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STUDY ON THE EFFECT OF pH ON BIOGAS PRODUCTION FROM FOOD WASTE
BY ANAEROBIC DIGESTION
Conference Paper · May 2014
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STUDY ON THE EFFECT OF pH ON BIOGAS PRODUCTION FROM FOOD
WASTE BY ANAEROBIC DIGESTION
1 1 2
S. Jayaraj , B. Deepanraj , V. Sivasubramanian
1
Department of Mechanical Engineering, National Institute of Technology, Calicut-673601, India
2
Department of Chemical Engineering, National Institute of Technology, Calicut-673601, India
sjayaraj@nitc.ac.in, babudeepan@gmail.com, siva@nitc.ac.in
ABSTRACT
Irrepressible urbanization has resulted in release of excessive and erratic food waste, which contains
abundant organic matter and can suitably be harnessed to produce biogas by controlled
decomposition with microorganisms. The yield of biogas apparently depends on parameters like solid
content, pH, temperature, C/N ratio, mixing/agitation, retention time, etc. In the present study, the
effect of pH on the biogas yield is experimentally analyzed in five laboratory-scale batch reactors
maintained at pH 5, 6, 7, 8 and 9. The reactors were operated at mesophilic temperature condition
with a hydraulic retention time of 30 days. The food wastes used in this experiment were subjected to
characterization studies before and after digestion. The daily biogas production, cumulative biogas
production, methane and carbon-di-oxide composition were measured. The results show that, biogas
yield and degradation efficiency were substantially higher for the substrate of pH 7 compared to other
pH values. The methane and carbon-di-oxide composition in the biogas produced with pH 7 was
determined as 60.8% (v/v) and 36.3% (v/v), respectively. For the same pH, the cumulative biogas
production for a retention period of 30 days was measured as 5655ml from a 2 liter batch reactor. The
TS, VS and COD removal efficiency of pH 7 were found to be 49.44, 50.91 and 38.93% respectively.
The lowest biogas yield and degradation efficiency was obtained with the substrate of pH 5. The
experimental cumulative biogas produced was compared with theoretical cumulative biogas predicted
from first order rate of decay equation.
Keywords: Anaerobic digestion, food waste, biogas, pH effect.
INTRODUCTION
The increasing demand for energy, hikes in oil prices, depletion of fossil fuel resources and the
increasing concern for environmental issues are confronted the researchers at present to look for new
technological processes to obtain clean and sustainable energy from alternate energy sources
(Chynoweth et al., 2000; Gurung et al., 2012). Rapid population growth and urbanization have led to
an enormous increase of solid waste generation. Under-developed and developing countries have
great challenges concerning appropriate solid waste management to minimize the risk to human
health and pollution problems. These problems of energy and environment could be simultaneously
handled by biogas production from waste. Biogas can be generated from a wide range of solid or
liquid wastes (Zamalloa et al., 2012; Weiland., 2010).
In anaerobic digestion process, microorganisms uses the organic matter available in the waste to
produce a mixture of gases known as biogas (predominantly containing CH4 and CO2), which will
result in the reduction of environmental pollution. The process is brought about by a consortium of
interdependent and symbiotic populations of heterotrophic microorganisms which are capable of
utilizing elements and compounds from a diverse spectrum of substrates for the synthesis of new
cellular materials (Murphy and Keogh., 2004; Ghaly et al., 2000). Anaerobic digestion process can be
described by a four-stage scheme namely hydrolysis, acidogenesis, acetogenesis and
methanogensis (Demirbasa Balat., 2009). First stage is hydrolysis, in which the complex organic
structure of the substrate is broken down into simpler structure. In acidogenesis, the simple organic
compounds formed at the end of first stage are converted into volatile organic acids (acetate,
propionate, butyrate and valeric acid), carbon dioxide and hydrogen. Subsequently, acetate, hydrogen
and carbon dioxide are synthesized from the organic acids during the acetogenesis. In
methanogensis, the products of third stage are converted into biogas which mainly consists of
methane and carbon dioxide as its major composition (Yebo et al., 2011; Chandra et al., 2012;
Geraradi., 2003).
The gas yield in anaerobic digestion process depends on number of operating parameters such as
solid concentration, temperature, pH, C/N ratio, retention time, etc. Many researchers investigated
799
the effects of operating parameters on biogas production and reported their findings. Budiyono (2010)
studied the effect of substrate concentration (2.6, 4.6, 6.2, 7.4, 9.2, 12.3, and 18.4% of total solids) on
biogas production from cattle manure with rumen fluid as inoculum and reported that the substrate
concentration of 9.2% of total solids yielded more biogas (186.28ml/g of VS) followed by 7.4% (184.09
ml/g of VS). Kim et al. (2006) investigated the influence of temperature and hydraulic retention time on
anaerobic digestion using food waste as feed. They reported that the performance of anaerobic
digestion and food waste digestion efficiency increased at 50°C with 12 days hydraulic retention time.
Sivakumar et al. (2012) studied the effect of pH on biogas production from spoiled milk. Experiments
were conducted with substrate of different pH values (5-8) and reported that the substrate with 7 pH
resulted better biogas yield. Achmad et al. (2011) conducted experimental study on the effect of C/N
ratios (20, 25 and 30) on cattle feces and water hyacinth and reported that C/N ratio of 30 produced
highest methane compared to other C/N ratios. Abdel Hadi and Abd El-Azeem (2008) investigated the
effect of mixing, heating, organic total solids and digester type on biogas production with buffalo dung
as a substrate. Experiments were conducted in a 22 liter bench scale batch anaerobic digester
(horizontal and vertical types) and reported that the amount of biogas production not only depends on
the type of digester but also depends on other parameters like mixing, heating, organic total solids
and C/N ratio of the substrate.
In the present study, the effect of pH (5, 6, 7, 8 and 9) on biogas production was investigated
experimentally from the food waste through anaerobic digestion process using lab bench- scale batch
system. The experiment was carried out for a retention period of 30 days.
MATERIALS AND METHODS
Feedstock
Food waste used in this experimental study was collected from a hostel mess of National Institute
of Technology Calicut, Kerala, India. The food wastes obtained were shredded, mixed and stored
at 5°C until it is introduced in the anaerobic digester. The solid concentration taken in all the
reactors were 7.5% total solids, found as optimum value from the previous experiments (Deepanraj
et al., 2014). Substrates with 5 different pH (5, 6, 7, 8 and 9) were prepared with the help of 1 N
sodium bicarbonate solution. The characteristics of the substrate used were determined before and
after digestion. For the entire experiments, cow dung was used as inoculum (10% inoculum to feed
ratio).
Experimental setup
Laboratory-scale anaerobic batch digesters made of glass vessel with total volume of 2 liter and
working volume of 1.6 liter was used in this experiment. The digesters were operated at ambient
(room) temperature in the mesophilic range with a hydraulic retention time of 30 days. Biogas
production from the digesters was measured daily by water displacement method. The volume of
water displaced from the flask was equivalent to the volume of gas generated. The reactor was
stirred using magnetic stirrer two times in a day.
Analytical methods
Characterization of feedstock is one of the most significant steps in the biogas production process.
Determining the general composition of the substrate (input feed) is essential for calculating the
quantity and composition of the biogas generated. The total solids (TS), volatile solids (VS), fixed
solids (FS) and chemical oxygen demand (COD) of the substrate and digestate were determined
as per the standard method (APHA., 1989). pH of the substrate and digestate was determined
using pH meter (pH-201, Lutron Electronic Enterprise, Taiwan). The methane and carbon-di-oxide
composition in the biogas were measured using Infrared gas analyzers (PIR-89, Technovation
Analytical Instruments, India).
Kinetic Study
The degradation of substrate in each reactor was assumed to follow a first order rate of decay. Thus,
the cumulative biogas production was assumed to follow the equation (Gunaseelan. 2004):
-kt
B = B0(1-e )
Where B is the cumulative biogas production at digestion time ‘t’ days. B0 is the biogas potential of the
substrate. k is the first order disintegration rate constant (biogas production rate constant). A nonlinear
least-square regression analysis was performed using Polymath 6.0 to determine B0, k, and the
2
predicted biogas yield. The coefficient of determination (R ) was also obtained from the analysis with
95% confidence level.
800
RESULTS AND DISCUSSION
The characteristics of substrate and digestate before and after digestion process are given in Table 1
and 2. The pH of the substrate was adjusted to the required value (5-9) by adding 1 N sodium
bicarbonate solution. Figures 1 and 2 show the daily and cumulative biogas production over 30 days
retention period for the substrate with different pH respectively. Figure 1 show that the biogas gas
production was higher during initial days, and decreasing gradually as the days passes. The
th
maximum gas yield of 473 ml was obtained for pH 7 on the 6 day, followed by 430ml for pH 8 on the
th
8 day. Compared to pH 7 and 8, pH 5, 6 and 9 produced lower biogas production and degradation
efficiency. The results show that pH of the substrate has a significant effect on biogas production,
because it affects the activity of bacteria to destroy organic matter into biogas. A low pH in the digester
inhibits the activity of microorganisms involved in the digestion process particularly methanogenic
bacteria. Figure 3 shows the maximum biogas yield with respect to pH of the substrate. The
maximum cumulative biogas yield obtained by the reactor with pH 5, 6, 7, 8 and 9 are 4594, 5021,
5673, 5347 and 4889 ml. This shows that pH 7 resulted in higher biogas production followed by 8, 7, 9
and 6. Similar trend was observed by Budiyono et al. (2013) who has studied the effect of pH on
biogas production from food waste. The methane composition in the biogas produced with pH 5, 6,
7, 8 and 9 were determined to be 56.7, 58.6, 60.8, 60.1 and 59.4% (v/v) respectively.
Table 1. Characteristics of substrate
TS (g/l) VS (g/l) FS (g/l) COD (g/l)
75 71.34 3.66 69.39
Table 2. Characteristics of digestate
pH TS (g/l) VS (g/l) FS (g/l) COD (g/l)
5 44.86 39.50 5.36 48.84
6 42.56 37.54 5.02 46.55
7 37.92 35.02 2.90 42.70
8 40.78 36.77 4.01 45.20
9 43.53 38.36 5.17 47.43
Fig.1. Daily biogas production
801
Fig.2. Cumulative biogas production
Fig.3. Maximum biogas production vs. pH of the substrate
Figure 4 shows the relationship between the experimental and predicted cumulative biogas
production with respect to retention time. The estimated kinetic parameters based on the first order
rate of decay were given in Table 3. All parameters were predicted from nonlinear least-square
regression analysis performed using Polymath 6.0 software with a confidence level of 95%. The
predicted maximum cumulative biogas production (B) for pH 5, 6, 7, 8 and 9 were 4765, 5250,
5948, 5603 and 5151 respectively. Maximum biogas production potential B0) predicted for pH 5, 6,
7, 8 and 9, were 5604, 6046, 6707, 6420 and 6116 respectively. The difference between the
experimental and predicted cumulative biogas production after 30 days for pH 5, 6, 7, 8 and 9 were
171, 229, 273, 256 and 263 ml respectively. For all the pH values, the predicted biogas production
is little bit higher than experimental value. The first order rate constant (k) and coefficient of
2
determination (R ) estimated from the analysis are given in Table 3.
Table 3. Results of kinetic study using first-order kinetic model
Cumulative Cumulative
biogas yield after biogas yield after Biogas Coefficient of
Rate
pH 30 days- 30 days- Potential determination
constant (k) 2
Experimental Predicted (ml) (R )
(ml) (ml)
5 4594 4765 5604 0.0633 0.9861
6 5021 5250 6046 0.0675 0.9852
7 5675 5948 6707 0.0726 0.9794
8 5347 5603 6420 0.0687 0.9790
9 4889 5152 6116 0.0615 0.9781
802
Fig.4. Comparison of cumulative biogas production
The VS, TS and COD removal efficiencies with respect to different pH is shown in Fig. 5. The TS
removal efficiency of the reactor with pH 5, 6, 7, 8 and 9 were 40.18, 43.25, 49.44, 45.62 and 41.96
respectively. The VS removal efficiency with pH 5, 6, 7, 8 and 9 were 44.63, 47.37, 50.91, 48.45 and
46.22% respectively. Similarly COD removal efficiency with pH 5, 6, 7, 8 and 9 were 30.14, 33.42,
38.93, 35.35 and 32.16 respectively. For all the cases, pH 7 achieved better degradation efficiency
followed by 8, 6, 9 and 5. Compared to TS and VS removal efficiencies, the COD removal efficiency
was low for all the cases because of the solid content available in the substrate.
Fig.5. TS, VS and COD removal efficiency
CONCLUSIONS
In the anaerobic digestion process, pH is a very important parameter. This study investigated the
effect of different pH (5, 6, 7, 8 and 9) on biogas production from food waste in an anaerobic batch
reactor with a retention time of 30 days. The results showed that pH 7 made favorable condition for
bacterial growth in the digester and produced better biogas yield compared to the others. Next to pH
7, pH 8 yielded better result followed by 6, 9 and 5. The degradation of TS, VS, and COD further
support and strengthen the reported results.
NOMENCLATURE
TS Total Solids
VS Volatile Solids
FS Fixed Solids
COD Chemical Oxygen Demand
DBP Daily Biogas Production
CBP Cumulative Biogas Production
803
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