Biogas
is a term used to describe a mixture of gases containing mostly CH­­4
(Methane) (50 – 70 %), and some of other gases like CO2 (Carbon dioxide) (30 – 40
%), H2 (Hydrogen),
and H2S (Hydrogen Sulphide)
produced by microbial activity
which may be used as fossil fuel and natural gas substitute. During growth
certain microbes can produce certain gases as waste of their metabolism, the
gas produced depends on the type of microbes and what organic matter digested
by them. For example at the making of kombucha, Acetobacter xylinum is used to ferment tea which will produce
CO­2 as metabolism waste of digesting sugar, however certain
anaerobic bacteria which can be found in cellulosic material can produce a significant amount of methane.
Bacteria with this trait are classified as Methanogens, a common example for
methanogens is Methenobacterium which can be found in cow’s rumen or in sewage
sludges, but before the organic matters can be metabolized by these
Methanobacterium, it has to go through several processes involving digestion of organic matter by Hydrolitic
bacteria, Acidogens, and Acetogens to make it suitable for Methanogenesis.

Hydrolitic bateria is a group of bacteria that play
one of the most important role in biogas production. Hydrolitic bacteria catabolise
complex polymers inside organic matters into simpler monomer, making it easier
for other group of bacteria to digest it., this kind of bacteria is common
inside digestive system, that is why biogas is usually made from organic matter
originating from manure or sewage instead of other wastes since it is more
effective.   Based on book written by
Prakash S. B. (2012), Organic matters in use are generally composed of complex polymers such
as lipids, proteins, cellulose, hemicelullose and lignin, these complex
polymers have to be broken down into simpler soluble monomers by catabolizing it
with the help of water to help the digestion of organic matter,
enzymes and facultative anaerobic bacteria such as Cellulomonas sp, Eubacteria sp, Clostridium sp, and Ruminococcus sp, which are usually be found inside cow’s
digstion system or in sewage sludge,
these bacteria turns the organic matter into a
slurry composed of simpler soluble monomers, this process
takes place inside an anaerobic bioreactor or digester which the organic matter
is sterilized at 70 oC to kill pathogenic microorganisms and then
cooled inside with temperature of 35 – 38 oC for optimal mesophilic
environment or 52 oC for optimal thermophilic environment. According
to Yuni Erlita this process starts roughly in the first 8 day in traditional
in-home bioreactor and the gas produced are mainly CO2 (Carbon Dioxide),
which should be thrown away.

Simple monomers formed during hydrolisis then must be digested
 by another group of bacteria named
Acidogens changing them into acids and carbon dioxide, this group of bacteria
is also commonly found inside manure or sewage waste making it easy to find.
Said process is also crucial in biogas production because the quantity of acid
produced will diectly affects the quantity of biogas end product. According on
book written by Prakash S. B (2012), Acidogenesis process utilizes acid-producing
facultative anaerobic bacteria such as Escheria
coli, Clostridium sp, Actinomyces sp, and Corynebacterium sp, turning the
soluble monomers in the slurry into acids such as, H2
(Hydrogen), CO2 (Carbon Dioxide), NH3 (Ammonia), H2S (Hydrogen
Sulphide), shorter fatty acids, carbonic acids, alcohols, as well as  trace
of other byproducts, this process is called Acidogenesis, according to Yuni
Erlita, this process occurs at the same time and environment of Hydrolysis, and
done by bacteria classified as Acidogens.

The next bacteria involved in biogas production is Acetogens,
this bacteria will digest the acids produced by Acidogens in the previous process
and turn them into acetate. Acetate is one of the most important substance
produced by microbe in biogas production. Said substance is the one that will
be turned into methane in the last stage of biogas production. Based on book
written by Paul Singleton and Diana Sainsbury (2006), before Methanoges can
produce methane, it has to went into digestion that turns derivative of carbons, and other energy source matters from
acidogenesis into Acetic Acid (CH3COOH), some traces Carbon Dioxide (CO2), and Hydrogen
(H2) which is synthesized by Acetogens autotrophically, for example Acetobacter woodii who synthesize acetate
in biogas production which next will be digested and used by Methanoges to synthesize
methane needed to make biogas. According to Prakash S. B. (2012), All the microbe
involved in biogas production, including Acidogens live in optimal pH of 6.5 –
8 which is a slightly lower than neutral or basic solution.

The last stage of 
biogas production is Methanogenesis, which means a synthesization of
methane. Methanogenesis is done by methanogens through its digestion of acetate,
hydrogen, and carbon dioxide which will produce methane. Methanogens are also
commonly present in manure and sewage waste, which is why people mainly use
manure as the main organic matter to produce biogas.  According to Thauer and Shima (2006),
Methanogens are a group of anaerobic microorganism that produce a significant
amount of CH­­4 (Methane) and other gasses as metabolic byproduct, the example
of Methanogens are Methanobacterium sp.,
and Methanococcus sp., who live inside cattle’s digestive system.
Methanogens can easily digest CH3COOH (acetic acid), and produce CH­­4 (Methane) and a trace of CO2 (Carbon dioxide),
Methanogens then will digest CO2 (Carbon dioxide)  and H2 (Hydrogen), producing CH­­4
(Methane) and H2O (Water). According
to IPCC, 1 Kilogram manure can produce 0.24 m3
per kilogram or roughly 250 gram CH­­4 (Methane)
(50 – 70 %), with traces of
other gases like CO2 (Carbon dioxide) (30 – 40 %), H2 (Hydrogen),
and H2S (Hydrogen sulphide) (20% – 30%) over a 24 hour period of time, this
result depends on microbial activity, dry matter, organic matter, and time.