VFA concentration in the reactor stabilized at itslowest level and the react phase could then be considered asfinished. The duration of one cycle was about 10.5 h. Thehighest biogas productivity (450 L per kg of added COD)with high methane content (71%) was obtained at run 2.Earlier, Raynal et al.[11] and Verrier et al.[1] obtainedsimilar productivity, using an upflow anaerobic filters.

The average residual total COD and soluble COD in thetreated effluent were about 1300 and 270 mg/L at run 3, cor-responding to a purification level of 93 and 97%, respec-tively. These results show that the organic matter of FVWafter acidification, constituted mainly of VFA, is highlybiodegradable. TSS in the methanogenic reactor was be-tween 13 and 15 g/L and settling was very good during thewhole experiment.

196 H. Bouallagui et al. / Biochemical Engineering Journal 21 (2004) 193–197

Table 2Average values of FVW at the liquefaction–acidification stage

Run 1 Run 2 Run 3

Inlet total COD (g/L) 11.2± 0.05 23± 0.11 30± 0.2Inlet soluble COD (g/L) 6.8± 0.09 14.2± 0.3 15.7± 0.4Organic loading rate (g COD/L.d) 3.7± 0.02 7.67± 0.02 10± 0.03Retention time (days) 3 3 3Outlet total COD (g/L) 7.16± 0.18 12.9± 0.78 16.5± 0.36Outlet soluble COD (g/L) 6± 0.14 11.2± 0.87 13.8± 0.26Acetic acid (g COD/L) 0.7± 0.06 3.43± 0.27 5.18± 0.28Propionic acid (g COD/L) 0.56± 0.05 0.71± 0.08 1.2± 0.09Butyric acid (g COD/L) 1.83± 0.14 3.45± 0.31 5.46± 0.38Valeric acid (g COD/L) 1.41± 0.03 1.29± 0.09 1.47± 0.12Total VFA (g COD/L) 4.5± 0.16 8.8± 0.64 13.3± 0.43TSS (g/L) 7.5 9.2 12.4Hydrolysis yield (%) 74± 1.5 81± 3.5 81± 3Acidification yield 40.3± 0.9 38.9± 1.2 44.4± 1.8CODVFA/CODsolubleoutlet (%) 75 ± 1.2 80± 2.5 97± 4.2Total COD removal (%) 36± 0.9 44± 1.2 45± 1.3Gas production (L/L.d) 0.3± 0.02 0.41± 0.01 0.52± 0.01

Table 3Average performances of the methanogenic reactor

Run 1 Run 2 Run 3

COD total input (g/L) 7.16± 0.18 12.9± 0.78 16.5± 0.36Organic loading rate (g COD/L.d) 0.72± 0.03 1.29± 0.02 1.65± 0.02Retention time (days) 10 10 10Output total COD (g/L) 2.3± 0.09 1± 0.07 1.2± 0.1Output soluble COD (g/L) 0.35± 0.02 0.32± 0.01 0.27± 0.01TSS in the reactor (g/L) 13.1 13.7 14.8pH in the reactor 6.9± 0.1 7.2± 0.1 7.49± 0.15Biogas productivity (L per L of reactor a day) 0.26± 0.01 0.58± 0.02 0.74± 0.02Biogas yield (L per kg of input COD) 363.1± 16.5 450.3± 22.3 448.5± 19Methane content (%) 69± 2 71 ± 2 70.6± 1.5COD removal (%) 67.9± 3.2 92.2± 2.1 92.7± 2.3COD removal of the total process (%) 79.46± 2.8 95.65± 2.1 96± 2.6

Fig. 2. Biogas production rate and pH shift during a typical cycle of the methanogenic reactor operated at the higher loading rate (run 3).

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4. Conclusion

The study of two-stage anaerobic digestion of FVW wasundertaken. The acidification step was influenced by thevariation in the OLR, especially the concentration of VFAand the percent of VFA in the form of soluble COD. Theresults obtained in this work show that the FVW is highlybiodegradable with a conventional two-phase reactor and96% of the total COD was converted to biomass and bio-gas. The total COD in the final effluent from the methanizerwas usually below 1500 mg/L and the soluble COD below400 mg/L. Compared with a previous study[4], phase sepa-ration between the two groups of micro-organisms involvedin anaerobic digestion is necessary to improve the yield oftotal process.

Acknowledgements

The authors wish to acknowledge the Ministry of SuperiorEducation and Scientific Research and Technology, whichhas facilitated the carried work.

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