Classification of Biogas Plants:
• Biogas plants are mainly classified as:
1. Continuous and batch type(as per the process).
a) Single stage process.
b) Double stage process.
2. The dome and drum type.
3. Different variation in the drum type.
(1)Schematic of Single stage and Double stage
• The main features of continuous plant are as follows:
1. It will produce gas continuously.
2. It require small digestion chambers.
3. It needs smaller period of digestion.
4. It has less problem compared to batch type and it is easier in operation.
• The main features of Batch type plant are:
1. The gas production is intermittent, depending upon the clearing of the digester.
2. It needs several digesters or chambers for continuous gas production, these are
3. Batch plants are good for long fibrous materials.
4. This plant needs addition of fermented slurry to start the digestion process.
5. This plant is expensive and has problems comparatively, the continuous plant will
have less problems and will be easy for operation.
(2) The Dome and the Drum Type.
• Mainly two types of biogas plants are normally used:
1. The floating gas holder plant (Also known as KVIC plant)and other is
2. Fixed dome digester. (Also known as Chinese Plant).
• In a floating gas holder, gas holder is separated from the digester. While in fixed
dome digester, the gas holder and the digester are combined.
• The fixed dome is best suited for batch process especially when daily feeding is
adopted in small quantities.
• The fixed dome type plant is generally built below the ground level and is suitable
for cooler regions.
(3) Different variations in the drum type:
• There are mainly two variation in floating type plant. One with water seal and
other without water seal.
• Water sealing makes the plant completely anaerobic.
• Cylindrical shape of the digester is preferred because cylinder has no corners and
so that there will be no chances of cracks due to faulty construction. This shape
also needs smaller surface area per unit volume, which reduces heat losses also.
• Moreover the scum formation may be reduced by rotating gas holder in the
Types of Biogas Plants:
Floating Gas Holder (KVIC digester)
Selection of Site for a Biogas Plant:
• Following factors must be considered while selecting the site for a biogas plant.
2. Minimum gradient
3. Open space
4. Water table
5. Seasonal run off
6. Distance from wells
7. Space requirements
8. Availability of water
9. Sources of cow dung/materials for biogas generation
Methods of Maintaining Biogas Production:
The techniques usually suggested for maintaining the biogas production are
briefly summarized as follows:
• Insulating the gas plant: Mineral wool, aluminum cladding, fiber glass.
• Hot water Circulation.
• Use of Chemicals.
• Solar energy System.
Problems Relate to Biogas Plants:
In biogas plants some problems are natural and some are created by humans
but all are controllable.
• Handling of effluent slurry is a major problem if the person is not having
sufficient open space to get the slurry dry.
• The gas forming methanogenic bacteria are very sensitive towards the
temperature compared to those of non-methanogenic . As in winter the
temperature falls there is a decrease in the activity of the bacteria. Which
indirectly decrease the gas production rate.
• Due to lack of proper training a lot of problems arises. It has been noticed
that many persons increase the loading rate and some also do not try to
mix the cattle dung with water, keeping in mind more gas production. Due
to this the slurry from inlet towards outlet is very slow or even stops. This
may cause accumulation of acids and drop in the PH and then failure of
• This can be reduced by use of solar heated hot water to make slurry,
manual or auto stirring of digester slurry, addition of various nutrients for
bacteria, covering of biogas plants by bags during night hours.
• Some persons add urea fertilizer in large quantities due to which toxicity
of ammonia nitrogen may cause a decrease in gas production.
• PH and acids play an important role in anaerobic digestion and should
remain under optimum range otherwise this may cause upsetting of
digester and even its failure.
• Leakage of gas from gas holder especially in case of Janta Plant is a major
and very common problem. When there is quite enough gas in the holder,
the leakage should be checked by using water and the points marked and
then get repaired.
Starting a Biogas Plant:
• Starting procedure is common to all types of Biogas plants. When the
construction is over then it is necessary to decide whether the digester is
to run on agriculture / forest residue or on animal / human waste.
• If the digester is to be fed with animal/ human waste then it is not
necessary to be inoculated because it contains necessary bacteria
although not in proper number as required.
• It has been found that if the cattle dung is kept for anaerobic digestion
mixing with water in 1:1 ratio at 37˚ C, the usual gas production starts on
20th day, but if the influent is mixed with effluent slurry obtained from
another biogas plant operating on some feedstock then the gas
production starts earlier.
• The gas production rate increases with increase in effluent slurry mixed
in with influent slurry.
• If the digester to be operated on agriculture waste then inoculation is
necessary otherwise there will not be any gas production.
Filling a Digester for Starting:
• For starting a digester its air should be taken out by filling it with influent
upto its over flow in every type of biogas plant.
• However in case of fixe dome (Janta Model) there are some chances that
the slurry may enter the gas outlet pipe whereas in floating type biogas
plant there will be some oxygen (air), at zero level of gas holder and there
may be explosion due to the mixture of evolved gas and air.
• To overcome this problem the space above the digester slurry be filled
with some inert gas such as nitrogen or with biogas.
• In the digester, in which influent has been inoculated with effluent slurry,
the gas production will be soon and the percentage of oxygen in digester
over headspace will become very less, thus minimizing the chances of
Utilization of Biogas:
• The main product of biogas are fuel gas and organic manure.
• Biogas is a flammable gas. Methane is the only combustible portion in the
gas and hence around 60% by volume is only usable for combustion.
• The biogas can be utilized effectively for household cooking, lighting,
operating small engines, utilizing power for pumping water by using
already known technology.
• There are millions of home today using this gas as fuel solely for cooking
purposes. The burning of cattle dung has not led to national wastage of
organic wastage of organic manure but has also caused health hazard
• The traditional chullas are amongst the major causes of high incidence of
respiratory dieses and trachoma to eyes of the women.
• For the use of biogas in domestic burning special types of burner and
nozzles are already designed by engineers.
• A biogas lamp of a 60 watts equivalent electric light can function for six to
seven hours if one cu. meter gas is available.
• It may be noted that biogas cannot be bottled into cylinders like L.P.G.
• Electricity could be produced directly by using bio organic matter as the
source of the in a biochemical fuel cell. The air is injected at the cathode
as an oxidizer and bioorganic matter at the anode as the fuel. The
electrolyte is usually an organic solution or an aqueous medium such as
• Biogas can be used to operate the both C.I (diesel) and S.I (petrol) engines.
The only requirement is that at the starting of engine pure form of Diesel
or Petrol is required.
• For the above purpose we have to do some modification in existing
engines. In petrol engine we have to modify the carburetor.
• Thee other main product of the biogas plant is the organic manure. This
comes out as slurry which is quite rich in nitrogen. It can be applied
directly to the farm by mixing with irrigation water.