Abstract: The main focus of this study was mesophilic anaerobic co-digestion of cow dung, chicken droppings and grass clippings using pilot bio-digesters. The biochemical methane potential (BMP) works under batch anaerobic digester operating in ambient mesophilic temperature of 35 oC and 37 0C and pH of 7 to generate biogas. The carbon/nitrogen (C/N) ratio for cow dung and chicken droppings was found to be 17.70 and 63.67 respectively and grass clippings to be 20.54. Through co-digestion in a ratio of 1:1, the C/N ratio for cow dung and grass clippings settled at 19.19 while that for chicken droppings and grass clippings settled at 20.49. The conversion rate of the reaction and biogas production increased with the increase in temperature and hydraulic retention time until an equilibrium state was achieved. At the temperature 37 OC, it was observed to be the suitable mesophilic temperature for anaerobic digestion due to high dissociation and collision leading to high rate of biogas production

Belaid, Mohamed

Matheri, Anthony Njuguna

Mbohwa, Charles

Ngila, Catherine Jane

Seodigeng, Tumisang

English

Name not available

  
Abstract— The main focus of this study was mesophilic 
anaerobic co-digestion of cow dung, chicken droppings and 
grass clippings using pilot bio-digesters.  The biochemical 
methane potential (BMP) works under batch anaerobic 
digester operating in ambient mesophilic temperature of 35 
oC and 37 0C and pH of 7 to generate biogas. The 
carbon/nitrogen (C/N) ratio for cow dung and chicken 
droppings was found to be 17.70 and 63.67 respectively and 
grass clippings to be 20.54. Through co-digestion in a ratio of 
1:1, the C/N ratio for cow dung and grass clippings settled at 
19.19 while that for chicken droppings and grass clippings 
settled at 20.49. The conversion rate of the reaction and 
biogas production increased with the increase in temperature 
and hydraulic retention time until an equilibrium state was 
achieved. At the temperature 37 OC, it was observed to be the 
suitable mesophilic temperature for anaerobic digestion due 
to high dissociation and collision leading to high rate of biogas 
production. 
Keywords— Anaerobic digestion, Co-digestion, Mesophilic 
Temperature 
I. INTRODUCTION 
HE energy consumption worldwide is spontaneously 
increasing due to industrialization, population growth, 
and state of development. The need for an alternative 
 
Manuscript received June 7, 2016; revised June 9, 2016. This work was 
supported in part by the University of Johannesburg, Process Energy 
Environmental station (PEETS) and City of Johannesburg, South Africa. 
Anthony Njuguna Matheri; is with Department of Chemical 
Engineering; University of Johannesburg; Johannesburg 2028 South Africa; 
Cell: +27616986335 (tonynjuguna22@gmail.com) 
Mohamed Belaid; Senior lecturer and HOD at Department of Chemical 
Engineering; University of Johannesburg; Doornfortein, Johannesburg 
2028 South Africa; Tel: +27115596402 (mbelaid@uj.ac.za) 
Tumisang Seodigeng; Senior lecturer at Department of Chemical 
Engineering; Vaal University of Technology; Private Bag X021-
Vanderbijilpark-1911, Andries Potgieter Blvd South Africa; Tel: 
+27169509734 (tumisangs@vut.ac.za) 
Catherine Jane Ngila; Professor and HOD at Department of Analytical 
Chemistry; University of Johannesburg; P.O. Box 17011, Doornfortein, 
Johannesburg 2028 South Africa; Tel: 27115596169 (jcngila@uj.ac.za) 
Charles Mbohwa; Professor and Vice Dean at Faculty of Engineering 
and Build Environment (FEBE); University of Johannesburg; Johannesburg 
2028 South Africa; Tel: +2711 559 6165  (cmbohwa@uj.ac.za) 
 
 
 
source of energy for centralized and decentralized power 
generation has led to researchers looking for alternative 
source of energy [1].     
With the fast depletion of non-renewable energy sources 
such as fossil fuel and coal which has led to human health 
problems and global climate change and environmental 
degradation, the commercial production of biogas and other 
alternative energy source such as solar energy, hydropower, 
wind energy, bioenergy, geothermal will definitely give a 
drive for the development of the economy [2, 3]. Fig. 1 
shows renewable energy conversion technologies.  Energy 
derived from biogas is used in the form of fuel, heat, and 
electricity. It is desirable to create sustainable and with zero 
carbon emissions worldwide energy system [4, 5]. This 
results in resource conservation and environmental 
protection [5, 6]. 
 
Radiation energy Photolyse Thermal energy 
heat 
Solar cell
Therm collector
Solar energy, Therm Power plant
radiation
Biomass Production Convertion plant Chemical energy
Warming of surface Sea heat power plant
 and atmosphere
Heat pump
Transpiration
 and precipitation Hydo power plant
melting Glacier ice power plant
wind Wind energy converter Mechanical energy 
mobility
Geothermal heat, Geotherm, heat plant
radioctive 
disintegration Geotherm, Power plant
Electrical energy 
Planet Gravitation Tides Tides power plant Electricity
 
Fig I: Renewable energy conversion technologies  
Biogas is produced by anaerobic decomposition process 
called Anaerobic Digestion (AD). It is the biological 
breakdown of organic matters in the absence of oxygen. The 
main product of biogas is methane and carbon dioxide [7, 
8]. 
Biogas production follows four fundamentals processes. 
These processes include hydrolysis, acidogenesis, 
Mesophilic Anaerobic Co-digestion of Cow 
dung, Chicken Droppings and Grass Clippings 
Anthony Njuguna Matheri1, Member, IAENG, Mohamed Belaid2, Member, IAENG, Tumisang 
Seodigeng3, Catherine Jane Ngila4, Charles Mbohwa5 
T
 acetogenesis and methanogenesis [9]. Fig. 2 shows a 
simplified generic anaerobic digestion process [10]. 
 
Inert particulate
Carbohydrates Proteins Lipids Inert soluble
Facultative anaerobic 
Sugars Amino Acids LCFC bacteria
Acidogenic bacteria
Propionate, 
Butyrate, 
Valerate (Alcohol, Lactate)
Acetate oxidizing bacteria Acetogenic bacteria
Acetate H2, CO2
Homoacetogenic bacteria
Methanogenic bacteria
CH4, CO2
Biomass
Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis
Disintegration
 
Fig 2: Degradation steps of the anaerobic digestion process. 
Hydrolysis is theoretically the first step of anaerobic 
digestion, during which the complex organic matter 
(Polymers) are decomposed into smaller units (mono- and 
oligomers). During hydrolysis long-chain molecules, such 
as carbohydrate, protein, and fat polymers, are broken down 
to monomers (small molecules). Different specialized 
microbial produce a number of specific enzymes which 
catalyze the decomposition, and the process is extracellular. 
During hydrolysis, polymers like carbohydrates, proteins, 
lipids and nucleic acids are converted into glucose, glycerol, 
purines, and pyridines [11]. In the acidogenesis process, the 
acidogenic bacteria transform the products of the hydrolysis 
into short chain volatile acids, alcohol, ketones, carbon 
dioxide and hydrogen. Some of the major acidogenesis 
stage products are acetic acid, propionic acid, formic acid, 
butyric acid, lactic acid, ethanol, and methanol. From these 
products, the carbon dioxide, hydrogen, and acetic acid will 
skip the third stage, acetogenesis, and be utilized directly by 
the methanogenic bacteria in the final stage [12, 13]. The 
third stage is known as the acetogenesis stage, the rest of the 
acidogenesis products, i.e. the butyric acid, propionic acid 
and alcohols are transformed by acetogenic bacteria into 
hydrogen, carbon dioxide, and acetic acid. Hydrogen plays 
an important role in this process, as the reaction will only 
occur if the hydrogen partial pressure is low enough to 
thermodynamically allow the conversion of all compounds 
of acids. Such lowering of the partial pressure is carried out 
by hydrogen scavenging bacteria thus, the hydrogen 
concentration of a digester is an indicator of its health [14]. 
Methanogenesis is the final stage of the biogas production. 
The production of methane and carbon dioxide from 
intermediate products is carried out by Methanogenic 
bacteria. 70% of the formed methane during AD originates 
from acetate, while the remaining 30% is produced from the 
conversion of carbon dioxide (CO2) and hydrogen (H2) [11]. 
The methanogens bacteria have the slowest growth rate 
involved in the process; they also the limiting factor for how 
quickly the process can proceed and how much material can 
be digested in AD. The growth rate of the methanogens is 
only around one fifth of the acid-forming bacteria [15]. 
The activity of biogas production depends on various 
parameters like temperature, partial pressure, pH, hydraulic 
retention time, C/N ratio, pre-treatment of feedstock, trace 
of metals and concentration of substrate [5, 16-18]. Table I 
shows different thermal stages and typical hydraulic 
retention times for the AD process. 
 
TABLE I:   
THERMAL STAGES AND TYPICAL HYDRAULIC RETENTION 
TIMES [19]. 
 
Thermal stages  Process temperature (0C) HRT(days) 
Psychrophilic  <20 From 70-80 
Mesophilic From 30-42 From 14-40 
Thermophilic From 43-55 From 14-20 
 
Biogas can be produced from co-digestion of various 
substrates. In the present study, anaerobic digestion of co-
digestion of cow dung/chicken droppings and grass 
clippings were studied in laboratory experiments in a 10 
liters digester under a constant temperature of 35 oC and 37 
0C.  
II. METHODOLOGY 
 
A. Substrate Collection 
Cow dung and Chicken droppings were collected at the 
farm in Gauteng province while grass clippings were 
collected from the University of Johannesburg. Waste 
characterization was done to ascertain the composition. 
These included physical and chemical composition with 
regards to C/N ratio, total solids, volatile solids and 
elemental analysis for Carbon, Nitrogen, Sulphur and 
Hydrogen in accordance with the standard method (APHA 
1995) [20]. 
To determine biogas production rate, a batch digester was 
fed with the co-digested substrates and inoculum under pre-
set conditions of 37 0C and pH of 7 as shown in Fig. 3. pH 
was neutralized by a solution of 8g NaOH in 100 ml and 
H2SO4. The digester was flushed with Nitrogen gas to expel 
the oxygen and create an anaerobic process. It was then 
immersed in the water bath and kept under constant 
temperature. The liquid sample collected was analyzed 
using a spectrophotometer. The gas produced was measured 
using downwards displacement method on a daily basis 
until the end of retention time. The gas collected was 
sampled by extracting it from the digester using a gas 
syringe and was analyzed using gas chromatography 
instrument with flame- ionization detectors. The operating 
conditions were; helium was used as carrier gas at 20 
ml/min, oven temperature 70 0C, detector temperature 150 
0C and injector port at 80 0C. 
 
  
Fig 3: Biogas production setup. 
Where: 1 – Thermostatic water bath, 2 - T-Glass bottle 
reactor, 3 – CO2 fixing unit and 4 – Gas volume measuring 
device.  
III. RESULTS AND DISCUSSION 
 
The purpose of this experiment was to determine the 
efficiency of biogas production under mesophilic 
conditions. In this study, co-digestion of Cow dung/Chicken 
droppings and grass clippings were evaluated for the 
purpose of getting the bio-methane potentials and 
biochemical kinetics at optimum temperature 35 oC, 37 0C 
and initial pH of 7. Table II shows the substrate 
characterization. Cow dung and chicken droppings were 
found to contain more volatile solids compared to grass 
clippings which had more nutrients. The elemental analysis 
of cow dung indicated low C/N ratio compared to grass 
clippings. Through co-digestion, the C/N ratio increased to 
19.19 while that one for co-digestion of chicken droppings 
and grass clippings the C/N ratio was 20.49. 
TABLE II:  
 SUBSTRATE CHARACTERIZATION  
Substra C H N S VS (%) 
TS 
(%) 
C/N 
ratio 
GC 19.10 1.04 0.93 0.00 64.08 87.88 20.54 
CM 63.67 0.85 3.11 2.25 11.75 18.74 20.47 
CD 14.87 1.65 0.84 3.66 78.72 91.55 17.70 
 
Where: Substrate, GC-Grass clippings, CM-Chicken 
manure, CD-Cow dung, C – Carbon, H – Hydrogen, N – 
Nitrogen, S – Sulphur, TS – Total Solids, VS – Volatile 
Solids. 
TS is the sum of dissolved solids and suspended solids. 
TS and pH are important to assess anaerobic digestion 
process efficiency [14, 19]. VS is the organic portion of TS 
that biodegrade in the anaerobic process. C/N ratio is an 
important factor in bacteria stability in the anaerobic 
process. The C/N ratio required for the production of biogas 
is from  15-30 [21, 22]. TS and VS are calculated using 
equation (1) and (2) respectively while C/N ratio is 
calculated using equation (3). 
 
M
MM
wet
burneddriedVS
(%)                                             (1) 
 
M
M
wet
driedTS (%)                                                                 (2) 
 
Where: 
 Mdried = Amount dried sample (mg) 
 Mwet = Amount of wet sample (mg) 
Mburned = Amount of burned sample (mg) 
 
)*()*(
)*()*(
NN
SC
ff
fF
FF
SF
N
C

                                           (3) 
                                     
Where: F = First substrate, S = Second substrate, Cf = 
Carbon composition for the first substrate, Cs = Carbon 
composition for the second substrate, Nf = Nitrogen 
composition for the first substrate, Ns = Nitrogen 
composition for the second substrate.  
 
In order for a chemical reaction to take place, the ions or 
particles, which are reactants, must physically come into 
contact with one another. Anything that increased the 
frequency of these encounters increased the rate at which 
products were formed. The rate of a chemical reaction can 
be increased by increasing the temperature at which the 
reaction occurs. Temperature affects the chemical properties 
of the components in the substrates and the growth rate and 
metabolism of the micro-organisms as reported by Fogler 
(2010) [23]. High temperature kills the microorganisms and 
thus decreasing methane produced. It affects the population 
dynamics in the digester. The temperature sensitivity and 
classification can be divided into psychrophilic, mesophilic 
and thermophilic. Acetotrophic methanogens and 
degradation of butyrate and propionate are sensitive to high 
temperatures. Temperature also affects the partial pressure 
of H2 in the digesters and thus the movements of the 
syntrophic metabolism. High temperature mesophilic 30-40 
0C favored the production of acetate, carbon dioxide and 
hydrogen from propionate. It was desirable to keep the 
operating temperature stable so as to avoid disturbing the 
livelihood of the bacteria, especially the methanogens. 
Fig. 4 and 5 shows the effect of mesophilic temperature 
on AD 35 0C and 37 0C. Between 1-10 days the rate of 
conversion increased with retention time. This was because, 
with time, the conversion rate/percent of reactants to 
products increased. Temperature played an important role in 
dissociating reactant particles to form new species. The 
conversion rate of the reaction and biogas production 
increased with the increase in temperature and hydraulic 
retention time until an equilibrium state was achieved. 
The average CH4 and CO2 contents for Cow dung and 
Chicken droppings co-digested with Grass clippings were 
reported as 60 %, 58 % and 35 %, 38 % respectively. From 
the analysis using gas chromatography instrument (GC), 
CH4 was observed to contain 60 % and 58 % in biogas. 
According to the literature, the energy content of biogas is 
 directly proportional to CH4 concentration. 1 m3 of CH4 has 
a calorific value of around 9.81 kWh, while CO2 had zero 
[24]. The biogas generated can be used to generate 
renewable (green) energy in the form of electricity and heat 
using combined heat and power (CHP) engine. 
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 2 4 6 8 10 12 14 16
G
as
 a
cc
um
ul
at
io
n 
m
l
Retention Time (days)
Gas Accumulation Vs Retention Time
Cow dung/Grass
clippings 35 0C
Cow 
dung/Grass
clippings 37 
0C
 
Fig 4: Effect of different temperature constant on rate of conversion with 
co-digestion of cow dung and grass clippings 
 
0
5000
10000
15000
20000
25000
30000
35000
0 2 4 6 8 10 12 14 16 18 20
G
as
 A
cc
um
ul
at
io
n 
(m
l)
Retention Time (days)
Gas Accumulation chic Vs Retention Time
Chicken 
droppings/Grass
clippings 35 0C
Chicken 
droppings/Grass
clippings 37 0C
 
Fig 5: Effect of different temperature constant on the rate of conversion 
with co-digestion of chicken dropping and grass clippings. 
 
 
IV. CONCLUSION 
Biogas production from co-digestion of cow dung and 
grass clippings was established to be feasible at a 
temperature of 37 0C. All co-digestion in a ratio of 1:1 
showed a good productivity of methane indicated as 58-60 
%. 
 
ACKNOWLEDGEMENT 
The author wish to express their appreciation to Process 
Energy Environmental and Technology Station (PEETS) 
funded by South Africa National Energy Development 
Institute (SANEDI) and Technology Innovation Agency 
(TIA), City of Johannesburg (COJ) through Prof. Charles 
Mbohwa, Chemical Engineering and Applied Chemistry 
Departments at the University of Johannesburg for allowing 
us to work in their laboratories.  
 
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Mesophilic anaerobic co-digestion of cow dung, chicken droppings and grass clippings

University of Johannesburg Institutional Repository

  Abstract— The main focus of this study was mesophilic anaerobic co-digestion of cow dung, chicken droppings and grass clippings using pilot bio-digesters.  The biochemical methane potential (BMP) works under batch anaerobic digester operating in ambient mesophilic temperature of 35 oC and 37 0C and pH of 7 to generate biogas. The carbon/nitrogen (C/N) ratio for cow dung and chicken droppings was found to be 17.70 and 63.67 respectively and grass clippings to be 20.54. Through co-digestion in a ratio of 1:1, the C/N ratio for cow dung and grass clippings settled at 19.19 while that for chicken droppings and grass clippings settled at 20.49. The conversion rate of the reaction and biogas production increased with the increase in temperature and hydraulic retention time until an equilibrium state was achieved. At the temperature 37 OC, it was observed to be the suitable mesophilic temperature for anaerobic digestion due to high dissociation and collision leading to high rate of biogas production. Keywords— Anaerobic digestion, Co-digestion, Mesophilic Temperature I. INTRODUCTION HE energy consumption worldwide is spontaneously increasing due to industrialization, population growth, and state of development. The need for an alternative  Manuscript received June 7, 2016; revised June 9, 2016. This work was supported in part by the University of Johannesburg, Process Energy Environmental station (PEETS) and City of Johannesburg, South Africa. Anthony Njuguna Matheri; is with Department of Chemical Engineering; University of Johannesburg; Johannesburg 2028 South Africa; Cell: +27616986335 (tonynjuguna22@gmail.com) Mohamed Belaid; Senior lecturer and HOD at Department of Chemical Engineering; University of Johannesburg; Doornfortein, Johannesburg 2028 South Africa; Tel: +27115596402 (mbelaid@uj.ac.za) Tumisang Seodigeng; Senior lecturer at Department of Chemical Engineering; Vaal University of Technology; Private Bag X021-Vanderbijilpark-1911, Andries Potgieter Blvd South Africa; Tel: +27169509734 (tumisangs@vut.ac.za) Catherine Jane Ngila; Professor and HOD at Department of Analytical Chemistry; University of Johannesburg; P.O. Box 17011, Doornfortein, Johannesburg 2028 South Africa; Tel: 27115596169 (jcngila@uj.ac.za) Charles Mbohwa; Professor and Vice Dean at Faculty of Engineering and Build Environment (FEBE); University of Johannesburg; Johannesburg 2028 South Africa; Tel: +2711 559 6165  (cmbohwa@uj.ac.za)    source of energy for centralized and decentralized power generation has led to researchers looking for alternative source of energy [1].     With the fast depletion of non-renewable energy sources such as fossil fuel and coal which has led to human health problems and global climate change and environmental degradation, the commercial production of biogas and other alternative energy source such as solar energy, hydropower, wind energy, bioenergy, geothermal will definitely give a drive for the development of the economy [2, 3]. Fig. 1 shows renewable energy conversion technologies.  Energy derived from biogas is used in the form of fuel, heat, and electricity. It is desirable to create sustainable and with zero carbon emissions worldwide energy system [4, 5]. This results in resource conservation and environmental protection [5, 6].  Radiation energy Photolyse Thermal energy heat Solar cellTherm collectorSolar energy, Therm Power plantradiationBiomass Production Convertion plant Chemical energyWarming of surface Sea heat power plant and atmosphereHeat pumpTranspiration and precipitation Hydo power plantmelting Glacier ice power plantwind Wind energy converter Mechanical energy mobilityGeothermal heat, Geotherm, heat plantradioctive disintegration Geotherm, Power plantElectrical energy Planet Gravitation Tides Tides power plant Electricity Fig I: Renewable energy conversion technologies  Biogas is produced by anaerobic decomposition process called Anaerobic Digestion (AD). It is the biological breakdown of organic matters in the absence of oxygen. The main product of biogas is methane and carbon dioxide [7, 8]. Biogas production follows four fundamentals processes. These processes include hydrolysis, acidogenesis, Mesophilic Anaerobic Co-digestion of Cow dung, Chicken Droppings and Grass Clippings Anthony Njuguna Matheri1, Member, IAENG, Mohamed Belaid2, Member, IAENG, Tumisang Seodigeng3, Catherine Jane Ngila4, Charles Mbohwa5 T acetogenesis and methanogenesis [9]. Fig. 2 shows a simplified generic anaerobic digestion process [10].  Inert particulateCarbohydrates Proteins Lipids Inert solubleFacultative anaerobic Sugars Amino Acids LCFC bacteriaAcidogenic bacteriaPropionate, Butyrate, Valerate (Alcohol, Lactate)Acetate oxidizing bacteria Acetogenic bacteriaAcetate H2, CO2Homoacetogenic bacteriaMethanogenic bacteriaCH4, CO2BiomassHydrolysisAcidogenesisAcetogenesisMethanogenesisDisintegration Fig 2: Degradation steps of the anaerobic digestion process. Hydrolysis is theoretically the first step of anaerobic digestion, during which the complex organic matter (Polymers) are decomposed into smaller units (mono- and oligomers). During hydrolysis long-chain molecules, such as carbohydrate, protein, and fat polymers, are broken down to monomers (small molecules). Different specialized microbial produce a number of specific enzymes which catalyze the decomposition, and the process is extracellular. During hydrolysis, polymers like carbohydrates, proteins, lipids and nucleic acids are converted into glucose, glycerol, purines, and pyridines [11]. In the acidogenesis process, the acidogenic bacteria transform the products of the hydrolysis into short chain volatile acids, alcohol, ketones, carbon dioxide and hydrogen. Some of the major acidogenesis stage products are acetic acid, propionic acid, formic acid, butyric acid, lactic acid, ethanol, and methanol. From these products, the carbon dioxide, hydrogen, and acetic acid will skip the third stage, acetogenesis, and be utilized directly by the methanogenic bacteria in the final stage [12, 13]. The third stage is known as the acetogenesis stage, the rest of the acidogenesis products, i.e. the butyric acid, propionic acid and alcohols are transformed by acetogenic bacteria into hydrogen, carbon dioxide, and acetic acid. Hydrogen plays an important role in this process, as the reaction will only occur if the hydrogen partial pressure is low enough to thermodynamically allow the conversion of all compounds of acids. Such lowering of the partial pressure is carried out by hydrogen scavenging bacteria thus, the hydrogen concentration of a digester is an indicator of its health [14]. Methanogenesis is the final stage of the biogas production. The production of methane and carbon dioxide from intermediate products is carried out by Methanogenic bacteria. 70% of the formed methane during AD originates from acetate, while the remaining 30% is produced from the conversion of carbon dioxide (CO2) and hydrogen (H2) [11]. The methanogens bacteria have the slowest growth rate involved in the process; they also the limiting factor for how quickly the process can proceed and how much material can be digested in AD. The growth rate of the methanogens is only around one fifth of the acid-forming bacteria [15]. The activity of biogas production depends on various parameters like temperature, partial pressure, pH, hydraulic retention time, C/N ratio, pre-treatment of feedstock, trace of metals and concentration of substrate [5, 16-18]. Table I shows different thermal stages and typical hydraulic retention times for the AD process.  TABLE I:   THERMAL STAGES AND TYPICAL HYDRAULIC RETENTION TIMES [19].  Thermal stages  Process temperature (0C) HRT(days) Psychrophilic  <20 From 70-80 Mesophilic From 30-42 From 14-40 Thermophilic From 43-55 From 14-20  Biogas can be produced from co-digestion of various substrates. In the present study, anaerobic digestion of co-digestion of cow dung/chicken droppings and grass clippings were studied in laboratory experiments in a 10 liters digester under a constant temperature of 35 oC and 37 0C.  II. METHODOLOGY  A. Substrate Collection Cow dung and Chicken droppings were collected at the farm in Gauteng province while grass clippings were collected from the University of Johannesburg. Waste characterization was done to ascertain the composition. These included physical and chemical composition with regards to C/N ratio, total solids, volatile solids and elemental analysis for Carbon, Nitrogen, Sulphur and Hydrogen in accordance with the standard method (APHA 1995) [20]. To determine biogas production rate, a batch digester was fed with the co-digested substrates and inoculum under pre-set conditions of 37 0C and pH of 7 as shown in Fig. 3. pH was neutralized by a solution of 8g NaOH in 100 ml and H2SO4. The digester was flushed with Nitrogen gas to expel the oxygen and create an anaerobic process. It was then immersed in the water bath and kept under constant temperature. The liquid sample collected was analyzed using a spectrophotometer. The gas produced was measured using downwards displacement method on a daily basis until the end of retention time. The gas collected was sampled by extracting it from the digester using a gas syringe and was analyzed using gas chromatography instrument with flame- ionization detectors. The operating conditions were; helium was used as carrier gas at 20 ml/min, oven temperature 70 0C, detector temperature 150 0C and injector port at 80 0C.    Fig 3: Biogas production setup. Where: 1 – Thermostatic water bath, 2 - T-Glass bottle reactor, 3 – CO2 fixing unit and 4 – Gas volume measuring device.  III. RESULTS AND DISCUSSION  The purpose of this experiment was to determine the efficiency of biogas production under mesophilic conditions. In this study, co-digestion of Cow dung/Chicken droppings and grass clippings were evaluated for the purpose of getting the bio-methane potentials and biochemical kinetics at optimum temperature 35 oC, 37 0C and initial pH of 7. Table II shows the substrate characterization. Cow dung and chicken droppings were found to contain more volatile solids compared to grass clippings which had more nutrients. The elemental analysis of cow dung indicated low C/N ratio compared to grass clippings. Through co-digestion, the C/N ratio increased to 19.19 while that one for co-digestion of chicken droppings and grass clippings the C/N ratio was 20.49. TABLE II:   SUBSTRATE CHARACTERIZATION  Substra C H N S VS (%) TS (%) C/N ratio GC 19.10 1.04 0.93 0.00 64.08 87.88 20.54 CM 63.67 0.85 3.11 2.25 11.75 18.74 20.47 CD 14.87 1.65 0.84 3.66 78.72 91.55 17.70  Where: Substrate, GC-Grass clippings, CM-Chicken manure, CD-Cow dung, C – Carbon, H – Hydrogen, N – Nitrogen, S – Sulphur, TS – Total Solids, VS – Volatile Solids. TS is the sum of dissolved solids and suspended solids. TS and pH are important to assess anaerobic digestion process efficiency [14, 19]. VS is the organic portion of TS that biodegrade in the anaerobic process. C/N ratio is an important factor in bacteria stability in the anaerobic process. The C/N ratio required for the production of biogas is from  15-30 [21, 22]. TS and VS are calculated using equation (1) and (2) respectively while C/N ratio is calculated using equation (3).  MMMwetburneddriedVS(%)                                             (1)  MMwetdriedTS (%)                                                                 (2)  Where:  Mdried = Amount dried sample (mg)  Mwet = Amount of wet sample (mg) Mburned = Amount of burned sample (mg)  )*()*()*()*(NNSCfffFFFSFNC                                           (3)                                      Where: F = First substrate, S = Second substrate, Cf = Carbon composition for the first substrate, Cs = Carbon composition for the second substrate, Nf = Nitrogen composition for the first substrate, Ns = Nitrogen composition for the second substrate.   In order for a chemical reaction to take place, the ions or particles, which are reactants, must physically come into contact with one another. Anything that increased the frequency of these encounters increased the rate at which products were formed. The rate of a chemical reaction can be increased by increasing the temperature at which the reaction occurs. Temperature affects the chemical properties of the components in the substrates and the growth rate and metabolism of the micro-organisms as reported by Fogler (2010) [23]. High temperature kills the microorganisms and thus decreasing methane produced. It affects the population dynamics in the digester. The temperature sensitivity and classification can be divided into psychrophilic, mesophilic and thermophilic. Acetotrophic methanogens and degradation of butyrate and propionate are sensitive to high temperatures. Temperature also affects the partial pressure of H2 in the digesters and thus the movements of the syntrophic metabolism. High temperature mesophilic 30-40 0C favored the production of acetate, carbon dioxide and hydrogen from propionate. It was desirable to keep the operating temperature stable so as to avoid disturbing the livelihood of the bacteria, especially the methanogens. Fig. 4 and 5 shows the effect of mesophilic temperature on AD 35 0C and 37 0C. Between 1-10 days the rate of conversion increased with retention time. This was because, with time, the conversion rate/percent of reactants to products increased. Temperature played an important role in dissociating reactant particles to form new species. The conversion rate of the reaction and biogas production increased with the increase in temperature and hydraulic retention time until an equilibrium state was achieved. The average CH4 and CO2 contents for Cow dung and Chicken droppings co-digested with Grass clippings were reported as 60 %, 58 % and 35 %, 38 % respectively. From the analysis using gas chromatography instrument (GC), CH4 was observed to contain 60 % and 58 % in biogas. According to the literature, the energy content of biogas is  directly proportional to CH4 concentration. 1 m3 of CH4 has a calorific value of around 9.81 kWh, while CO2 had zero [24]. The biogas generated can be used to generate renewable (green) energy in the form of electricity and heat using combined heat and power (CHP) engine. 05001000150020002500300035004000450050000 2 4 6 8 10 12 14 16Gas accumulation mlRetention Time (days)Gas Accumulation Vs Retention TimeCow dung/Grassclippings 35 0CCow dung/Grassclippings 37 0C Fig 4: Effect of different temperature constant on rate of conversion with co-digestion of cow dung and grass clippings  050001000015000200002500030000350000 2 4 6 8 10 12 14 16 18 20Gas Accumulation (ml)Retention Time (days)Gas Accumulation chic Vs Retention TimeChicken droppings/Grassclippings 35 0CChicken droppings/Grassclippings 37 0C Fig 5: Effect of different temperature constant on the rate of conversion with co-digestion of chicken dropping and grass clippings.   IV. CONCLUSION Biogas production from co-digestion of cow dung and grass clippings was established to be feasible at a temperature of 37 0C. All co-digestion in a ratio of 1:1 showed a good productivity of methane indicated as 58-60 %.  ACKNOWLEDGEMENT The author wish to express their appreciation to Process Energy Environmental and Technology Station (PEETS) funded by South Africa National Energy Development Institute (SANEDI) and Technology Innovation Agency (TIA), City of Johannesburg (COJ) through Prof. Charles Mbohwa, Chemical Engineering and Applied Chemistry Departments at the University of Johannesburg for allowing us to work in their laboratories.   REFERENCE [1] Matheri, A.N., et al., Modelling the Kinetics of Biogas Production from Co-digestion of Pig Waste and Grass Clippings. 24th World Congress on Engineering (WCE 2016) London-UK, 2016. [2] Abbasi, T., S.M. Tauseef, and S.A. Abbasi, Anaerobic digestion for global warming control and energy generation—an overview. Renewable and Sustainable Energy Reviews, 2012. 16(5): p. 3228-3242. 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Mesophilic anaerobic co-digestion of cow dung, chicken droppings and grass clippings

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