Biogas is produced after organic materials (plant and animal products) are broken down by bacteria in an oxygen-free environment, a process called anaerobic digestion. Biogas systems use anaerobic digestion to recycle these organic materials, turning them into biogas, which contains both energy (gas), and valuable soil products (liquids and solids).

1. Biological Conversion & Anaerobic
Digestion

2. INTRODUCTION
Biological Conversion
 Conversion of the biomass to fuel by exposing biomass to
certain microorganisms is called biological conversion.
 The secondary fuels are produced as a result of metabolic
activity of the microorganisms.
 Fermentation and anaerobic digestions are the two most
common biological conversion processes and products of these
processes are ethanol and biogas.

3. Biogas
 Biogas originates from the bio-degradation of organic material
under anaerobic conditions.
 Today biogas has several applications such as industrial and
household cooking, lighting, radiant heaters and incubators for
agricultural purposes and absorption refrigerators.
 Biogas system provides a whole range of benefits for their users,
the society and the environment in general.

4. Biogas
Benefits:
 Production of energy (heat, light, electricity),
 Transformation of organic waste into high quality fertilizer,
 Improvement of hygienic conditions via reduction of
pathogens,worm eggs and flies,
 Increase of productivity, mainly for women, in firewood
collection and cooking,
 Environmental advantages through protection of soil, water,
air and woody vegetation,
 Micro-economical benefits through energy and fertilizer
substitution,
 Additional income sources and increasing yields of animal
husbandry and agriculture,
 Macro-economical benefits through decentralized energy
generation, import substitution and environmental
protection.

5. Overview
 Fermentation is a natural process initiated by microorganisms,
similar to common yeast cultures, under anaerobic conditions.
 Ethanol can derive from any material which contains sugar.
 In the Fermentation process, sugar elements such as glucose,
fructose and sucrose are converted into ethanol and carbon
dioxide as metabolic waste products.
 The net chemical equation for the production of ethanol from
glucose is:
2
5
2
6
12
6 2CO
OH
H
C
2
O
H
C 


6. ANAEROBIC DIGESTION
Basic Process
 A simplified stoichiometry for anaerobic digestion of
biomass is;
 The whole process of biogas production from organic
wastes occurred in main three steps namely hydrolysis,
acidification, and methane formation.
 Three types of bacteria namely fermentative, acetogenic
and methanogenic are involved in hydrolysis, acidification
and methane formation, respectively.
2
4
2
5
10
6 3CO
CH
3
O
H
O
H
C 



7. Basic Process
 The three stage anerobic fermentaton of biomass
Organic waste
(Cellelose, Starch, Protein,
Lipid)
H2, CO2, Acedic Acid
Alcohol, organic acids, amino
acids , hydrogen sulphide and
other compounds
H2, CO2, Acedic Acid
CH4, CO2
Hydrolysis
Acidification
Methane
formation
Fermentative
bacteria
Acetogenic
bacteria
Methanogenic
bacteria

8. Hydrolysis
 In hydrolysis aerobic micro-organisms convert complex organic
compounds (carbohydrates, proteins and lipids) into simple forms
which are soluble and can be consumed by the micro-organisms.
 As an example, Polysaccharides are converted into
monosaccharides, lipids to fatty acids, proteins to amino acids and
peptides.
Acidification
 In the second step, acid-producing bacteria (acetogenic bacteria),
convert the intermediates of fermenting bacteria into mixture of acetic
acid (CH3COOH), H2, CO2, alcohols, organic acids, amino acids and
hydrogen sulphide.

9. Acidification
 The oxygen requirement for producing acetic acid is fulfilled by the
oxygen solved in the solution or bounded-oxygen.
 The acid-producing bacteria create an anaerobic condition which is
essential for the methane producing microorganisms.
Methane formation
 In the third step, methane-producing bacteria utilize
hydrogen, carbon dioxide and acetic acid formed in
acidification process to form methane and carbon dioxide.

10. Factors influencing the biogas production
Substrate temperature
 Optimal temperature range for biogas production: 20-28°C.
 This can be achieved in a satisfactory level only where mean
annual temperatures are around 20°C or above or where the
average daily temperature is at least 18°C.
 If the temperature is below 15°C, gas production will be so low
that the biogas plant is no longer economically feasible.
Changes in temperature
 The process is very sensitive to changes in temperature.
 Most of the biogas plants are builds in underground in order to
overcome this issue.
 The temperature fluctuations between day and night are no great
problem for plants built underground, since the temperature of
the earth below a depth of one meter is practically constant.

11. Available nutrient
 In order to grow, bacteria need organic substances as a source
of carbon and energy.
 In addition to carbon, oxygen and hydrogen, the generation of
bio-mass requires an adequate supply of nitrogen, sulfur,
phosphorous, potassium, calcium, magnesium and a number of
trace elements such as iron, manganese, molybdenum, zinc,
cobalt, selenium, tungsten, nickel etc.
 "Normal" substrates such as agricultural residues or municipal
sewage usually contain adequate amounts of the mentioned
elements.
 Higher concentration of any individual substance usually has an
inhibitory effect, so that analyses are recommended on a case-to-
case basis to determine which amount of which nutrients, if any,
still needs to be added.

12. Retention time
 The effective retention time may vary widely for the individual
substrate constituents depending on the vessel geometry, the
means of mixing, etc.
 Selection of a suitable retention time depends on process
temperature as well as on the type of substrate used.
 For liquid manure undergoing fermentation, the following
approximate values apply:
- liquid cow manure: 20-30 days
- liquid pig manure: 15-25 days
- liquid chicken manure: 20-40 days
- animal manure mixed with plant material: 50-80 days
 If the retention time is not maintained properly and it is too short,
the bacteria in the digester are "washed out" faster than they can
reproduce, and fermentation practically comes to a standstill.
This problem rarely occurs in WAB systems.

13. pH Value
 The best condition for the methane-producing bacteria is neutral
to slightly alkaline conditions.
 Once the process of fermentation has stabilized under anaerobic
conditions, the pH will normally take on a value of between 7 and
8.5.
 If the pH value drops below 6.2, the medium will have a toxic
effect on the methanogenic bacteria.
Nitrogen inhibition
 Nitrogen in the substrate inhibits the process of fermentation.
 Noticeable inhibition occurs at a nitrogen concentration of roughly
1700 mg ammonium-nitrogen (NH4-N) per liter substrate.
 The main prerequisite is that the ammonia level does not exceed
200-300 mg NH3-N per liter substrate.

14. C/N ratio
 Microorganisms required both nitrogen and carbon for
assimilation into their cell structures.
 Various experiments have shown that the metabolic activity of
methanogenic bacteria can be optimized at a C/N ratio of
approximately 8-20, whereby the optimum point varies depending
on the nature of the substrate.
Substrate solid content
 Solid content of the substrate impaired the mobility of the
methanogens within the substrate.
 Therefore the biogas yield decreases with the increase of solids
content.
 No generally valid guidelines can be offered with regard to
specific biogas production for any particular solids percentage.

15. Types of Biogas Plants
 There are various types of biogas plants available in the world and
they are classified mainly based on feeding method and construction.
 Based on the feed method, three different forms can be
distinguished:
 Batch plants,
 Continuous plants,
 Semi-batch plants.
 Based on the construction, two main types of simple biogas plants
can be distinguished:
 Fixed-dome plants,
 Floating-drum plants.

16. Batch type digesters.
 In batch type plants, materials fed into the digester at a time and
sealed only allowing the gas to exit and then emptied completely
after a fixed retention time.
 Each design and each fermentation material is suitable for batch
filling, but batch plants require high labor input.
 The major disadvantage of this type is unsteady gas-output.
1 - Digester.
2 - Gasholder.
3 - Gas pipe.
1
3
2

17. Continuous type digesters.
 Once the process started, regular quantity of waste are fed and
regular quantity of material discharged, continuously.
 They empty automatically through the overflow whenever new
material is filled in.
 Therefore, the substrate must be fluid and homogeneous. This
technology is suitable for both medium and large scale waste
treatment and large scale biogas production.
 Advantages of this type are constant and higher gas production

18. Semi-batch type digesters.
 If the two materials which have completely different digestion
rates (such as straw and dung) are to be digested together, a
biogas plant can be operated on a semibatch basis.
 The slowly digested straw-type material is fed in about twice a
year as a batch load. The dung is added and removed regularly.

19. Fixed Dome type digesters.
 A fixed-dome plant comprises of a closed, dome-shaped digester
with an immovable, rigid gas-holder and a displacement pit.

20. Fixed Dome type digesters.
 The gas is stored in the upper part of the digester.
 When gas production commences, the slurry is displaced into the
displacement tank.
 Gas pressure increases with the volume of gas stored, i.e. with
the height difference between the two slurry levels.
 If there is little gas in the gasholder, the gas pressure is low.
 The digesters of fixed-dome plants are usually masonry
structures, structures of cement and ferro-cement exist.
 Main parameters for the choice of material are technical
suitability (stability, gas- and liquid tightness), cost-effectiveness,
availability in the region and transport costs and availability of
local skills for working with the particular building material.

21. Fixed Dome type digesters.
 Currently, various types of fixed dome plants are available such
as Chinese fixed dome plant, Janata model, Deenbandhu and
CAMARTEC model.
rious types of fixed dome plants are available such as Chinese fixed dome plant,
l, Deenbandhu and CAMARTEC model. Chinese fixed-dome plant, shown in
s the archetype of all fixed dome plants and extensively use in China. The
ists of a cylinder with round bottom and top.
Chinese fixed dome type biogas digester
Fixed dome biogas digester: CAMARTEC model

22. Fixed Dome type digesters.
Fixed dome biogas digester - Nicarao design
Automatic
overflow
Waste
Biogas Gas collector,
fixed dome
Slurry
10
4
5
6
7
8
9
3
1
2
1. Mixing tank with inlet pipe and
sand trap.
2. Digester.
3. Compensation and removal tank.
4. Gasholder.
5. Gas pipe.
6. Entry hatch, with gastight seal.
7. Accumulation of thick sludge.
8. Outlet pipe.
9. Reference level.
10. Supernatant scum

23. Floating drum type digesters.
 Major difference between fixed dome and floating drum type plant
is, a floating-drum plant consists of a floating gas-holder, or drum.

24. Floating drum type digesters.
 This floats either directly in the fermenting slurry or in a separate
water jacket.
 The drum in which the biogas collects has an internal and/or
external guide frame that provides stability and keeps the drum
upright. If biogas is produced, the drum moves up, if gas is
consumed, the gasholder sinks back.
 Floating-drum plants are used mainly in continuous feed mode of
operation.
 They are used most frequently by small- to middle-sized farms
(digester size: 5-15m3) or in institutions and larger agro-industrial
estates (digester size: 20-100m3)

25. Advantages of fixed dome type.
 Produce just as much gas as floating-drum plants, if they are
gas-tight.
 Low cost operation
 Simple design
 Long life of the plant (20 years or more)
Disadvantages of fixed dome type.
 Utilization of the gas is less effective as the gas pressure
fluctuates substantially.
 Labor-intensive design
 Not easy to build.
 Difficult to achieve gas tightness.

26. Advantages of floating drum type.
 Simple operation
 Provide gas at a constant pressure
Disadvantages of floating drum type.
 The steel drum is relatively expensive and maintenance-
intensive.
 Removing rust and painting has to be carried out regularly.
 The life-time of the drum is short (up to 15 years; in tropical
coastal regions about five years).

27. Biogas Yield
 Biogas yield of a biomass material depends on the organic fraction of
dry matter in the material and the waste management system
associated with it.
 The dry matter (DM) of the waste is the matter left after removal of its
moisture content. It may be obtained as the weight loss on heating to
a temperature of 105 C.
 Whereas, Volatile Solids (VS) are defined as the organic fraction of
dry matter in waste.
 Around 50-60% of the initial energy content in the organic material
can be converted to biogas in a properly operated digester.

28. If you would like to donate us?
Scan below and donate us 0.013$ (US dollar) (5Rs Indian rupee)
Contact: If you want PPT/PDF files, please contact below.
Email: gnccmysore@gmail.com
Telegram:+919738137533(only for Chat)