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Copyright: © 2015, Thanikal JV, Yazidi H. This is an open-access article distributed under the terms of the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any site, provided the original author and
source are credited.
Vol. 3, No. 2: 131-134
(2015).
ISSN: 2313-3937
Anaerobic Co-digestion of fruit and vegetable waste:
Bio-reactor performance
Joseph V.Thanikal1, HatemYazidi1*
INTRODUCTION
The central fruit and vegetable market located at Al-
Mawaleh (Local entity) in Muscat, sultanate of Oman, is
a whole sale market. The market data and the survey
conducted over a period of one year quantified 50
tonnes of fruit and vegetable wastes (FVW) being
generated per day from the market. This waste is
currently sent to the landfill. Large quantity of waste
disposed to the landfill constitutes a source of nuisance
because of their high biodegradability [1]. Many
researchers have found that the anaerobic digestion
process is one of the most sought technology for
treatment these wastes [2,3,4]. The anaerobic digestion
is a process by which almost any organic waste can be
biologically converted in the absence of oxygen. This
process requires specific environmental conditions and
different bacterial populations. Mixed bacterial popula-
tions degrade organic compounds and produce as end-
product a valuable high energy mixture of gases (mainly
methane – CH4 and carbon dioxide – CO), termed
biogas [5]. The anaerobic digestion of FVW is
accomplished by a series of biochemical transfor-
mations, which can be roughly separated into four
metabolic stages namely hydrolysis, acidogenesis,
acetogenesis and methanogenesis [11]. First, particulate
ABSTRACT
The market waste at Al-Mawaleh (Sultanate of Oman) central fruit and vegetable market has been facing a
problem of disposal. To resolve the problem of market waste a 6L laboratory scale anaerobic batch reactor
was studied for co-digestion of fruit and vegetable wastes at mesophilic condition. The effect of increasing
load rate (0.5 – 6 g/L) has shown increase in methane productivity. The total COD in the final effluent was
below 900 mg/L and soluble COD was below 400 mg/L. The concentration of Volatile fatty acid (VFA)
increase was not significant at increased loading rate and pH remained at 7.56±0.25 throughout the
experimental phase. The 6L reactor operation was stable with significant biogas productivity.
Keywords: Anaerobic co-digestion, Biogas production, Mesophilic conditions.
Citation: Thanikal JV, Yazidi H. (2015) Anaerobic Co-digestion of fruit and vegetable waste: Bio-reactor
performance. World J Exp Biosci 3: 131-134.
Received November 22, 2015; Accepted December 15, 2015; Published December 20, 2015.
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*Correspondence: HatemYazidi.hatem.yazidi@gmail.com.
Caledonian College of Engineering, PO Box 2322, CPO SEEB, 111, R301, Muscat, Sultanate of Oman
Full list of author information is available at the end of the article
Research article
WORLD JOURNAL OF
EXPERIMENTAL
BIOSCIENCES

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organic materials of FVW like cellulose, hemicellulose,
pectin, and lignin, must undergo liquefaction by
extracellular enzymes [6,7]. Second the acidogenic
bacteria transform the liquefied product (monomers) into
short chain volatile acids, ketones, alcohols, hydrogen
and carbon dioxide. Third the acidogenesis products are
transformed by acetogenic bacteria into hydrogen,
carbon dioxide and acetic acid. During the Fourth stage,
microorganisms transform the hydrogen and acetic acid
to methane gas and carbon dioxide.
The anaerobic co-digestion process was found reliable
in treating two or more organic wastes which are
complementary to each other. The co-digestion helps in
dilution of potential toxic compounds, improved balance
of nutrients, synergetic effect of microorganisms and
better biogas yield [8,9,11].
In practice, there are a large number of factors which
affect anaerobic biogas production efficiency such as
environmental conditions like pH, temperature, type and
quality of substrate, mixing as well as process inhibitory
parameters likewise high organic loading, formation of
high volatile fatty acids and inadequate alkalinity [10].
Anaerobic batch reactor is found to be more flexible in
operation considering the cycle operation and do not
require separate clarifier and helps retain high
concentration of bacteria within the reactor [7,8]. This
study aims at treating vegetable and fruit wastes by
method of anaerobic co-digestion and study the bio-
reactor performance.
MATERIALS AND METHODS
Substrate preparation and characterization
Fruit and vegetable wastes collected randomly from
market representative samples, which were significant in
the survey conducted, were: Potato, Carrot, Spinach,
Onion, Tomato, Lettuce, Apple, Orange, Grapes,
Pomegranate and Watermelon. It was important to
characterize each of the vegetable and fruit substrate for
its, moisture content, Total Suspended Solids (TSS),
Volatile Suspended Solids (VSS), Chemical Oxygen
Demand (COD), and other significant characteristics that
would influence the co-digestion and the results are
tabulated in table1. The mixture of both fruit and
vegetable substrates were prepared by shredding them
into small pieces using a vegetable shredder for easy
feeding to the reactor. The substrates homogenised
mixture was then characterized before fed into the bio-
reactor and the results are shown in table 2.Total initial
COD was around 100g/kg (humid weight), with soluble
COD 68g/kg. The mixture was then stored at 4°C.
Table 2.Characteristics of shredded mixture of FVW.
Parameters Average value
pH 5.6
COD (g/Kg) 185
Moisture content (%) 95.5
Total Solids (g/g) 220
Volatile Solids (g/g) 88
Suspended Solids (g/g) 120
Total Nitrogen Kjedahl(g/kg) 5.6
Reactor design and operational conditions
Double walled glass reactor of effective volume 6L was
used for the study. Hot water was circulated
continuously around the reactors to maintain a
temperature of 36±2°C. The reactor was fitted with
Metler Toledo pH probe Inpro 4260i to measure pH
online. The reactor was continuously mixed using
magnetic stirring system. The reactor was fed with
vegetable and fruit mixture separately and also in co-
digestion at an organic loading rate, gram of VSS per
liter volume of reactor (OLR), ranging between 0.5 –
6g/L. The end of reaction in a batch was marked when
the flow rate of biogas fall to 5-10 ml/h. The reactor was
inoculated using anaerobic sludge from UASB reactor
treating winery effluent.
Start-up of experiments
The reactor was filled with inoculum of VSS
concentration 6g/L. The reactor was initially fed with
three cycles of 5 ml of ethanol as sole carbon source to
check the activity of the inoculum. The pH was
maintained at 7.56±0.25, by adding NaOH solution in
case of drop in pH. The reactor was fed with FVW
substrate starting with low organic loading rate 0.5g
VSS/L. The reactor was characterised for TS, VSS,
COD, Volatile Fatty Acid (VFA) and pH before and after
end of the cycle.
Table 1.Characteristics of raw vegetable and fruit substrates
Parameters Potato Carrot Spinach Onion Tomato Lettuce Apple Orange Grapes POM* WAT
**
pH 4.5 4.8 5.6 7.82 7.78 7.70 5.74 3.89 3.42 5.31 6.79
COD (g/Kg) 220 180 90 52 310 145 120 60 80 210 150
Moisture content (%) 99.3 89.69 99.7 85.56 99.5 88.54 90.47 95.92 98.8 72.63 95.35
Total Solids (g/g) 0.34 0.17 0.13 0.25 0.09 0.11 0.21 0.25 0.36 0.38 0.14
Volatile Solids (g/g) 0.16 0.07 0.05 0.12 0.04 0.05 0.12 0.12 0.16 0.20 0.08
Suspended Solids (g/g) 0.18 0.09 0.08 0.13 0.05 0.06 0.13 0.14 0.20 0.25 0.09
*POM: Pomegranate; **WAT: Watermelon
Thanikal JV, Yazidi H. (2015) World Journal of Experimental Biosciences. Vol. 3, No. 2: 131-134.

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The OLR was increased systematically at 1 – 6 g/L. The
reactor was operated for 180 days. During this operation
pH remained at 7.5 inside the reactor.
Analytical Method
Total solids (TS) and volatile suspended solids (VSS)
were measured according to the standard methods [12].
The biogas production was measured on-line every 2
min by Milligascounter MGC-1 flow meters
manufactured by Ritter gas meters fitted with a 4-20 mA
outputfor acquiring the data [15]. The software supplied
by Ritter Company was used to log the gas output. The
samples were centrifuged at 15,000 rpm for 15 min at
20°C; 2 ml of the clear supernatant is then added to
HACH COD vial and digested in a COD digester for 2h.
The CODsoluble was determined by spectrophotometry at
620 nm according to the APHA Standard Methods [12].
The VFA was determined by titration method. Methane
content in the biogas is measured using online methane
analyser supplied by BlueSens, Germany.
RESULTS AND DISCUSSION
Biogas production and organic matter
degradation
The biogas production and the pH were monitored
continuously throughout the experiment. The reactor
characterisation was performed before and after feed of
substrate to the reactor. Towards the end of the batches,
the biogas production rate became low as much as 10 –
5 ml/h and this was assumed as the starting time for
sludge returning back to its endogenous activity, that is
to say the time when it could be assumed that the
reaction was over and the organic matter added was
eliminated. Volatile fatty acid was measured for alternate
cycles. The typical cycle of biogas production at higher
organic loading for co-digestion of FVW is shown in fig
1. The faster degradation of available organic matter
Fig 1. Biogas production and flow rate plots vs. time (h) during a
typical cycle at higher organic loading rate
was completed in few hours from start of the cycle.
However the gas flow rate was continued and this may
be because of the endogenous phase [4]. The duration
of the cycle was typically 15 h for an organic loading rate
of 6.0 g/L, producing maximum biogas volume of 335 L
per kg of VSS with a methane content of 68%. These
results are in agreement with those obtained by
Fernandez et al. [13] and Bouallagui et al. [1]. It is very
likely that the high degradation efficiency in the co-
fermentation was due to an improved ratio of nutrients
and better availability of the organic substances, which
facilitate their assimilation by anaerobic flora and increa-
ses the degree of degradation. The loading of bio-
reactor was done systematically. The typical COD at the
outlet was 1300-900 mg/L (fig. 4). The percentage of
solids reduction was 80% indicating good biodegrade-
ation of FVW. Table 3 shows the results of reactor
characterisation. Biogas production was exponential to
the increase in OLR (fig. 2). The rate of biogas produ-
ction was more between OLR of 1 - 4.5gVS/L and
thereafter it was less at high OLR (fig.2). The maximum
organic loading rate (OLR) tested was below 3 kg
TVS/(m3 /day). The OLR of 6 kg TVS/(m3/day) was
found to be a limit condition for a similar waste digestion
[14].
Table 3.Reactor characterization.
Parameters Average value
pH 9.34
COD input (g/L) 6 g/L VSS
Organic loading rate Max. (g VS/L) 15
Retention time (h) 7.92
Total solids (g/kg) 1.2
COD output (g/L) 333
Fig 2. Cumulative Biogas production (black line) and Organic
loading rate for FVW (vertical bars) vs. time (h) during the all
experiment.
The effect of pH and volatile fatty acid is shown in fig 3
and that the variation in pH and volatile fatty acid was
during the phase of degradation and there after became
to a steady state. The reactor experienced the problem
of mixing when higher quantities of solids are added to
the reactor.The cumulative biogas volume showed an
increase in gas production with regard to a systematic
loading of reactor.
Thanikal JV, Yazidi H. (2015) World Journal of Experimental Biosciences. Vol. 3, No. 2: 131-134.

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Fig 3.pH and volatile fatty acid time variation in the reactor at typical
cycle of high influent OLR.
Fig 4.Total effluent COD time variation (black line) measured at the
outlet vs. the influent Organic loading rate (vertical gray bars).
A co-digestion process of FVW was studied at the
laboratory scale. Each of the substrates had different
characteristics. The reactor performed with a biogas
production of 335 L/ kg of VSS and the biogas
production was not affected by variation in pH during the
process. There was no accumulation of VFA throughout
the experiment and the COD removal was up to 87%.
The results show that FVW is highly biodegradable with
conventional anaerobic co-digestion complementing
each other’s characteristics.
Acknowledgments
The authors acknowledge The Research Council of Oman for
the research grant extended to conduct this research and the Al
Mawaleh market to support the conduct of survey
Conflict of interest
The authors declare that they have no conflict of interests.
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Author affiliation:
Caledonian College of Engineering, PO Box 2322, CPO
SEEB, 111, R301, Muscat, Sultanate of Oman.
Thanikal JV, Yazidi H. (2015) World Journal of Experimental Biosciences. Vol. 3, No. 2: 131-134.