• Part of the carbon is oxidized and another part
reduced to produce CO2 and CH4.
• These bacteria live and grow without oxygen.
• They derive the needed oxygen by
• The process is favored by wet, warm and dark
• The airtight equipment used for conversion is
known as a biogas plant or digester, which is
constructed and controlled to favour methane
i. Stage I:
• The original organic matter containing comlex
compounds, e.g., carbohydrates, protein and fats
is broken down through the influence of water
(known as hydrolysis) to simple water soluble
• The polymers (large molecules) are reduced to
monomers (basic molecules).
• The process takes about a day at 25oC in an active
Biogas production from waste biomass:
• Biomass if left to decompose in open air, is acted
upon by aerobic bacteria (bacteria that require
oxygen for their survival and growth) to produce
mainly CO2 and NH3.
• The total carbon component completely gets
oxidized to produce CO2 and no fuel is produced.
• Some form of nitrogen is also lost in the form of
• Biogas is produced from wet biomass with about
90-95% water content by the action of anaerobic
• The conversion process is known as anaerobic
fermentation (or biodigestion).
• Nutrients such as soluble nitrogen compounds
remain available in solution and provide
excellent fertilizer and humus.
• The energy available from the combustion of
biogas is 60-90% of the input dry matter heat
• The energy conversion efficiency of the
process is 60-90%.
• The biochemical processes proceed in three
stages as shown in figure (next slide).
ii. Stage II:
• The micro-organisms of anaerobic and
facultative (that can live and grow with or
without oxygen) groups, together known as
acid formers, produce mainly acetic and
• This stage also takes about one day at 25oC.
• Much of CO2 is released in this stage.
iii. Stage III:
• Anaerobic bacteria, also known as methane
formers slowly digest the products available
from the second stage to produce methane,
carbon dioxide, a small amount of hydrogen
and a trace amount of other gases.
• The process takes about two weeks time to
complete at 25oC.
• This third stage, i.e., methane formation stage
is carried out strictly by the action of
• Biogas plants are mainly classified as:
1. Continuous and batch types (as per the
2. The dome and drum types.
1. Continuous and batch types:
a. Continuous plant:
• There is a single digester in which raw material
are charged regularly and the process goes on
without interruption except for repair and
• In this case the raw material is self buffered
(like cow dung) or otherwise thoroughly mixed
with the digesting mass where dilution
prevents souring and the biogas production is
• The continuous process may be completed in
a single stage or separated into two stages.
i. Single stage process:
• The entire process of conversion of complex
organic compounds into biogas is completed
in a single chamber.
ii. Double stage process:
• This chamber is regularly fed with the raw
materials while the spent residue keeps moving
• Serious problems are encountered with
agricultural residues when fermented in a single
• The acidogenic stage and methanogenic stage
are physically separated into two chambers.
• Thus the first stage of acid production is
carried out in a separate chamber and only
the diluted acids are fed into the second
chamber where bio-methanation takes place
and the biogas can be collected from the
• Considering the problems encountered in
fermenting fibrous plant waste materials the
two stage process may offer higher potential
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