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Vol. 8(11), pp. 443-450, 23 March, 2013
DOI: 10.5897/IJPS2013.3863
ISSN 1992-1950 © 2013 Academic Journals
http://www.academicjournals.org/IJPS
International Journal of Physical
Sciences
Full Length Research Paper
Co-digestion of cattle manure with organic kitchen
waste to increase biogas production using rumen fluid
as inoculums
Tamrat Aragaw1, Mebeaselassie Andargie1* and Amare Gessesse2
1Department of Biology, College of Natural and Computational Sciences, Haramaya University, P. O. Box 138,
Haramaya, Ethiopia.
2Department of Biology, College of Natural and Computational Sciences, Addis Ababa University, P. O. Box 1176,
Addis Ababa, Ethiopia.
Accepted 18 March, 2013
Anaerobic co-digestion strategies are needed to enhance biogas production when treating certain
residues such as cattle/pig manure. Co-digestion of food waste with animal manure or other feedstocks
with low carbon content can improve process stability and methane production. In this study, anaerobic
digestion and co-digestion of cattle manure with organic kitchen waste using rumen fluid as inoculums
have been experimentally tested to determine the biogas potential. Co-digestion substantially increased
the biogas yields by 24 to 47% over the control (organic kitchen waste and dairy manure only). The
highest methane yield of 14,653.5 ml/g-VS was obtained with 75% organic kitchen waste (OKW) and
25% cattle manure (CM) additions. In contrast, addition of 75% cattle manure caused inhibition of the
anaerobic digestion process, and its cumulative methane yield was 23% lower than that with 25% cattle
manure addition.
Key words: Cattle manure, co-digestion, methane, organic kitchen waste, rumen fluid.
INTRODUCTION
Energy is one of the most important factors for human
development and to global prosperity. The dependence
on fossil fuels as primary energy source has led to global
climate change, environmental degradation, and human
health problems. 80% of the world’s energy consumption
still originates from combusting fossil fuels (Goldemberg
and Johansson, 2004). Yet the reserves are limited;
means do not match with the fast population growth, and
their burning substantially increases the greenhouse gas
(GHG) concentrations that contributed for global warming
and climate change (Schamphelaire and Verstraete,
2009). So, bio-energy (energy production from biomass)
can be seen as one of the key options. Among the many
bio-energy related processes being developed, those
processes involving microorganisms are especially
promising, as they have the potential to produce
renewable energy on a large scale, without disrupting
strongly the environment or human activities (Rittmann,
2008).
Anaerobic digestion (AD) is a technology widely used
for treatment of organic waste for biogas production.
Anaerobic digestion that utilizes manure for biogas
production is one of the most promising uses of biomass
wastes because it provides a source of energy while
simultaneously resolving ecological and agrochemical
issues. The anaerobic fermentation of manure for biogas
production does not reduce its value as a fertilizer
supplement, as available nitrogen and other substances
*Corresponding author. E-mail: mebhel@yahoo.com. Tel: +251921655912.
444 Int. J. Phys. Sci.
remain in the treated sludge (Alvarez and Liden, 2007).
Ethiopia has a large population of dairy and beef cattle,
generating large amounts of surplus manure that can be
used in biogas plants to produce renewable energy.
However, the high water content, together with the high
content in fibers, are the major reasons for the low
methane yields when cattle manure is anaerobically
digested, typically ranging between 10 and 20 m3 CH4
per tone of manure treated (Angelidaki and Ellegaard,
2003).
Studies demonstrated that using co-substrates in
anaerobic digestion system improves the biogas yields
due to the positive synergisms established in the
digestion medium and the supply of missing nutrients by
the co-substrates (Wei, 2000). In a study carried out by
Adelekan and Bamgboye (2009) on the different mixing
ratios of livestock waste with cassava peels, the average
cumulative biogas yield was increased to 21.3, 19.5, 15.8
and 11.2 L/kg TS, respectively for 1:1, 2:1, 3:1 and 4:1
mixing ratios when cassava peel was mixed with cattle
waste. In another report, co-digestion of cow dung with
pig manure increased biogas yield as compared to pure
samples of either pig or cow dung. Comparing to samples
of pure cow dung and pig manure, the maximum increase
of almost seven and three fold was respectively achieved
when mixed in proportions of 1:1 (Muyiiya and Kasisira,
2009). Co-digestion with other wastes, whether industrial
(glycerin), agricultural (fruit and vegetable wastes) or
domestic (municipal solid waste) is a suitable option for
improving biogas production (Amon et al., 2006; Macias-
Corral et al., 2008; El-Mashad and Zhang, 2010;
Marañón et al., 2012).
Food waste is a desirable material to co-digest with
dairy manure because of its high biodegradability (Zhang
et al., 2006, 2011; Li et al., 2010; Wan et al., 2011).
Study on the biogas production potential of unscreened
dairy manure and different mixtures of unscreened dairy
manure and food waste using batch digesters at 35°C
showed that the methane yield of unscreened manure
and two mixtures of unscreened manure and food waste
(68/32 and 52/48), after 30 days of digestion, was 241,
282 and 311 L/kg VS, respectively (El-Mashad and
Zhang, 2010).
In a study conducted by Zhu et al. (2011), they used
different food wastes, including expired creamer; expired
beer; slaughterhouse waste (SW); and fat, oil, and
grease (FOG), and these food substances were co-
digested with dairy manure to determine the methane
potential. According to the result, co-digestion
substantially increased the methane yields by 2.0 to 4.6
times over the control (dairy manure only).
This study was initiated to investigate the feasibility of
biogas production from the different wastes that are
generated from Haramaya University and the aims of the
present research work were to determine the optimal
conditions and mixing ratios for improved production of
biogas using co-digestion of cattle manure and solid
organic kitchen waste and also identify the key
parameters influencing the increase of biogas and
methane yield.
MATERIALS AND METHODS
Sample collection and preparation
Fresh cattle manure (CM) from beef and dairy farm, fresh organic
kitchen waste (OKW) from staff lounge, and rumen fluid (RF) from
the slaughterhouse were collected from Haramaya University
compound. 2 kg of fresh cattle manure was collected from eight
randomly selected cattle from beef and dairy farms for five
consecutive days. In these sites there are special feeds and normal
grazing cattle. The special feeds are provided with special type of
feeding program that includes silage, concentrate, hay forage,
agricultural residues and different grass types, byproducts from
Harar Brewery and Hamaressa Food Complex, etc. On the other
hand normal grazers are not provided with special type of feeding
program rather they graze grasses in the field and get only fodder
and agricultural residues. Finally the CM from both types of cattle
(special and normal grazers) was sorted and dried separately on a
plastic tray using direct sunlight for two days. 3 kg of fresh organic
kitchen wastes were also collected from the staff lounge similarly for
five consecutive days. The OKW was sorted manually to prevent
the inclusion of unwanted and possibly contaminant materials (such
as detergents, sand, bones etc.) and then dried with direct sunlight
for two days.
Following the methods suggested by Wendland et al. (2006),
separately dried cattle manure from special feeds and normal
grazers were mixed by weighing equal amount from each source
and shredded using shredder (Fritsch- Adam Baumuler model 80a-
4S114 type) to an average particle size of 2 mm and kept in a
refrigerator at 4°C. The shredded small sized cattle manure and
organic kitchen waste were mixed separately with water in 1:5 (solid
waste: water) volume ratio, in order to maintain the total solid in the
digester between 8 to 15%, which is the desired value for wet
anaerobic digestion.
Inoculum preparation
Following the recommendation of Aurora (1983), due to the
presence of higher content of anaerobic bacteria in the rumen of
the ruminant animals and the abundance of rumen waste disposal
from the nearby slaughterhouse, rumen fluid was used as inoculum
for anaerobic co-digestion of cattle manure and organic kitchen
waste.
Experimental set-up and design
A completely randomized experimental design was used in a 5 × 4
replicated laboratory experiment and it was conducted in a series of
five plastic tanks with 2 L capacity which was used as a laboratory
scale anaerobic digesters at mesophilic temperature (30 ± 8°C).
The working volume of each digester was 1.6 L. In each digester,
rumen fluid was used as inoculum. The TS and VS/ TS of the
inoculum used were 1.03% (wet basis) and 63.9%, respectively.
Each digester was purged for 5 min (300 mL/min) with inert gas
(N2) to create an anaerobic environment. Food waste, cattle
manure and their mixtures were separately examined in mono and
co-digestion respectively. The characteristics of the different
experiments are shown in Table 1. In co-digestion, the amount of
organic kitchen waste as well as that of cattle manure in each
Aragaw et al. 445
Table 1. Properties of organic kitchen waste, cattle manure and rumen fluid (mean ±SD).
Parameter
Organic kitchen waste
Cattle manure
Rumen fluid
pH
5.51±0.129
7.19±0.215
7.45±0.114
MC (%)
82.95±0.169
84.59±0.40
98.98±0.01
TS (Wt %)
17.05±0.169
15.42±0.40
1.03±0.01
VS (Wt %)
15.89±0.52
12.68±0.63
0.66±0.01
VS/TS ratio
93.18±2.54
82.23±2.04
63.9±0.45
Table 2. Properties of cattle manure and organic kitchen waste before digestion (mean value ± SD).
Parameter (before digestion)
Mixture
pH
MC (%)
TS (%)
VS (%)
VS/TS (%)
A1
6.95±0.030
86.15±0.128
13.85±0.128
12.85±0.403
92.75±2.398
A2
7.45±0.071
87.46±0.314
12.54±0.314
10.27±0.503
81.9±1.403
A3
7.32±0.065
87.09±0.490
12.83±0.353
10.81±0.470
84.2±1.354
A4
7.19±0.051
86.83±0.358
13.17±0.358
11.34±0.445
86.03±1.159
A5
7.09±0.025
86.42±0.274
13.58±0.274
11.89±0.389
87.5±1.587
digester was varied when it was added. The FW/CM ratios (based
on VS) of digestion A3, A4, A5 were designed as 0.3, 1 and 3,
respectively, corresponding to the organic kitchen waste and cattle
manure amounts of 25:75, 50:50 and 75:25 g-VS/L. In digestion A1,
organic kitchen waste was digested alone at the load of 100 g-
VS/L, whereas in digestion A2, cattle manure was digested alone at
the load of 100 g-VS/L as a control group. Thus, to determine the
performance of co-digestion, the co-digestion of A3, A4 and A5 was
compared with mono-digestion groups of A1 and A2. In addition, to
provide mixing of the digester contents, all digesters were shaken
manually for about 1 min once a day prior to measurement of
biogas volume.
Measurement of biogas yield
Biogas was collected by water displacement method. In order to
prevent the dissolution of biogas in the water, brine solution was
prepared. Following the method suggested by Elijah et al. (2009),
an acidified brine solution was prepared by adding NaCl to water
until a supersaturated solution was formed. Three to five drops of
sulphuric acid were added to acidify the brine solution. As biogas
production commenced in the fermentation chamber, it was
delivered to the second chamber which contained the acidified
brine solution. Since the biogas is insoluble in the solution, a
pressure build-up and provides the driving force for displacement of
the solution. Thus the displaced brine solution was measured to
represent the amount of biogas produced. The biogas volume was
calculated daily and was transformed into the volume at Standard
Temperature and Pressure (STP) condition.
Chemical analysis
The pH, TS and VS of organic kitchen waste and cattle manure
samples were measured according to the standard methods
(APHA, 1998). The pH values of each digester were monitored in
five days interval using digital pH meter (HANNA Model pH-211).
Following the method of Radtke et al. (1998) and Yu and Fang
(2002), the pH values of the contents of digesters were buffered
between 6.8 and 7.4 by the addition of hydrated calcium carbonate.
The VS content of the liquor was subsequently measured. The
values of VS destructions were calculated based on total mass
balances of VS in each digester before and after the digestion test
with subtracting the VS contents of the control digesters from that of
the testing digester.
RESULTS AND DISCUSSION
Pre-digestion characteristics of substrates
Table 2 summarizes the values obtained in the pre-
digestion characteristics of the five feed stocks. As it is
shown, there is a considerable amount of variation in the
composition of feed mixtures, which is due to the
variability in the composition of the samples of the
different substrates taken over the experimental period.
The content in volatile solids of cattle manure and organic
kitchen waste ranged between 9.8-10.8% and 12.4-
13.3%, respectively (average values of 10.3 and 12.9%,
respectively). On a dry matter (TS) basis, organic kitchen
waste contained higher VS than cattle manure. The
higher VS content of organic kitchen waste (13 g/kg),
compared with that of manure (10 g/kg), means relatively
higher energy content, which is desirable from an
economic standpoint with regards to biogas energy
production. The VS/TS ratios were 82 and 93% for cattle
manure and organic kitchen waste, respectively.
Before inoculation the mean pH values of CM and
OKW were 7.19 and 5.51, respectively; however, after
they are inoculated with rumen fluid, the inoculum mean
pH values of the two control groups (A1 and A2) were
446 Int. J. Phys. Sci.
A (100% CM and 0% OKW)
B (75% CM and 25% OKW)
C (50% CM and 50% OKW)
D (25% CM and 75% OKW)
E (0% CM and 100% OKW)
Figure 1. Daily mean biogas yield of digester D in 45 days.
increased. This indicates that the rumen fluid used for
this study have had a good buffering capacity as it was
also reported earlier (Girma et al., 2004; Forster-Carneiro
et al., 2008; Montusiewicz et al., 2008; Uzodinma and
Ofoefule, 2008).
Biogas production rate
On average, biogas productions from digesters A2, A3,
A4, A5, and A1 were detected on the 7th, 6th, 5th, 6th, and
8th days respectively. The results showed that the co-
digestion of samples with the three mix ratios (A4, A5,
and A3) produced biogas earlier than the two pure
substrates (A1 and A2) that were used as control groups.
From the three mix groups, digester A4 produced biogas
much faster, followed by digester A5 and A3. This might
be due to the attribution of the positive synergetic effect
of the co-digestion of CM and OKW in providing more
balanced nutrients, increased buffering capacity, and
decreased effect of toxic compounds. Digestion of more
than one kind of substrate could establish positive
synergism in the digester (Mata-Alvarez et al., 2000; Li et
al., 2009; Jianzheng et al., 2011). The rapid initial biogas
production in digester A4 might be also due to shorter lag
phase growth, the availability of readily biodegradable
organic matter in the substrate, and the presence of high
content of the methanogens.
Biogas production
Biogas production was used mainly as an indication of
optimum production and the development of favorable
conditions for microbial activity during the digestion
process. The daily methane production from the control
and digesters are shown in Figure 1. The average daily
biogas yield observed from the five digesters (A1, A5, A4,
A3, and A2) were 176.77, 237.85, 284.76, 325.63, and
236.18 mL/g-VS, respectively. As compared to digesters
A1 and A2, digesters A5, A4, and A3 produced the 1st,
2nd, and 3rd highest volume of biogas on each day during
the 45 days of experiment, respectively (Figure 1). The
higher biogas production from these mixtures could be
due to the balanced (nutrient to microorganism)
composition, and stable pH which was attained from the
inoculation with rumen fluid and mixing ratios used. On
the other hand low average daily biogas production
observed from digesters A1 and A2 containing pure
100% OKW and 100% CM, attributed to the unbalanced
nutrient to microorganism ratio, and unstable pH value.
After the gas production was started and stabilized,
digesters A4, A5, and A1 produced the least amount of
daily biogas on the 5th, 6th, and 8th, days of the run,
respectively. The observed least gas yield from these
digesters might be due to the production of volatile fatty
acids by the microorganism which hinders the releasing
of the biogas. This is in agreement with the report of
Budiyono et al. (2010) who also observed low level of
Aragaw et al. 447
A (100% CM and 0% OKW)
B (75% CM and 25% OKW)
C (50% CM and 50% OKW)
D (25% CM and 75% OKW)
E (0% CM and 100% OKW)
Figure 2. Mean cumulative biogas yield of all samples within 45 days.
biogas production due to the lag phase of microbial
growth during these periods of the run.
The cumulative biogas productions of the five samples
in all experiments were averaged and the mean
cumulative biogas production and total gas production
were summarized in Figure 2. As compared to the single
anaerobic digestion of the two pure samples, the co-
digestion of the three mix ratios produced higher volume
of biogas. The total gas produced from the co-digestion
of the three mixed samples (A3, A4, and A5) was
indicated in Figure 2. From the co-digestion of A3, A4
and A5; 24.12, 37.91 and 47.13% more biogas was
produced respectively than the two pure samples used as
control. This might be due to mixing of cattle manure with
organic kitchen waste provided balanced nutrients,
buffering capacity, appropriate C/N ratio and sufficient
anaerobic microorganisms. Moreover, the cumulative
biogas yield of sample A5 is greater than sample A4
which is greater than sample A3. This might be attributed
to the increased content of organic kitchen waste from 25
to 50% and to 75% (Amirhossein et al., 2004; Jianzheng
et al., 2011). This result was in accordance with those
obtained with co-digestion of 75% brewery waste and
25% sewage sludge (Babel et al., 2009).
Biodegradation during anaerobic digestion
In order to determine which matter in what amount was
utilized from the initial feed during the 45 days of
retention time and to correlate with the rate and amount
of biogas produced, the digestate from each digester
were characterized (Table 2). It is important to maintain
the pH of an anaerobic digester between 6 and 8;
otherwise, methanogen growth would be seriously
inhibited (Gerardi, 2003). In this study, the initial pH of all
the digesters was in the range of 6.95 to 7.45 even with
the addition of acidic food wastes (like injera) indicating
the buffering capacity of the cattle manure. But finally the
pH showed a significant increase and it was in the range
of 7.66 to 8.47. This was predicted because the VFAs
produced by acidogens during the start up phase were
consumed by methanogens and transferred to the
methane. Generally, pH increase accompanies
increasing biogas production because methanogens
consume VFAs and generate alkalinity. In addition there
occurs a decrease in VS and VS/TS ratio and this might
be due to the biodegradation and conversion of VS into
biogas through the microbial acidogenesis and
methanogenesis. At the beginning of the digestion
process the average total solids (TS) and volatile solids
(VS) content of substrates in all digesters were high
(Table 3). But, at the end of the 45 days anaerobic
digestion period the contents of both TS and VS were
highly reduced and this is attributed to their consumption
by fermenting and methanogenic bacteria.
The efficiency of anaerobic co-digestion of cattle
manure and organic kitchen waste was evaluated in
terms of TS and VS reduction as the amount of dry
matter and organic compounds. Table 4 presents the
amount of TS, and VS biodegradation and conversion
into biogas per mg, TS and VS removed in the anaerobic
co-digestion processes of cattle manure with organic
kitchen waste at an ambient temperature of 30 ± 8°C.
448 Int. J. Phys. Sci.
Table 3. Properties of cattle manure and organic kitchen waste after digestion (mean value ± SD).
Parameters (after digestion)
Mixture
pH
MC (%)
TS (%)
VS (%)
VS/TS (%)
A1
7.66 ± 0.264
94.05 ± 1.067
5.95 ± 1.067
4.71± 0.721
79.16 ± 5.041
A2
8.47 ± 0.173
96.76 ± 0.462
3.24 ± 0.462
0.89 ± 0.307
27.47 ± 6.322
A3
8.27 ± 0.191
96.58 ± 0.486
3.42 ± 0.486
1.28 ± 0.369
37.43 ± 5.824
A4
8.04 ± 0.174
95.96 ± 0.539
4.05 ± 0.539
2.05 ± 0.394
50.62 ± 3.495
A5
7.86 ± 0.236
95.28 ± 0.788
4.72 ± 0.788
2.83 ± 0.568
59.96 ± 4.403
Table 4. Organic matter degradation and biogas yield from each digester.
Treatments
Organic matter composition and its removal
Biogas yield
Total solids (mg/vol)
Volatile solids (mg/vol)
Total
(ml)
ml/mg TS
removed
ml/mg VS
removed
Initial TS
(mg)
Removed
Initial
VS
(mg)
Removed
Mg/Vol.
%/Vol.
Mg/Vol.
%/Vol.
A1
22,160
12,640
57.04
20,560
13,024
63.35
10,628.3
0.84
0.82
A2
20,064
14,880
74.16
16,432
15,008
91.33
7,954.8
0.54
0.53
A3
20,528
15,056
73.34
17,296
15,248
88.16
10,703.3
0.71
0.70
A4
21,072
14,592
69.25
18,144
14,864
81.92
12,814.3
0.88
0.86
A5
21,728
14,176
65.24
19,024
14,496
76.20
14,653.5
1.03
1.01
Biodegradation of TS and VS was high in samples
containing high proportion of CM and decreases as the
proportion of OKW in the mix ratio increases. With gas
production rate of 1.03 ml/mg TS or 1.01 ml/mg VS
removed from the biodegradation of 14,176 mg (65.24%)
of the initial TS, or 14,496 mg (76.20%) of the initial VS,
digester A5 gave the 1st highest cumulative biogas yield
of 14,653.50 ml/g-VS. The result showed that in digester
A4 and A5 there was a direct relationship between total
biogas yield and gas production rate per each milligram
of total solids and volatile solids removed. This might be
because, the digestion process in these two digesters
had more balanced acidogenesis and methanogenesis
and the VS removed were utilized for biogas produce
more efficiently than the other levels. Similar results were
reported by Joung et al. (2008) from the anaerobic co-
digestion of swine manure and food waste.
Digester A2 was observed with the highest percentage
of TS and VS removal; however, it produced the least
cumulative biogas yield of 7,954.75 ml/g-VS. This might
be because of the presence of only cattle manure that is
inoculated with rumen fluid. Since both cattle manure and
rumen fluid are partially digested in the guts of the
ruminants less biogas production from cattle manure
within short retention period can be attributed to its
relatively lower organic content than organic kitchen
waste. Generally, it was observed that the TS and VS
removal rates were affected by the different mixing ratios
of cattle manure with organic kitchen waste and the
hydraulic retention time. This suggests that high
concentration of anaerobic bacteria content in rumen fluid
and cattle manure works effectively to degrade organic
matter composed in organic kitchen waste. So the results
of this study imply that the biodegradability of organic
matter and cumulative biogas yield was improved by co-
digesting cattle manure with organic kitchen waste using
rumen fluid as inoculum.
Co-digestion performance and synergistic effect
The co-digestion of three mix ratios (75:25, 50:50 and
25:75) of rumen fluid inoculated CM with OKW was
performed and biogas productions from the biodegra-
dation of organic matter were compared with pure cattle
manure and organic kitchen waste as the controls. As the
result indicated, the co-digestions of the three mixes
showed improved biogas production rates and achieved
higher cumulative biogas production than the two pure
samples. This higher biogas production from digesters
A3, A4, and A5 with mixed substrates of rumen fluid
inoculated cattle manure and organic kitchen waste was
due to the increased carbon content of OKW and high
concentration of anaerobic bacteria content of cattle
Aragaw et al. 449
Table 5. Synergistic effect of co-digestion of cattle manure and organic kitchen waste.
Treatments
Percentage of
CM /OKW
Cumulative biogas yield
Cattle manure
(ml)
Co-digestion
(ml)
Organic kitchen waste
(ml)
Increase
(ml)
Increase
(%)
A1
0:100
0.00
10,628.25
A2
100:0
7,954.75
0.00
A3
75:25
5,966.06
10,703.25
2,657.06
2,080.13
24.12
A4
50:50
3977.38
12,814.25
5314.13
3522.74
37.91
A5
25:75
1988.69
14,653.5
7971.19
4693.62
47.13
manure and rumen fluid. In other words this might be due
to synergistic effect of CM to OKW (Table 5). The
synergistic effect is mainly attributed to more balanced
nutrients, increased buffering capacity, and decreased
effect of toxic compounds (Li et al., 2009; Danqi, 2010;
Jianzheng et al., 2011). More balanced nutrients in co-
digestion would support microbial growth for efficient
digestion, while increased buffering capacity would help
maintain the stability of the anaerobic digestion system.
As it is shown on Table 5, from the co-digestion of
cattle manure and organic kitchen waste with 75:25,
50:50, and 25:75 mix ratios 24.12, 37.91 and 47.13%
additional biogas production was obtained, respectively
when it is compared with that of the mono-digestions. It is
evident from this result that digestion of more than one
kind of substrate could establish positive synergism in the
digester and provides more balanced nutrients as well as
buffering capacity thus enhance the anaerobic digestion
process and bio-energy production.
Identification of mix ratio for highest biogas
production
As the proportion of OKW in the mix ratio increases from
0 to 25% to 50% and to 75% biogas yield was increased
by 24.12, 37.91 and 47.13%, respectively. Thus, digester
A5 with mix ratio of 25% CM and 75% OKW produced
the highest volume of biogas (Figure 2). This might be
due to the high organic content of OKW coupled with the
supply of suitable microorganisms and missing nutrients
by the rumen fluid and CM make the carbon to nitrogen
ratio within the desired range.
Conclusions
Organic kitchen wastes co-digested with cattle manure
improved the biogas potential compared to cattle manure
alone. The co-digestion of rumen fluid inoculated CM and
OKW with mix ratio of 50:50, gives biogas yield earlier
and highest average daily and cumulative biogas yield
were obtained from the co-digestion of rumen fluid
inoculated CM and OKW with 25:75 ratio. The 25:75,
50:50 and 75:25 mix ratios of CM and OKW gave from
24.12 to 47.13% additional biogas yield and cumulative
gas production was enhanced by 1.01-1.84 times. Thus,
as compared to the mono-digestions of pure CM and
pure OKW anaerobic co-digestion of rumen fluid
inoculated CM and OKW in 25:75, 50:50, and 75:25 mix
ratios enhances both the rate and amount of biogas yield.
ACKNOWLEDGEMENTS
This research was supported by the Ethiopian Ministry of
Education. The authors would like to thank Haramaya
University Staff Lounge and Animal Farming for their
cooperation during sample collection and preparation.
The authors also appreciate the support of the
Department of Biology Lab Technicians specifically Mr.
Samuel Tesfaye and Ms. Yodit.
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