methane and 30%-50% carbon dioxide, depending on
the substrate as well as small amounts of other gases
including hydrogen sulphide. Methane is the
component chiefly responsible for a typical caloric
value of 21-24 MJ/m3 or around 6 kWh/m3. Biogas is
often used for cooking, heating, lighting or electricity
generation [3]. AD, which can be divided into 6
parallel or series reactions completed by a variety of
bacterial species, as followed [2]: (1) hydrolysis of
solid organic particles and high molecular weight
compounds such as polymers which are too large to
permeate the cell membrane into soluble and
metabolizable molecules small enough to diffuse
across the membrane. Bacteria is catalyzed by
extracellular enzymes released by fermentative and/or
acid forming on hydrolysis process; (2) the sugars,
lipids, and amino acids resulting from carbohydrate
and protein hydrolysis are transformed into VFA
(volatile fatty acids), H2 (hydrogen) and CO2 by
fermentative bacteria; (3) H2 (hydrogen) producing
acetogens oxidize VFA with more than two carbon,
and long with methanogens which consume the H2
produced during oxidation; (4) homoacetogenic
bacteria transform CO2 and H2 into actic acid; (5)
acetoclastic methanogens degrade acetic acid into CH4
and CO2; (6) H2 utilizing hydrogenotrophs
(methanogens) reduce CO2 to CH4. The latter
population plays an important role in AD as they
regulate the process by maintaining a low
concentration of dissolved H2 making degradation of
organic matter thermodynamically feasible.