




































  
African Journal of Pig Farming Vol. 1 (2), pp. 023-027, October, 2013. Available online at 
www.internationalscholarsjournals.org © International Scholars Journals 
 
 
 

 

Full Length Research Paper 
 

Biodigestion of cassava peels blended with pig dung 
for methane generation 

 
Oparaku, N. F., Ofomatah, A. C* and Okoroigwe, E. C. 

 
National Center for Energy Research and Development, University of Nigeria, Nsukka, Enugu State, Nigeria. 

 
Accepted 16 September, 2013 

 
Biogas production from cassava (Manihot esculentus) peels and pig dung under a mesophilic 
temperature condition was investigated. Three blends of the wastes and a control labeled as B1, B2, B3 
and C representing blend 1 (50:50 peel/dung), blend 2 (30:70 peel/dung), blend 3 (10:90 peel/dung) and 
control (pig dung alone) were used, respectively. Biodigestion of the wastes blends and control was 
carried out simultaneously under the same environmental and operational conditions of 30 days 
retention period using four metallic biodigesters of 32 L capacity each. The biogas yield result shows 
that blend 2 yielded the highest cumulative biogas of 78.5 L, while the least yield of 61.7 L was obtained 
by blend 3. When compared with the control set up and biodigestion of cassava waste alone from 
literature, there was blending effect resulting in increase in yield of biogas over the sole digestion of 

cassava peel or pig dung. Methane production leading to the combustibility of the biogas started at 6
th

, 

5
th

, 5
th

 and 4
th

 days for B1, B2, B3 and C, respectively. This, in agreement with earlier studies show that 
better handling of cassava peels for energy production would be achieved by blending it with animal 
wastes in the right proportion. 

 
Key words: Cassava peel, biogas, co-digestion, anaerobic digestion, wastes blends, lag days. 

 
 
INTRODUCTION 
 
One of the main environmental problems of today’s 
society is the continuous increase in production of 
organic wastes which are harmful to human existence. In 
many developing countries, sustainable waste manage-
ment and reduction have become major issue due to lack 
of adequate technology and methodology to handle 
wastes generated from daily living activities. Some of the 
waste types which are posing serious environmental 
threats to human and animal existence in these nations 
come from agriculture due to their degradable nature and 
lack of profitable technique to convert these ‘wastes’ to 
better manure quality or other useful means such as 
energy. With increase in farm (agricultural) operations, 
greater waste production is proposed.  

Cassava is one of the major root crops produced in sub-
Saharan Africa. World cassava production in 2002 was 

 
 
 

 
estimated at 184 million tonnes (Odoemenem and 
Otanwa, 2011). As at 2002, Africa exported only one ton 
of cassava annually (FAO, 2001) but by 2007, out of  
“more than 228 million tonnes of cassava produced 
worldwide, Africa accounted for 52% and Nigeria 
produced 46 million tons making it the world's largest 
producer of cassava” (IITA -1). It has been projected that 
total world cassava utilization would hit 275 million tons 
by 2020 (IFPRI, 2008; Arowolo and Adaja, 2012) while 
some researchers estimate this number closer to 291 
million tons.  

Currently, there is increase in campaign for enlarging 
the cassava production scale in Nigeria. The implication 
of this is increased waste production from cassava pro-
cessing. According to FAO (2001), about 250 to 300 kg of 
cassava peels is produced per tonne of fresh cassava 

 
                                *Corresponding author. E-mail: ofomatony@yahoo.co.uk. Tel: +2348034010807.  



Oparaku  et al.         023 
 
 

 
root processed. This suggests huge sum of waste 
production in form of peels from cassava production and 
processing. Hence, there is need to design and adopt a 
system capable of handling the huge waste accruing from 
this development and anticipated problems such as 
unpleasant odour production.  

One of the ways by which cassava peels can be 
managed in addition to using it as animal diet (Okeudo 
and Adegbola, 1993) is by anaerobic digestion for 
methane and bio-fertilizer production. From the literature, 
it is obvious that attempts have been made to convert 
cassava peels to energy. Adeyanju (2008) demonstrated 
the effect of adding wood ash to the biodigestion of 
mixture of piggery wastes and cassava peels in a 
laboratory scale. It was found that the wood ash addition 
increased the biogas production of either the biodigestion 
of piggery wastes and cassava peels only or in 
combination of both wastes in different proportions. The 
period it takes to produce methane was not stated 
probably because it was only a laboratory set up. 
Ofoefule and Uzodimma (2009) compared the biogas 
production potential of cassava peels alone to the blends 
of the waste with animal wastes. It was found that 
cassava peels alone produced a total of 68.70 L of biogas 
which could burn after 58 days of digestion. When 
blended with cow dung, poultry droppings and pig dung, 
the volume of biogas produced increased to 146.5, 
166.50, and 169.60 L, respectively, while the flammable 
gas was produced after 9 days in the first 2 and 11 days 
in the later. These imply that anaerobic digestion of 
cassava peels alone is by no means economical. 
However, in this work, the blending with animal waste 
was on equal ratio. The effect of different mix ratios in 
biogas production potential and flammability of the biogas 
is a gap to be filled up by further investigation. Ezekoye 
and Ezekoye (2009) combined cassava peels with rice 
husk in the ratio of 1:5 for biodegradation of the wastes. It 
is also gathered that this set up was inoculated with cow 
dung mixed with water. From the result, flammable 
biogas was produced after 30 days with a total of volume 

of 3.450 m
3
. It is not certain which of the substrates is 

responsible for the long retention period of 70 days and 
large volume of biogas recorded. However, it is obvious 
that combination of two plant biomass is not a favourable 
combination in anaerobic digestion since animal protein is 
important for microbial activity of the methanogens.  

Adelekan and Bamgboye (2009) have done a more 
detailed investigation from the existing works. The biogas 
productivity of cassava peels mixed in different ratios with 
fixed amount of animal wastes was investigated. In this 
work, 1, 2, 3 and 4 parts of cassava peels were mixed 
with one part each of poultry, piggery and cattle wastes. It 
was found that there was statistically significant effect of 
the mix ratios over biogas production. In this work also, it 
was found that the equal mass combination in all the 
animal waste types produced the highest biogas by 
volume when compared with other ratios. The effect of 

 

  
 
 

 
mix ratios on how long it takes to produce flammable 
biogas was not covered.  

Combination of cassava waste with other biomass 
species of agricultural origin has continued to attract the 
interest of scholars. In addition to the above, Ilaboya et al. 
(2010) blended cassava peels with pineapple and 
plantain peels in a laboratory set up. One of the aims of 
their work was to monitor the effect of alkaline addition to 
the substrate in biogas generation potential of the 
mixture. It was observed that addition of NaOH (alkaline) 
solution resulted in increase in biogas production over no 
alkaline addition. Also, there was positive effect in 
increase of biogas generation by different ratios of 
alkaline mixture. However, this work did not also address 
the effect of mixing the cassava peels in different ratios of 
other wastes.  

In order to fill some gaps existing in previous co-
digestion of cassava peel with other waste types, there is 
need to study the effect of varying the mix ratio of 
cassava peels vis-a-vis mix ratio of other wastes. In this 
work, quantity of cassava peel is varied with varying 
quantity of piggery dung in a batch anaerobic digestion. It 
is aimed at determining the mix combination that will yield 
the largest volume of biogas and the number of days it 
would take to produce flammable biogas while working 
with a particular waste. However, varying cassava peels 
in blends of multiple animal waste types can be 
investigated in another work.  

Biodigestion of agricultural wastes is important as a 
way of maximizing the output of agricultural practice of 
developing nations. Multiple products (benefits) can be 
derived. This includes energy and biofertilizer production. 
It has been found that biodigestion of agricultural wastes 
produces better manure than using the wastes as raw 
manures in farm practice (Okoroigwe, 2007; Okoroigwe 
et al., 2008). 

 
MATERIALS AND METHODS 
 
The raw materials (cassava peels and the piggery waste) were 
obtained within University of Nigeria, Nsukka in Enugu State of 
Nigeria. Prior to the biodigestion of the wastes, the cassava peels 
were partially fermented by soaking in water for 7 days in order to 
reduce the acid content of cassava peels which could be 
detrimental to the microbes. Cyanide is an inhibitor to microbial 
activities (Cuzin and Labat, 1992; Cuzin et al., 1992). Standard 
methods were used to determine the proximate (moisture, ash) and 
chemical analyses of the samples prior to digestion. The cassava 
peels and the piggery dung were mixed in the ratio of 50% by 
weight of cassava peels to 50% by weight of piggery dung. This 
serves as blend 1 (B1). The second and third blends were achieved 
by 20% reduction and 20% increase in weight of cassava peel and 
piggery dung, respectively, in preceding blend. Hence, B2 and B3 
were 30:70 and 10:90 (cassava peels: piggery dung), respectively. 
A control set up contained only piggery dung mixed with water at 
equal weight. All the samples were added with equal weight of 
water to the combined weights of the waste types. This is required 
to achieve 5 to 10% total solids (TS) concentration. The set up was 
monitored for daily gas production by measuring the volume of gas 
using downward displacement method. In order to achieve 



024          Afr. J. Pig Farming 
 
 

 
Table 1. Proximate composition and 
physicochemical properties of the various wastes. 

 

Parameter 
 Composition (%)  

 

B1 B2 B3 C  

 
 

Crude protein 2.89 3.23 3.57 3.74 
 

Crude fat 1.17 0.87 0.57 0.42 
 

Crude fibre 1.17 0.87 15.05 0.42 
 

Ash 18.45 18.79 19.13 19.30 
 

Moisture 26.65 21.79 16.93 14.50 
 

Carbohydrate 37.69 41.22 44.75 46.52 
 

Total solids 63.43 67.86 72.29 74.50 
 

Volatile solids 35.00 35.07 35.15 35.19 
 

Carbon 22.72 21.22 19.71 18.96 
 

Nitrogen 0.46 0.52 0.87 0.60 
 

C:N 49 41 23 32 
 

 

 
homogeneity of the slurry and discourage scum formation in the 
system, daily stirring was carried out using the inbuilt stirring 
mechanism in the digesters.  

Both slurry and ambient temperatures were measured at morning 
and afternoon hours. The average of both readings for any day 
becomes the reported slurry or ambient. 
 
 
RESULTS AND DISCUSSION 
 
The physico-chemical parameters of the waste streams 
are presented in Table 1. It shows that these wastes are 
good bioresources for methane generation. These values 
are close to values obtained by other researchers. The 
values however differ slightly because different waste 
combinations will give different composition of nutrients. It 
has been shown that biogas yield from AD of wastes 
depends on a number of factors such as pH, HRT and 
CN ratio (Yadika et al., 2004). CN ratio is an important 
indicator for controlling biological treatment systems 
(Wang et al., 2012). It has been pointed out that high C/N 
ratio indicates rapid nitrogen consumption by 
methanogens and leads to lower gas production while 
low C/N ratio results in ammonia accumulation and an 
increase in pH values, which is toxic to methanogenic 
bacteria (Zhang et al., 2013). This is partly reflected in the 
biogas yield of the blends as B3 with the lowest C/N ratio 
yield the least gas among the blends and the B1 with 
largest C/N ratio yields moderate biogas as compared to 
B2 according to the explanation above but Yadika et al. 
(2004), pointed out that during anaerobic digestion, 
microorganisms utilize carbon 25 to 30 times faster than 
nitrogen. Thus, to meet this requirement, microbes need 
a 20 to 30:1 ratio of C to N. This may justify the values 
presented in Table 1 as good for biogas generation. 
 

The pH of the blend substrates and the control are 
presented in Figure 1. The result shows that the range of 

the pH values was 5 to 7.3 from the 3
rd

 day to the 30
th

 

 
 

 
day. It shows that the B1 and C had initial acidic 
condition. This agrees with Zhang et al. (2013) 
explanation on relationship between high C/N ratio and 
toxicity of the reacting medium. Similarly, the pH values 
of other blends were responsible for the high biogas yield 
experienced by B2 and B3. Figure 1 also shows that the 
reactions in all the blends and control have initial acid  
condition which neutralized as the reactions progressed 
to the 30

th
 day.  

Table 2 presents the cumulative biogas yield of the 
different blends and the control, while Figure 1 shows 
daily biogas production pattern of the waste blends and 
the control. From the result, blend 2 (B2) yielded the 
largest quantity of biogas (78.5L) followed by B1 (73.5L) 

while the least yield of 61.7L was obtained from the 3
rd

 

blend (B3). When compared with the control set up of 
biodigestion of piggery dung alone, there was blending 
effect leading to the increase in biogas yield. This shows 
that anaerobic digestion of piggery waste can be 
enhanced by combination with other wastes such as plant 
biomass. The result is different if combustibility of the 
generated gas is of interest. Biogas is not useful if it 
cannot be burnt in a combustor for energy and power 
production. Even though, the lag day (period to produce 
flammable gas) is shortest in the control experiment, it is 
necessary to blend it with other wastes as there is not 
much difference in the lag day of B2 which produced the 
largest volume of biogas. The low volume of biogas in 
biodigestion of pig dung alone is due to high 

concentrations of NH4-N which inhibits the process of 

degradation of organic matter, causing a decrease in 
volume of biogas produced.  

When compared with literature value of anaerobic 
digestion (AD) of cassava peels only, the least blending 
yield of 61.7 L (B3) obtained in this work was close to 
68.70 L obtained by Ofoefule and Uzodimma (2009). The 
higher yield however, could be because of larger volume 
of biodigester and pretreatment used by them. But it has 
been pointed out that cassava peels alone is a poor 
feedstock for AD due to its high acid content (Cuzin and 
Labat, 1992; Cuzin et al., 1992). The results obtained 
here conform with other reports from other scholars 
(Adeyanju, 2008; Adelekan and Bamgboye, 2009) 
showing blending effect. This result has shown that 
blending the cassava peel with animal wastes should be 
better done at co-variation of both waste types to obtain 
an optimum mix ratio that will yield larger volume of gas.  

The effect of microbial load (Table 3) can be observed 
in the pH stabilization (Figure 2) as the reactions 

progressed from 6
th

 to 30
th

 day. The multiplication of 

microbes in the system from 6
th

 to 21
st

 day is responsible 
for acid neutralization as there was progressive con-
sumption of nutrients in the substrates. This was also 
responsible for the increase in the biogas yield in all the 
blends as microbial count increased. The peak pro-
duction of biogas (Figure 1) is observed around the 21st 
day in blend 2 due to peak microbial count about that 



Oparaku  et al.         025 
 

 
 
 8         

 

 7         
 

 6         
 

ga
s 

(L
)o

fg
as

(L
) 

5        
B1  

        
 

4 
       B2 

 

         

Vo
l o

fV
ol

um
e        

B3  

        
 

3 
       C 

 

        
 

         
 

 2         
 

 1         
 

 0         
 

 0 5 10 15 20 25 30 35 40 
  

Time (days) 
 
Figure 1. Daily biogas production pattern in the wastes. 
 
 
 

Table 2. Cumulative biogas yield pattern of the wastes. 
 

 
Parameter 

 Value  
 

 

B1 B2 B3 C  

  
 

 Lag days 6 5 5 4 
 

 Cum biogas yield (L/gS) 73.50 78.5 61.7 55.1 
 

 Mean biogas yield 2.45 2.62 2.05 1.83 
 

 
 
 

Table 3. Microbial load of the different waste blends. 
 

 
Day 

 Microbial count (x10
4
)  

 

 

B1 B2 B3 C  

  
 

 1 2.0 2.2 1.9 1.9 
 

 6 2.2 2.3 2.1 2.0 
 

 11 2.3 2.5 2.3 2.2 
 

 16 3.8 3.9 3.2 3.0 
 

 21 2.0 22.0 1.3 12.0 
 

 26 3.2 3.3 2.2 2.0 
 

 30 2.5 1.7 1.9 1.9 
 

 
 

 
time (Table 3). The microbial load began to reduce from 

the 21
st

 day until the 30 
th

 day due to extensive 
consumption of nutrients in all the blends as well as in the 
control.  

Figure 3 shows the temperature distribution pattern in 
both waste slurries and the ambient. The temperature 

 
 

 
values were with 25 to 35°C in all cases within the 30 day 
test period. This is a mesophilic temperature range which 
enabled the micro-organisms to thrive favourably in the 
system for maximum performance. It therefore means 
that the volumetric result of biogas production is at its 
utmost yield condition. 

  



        
 

  8        
 

  7        
 

  6        
 

  5        
 

 sc a
le

 

       B1 
 

 
4 

      B2 
 

 p H
 

      

B3  

        
 

          

         C 
 

  3        
 

  2        
 

  1        
 

  0        
 

  0 5 10 15 20 25 30 35 
  

Time (days) 
 

Figure 2. pH variation in the wastes. 
 
 
 

 40         
 

 35         
 

 30         
 

Te
m

pt
 (o

C
)T

em
pe

ra
ur

e°
C

) 

25        B1 
 

        B2 
 

20        B3 
 

         

        C 
 

15 
       Ambient 

 

        
 

          

 10         
 

 5         
 

 0         
 

 0 5 10 15 20 25 30 35 40 
 

 
Time (days) 

 
Figure 3. Average slurry and ambient temperature variation. 

026          Afr. J. Pig Farming 



Oparaku  et al.         027 
 
 

 
Conclusion 
 
In this work, co-biodigestion of cassava peels with 
piggery waste was carried out in varying quantities of 
both wastes; there was blending effect over single 
biodigestion either of the plant and animal wastes. The 
B2 blend showing the combination of 30% by weight of 
plant waste (cassava peels) with 70% by weight of animal 
waste (piggery dung), yielded the largest volume of 
biogas on cumulative basis. Flammable biogas (methane 

rich biogas) was produced on the 5
th

 day even though 
biodigestion of piggery waste alone produced flammable 

biogas on the 4
th

 day. The blending improved the 
methane production of cassava peels alone from about 
59 days (literature value) to five days in combination with 
piggery dung. 
 
 
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