biogas production methods

1. Lecture 3
Biogas technology – classification - types of
biogas plants – KVIC and Deenabandhu model
biogas plants – factors affecting biogas plants.

2. Biogas technology
• The biogas technology offers an appropriate
technique to convert non-conventional energy
into conventional energy.
• The gas thus produced is a neat, combustible
and pollution-free fuel.
• India has made a significant contribution in
the field of biogas.

3. Applications of biogas plant
• Cooking fuel
• Fuel for lighting
• Fuel for motive power to replace diesel oil
• Enriched organic manure of agriculture and
aquaculture
• Manure for mushroom growing
• Manure for seed coating
• Use of light to trap insects at the farm
• To treat human excreta and for pollution control

4. Properties of biogas
• The decay of organic matter, particularly
human, animal and plant wastes, in the
absence of air produces an inflammable gas
which consists mainly of methane (CH4) and
carbon dioxide (CO2).
• Methane which is a principal constituent of
biogas is a colouless, odourless and tasteless
gas.

5. • Biogas unlike LPG cannot be converted to
liquid state under normal temperature. A
temperature of -162oC is needed to liquefy
methane at atmospheric pressure.
• Liquefaction of methane requires nearly
5000 psi (LPG liquefies at 160 psi) at a
temperature of -83oC.

6. Science of production of biogas
• It is a three-stage process in which three groups
of bacteria are involved.
a. Hydrolytic and acidogenic bacteria
b. Acetogenic bacteria
c. Methanogenic bacteria

8. Hydrolysis
• Starch and Glycogen are hydrolysed to disaccharides
by the action of amylases.
• Subsequently the disaccharides are cleaved to
monosaccharides by a glycosidase.
• Cellulose is hydrolysed to cellobiose and
subsequently to glucose by cellulase and cellobiase
which include both exo and endo glucanases.
• Lipases and esterase’s hydrolyse fats and lipids.
• Proteases catalyse the cleavage of peptide bonds of
proteins.

9. Acidogenesis and Acetogenesis
• Degradation of Monosaccharides
• Degradation of Fatty acids
• Decomposition of amino acids

10. Degradation of Monosaccharides
• A typical metabolic pathway for the
degradation of monosaccharides is the
Embden- Mayerhof-Pamas (EMP) pathway or
Glycolysis.
• The end product of Glycolysis in Yeast is
ethanol and in bacteria is acetic acid.

11. Degradation of Fatty acids
• It is generally agreed that the anaerobic
degradation of long chain fatty acids and
acetic acid can then either be liberated or the
acetate can be transferred to other functional
compounds.

12. Decomposition of amino acids
• In nature and in anaerobic biological
processes, proteolytic species of Clostridium
are largely responsible for the anaerobic
decomposition of amino acids.
• From a molecule of glutamic acid a molecule
of pyruvic acid and a molecule of acetic acid
are formed.

13. Methanogenisis
The low molecular weight acids produced in the acid
production stage are further degraded to methane and
carbon dioxide by the highly specialised group of
bacteria referred to as methane producing bacteria or
methanogens.
CH3COOH CH4 + CO2
CO2 + 4H2 CH4 + H2O
About two third of methane is derived from acetate
conversion and the one third as a result of CO2
reduction.

14. Types of biogas plants
• Continuous and batch types as per the process
• Dome and drum types
• Different variations in the drum type

15. Continuous and batch types

16. Continuous plant
• There is a single digester in which raw materials are
charged regularly and the process goes on without
interruption except for repair and cleaning etc.
• "The continuous process may be completed in a single
stage or separated into two stages.
The main features of continuous plant are that:
• It will produce gas continuously
• It requires small digestion chambers and it needs
lesser period for digestion

17. Single stage process
• The entire process of conversion of complex
organic compounds into biogas is completed
in a single chamber.
• The chamber is regularly fed with the raw
materials while the spent residue keeps
moving out.

18. Double stage process
• The acidogenic and methanogenic stage are
physically separated into two chambers.
• The first stage of acid production is carried out
in a separate chamber and only the diluted
acids are fed in to the second chamber where
biomethanation takes place and the biogas is
collected from the second chamber.

19. Batch Plant
• The feeding is between intervals, the plant is emptied
once the process of digestion is complete.
The main features of the batch plantare:
• The gas production is intermittent, depending upon the
clearing of the digester.
• It needs several digesters or chambers for continuous
gas production, these are fed alternately.
• This plant needs addition of fermented slurry to start
the digestion process.
• This plant is expensive.

20. The dome and the drum types

21. Floating gas holder plant
• The floating gas holder digester which is used in India
is known as KVIC plant.
• The floating gas holder digester developed in India is
of masonry construction with the gas holder made of
M.S. plates.
• The gas holder is separated from the digester.
Rusting of the gas holder as well as the cost of the
gas holder are the main drawbacks of this system.
Example: KVIC model biogas plant

22. Fixed dome digester
• In the fixed dome digester the gas holder and the
digester are combined.
• The fixed dome digester is usually built below ground
level.
• Local materials can be used in this construction.
• The pressure inside the digester varies as the gas is
collected.
• Example: Deenbhandu model and TNAU Sakthi
model.

23. Different variations in the drum type

24. • There are two main variations in the floating
drum design. One with water seal and the other
without water seal.
• Water sealing makes the plant completely
anaerobic. The other variations are of materials
used both in construction of the digester and the
gas holder.
• Bricks and stones are the commonly used
materials. Ferro cement rings are also used in
the construction of digester.

25. • Gas holders are also manufactured out of
ferro cement, as M.S. sheets get corroded.
• Polyethylene is also used in the construction
of gas holder.
• The latest design uses fibre glass reinforced
plastic.

26. Capacity determination
• Volume of waste to be digested daily
• Type and amount of waste available for
digestion, consistently
• Period of digestion
• Methods of stirring, the contents if any
• Method of adding the raw waste and
removing digested slurry
• Efficiency of the collection of the raw waste

27. • Climate condition of the region
• Availability of other cellulosic fermentable waste
in that area
• Information about sub-soil condition and water
table, and
• Type of the cover.
Generally no separate heating and stirring of the
contents are provided for digesting cattle waste
(Gobar Gas digester)
Stirring arrangement is provided for farmyard
waste and plant wastes as already discussed.

28. t
V
V


2
2
1
Where V1 = the volume of raw waste added daily.
V2 = volume of the waste after digestion.
t = period of digestion, in days.
Capacity =

29. Selection of site for biogas plant
• Distance – the distance between the plant and
the site of gas consumption should be less to
achieve economy in pumping of gas and
minimizing the gas leakage. For 2 m3 plant the
optimum distance is 10 m.
• Minimum gradient – for conveying the gas a
minimum gradient of 1% must be made available
for the line.

30. • Open space – the sunlight should fall on the
plant as temperature around 30oC is essential
for gas generation at good rate.
• Distance from wells – the seepage of fermented
slurry may pollute the well water. Hence a
minimum of 15 m should be maintained from
the wells.
• Availability of water
• Seasonal runoff – proper care has to be taken to
prevent the interference of runoff water during
the monsoon.

31. • Space requirement – Sufficient space must be
available for day to day operation and
maintenance. As a guideline 10 to 12 sq.m
area is needed per cu.m of the gas
• Sources of cow dung/materials for biogas
production – the distance between the
material for biogas generation and the gas
plant should be minimum to economize the
transportation cost.

32. Components of biogas plants
• Digester or fermentation chamber
• Gas holder or gas storage chamber
• Inlet pipe
• Outlet pipe
• Mixing tank
• Gas outlet pipe
• Inlet and outlet displacement Chambers and
• a Manhole or Digester cover

33. KVIC model biogas plant
• The design was developed and perfected in
India in the year 1954.
• This was taken up for propagation in the
villages the year 1962, by Khadi village
Industries Commission, Bombay - KVIC design.
It mainly consists of two main parts viz.,
(i) Digester or fermentation chamber or pit
(ii) Gas holder or the gas collector.

36. Digester
• Also called as the fermentation plant, it is a sort of
well of masonry work, dug and built below the ground
level.
• The depth of this well varies from 3.5-6 m and
diameter from 1.35- 6 m, depending upon the gas
generating capacity and the quantity of raw material
fed each day.
• The digester well is divided vertically into two semi-
cylindrical compartments by means of partition wall in
the centre.

37. • Two slanting cement pipes reach the bottom of the
well on either side of the partition wall.
• One pipe serves as the inlet and the other as outlet.
• An inlet chamber near the digester at surface level
serves for mixing dung and water which is done
mechanically or manually.
• The mixture of dung and water in proportion of 1: 1
or 4:5 by volume, called slurry, flows down the inlet
pipe to the bottom of the primary compartment of
the digester.
• The digester is designed to hold 60 days raw
material.

38. • The partition wall is lower than the level of the
digester rim and hence it is submerged in slurry
when the digester is full.
• The outlet chamber is again at surface level, just a
few centimeters below the level of the inlet
chamber.
• If both compartments of the digester are full and if
more slurry is added from the inlet, then an
equivalent amount of fermented slurry flows out
of the outlet and discharged into the compost pit.

39. Gas holder
• It is a drum constructed of mild steel sheets,
cylindrical in shape with a conical top and radial
support at the bottom.
• It fits into the digester like a stopper.
• It sinks into the slurry due to its own weight and
rests upon the ring constructed for this
purpose.
• As the gas is generated the holder rises and
floats freely on the surface of the slurry.

40. • A pipe is provided at the top of the holder for flow of
gas for usage.
• To prevent the holder from tilting a central guide pipe
is fitted to the frame and is fixed to the bottom in the
masonry work.
• The pressure, under which the gas is generated in this
arrangement, varies between 7-9cm of water column.
• The cost of the holder constitutes almost 40 per cent
of the total cost of the plant.
• The only maintenance required is the painting of the
gas collector at regular intervals to avoid rusting.

42. Materials for Gas holder
• Ferrocement
• Fibre
• Glass
• Reinforced polymer
All are quite expensive.

43. Working
• After the initial filling of the digester tank, drum
moves upwards due to gas pressure. Regular
loading of the plant should be commenced
after this.
• Rotate the gas holder once or twice every day
in order to break the scum. Once in a year the
gas holder should be painted with oil paint or
black paint to avoid rusting.

44. Fixed dome digester

45. • Fixed dome digester, which was developed and is widely
used in China, runs on a continuous batch basis.
• It could digest plant waste as well as human and animal
wastes.
• built below ground level - easier to insulate in a cold
climate.
• The digester can be built from several materials like bricks,
concrete, lime concrete and lime clay.
• This facilitates the introduction and use of local materials
and manpower.
• The variable pressure inside the digester was found to
cause no problems in China in the use of the gas.

46. Modified fixed dome digesters

47. In the Chinese fixed dome digester, a manhole is
provided on the top of the plant.
By modifying this design, two designs have been
developed
• Janata and
• Deenbandhu

48. In the Janata fixed dome digester
• the bottom is flat concrete
• the digester is cylindrical wall and
• the dome is a segment of sphere with the brick
masonry.
In the Deenbandhu fixed dome digester
• the lower part of concrete (digester) is a segment of
sphere and
• the upper part of brick masonry (dome) is a hemisphere.

49. Construction and working principle of
Janta model biogas plant

50. • This was first developed by the Planning,
Research and Action Division, Lucknow in 1978.
• It is an improved version of the chinese
fixed-dome biogas plant.
• It is a drumless type similar in construction to the
KVIC model except that the steel drum is replaced
by a fixed dome roof of masonry construction.
• The dome roof in the Janta model requires
specialized design and skilled masonry
construction.

51. • The foundation of Janta biogas plant is
laid at the base of the underground pit on
a levelled ground which bear the load of
the slurry as well as the digester walls.
• Digester is cylindrical in shape constructed
with bricks and cement.
• It should be noted that the diameter and
height ratio of the digester is kept 1.75:1.

52. • The gas is stored in gas portion, which is an
integral part of plant, between dome and
digester.
• The height of the gas portion is above the inlet
and outlet openings to beginning of dome,
and is equal to 30 to 40 per cent of plant
capacity.
• Dome is constructed over the gas portion,
with volume of 60 per cent of the plant
capacity.

53. • It must be constructed very carefully
integrating it with digester and gas portion so
that no leakage of gas can takes place.
• The gas outlet pipe is fixed at the top of dome
for laying the line.
• Inlet and outlet portions are constructed for
putting the fresh slurry inside the plant and to
take the digested slurry out.

54. • The opening to the digester for feeding
the waste laterial and effluent outlet
from it are also of large sizes.
• The discharge of slurry out of the plant is
due to pressure of the gas in the plant.
• Over the inlet portion, an inlet mixing
tank is also constructed to mix the dung
and water.

56. Construction and working principle of
Deenbandhu model biogas plant

57. Sections of Deenbandhu plant
• Foundation
• Digester cum fixed dome
• Inlet cum mixing tank
• Outlet slurry chamber
• Biogas outlet pipe

58. • The advent of Deenbandhu biogas plant
is mainly to reduce the initial cost of
installation as well as the subsequent
maintenance cost.

59. • The design essentially consists of segments of two
spheres of different diameters joined at their
bases.
• The structure thus formed acts as the digester and
the gas storage chamber and also provides for
empty sphere over the contents of the digester.
• The construction of the plant needs the services of
a trained mason to ensure that the construction is
done as per the design.
• The excavation of the pit should be in such a way
that it suits for the bottom concrete.

60. • Before laying the foundation it should be
confirmed whether the top of the plant /
slurry ejection level comes above the ground
level or below the ground level.
• Cement mortar of 1:3 mix is used for the
construction work.
• For laying the bricks, mortar is spread on the
base and bricks are placed on it.

61. • The outlet gate opening is left in the spherical
wall and after the wall has been built up the
level of the top or the opening an arch is laid
on it by making a form work.
• The wall upto the level of the top of the outlet
opening is plastered from outside with 1:3
mortar.
• Back filling is done with soil or sand by
ramming it hard so that no voids remain in the
outside soil.

62. • The inlet cum mixing tank is constructed as
per the design using 1:4 mortar and it is
connected to the inlet pipe fixed at a height of
30 cm from the bottom of the outlet tank and
it is plastered with 1: 4 mortar.
• The outlet tank is constructed in the size of
60 x 60 cm to a height of 57cm for 2m3 plant
and then the tank is expanded to a size of
100 x 165 after laying the proper base
concrete for the widened area.

63. • After curing the plant for 8-10 days, the plant
is left to dry for 2-3 days and then the ceiling
is painted with two coats of black synthetic
enamel paint.
• The plant is covered with soil to a height of 15
cm above the crown of the dome.
• The outlet tank is closed with the wooden
plank or reinforced concrete.

64. Working
• During the initial filling, fill the plant with slurry up
to second step level of the outlet (bottom of the
gas chamber).
• The regular loading of the plant should be
commenced only after automatic ejection of the
slurry through the outlet opening.
• Proper loading of the plant will avoid the scum
formation because of the slurry movement.
• The entire biogas plant should be covered with soil
to a minimum thickness of 15 cm.

67. Comparison between fixed dome and
floating drum models

68. a. Merits of Fixed-Dome Type

69. Floating gas holder type Fixed-dome type
Capital investment is high Capital investment in the corresponding size
of biogas unit is low
Steel gas holder is a must which needs to
be replaced after few years due to
corrosion damage
Steel gas holder is not required
Cost of maintenance is high As there is no moving part, the maintenance
cost is minimised
Life span of the digester is expected to be
30 years and that of gas holder is 5 to 8
years
Life span of the unit is expected to be
comparatively more
Movable drum does not allow the use of
space for other purposes
As the unit is an underground structure, the
space above the plant can be used for other
purposes.
Effect of low temperature during winter is
more
Effect of low temperature will be less
It is suitable for processing of dung and
night-soil slurry. Other organic materials
will clog the inlet pipe
It can be easily adapted / modified for use of
other materials along with dung slurry

70. Merits of Floating Gas-Holding Type

71. Floating gas holder type Fixed-dome type
Release of gas is at constant
pressure
Release of gas is at variable pressure which
may cause slight reduction in the efficiency of
gas appliances. To operate a diesel engine,
attachment of a gas pressure regulator in the
pipeline is a must.
Construction of digester is
known to masons but
fabrication of gas holder
requires workshop facility
Construction of the dome portion of the unit
is a skilled job and requires thorough training
of masons.
Location of defects in the gas
holder and repairing are easy
Location of defects in the dome and repairing
are difficult
Requires relatively less
excavation work
Requires more excavation work
In areas having a high water
table, horizontal plants could
be installed.
Construction of the plant is difficult in high
water table areas

72. Biogas Plant Models (India)
• KVIC (Khadi and Village Industries Commission) design.
• PRAD (Planning, Research and Action Division) design
• Murugappa Chettiar Research Centre design.
• Tamil Nadu Agricultural University dome type design
• ASTRA (Application of Science and Technology to Rural
Areas) design
• Himachal Pradesh Capsule design
• Kacha-Pucca model of Punjab Agricultural University
• Plug-flow design

73. • AFPRO (Action for Food Production) design
• Roorkee design
• Deen Bhandhu design
• Fibreglass fixed dome design (Underground model)
• Mobile biogas plants
• Plastic emulsion coated, heavily insulated,
temperature controlled Switzerland biogas plants.
• IARI (Indian Agricultural Research Institute) design
• Ganesh Model
• Ferro-cement Digester Biogas Plant

74. Feed quantity
• Cows - 10 kg per day
• Oxes - 12 kg per day
• Buffalo - 15 kg per day
• Calves - 5 kg per day
• Horses - 10 kg per day
• Goat/sheep - 5 kg per day
• Pigs - 2 kg per day
• Human excreta per person - 0.4 kg per day
• Chicken - 0.18 kg per day

75. Retention time
• Cow and Buffalo dung - 50 days
• Pig dung - 20 days
• Poultry droppings - 20 days
• Night soil - 30 days

76. Factors affecting biogas production

77. • pH or Hydrogen ion concentration – Micro-organisms
will be very active and biodigestion will be very efficient
in the pH range of 6.5 to 7.5
• Temperature – Methane bacteria work best at a
temperature between 35- 38 oC. The fall in gas
production starts at 20 oC and stops at a temperature
of 10 oC. The optimum mesophilic temperature lies at
about 35 oC, while the optimum thermophilic
temperature is around 55 oC.

78. • Total solid content of the feed material – Total solid
content of 8 – 10% helps in biodigesting the material at
a faster rate. Raw cow dung contains 80-82% of
moisture and the balance 18-20% is termed as total
soilds. Cow ding is mixed usually in the proportion of 1:1
in order to bring the TS to 8-10%
• Loading rate – it is defined as the amount of raw
material fed to the digester per day per unit volume.
Most municipal sewage treatment plants operate at
aloading rate of 0.5-1.6 kg of volatile solids per m3 per
day.

79. • Seeding – digested sludge rich in methane
formers is added as seeding to increase the
number of methane formers
• Uniform feeding
• Diameter to depth ratio – gas production per
unit volume of didigester capacity was
maximum when the Dia to depth ratio was in
the range of 0.66 to 1.00

80. • Carbon to nitrogen ratio – The optimum C/N
ratio that best suits for maximum
microbiological activity is 30:1 because during
the process of biomethanation anaerobes use
carbon 25 to 30 times more than that of
nitrogen
• Nutrients – The major nutrients required by the
bacteria in the digester are C, H2, O2, N2, P and
S. To maintain proper balance of N2 and P,
chopped leguminous plants and night soil
should be added

81. • Mixing or stirring - mixing improves
biomethanation
• Retention time – It is defined as the period of
retention of material for biogas generation,
inside the digester. This period will depends
on type of feedstocks and the temperature.
Normal value of retention period is between
30 and 45 days and in some cases 60 days

82. • Pressure – gas production will be better at
low pressure
• Acid accumulation inside the digester – Neem
cake is added to convert intermediate
products like acetic, propionic and butyric
acids to methane to avoid acid accumulation