Environmental Engineering Reference
In-Depth Information
The process brings about the conversion of organic raw materials to
biogas. This biogas is dominated by methane (50-70%) and carbon dioxide
(30-50%). Considering methane has a higher heating value (HHV) of
ca. 37.8MJ/m
n
3
and carbon dioxide has no energy associated with it (as it is
the product of complete combustion), biogas has an energy content of
between 19 and 26MJ/m
n
3
. Hydrogen sulphide is also present in the biogas;
the proportion depends on the characteristics of the feedstock. Typically,
biogas from animal slurry has a higher content of H
2
S than biogas from
crops. The bulk of the energy content of the material undergoing
biodegradation is conserved in the methane content, and only a minor
fraction is made available for bacterial growth and reproduction.
5.1.2 History
The first documented account of an anaerobic reactor was in France in
1891. The Mouras Automatic Scavanger was an airtight chamber in which
organic material was liquefied. In 1895 in Exeter, England, an engineer by
the name of Cameron invented the septic tank. It was utilised for the
preliminary treatment of municipal wastewater downstream of the coarse
and fine screens. The methane gas produced was used to heat and light the
wastewater treatment plant.
The septic tank evolved firstly to the Travis tank (1904) and then to the
Imhoff tank (1905). The Imhoff tank was utilised as a primary sedimenta-
tion step in wastewater treatment plants, with sludge storage within the
tank. This sludge was stored for months in the chamber where it was
biodegraded by anaerobic bacteria. After storage, it was inoffensive and
easy to dispose of. In 1927, Ruhrverband installed the first sludge heating
apparatus in a separate digestion tank. In the 1930s, a detailed description
of anaerobic digestion was published in the USA (Buswell and Hatfield,
1936). Much of the commercial applications of anaerobic digestion in the
latter half of the 20th century were applied to either high-strength
wastewaters (brewery and creamery wastewater with high organic loading
rates) as a precursor to aerobic treatment or to sewage sludge and
agricultural slurries. Anaerobic digestion is now viewed as a mature
technology for the treatment of wastewater, for the treatment of slurries and
sludges, for digestion of the organic fraction of municipal solid waste
(OFMSW) and, more recently, for renewable energy production through
digestion of crops (Murphy et al., 2011).
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