Contents
2
LITERATURE REVIEW................................................................................................................... 2
2.1
Problem Statement ....................................................................................................................... 2
2.2
Background ................................................................................................................................... 2
2.3
Bio-Methane Production .............................................................................................................. 2
2.4
Factors Affecting Bio-Methane Production .................................................................................. 3
2.4.1
pH .......................................................................................................................................... 3
2.4.2
Temperature ......................................................................................................................... 3
2.4.3
Organic Loading Rate (OLR) .................................................................................................. 4
2.5
Types of Feedstocks Used for Bio-Methane Production .............................................................. 4
2.5.1
Sewage sludge....................................................................................................................... 5
2.5.2
Animal manure...................................................................................................................... 5
2.5.3
Crops and their residues ....................................................................................................... 5
2.6
Bio-Methane Potential Test .......................................................................................................... 6
2.6.1
2.7
Methods of determining BMP .............................................................................................. 6
Factors Affecting BMP................................................................................................................... 7
2.7.1
Raw material composition .................................................................................................... 7
2.7.2
Total and volatile solids......................................................................................................... 7
2.7.3
Chemical and biological oxygen demand.............................................................................. 8
2.7.4
Carbon/nitrogen ratio ........................................................................................................... 8
2.7.5
Agronomic practices ............................................................................................................. 8
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2 LITERATURE REVIEW
2.1
Problem Statement
Over centuries fossil fuels have been a source of energy and fuel. An increase in the population
comes with an increasing demand of energy and fuel. The depreciating fossil fuels and the
environmental threat posed by the gases emitted to the atmosphere during the production of the
heat and energy, alternative sources has to be explored. Bio-methane, which is energy produced
by anaerobic digestion of organic waste is a sustainable alternative that is compatible to fossil
energy [1]. This research project presents the effect of organic ratios of co-digestion of sludge and
saw dust in bio-methane production.
2.2
Background
In today’s energy demanding lifestyle, the need for an alternative source of energy that is
compatible with the energy produced with fossil fuels is predictable. Industries depend on fossil
fuels as the main source of energy, this affects the climate change, increases environmental
pollution and degradation, thus leading to health hazards. Exploring renewable, sustainable and
environmentally friendly sources is foreseeable. The use of wood as a source of energy has
resulted in depleting forests. Bio-methane is a well-established fuel that can be used for the same
applications of wood or fossil energy. Fossil-based fuels is becoming scarce and more expensive,
the economics of bio-methane production is turning out to be more favorable. Biogas is a readily
available energy resource that significantly reduces greenhouse-gas emissions compared to the
emission of landfill gas to the atmosphere [1]. One huge problem faced by the country or the
world as a whole is waste and energy management. The uncontrollable growth in population has
created serious problems of energy management and solid waste disposal.
2.3
Bio-Methane Production
Bio-methane is produced by anaerobic digestion of biodegradable organic matter. Methane (CH4),
carbon dioxide (CO2) and trace of other gases including nitrogen (N2), hydrogen (H2), hydrogen
2
sulphide (H2s) and ammonia (NH3) make up Bio-methane. Bio-methane is an alternative to fossil
fuel, because it can be used for heat and energy generation, and can be used as fuel for vehicles.
Auto-manufactures are already using bio-methane as to power their vehicle [2]. Anaerobic
digestion is a biochemical process under anaerobic conditions of degradation of organic matter
using consortia of bacteria [2]. It is considered to be one of the most efficient, eco-friendly and
naturally occurring in oxygen-free environment process of producing bio-methane [2]. It
comprises of four stages, which are: hydrolysis, acedogenesis, acetogenesis and methanogenesis
[3].
2.4
Factors Affecting Bio-Methane Production
Studying and controlling the variations in some of the parameters is important in the performance
of biogas plant. Parameters like pH, temperature, loading rate, particle size, digester nature of
substrate and agitation affect bio-methane production. Any drastic changes in these parameters can
adversely affect the bio-methane production, thus they can only be varied within a desirable range
to operate the biogas plant efficiently.
2.4.1 pH
pH influences anaerobic processing, the coveted pH go for the digester is 6.8 to 7.2 by sustaining
it an ideal stacking rate. The pH of the digester substance is influenced by the measure of carbon
dioxide and unstable unsaturated fats delivered amid the anaerobic procedure. For a fruitful
anaerobic processing to happen, centralization of unpredictable unsaturated fats, acidic corrosive
specifically ought to be beneath 2000 mg/l [4]. pH esteems underneath 6.0 to 6.5 keep the methane
microscopic organisms action. To stay away from drops in pH, chemicals are added to the natural
substrate to supply a cradle limit. Sodium bicarbonate, sodium hydroxide, sodium carbonate and
sodium sulfide are the most utilized chemicals [4].
2.4.2 Temperature
The operating temperature inside the digester majorly affects the bio-methane production. The
anaerobic assimilation can be done in various temperature ranges: psychrophilic (<30°C),
mesophilic (30 to 40°C) and thermophilic (50 to 60°C). Be that as it may, anaerobes are most
dynamic in 30°C to 60°C, which are mesophilic and thermophilic temperature ranges. The length
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of assimilation period is dependent on temperature [4]. The methanogenic microscopic organisms
encourage the arrangement of bio-methane and are exceptionally delicate to temperature changes,
the ideal temperature for the microorganisms to work is between 33 to 38°C. Temperatures
underneath this slows the bio-methane generation process, while a higher temperature than would
normally kill the bio-methane delivering microbes. This is the reason the structure for biogas
creation is by and large fabricated underground, to keep the temperature as consistent as possible
[5]. Sharp temperature changes can cause a reduction in bio-methane generation because of the
death of particular microbes’ strains. Reactors should be submerged in a thermostatic water bath,
or emerged in thermostatically controlled space to keep up the temperature within bio-methane
creation tests [5]. The sludge and saw dust clean temperature esteems will be checked in deciding
the rate of absorption and maintenance of the procedure, since temperature is imperative. The
temperature influences the rate of absorption, because of the outside walls of the digester surface
reach the climate, consequently the digester walls retain or free warmth relying upon the
temperature slope between the digester and its prompt condition [5].
2.4.3 Organic Loading Rate (OLR)
The loading rate determines bio-methane production. Reduction in loading rate increases the
methane yield. As study was carried out in Pennsylvania, where manure was used as a substrate
on a 100m3 biogas plant, when organic loading rate was varied from 346 to 1030 kg VS/day, biomethane yield increased from 67 to 202 m3/day [6]. There is a specific feed rate for a particular
plant size, will be the most efficient , give maximum bio-methane yield , also increasing the
substrate quantity will produce more bio-methane. According to Yadvika (2004), a daily loading
rate of 16 kg VS/m3 of digester capacity produced 0.04 to 0.074 m3 of gas/kg of dung fed [8]. A
lab-scale digester operating at different organic loading rate produces more bio-methane than at
an organic loading rate being varied daily. Based on pilot plant studies, maximum gas yield was
observed for a loading rate of 24 kg dung/m3 digester/day.
2.5
Types of Feedstocks Used for Bio-Methane Production
Almost all types of organic waste cane be used for bio-methane production using anaerobic
digestion.
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2.5.1 Sewage sludge
In urban areas, sewage sludge is produced in large quantities and is a vast resource with high
biodegradability. Anaerobic stabilization of sewage sludge is the most prevalent anaerobic
digestion activity worldwide. The anaerobic digestion of sewage sludge helps with waste
management whilst providing a significant benefit as it leads to the production of energy in a form
of bio-methane. In Europe, between 30 and 70% of sewage sludge is treated by anaerobic digestion
[7]. South Africa is a developing country lacking sanitation facilities, thus anaerobic digestion can
also be used as wastewater treatment and provide heat and energy.
2.5.2 Animal manure
Historically, the most common substrate for bio-methane or biogas is animal manure. It is the
most common feedstock of anaerobic digestion. South Africa has a significant population of
livestock which produces large quantities of manure which is a suitable substrate for bio-methane
production. Animal manure comprise huge amounts of lignocellulose, polysaccharides, proteins
and biomaterials which are suitable for anaerobic digestion to produce an environmentally
friendly gas.
2.5.3 Crops and their residues
Crops demonstrate good biogas production potential. Crops produce large quantities of easily
degradable biomass which is necessary for high bio-methane yield [7]. The net energy yield per
hectare is the most important parameter for choosing energy crops [7]. The optimum harvesting
time and methane yield also play a role in the selection of crops for energy production [7]. Nonfood crops can also be used for bio-methane production. Several crop residues including cotton,
maize, and rice residues have showed high potential for anaerobic digestion [7]. However, high
lignin content of some straws and other residues can lead to poor biodegradability and low biomethane production. In an investigation conducted in 2012, Chandra provided a good review of
bio-methane production from lignocellulosic agricultural wastes [9]. They obtained 87.5% higher
biogas yield and 111.6% higher CH4 yield from NaOH pre-treated wheat straw compared with
non-treated wheat straw substrate. A similar study by Nges produced viable bio-methane yield
through pre-treatment, nutrient addition, and co-digestion of crop and waste biomass [10].
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2.6
Bio-Methane Potential Test
Bio-methane potential test (BMP) is a parameter used in evaluating biogas and methane potential
of organic materials [7]. The BMP is often defined as the maximum volume of CH4 produced per
g of substrate [7]. It provides an indication of the biodegradability of a substrate and its potential
to produce methane in anaerobic digestion [7]. This allows assessment of bio-methane yields
achieved by the anaerobic digestion process. The BMP also allows establishment baseline
performance data of anaerobic digestion. In addition, BMP assays have been useful tools for
determining the best substrates for co-digestion configurations [7].
2.6.1 Methods of determining BMP
The BMP test is a technique used to determine BMP and ascertains the effectiveness of anaerobic
digestion process and the biodegradability of substrate. It is a laboratory-scale batch assay of 30–
100 d of sample’s anaerobic digestion [8]. BMP test can be used to develop mathematical models
for prediction of BMP of feedstocks referencing to the information given by this test [7]. There
have been several trials to define a standard protocol for ultimate BMP test in imperative to
achieve comparable results. As of the year 2012, standardization was not met [8]. More recently,
Koch et al. reported, no commonly accepted experimental procedure yet exists that is based on a
standardized protocol for the execution of the test. This can be attributed because anaerobic
digestion is a very complex and dynamic system in which microbiological, biochemical, and
physio-chemical characteristics are closely related [8]. Even though there is no agreement on
standard manual, some BMP procedures exist that are used by various researchers. The three
commonly used methods include the German standard procedure, Verein Deutscher Ingenieure
(VDI) 4630 (VDI method), the Møller method and the Hansen method [8]. The VDI method
provides information regarding the methodology for performing fermentation tests in batch and
continuous mode [8].
Several techniques including manometric, volumetric, and gas chromatography methods can be
used to measure gas production [7]. Volumetric methods measure the amount of biogas or only
the volume of methane generated during anaerobic digestion of organic material. The volume is
either measured by water displacement or displacement of a piston of syringe or as part of the
reactor [7]. The manometric techniques derived from Warburg’s respirometer measure the gas
produced in constant volume by pressure increase by a differential manometer [7]. The amount
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of CH4 and CO2 of the bio-methane that ends up in the headspace of closed vials is measured by
means of gas chromatography (GC) [8]. Standard protocols provide information that clearly
demonstrate the setting of parameters that considerably influence the test results. These
parameters include temperature, pH, stirring intensity, physico-chemical characteristics of
substrates, and substrate/inoculum (S/I) ratio [8]. Most of the BMP tests described in the literature
are conventional tests. Nowadays, automatic BMP tests with greater precision, and minimum time
and labor requirements have been developed [8].
2.7 Factors Affecting BMP
2.7.1 Raw material composition
The origin, content of organic substrate and substrate composition of individual substrate varies
and also determines bio-methane yield. Feedstocks differ markedly in their chemical composition.
As such, the amount and the composition of biogas vary from one substrate to another [7].
Methane yield vary for different chemical constituents of the same feedstock. Fats and proteins
produce more methane than carbohydrates and lignin is not biodegradable under anaerobic
digestion.
2.7.2 Total and volatile solids
Organic and in-organic portion of matter is the total solids. Organic matter is measured by the
total amount carbon in a feedstock [7].
Anaerobic digestion and bio-methane production
efficiency is influenced by the total solids content of the feed stock. Systems set in anaerobic
digestion are classified by the percentage of the total solids (TS) of the feed stock [15]. Yi et al.
classified three main types of anaerobic digestion technologies that work according to the total
solids content of feedstocks. These are: conventional wet (≤10% TS), semi-dry (10–20% TS) and
modern dry (≥20% TS) processes. Abbassi-Guendouz et al. (2004), showed that total methane
yield decreased with TS contents increasing from 10% to 25% in batch anaerobic digestion of
cardboard under mesophilic conditions [10]. Similarly, Forster-Carneiro et al. (2008) showed that
biogas and methane production decreased when the TS contents increased from 20% to 30% in
dry batch AD of food waste [11]. Volatile solids affects the bio-methane yield, thus the high
correlation of volatile solids and bio-methane potential. This is evident that methane yield
increases with volatile products and less hemicellulose [8].
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2.7.3 Chemical and biological oxygen demand
The amount of organic matter in feedstocks and potential for bio-ethane production is quantified
by the chemical and biological demand [8]. The chemical oxygen demand substrate can be used
to calculate theoretical methane production yield. Biogas production in relation to COD is about
0.5 L g-COD removed, corresponding to a methane production of COD removed [8]. Biological
oxygen demand (BOD) is another widely used parameter, which is a measure of the oxygen used
by microorganisms to decompose organic matter. BOD is similar to COD in that both measure the
amount of organic matter. Typical examples of BOD values are: pig slurry 20,000 – 30,000, cattle
slurry 10,000 – 20,000 and wastewater 1000 – 5000 mg L-1 [8].
2.7.4 Carbon/nitrogen ratio
The relationship between the amount of nitrogen and carbon in a feedstock is the C/N ratio. A
feedstock C/N ratio of 25:1 produces optimal gas production [8]. The optimum range of C/N ratio
for anaerobic digestion is 20-35:1 [8]. Materials that are protein rich have the low ratio. AD of
such material results in increased content of free ammonia that causes high pH leading to
methanogenic inhibition [8]. Lower gas production is caused by a high ratio that causes rapid
depletion of nitrogen decreasing the production rate. Wang et al. reported an interactive effect
between temperature and C/N on anaerobic digestion performance. They reported that when
temperature was increased, a higher C/N ratio would be required in order to reduce the risk of
ammonia inhibition. Typical C/N ratios for some feedstocks are: cattle manure 13:1, chicken
manure 15:1, grass silage 25:1 and rice husks 47:1 [8].
2.7.5 Agronomic practices
Agro-climatic conditions, soil characteristics, plant varieties, and agronomic practices influence
both the composition and yield of crops. The biomass composition then influences the methane
content in the biogas leading to various BMP values [18]. A study done by Mahmood et al. showed
significant variations across plant variety on specific methane yield and maize. Agrogas reportedly
had the highest methane yield of 367 NL kg VS-1. This was attributed to high starch and low lignin
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content of maize cv. Agrogas. In a study by Sepalla to evaluate the methane production potential
of traditional and novel energy crops in boreal conditions, maize attained the greatest methane
potential ranging from 4,000 to 9,200 m3 CH4 ha–1 a–1 [11]. Bio-methane yield per unit of cropped
area can be improved provided Equation 1 for calculating the bio-methane yield from crop
production systems as:
Bio-methane yield (m3 CH4ha -1) = BMP (m3CH 4t-1 ) ×biomass yield (t ha-1)
(Eq. 1)
In a study with maize, Mayer et al. achieved an average bio-methane yield per hectare of 7,266
m3 ha–1. It was concluded that the cropping environment was responsible for most of the variation
of the bio methane yield per hectare [8]. They also reported a decrease in bio-methane yield with
stage of maturity of the maize crop. This can be ascribed to increased content of structural
carbohydrates and lignin in the plants. The chemical composition is affected by the stage of
maturity of a crop, which in turn affects BMP. However, it was reported that despite substantially
different nutrient concentration among some maize hybrids used in a trial, no clear-cut association
existed between chemical composition and specific methane yield. This variation could be due to
variation in chemical content within critical limits to affect BMP.
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2.8
References
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R.O., & Dike, D.O., 201). Estimation Of The Electric Power Potential Of Human Waste Using
Students Hostel Soak-Away Pits. Am. J. Eng. Res. 2(9):198-203.
[4] Yadvika,, Santosh, Sreekrishnan, T..R., Kohli S., & Rana, V., 2004. Enhancement of biogas
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rural school in Maphephetheni. KwaZuluNatal. J. Energy South. Afr. 16(3): 45-55.
[7] Chen ,Y., Cheng, J., & Creamer, K., 2008. Inhibition of anaerobic digestion process: A
review. Bioresour. Technol. 99:4044-4064.
[8] Jingur, R.M.,& Kamusoko, R., 2017. Methods of dertemination of bio-methane potential of
feed stocks: a review, Biofuel Reasearch Journal 14.
[9] Chandra, R., Takeuchi, H., Hasegawa, T., 2012. Methane production from lignocellulosic
agricultural crop wastes: a review in context to second generation of biofuel production. Renew.
Sust. Energy Rev. 16(3), 1462-1476.
[10] Koch, K., Fernández, Y.B., Drewes, J.E., 2015. Influence of headspace flushing on methane
production in biochemical methane potential (BMP) tests. Bioresour. Technol. 186, 173-178.
[11] Sepalla, M., 2013. Biogas production from high-yielding energy crops in boreal conditions.
Academic Thesis, University of Jyväskylä, Jyväskylä, Finland.
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