Biomethanation
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Biomethanation involves the
biodegradation of organic wastes under strict anaerobic conditions to yield
methane-rich biogas.
Biomethanation of aqueous wastes
involves hydrolysis, acidogenesis, acetogenesis and methanogenesis reactions,
which generates a mixture of methane, carbon dioxide and other gases.
Biomethanation can be carried out in
a single step or two steps.
In the process of hydrolysis,
anaerobic bacteria breakdown complex organic molecules such as protein,
cellulose, lignin and lipids into soluble monomer molecules such as amino
acids, glucose, fatty acids and glycerol. Hydrolysis phase is relatively slow.
Acidogenic bacteria convert sugar,
amino acids and fatty acids to organic acids, alcohols and ketones, acetate, Co2
and H2. The products formed vary with type of bacteria as well as
with the temperature, PH etc.
Acetogenic bacteria convert fatty
acids and alcohols into acetate, hydrogen and carbon dioxide. Acetogenic
bacteria require low hydrogen for fatty acids conversion.
Methanogenesis is the formation of
methane by microbes known as methanogens. Methanogenesis in microbes is a form
of anaerobic respiration. Methanogens do not use oxygen to respire;
in fact, oxygen inhibits the growth of methanogens. The terminal electron
acceptor in methanogenesis is carbon.
The two best described pathways
involve the use of carbon dioxide and acetic acid as
terminal electron acceptors:
CO2 + 4 H2 →
CH4 + 2H2O
CH3COOH → CH4 +
CO2
The process of small-scale
Biomethanation includes feedstock colleting, pretreatment, fermentation,
treatment and purifying, storage and transportation.
Batch fermentation and
semi-continuous fermentation are usual technologies for small-scale
biomethanation. In batch fermentation technology, all the feedstock is added at
the first. The biomethane generates fast at the beginning and then decreases.
In semi-continuous fermentation technology, ¼ - ½ feedstock was added at the
first. When the biomethane generation slows down, more feedstock is added to
make the biomethane generation work in order.