The process of breakdown the organic materials
in the absence of oxygen produces methane is known as
anaerobic digestion and performed the biological activity of microorganisms. There are three
steps of anaerobic digestion of biogas production from orange peel; hydrolysis,
acidification, and methane formation. The
experimental studies using treated orange peel waste can be a suitable source
for biogas production when pre-treatment is performed to the digestion process.

c.      
Bacteria that
catabolize furfural and sulfates (short and curved rods)

b.      Methanothrix bacteria (long and tubular)

a.       Methanosarcina genus (spherically shaped)

In
the third stage named methane formation, the
methane producing bacteria decompose further the compounds with a low weight.
For example, in order to form methane and carbon dioxide, the methane procucing
bacteria utilise hydrogen, carbon dioxide and acetic acid. The methane
producing bacteria exist under natural conditions under water where anaerobic conditions are present. These
microorganisms are very sensitive to environmental variations since they are
obligatory anaerobic. The methanogenic bacteria are included in the
archeabacter  to acidogenic and acetogenic
bacteria. There
are three types of methanogenic
bacteria involved in the metane producing process (Mustafa, 2015);

Acidification is the second step in the
process where acid-producing bacteria transforms the monomers and fermentation
products produced in stage I, into acetic acid, hydrogen, and carbon dioxide. Since the bacteria are facultative
anaerobic they are able to grow in acidic environment. For production of acetic
acid, these bacteria need oxygen and carbon. The solved oxygen is used for this
propose. An anaerobic environment is obtained by acid producing bacteria, which
is vital for the methane-producing microorganisms. Furthermore, the acid
producing bacteria also reduce the compounds with a low weight into organic
acids, alcohols, amino acids, carbon dioxide, traces of methane and hydrogen
sulphide. The main acids produced in this stage are acetic acid, propionic
acid, and butyric acid, for also ethanol is produced (Mustafa, 2015).

In hydrolysis, which is the first step of
biogas production, the extracellular enzymes, such as celluloses, amylases,
proteases and lipases released by the bacteria, hydrolyse the organic material.
The hydrolysis stage is also known as the polymer breakdown stage. The complex
chain of carbohydrates, proteins and lipids are decomposed into shorter parts.
Polysaccharides are at this stage transformed into monosaccharides and proteins
are converted into peptides and amino acids. Large amount of
organic acids are also produced by acid forming bacteria in this fermentation (Mustafa, 2015).

Wherever there is human habituation,
organic waste is produced, which consist of household food waste, agricultural
waste, human and animal waste. For each year the amount of produced organic waste
is increasing, source-separation, composting and anaerobic digestion with
related biogas production is increasingly as a substitute
for waste management as land-filling of municipal solid waste (MSW). The process of breakdown the organic materials
in the absence of oxygen produces methane is known as
anaerobic digestion and performed through the biological activity of
microorganisms (Uddin, 2011).