1

CHAPTER 1

INTRODUCTION

Due to scarcity of petroleum and coal it threatens supply of fuel throughout the world also

problem of their combustion leds to research in different corners to get access the new

sources of energy, like renewable energy resources. Solar energy, wind energy, different

thermal and hydro sources of energy, biogas are all renewable energy resources. But, biogas

is distinct from other renewable energies because of its characteristics of using, controlling

and collecting organic wastes and at the same time producing fertilizer and water for use in

agricultural irrigation. Biogas does not have any geographical limitations nor does it requires

advanced technology for producing energy, also it is very simple to use and apply.

Kitchen waste is organic material having the high calorific value and nutritive value to

microbes, that’s why efficiency of methane production can be increased by several order of

magnitude as said earlier .It means higher efficiency and size of reactor and cost of biogas

production is reduced. Also in most of cities and places, kitchen waste is disposed in landfill

or discarded which causes the public health hazards and diseases like malaria, cholera,

typhoid. Inadequate management of wastes like uncontrolled dumping bears several adverse

consequences: It not only leads to polluting surface and groundwater through leachate and

further promotes the breeding of flies , mosquitoes, rats and other disease bearing vectors.

Also, it emits unpleasant odour & methane which is a major greenhouse gas contributing to

global warming.

Mankind can tackle this problem(threat) successfully with the help of methane , however till

now we have not been benefited, because of ignorance of basic sciences – like output of work

is dependent on energy available for doing that work. This fact can be seen in current

practices of using low calorific inputs like cattle dung, distillery effluent, municipal solid

waste (MSW) or sewage, in biogas plants, making methane generation highly inefficient. We

can make this system extremely efficient by using kitchen waste/food wastes

1.1 OBJECTIVES OF STUDY

How the yield of biogas is increased by introducing gas recycling to the reactor

How the reaction rate effected while increasing the amount of CO2 in the system

2

How much cost effective is the system

How the reaction rate is enhanced by providing feed in the form of slurry instead

of solid feed

1.2 BIOGAS

BIOGAS is produced by bacteria through the bio-degradation of organic material under

anaerobic conditions. Natural generation of biogas is an important part of bio-geochemical

carbon cycle. It can be used both in rural and urban areas.

Composition of biogas depends upon feed material . Biogas is about 20% lighter than air has

an ignition temperature in range of 650 to 750 0C.An odorless & colorless gas that burns with

blue flame similar to LPG gas. Its caloric value is 20 Mega Joules (MJ) /m3 and it usually

burns with 60 % efficiency in a conventional biogas stove.

This gas is useful as fuel to substitute firewood, cow-dung, petrol, LPG, diesel, & electricity,

depending on the nature of the task, and local supply conditions and constraints.

Biogas digestor systems provides a residue organic waste, after its anaerobic digestion(AD)

that has superior nutrient qualities over normal organic fertilizer, as it is in the form of

ammonia and can be used as manure. Anaerobic biogas digesters also function as waste

disposal systems, particularly for human wastes, and can, therefore, prevent potential sources

of environmental contamination and the spread of pathogens and disease causing bacteria.

Biogas technology is particularly valuable in agricultural residual treatment of animal excreta

and kitchen refuse(residuals).

Many factors affecting the fermentation process of organic substances under anaerobic

condition are,

The quantity and nature of organic matter

The temperature

Acidity and alkanity (PH value) of substrate

The flow and dilution of material

Reducing usage of LPG for cooking

3

CHAPTER 2

LITERATURE REVIEW

ARTI – appropriate rural technology of India, pune (2003) has developed a compact biogas

plant which uses waste food rather than any cow dung as feedstock, to supply biogas for

cooking. The plant is sufficiently compact to be used by urban households, and about 2000

are currently in use – both in urban and rural households in Maharashtra. The design and

development of this simple, yet powerful technology for the people.

Dr. Anand Karve (ARTI) developed a compact biogas system that uses starchy or sugary

feedstock (waste grain flour, spoilt grain, overripe or misshapen fruit, non edible seeds, fruits

and rhizomes, green leaves, kitchen waste, leftover food, etc). Just 2 kg of such feedstock

produces about 500 g of methane, and the reaction is completed with 24 hours. The

conventional biogas systems, using cattle dung ,Sewerage, etc. use about 40 kg feedstock to

produce the same quantity of methane, and require about 40 days to complete the reaction.

Thus, from the point of view of conversion of feedstock into methane, the system developed

by Dr. Anand Karve is 20 times as efficient as the conventional system, and from the point of

view of reaction time, it is 40 times as efficient. Thus, overall, the new system is 800 times as

efficient as the conventional biogas system.

Hilkiah Igoni (2008) studied the Effect of Total Solids Concentration of Municipal

SolidWaste on the Biogas Produced in an Anaerobic Continuous Digester. The total

solids (TS) concentration of the waste influences the pH, temperature and effectiveness of the

microorganisms in the decomposition process. They investigated various concentrations of

the TS of MSW in an anaerobic continuously stirred tank reactor (CSTR) and the

corresponding amounts of biogas produced, in order to determine conditions for optimum gas

production. The results show that when the percentage total solids (PTS) of municipal sold

waste in an anaerobic continuous digestion process increases, there is a corresponding

geometric increase for biogas produced. A statistical analysis of the relationship between the

volume of biogas produced and the percentage total solids concentration established that the

former is a power function of the latter, indicating that at some point in the increase of the

TS, no further rise in the volume of the biogas would be obtained.

Kumar et al., (2004) investigated the reactivity of methane. They concluded that it has more

than 20 times the global warming potential of carbon dioxide and that the concentration of it

in the atmosphere is increasing with one to two per cent per year. The article continues by

4

highlighting that about 3 to 19% of anthropogenic sources of methane originate from

landfills.

Shalini Singh et al. (2000) studied the increased biogas production using microbial

stimulants. They studied the effect of microbial stimulant aquasan and teresan on biogas

yield from cattle dung and combined residue of cattle dung and kitchen waste respectively.

The result shows that dual addition of aquasan to cattle dung on day 1 and day 15 increased

the gas production by 55% over unamended cattle dung and addition of teresan to cattle

dung: kitchen waste (1:1) mixed residue 15% increased gas production.

Lissens et al. (2004) completed a study on a biogas operation to increase the total biogas

yield from 50% available biogas to 90% using several treatments including: a mesophilic

laboratory scale continuously stirred tank reactor, an up flow biofilm reactor, a fiber

liquefaction reactor releasing the bacteria Fibrobacter succinogenes and a system that add

water during the process. These methods were sufficient in bringing about large increases to

the total yield; however, the study was under a very controlled method, which leaves room

for error when used under varying conditions. However, Bouallagui et al. (2004) did

determine that minor influxes in temperature do not severely impact the anaerobic digestion

for biogas production.

As Taleghani and Kia (2005) observed, the resource limitation of fossil fuels and the

problems arising from their combustion has led to widespread research on the accessibility of

new and renewable energy resources. Solar, wind, thermal and hydro sources, and biogas are

all renewable energy resources. But what makes biogas distinct from other renewable

energies is its importance in controlling and collecting organic waste material and at the same

time producing fertilizer and water for use in agricultural irrigation. Biogas does not have any

geographical limitations or requires advanced technology for producing energy, nor is it

complex or monopolistic.

Murphy, McKeog, and Kiely (2004) completed a study in Ireland analyzing the usages of

biogas and biofuels. This study provides a detailed summary of comparisons with other fuel

sources with regards to its effect on the environment, finical dependence, and functioning of

the plant. One of the conclusions the study found was a greater economic advantage with

utilizing biofuels for transport rather than power production; however, power generation was

more permanent and has less maintenance demands.

Thomsen et al. (2004) found that increasing oxygen pressure during wet oxidation on the

digested biowaste increased the total amount of methane yield. Specifically, the yield which

is normally 50 to 60% increased by 35 to 40% demonstrating the increased ability to retrieve

5

methane to produce economic benefits.

Carrasco et al. ( 2004) studied the feasibility for dairy cow waste to be used in anaerobic

digestive systems. Because the animal’s wastes are more reactive than other cow wastes, the

study suggests dairy cow wastes should be chosen over other animal wastes .

Jantsch and Mattiasson (2004) discuss how anaerobic digestion is a suitable method for the

treatment of wastewater and organic wastes, yielding biogas as a useful by-product.

However, due to instabilities in start-up and operation it is often not considered. A common

way of preventing instability problems and avoiding acidification in anaerobic digesters is to

keep the organic load of the digester far below its maximum capacity. There are a large

number of factors which affect biogas production efficiency including: environmental

conditions such as pH, temperature, type and quality of substrate; mixing; high organic

loading; formation of high volatile fatty acids; and inadequate alkalinity.

Jong Won Kang et al (2010) studied the On-site Removal of H2S from Biogas Produced

by Food Waste using an Aerobic Sludge Biofilter for Steam Reforming Processing. They

show that A biofilter containing immobilized aerobic sludge was successfully adapted for the

removal of H2S and CO2 from the biogas produced using food waste. The biofilter

efficiently removed 99% of 1,058 ppmv H2S from biogas produced by food waste treatment

system at a retention time of 400 sec. The maximum observed removal rate was 359 g-

H2S/m3/h with an average mass loading rate of 14.7 g-H2S/m3/h for the large-scale biofilter.

The large-scale biofilter using a mixed culture system showed better H2S removal capability

than biofilters using specific bacteria strains. In the kinetic analysis, the maximum H2S

removal rate (Vm) and half saturation constant (Ks) were calculated to be 842.6 g-H2S/m3/h

and 2.2 mg/L, respectively. Syngas was generated by the catalytic steam reforming of

purified biogas, which indicates the possibility of high efficiency electricity generation by

SOFCs and methanol manufacturing.

6

2.1 PRODUCTION PROCESS

In biogas plants, organic substrates are converted in the liquid phase to methane and carbon

dioxide which accumulate in the gas phase. The degradation of organic matter to biogas

follows a complex reaction network and it is grouped into five degradation steps, which are

(1) disintegration of complex substrates to macromolecules, (2) hydrolysis of these to smaller

molecules, (3) acidogenesis and (4) acetogenesis, where acids are produced, and finally (5)

methanogenesis. In the last degradation step, methane is produced within two different

pathways. In the acetoclastic path, acetic acid is degraded to methane and carbon dioxide.

The latter can be consumed by hydrogenotrophic microorganisms and converted to methane,

as well. Both pathways are present in a microbial community of a biogas plant.

Organic substances exist in wide variety from living beings to dead organisms . Organic

matters are composed of Carbon (C), combined with elements such as Hydrogen (H), Oxygen

(O), Nitrogen (N), Sulphur (S) to form variety of organic compounds such as carbohydrates,

proteins & lipids. In nature MOs (microorganisms), through digestion process breaks the

complex carbon into smaller substances. There are 2 types of digestion process :

Aerobic digestion.

Anaerobic digestion.

The digestion process occurring in presence of Oxygen is called Aerobic digestion and

produces mixtures of gases having carbon dioxide (CO2), one of the main “green houses”

responsible for global warming.

The digestion process occurring without (absence) oxygen is called Anaerobic digestion

which generates mixtures of gases. The gas produced which is mainly methane produces

5200-5800 KJ/m3 which when burned at normal room temperature and presents a viable

environmentally friendly energy source to replace fossil fuels (non-renewable).

2.1.1 Anaerobic digestion

It is also referred to as biomethanization, is a natural process that takes place in absence of air

(oxygen). It involves biochemical decomposition of complex organic material by various

biochemical processes with release of energy rich biogas and production of nutrious

effluents.

Biological processes involved,

Hydrolysis: In the first step the organic matter is enzymolysed externally by extracellular

enzymes, cellulose, amylase, protease &lipase, of microorganisms. Bacteria decompose long

chains of complex carbohydrates, proteins, & lipids into small chains. For example,

7

Polysaccharides are converted into monosaccharide. Proteins are split into peptides and

amino acids.

Acidification: Acid-producing bacteria, involved this step, convert the intermediates of

fermenting bacteria into acetic acid, hydrogen and carbon dioxide. These bacteria are

anaerobic and can grow under acidic conditions. To produce acetic acid, they need oxygen

and carbon. For this, they use dissolved O2 or bounded-oxygen. Hereby, the acid-producing

bacteria creates anaerobic condition which is essential for the methane producing

microorganisms. Also, they reduce the compounds with low molecular weights into alcohols,

organic acids, amino acids, carbon dioxide, hydrogen sulphide and traces of methane. From a

chemical point, this process is partially endergonic (i.e. only possible with energy input),

since bacteria alone are not capable of sustaining that type of reaction.

Methanogenesis: (Methane formation) Methane-producing bacteria, which were involved in

the third step, decompose compounds having low molecular weight. They utilize hydrogen,

carbon dioxide and acetic acid to form methane and carbon dioxide. Under natural

conditions, CH4 producing microorganisms occur to the extent that anaerobic conditions are

provided, e.g. under water (for example in marine sediments),and in marshes. They are

basically anaerobic and very sensitive to environmental changes, if any occurs. The

methanogenic bacteria belongs to the archaebacter genus, i.e. to a group of bacteria with

heterogeneous morphology and lot of common biochemical and molecular-biological

properties that distinguishes them from other bacterias. The main difference lies in the

makeup of the bacteria’s cell walls.

Symbiosis of bacteria:

Methane and acid-producing bacteria act in a symbiotical way. Acid producing bacteria

create an atmosphere with ideal parameters for methane producing bacteria (anaerobic

conditions, compounds with a low molecular weight). On the other hand, methane-producing

microorganisms use the intermediates of the acid producing bacteria. Without consuming

them, toxic conditions for the acid-producing microorganisms would develop. In real time

fermentation processes the metabolic actions of various bacteria acts in a design. No single

bacteria is able to produce fermentation products alone as it requires others too.

8

2.2 CONVENTIONAL BIOGAS PLANT

A typical biogas system consists of the following components:

(1) Waste collection

(2) Anaerobic digester

(3) Effluent storage

(4) Gas handling

(5) Gas use

For the methanation process, carbon sources are required .

4H2+CO2→ CH4 + 2H2O

Since biogas consists of up to 50% carbon dioxide, it is commonly used for this purpose as a

carbon source. In most discussed processes the carbon dioxide of the biogas is separated from

the methane. In a Sabatier reactor, it can be methanized chemically under high temperatures

and pressures . Another option is the biological methanation, called methanogenesis. Its mild

conditions regarding temperature and pressure are advantageous, but these processes suffer

from the complexity involved in the use of a biological system, and insufficient process

knowledge

Biogas typically refers to a mixture of different gases produced by the breakdown of organic

matter in the absence of oxygen. Biogas can be produced from raw materials such as

agricultural wastes, manure municipal waste, plant material, sewage, green waste or food

waste. It is a renewable energy source and in many cases exerts a very small carbon footprint.

Biogas is primarily methane and carbon dioxide and many have small amounts of hydrogen

sulphide, moisture and siloxanes. The gases methane, hydrogen and carbon monoxide can be

combusted or oxidized with oxygen. This energy release allows biogas to be used as fuel; it

can be used for any heating purpose, such as cooking. It can also be used in a gas engine to

convert the energy in the gas into electricity and heat.

9

Compound Formula %

Methane CH4 50-75

Carbon

dioxide

CO2 25-50

Nitrogen N2 0-10

Hydrogen H2 0-1

Hydrogen

sulphide

H2S 0-3

Oxygen O2 0-0.5

A biogas plant is the name often given to an anaerobic digester that treats farm wastes or

energy crops. It can be produced using anaerobic digesters. These plants can be fed with

energy crops such as maize, silage or biodegradable waste including sewage sludge and food

waste. During the process, the microorganisms transform biomass waste into biogas and

digestate.

Anaerobic digesters can be designed and engineered to operate using a number of different

configurations and can be categorized into batch vs. continuous process mode, mesophilic vs.

thermophilic temperature conditions, high vs. low portion of solids, and single stage vs.

multistage processes.

Batch or continuous

In a batch system, biomass is added to the reactor at the start of the process. The reactor is

then sealed for the duration of the process. In its simplest form batch processing needs

inoculation with already processed material to start the anaerobic digestion. As the batch

digestion is simple and requires less equipment and lower levels of digestion work, it is

typically a cheaper form of digestion.

In continuous digestion processes, organic matter is constantly added or added in stages to

the reactor. Here, the end products are constantly or periodically removed, resulting in

constant production of biogas. A single or multiple digesters in sequence may be used.

Examples of this form of anaerobic digestion include continuous stirred-tank reactors, up

flow anaerobic sludge blankets, expanded granular sludge beds and internal circulation

reactions.

Temperature

The two conventional operational temperature levels for anaerobic digesters determine the

species of methanogens in the digesters:

10

Mesophilic digestion takes place optimally around 30 to 380C, or at ambient

temperature between 20 to 450C, where mesophiles are the primary microorganisms

present.

Thermophilic digestion takes place optimally around 49 to 570C, or at elevated

temperatures upto 700C, where thermophiles are the primary microorganisms present.

Solids content

In a typical scenario, three different operational parameters are associated with the solids

content of the feedstock to the dogesters:

High solids (dry-stackable substrate)

High solids (wet-pumpable substrate)

Low solids (wet- pumpable substrate)

Complexity

Digestion system can be configured with different levels of complexity. In a single stage

digestion system, all of the biological reactions occur within a single, sealed reactor or

holding tank. Using a single stage reduces construction costs, but results in less control of the

reactions occurring within the systems. Here the anaerobic reactions are contained within the

natural anaerobic sludge contained in the pool.

In a two stage digestion system, different digestion vessels are optimized to bring maximum

control over the bacterial communities living within the digesters. Acidogenic bacteria

produce organic acids and more quickly grow and reproduce than methanogenic bacteria.

Methanogenic bacteria require stable pH and temperature to optimize their performance

2.3 APPLICATIONS

Using anaerobic digestion technologies can help to reduce the emission of green house gases

in a number of key ways:

Replacement of fossil fuels

Reducing or eliminating the energy footprint of waste treatment plants

Reducing methane emission from landfills

Displacing industrially produced chemical fertilizers

Reducing vehicle movements

Reducing electrical grid transportation losses

11

CHAPTER 3

REACTOR OVERVIEW

3.1 REACTOR DESIGN

Fig 3.1.1.Design of reactor

This biogas system consist of two tank with gas recirculation and each tank contain 40L

volume. The first tank is the one in which feed is admitted. Initially the feed is grinded using

a pulverizer and fed through an inlet pipe of 10 cm diameter. The first tank consist of a dome

at the top, at which biogas get collected after the process. There are also connections

provided for transferring the partially reacted feed to the second tank and for gas

recirculation. The second tank consist of a hand operated mixer at the top. The gas is

recirculated to first tank from second tank using a small pipe of 1.2 cm diameter. The outlet

pipe level designed as little below than the inlet to ensure there will not be any back mixing.

12

Fig3.1.2.Reactor

Fig 3.1.3. Inlet

13

Fig 3.1.4.Reactor tank 1

Fig 3.1.5. inner view of reactor tank 1

14

Fig 3.1.6. The gas holder

Fig 3.1.7. The gas outlet

15

Fig 3.1.8. The reactor tank 2

Fig 3.1.9.The inner view of reactor tank 2

16

Fig 3.1.10. The hand operated agitator unit

3.2 WORKING

As described in the design, feed which is mainly kitchen waste is provided in the grinded

form using a grinding motor fitted outside the reactor, through the inlet to the first tank. The

grinding of the feed can enhance reaction rate by increasing the surface area exposed for

microbial action. Feed is always maintained to a particular level in tank and is in a semisolid

form and small amount of water is added in order to avoid foul smell. In the first tank mainly

acidogenesis takes place due to the presence of archeal population that is formed from the

kitchen waste itself. Along with CO2 some amount of methane is also is produced. When the

feed reaches a particular level in the tank the slurry moves to second tank through the

connection provided as in figure. The connection is made in such a way that there will not be

any back-mixing of the feed or slurry. And in the second tank methanogenisis is the main

process taking place. To accelerate this process an initial feed of cow dung is provided in the

second tank which acts as an inoculum. Most of the co2 is converted to methane in this tank

and this tank has also provision for the removal of feed or addition of cow dung as per the

requirement. Also a hand operated motor is provided at the top of the second tank which

provides adequate mixing of the slurry. Now most of the gases along with co2 and methane

produced in the second tank is re -circulated back to the first tank through a connection pipe.

This occurs naturally when the pressure reaches a particular level. This pipe is connected

17

such that it reaches the bottom of the first tank and the gas is bubbled into the slurry. This in

turn help in the overall mixing of the feed and increases the rate of the reaction. The

propionic acid production and the anaerobic condition in turn enhances the rate of reaction

which would reduce the initial time taken for the production of gas. The gas formed is

collected in a dome and can be taken out through the outlet provided.

3.3 MICROBIAL COMMUNITY ANALYSIS

The reactor was initially fed with an inoculum enriched in methanogenic bacteria before

adding the sludge. The quantitative changes of the bacterial and archaeal population in the

raw sludge and in the inoculum enriched in methanogens. A significant difference was found

for the bacterial and archeal populations between raw sludge and inoculum enriched

methanogens. The bacterial population dominated the Archaea in raw sludge. In contrast, the

archaeal population was drastically increased in the inoculum enriched in methanogens to

4.64 _ 1013 rRNA gene copies mL_1 while the bacterial population dropped to 1.08 _ 109

rRNA gene copies mL_1. The increase in the archaeal population after enrichment shows that

the culture was dominated with methanogens which were ready to use the supplemented

gases as a substrate for CH4 production. Thus, CH4 evolution from the supplemented CO2

was from the assimilation of CO2 by the enrichment of WAS under anaerobic conditions

.The archaeal population was dominated by members of the Methanobacteriaceae,

Methanospirillaceae and Methanosarcinaceae family.

18

CHAPTER 4

DISCUSSION

Biological methane production from CO2 sequestration by enriched methanogens from waste

activated sludge (WAS) was demonstrated in this study. This approach appears to be

reasonable to mitigate global warming. The WAS that was dominated by an active bacterial

community was changed to an active archaeal community after the enrichment for

methanogens Methane was not produced without CO2 supplementation to the enriched

methanogens due to the lack of available carbon for the archaeal community, and the carbon

in the methane was shown to come from utilization of the CO2 based on the use of 13CO2.

Therefore, application of this research can contribute to improving the environment by

reducing the greenhouse gas CO2.

19

CHAPTER 5

CONCLUSION

Biogas systems are those that take organic material (feedstock) into an air-tight tank, where

bacteria break down the material and release biogas, a mixture of mainly methane with some

carbon dioxide. The biogas can be burned as a fuel, for cooking or other purposes, and the

solid residue can be used as organic compost. Through this compact system, it has been

demonstrated that by using feedstock having high calorific and nutritive value to microbes,

the efficiency of methane generation can be increased by several orders of magnitude. It is an

extremely user friendly system.

20

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Reichl ,K. Sundmacher

[2] BIOGAS PRODUCTION FROM KITCHEN WASTE ,Department of Biotechnology and

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Ping Yu , Thomas K. Wood

[4] Hilkiah Igoni, M. F. N. Abowei, M. J. Ayotamuno and C. L. Eze (2008), Effect of

Total Solids Concentration of Municipal Solid Waste on the Biogas Produced in

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[7] Karve of Pune A.D (2006). Compact biogas plant compact low-cost digester from

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