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12 NO 3 þ 5C 2 H 5 OH ! 6N 2 10 CO 2 9H 2 O þ 12 OH ;
ð 2 : 12 Þ
6Fe 2 þ þ 12 OH ! 6Fe O ð 2 # :
ð 2 : 13 Þ
12 NO 3 þ 5C 2 H 5 OH þ 6Fe 2 þ ! 6Fe O ð 2 6N 2 10 CO 2
9H 2 O :
ð 2 : 14 Þ
2.5.3 Biogas Production in Situ for Mitigation of Soil Liquefaction
Earthquake is one of the most devastating types of geohazards on Earth causing
great economic losses including damages to infrastructures and properties. Many
of the damages were related to soil liquefaction—a phenomenon whereby soil
substantially loses strength and stiffness. Conventional ground improvement for
mitigating liquefaction-induced geotechnical hazards are vibroreplacement, com-
paction grouting, and deep dynamic compaction methods. However, these methods
are energy-consuming and expensive. Furthermore, dynamic compaction cannot
be used for retrofitting or in the city or built-up areas. Recent fundamental studies
in soil mechanics showed that inclusion of gas bubbles in saturated sand can
reduce its susceptibility for liquefaction substantially (Xia and Hu 1991 ; Yang
et al. 2004 ; Yegian et al. 2007 ; Eseller-Bayat et al. 2012 ). This finding has paved
the way for the development of one of the best solutions to the mitigation of
liquefaction disasters. It has been demonstrated that the liquefaction resistance of
saturated sand can be significantly increased when the sand is slightly de-saturated
with some voids displaced by nitrogen gas produced by denitrifying bacteria (Chu
et al. 2009a , ; Rebata-Landa and Santamarina 2012 ; He et al. 2013 ). The biogas
production in situ (Eqs. 2.15 - 2.20 ) has three major advantages over the other
methods: (1) the distribution of gas is uniform because bacteria and reagents are in
the liquid form and can be distributed evenly in sand; (2) the gas bubbles generated
by bacteria are tiny and thus the gas bubbles are relatively stable; and (3) nitrogen
gas is inert and has low solubility.
Different organic and inorganic substances can be biooxidized by nitrate but
ethanol (C 2 H 5 OH), acetic acid (CH 3 COOH), or glucose (C 6 H 12 O 6 ) that can be as
used as 75 % (w/v) syrup from corn, are most suitable electron donors because of
their low cost, availability, and high solubility in water. Their biooxidation by
nitrate (denitrification), is shown below:
1 : 67 C 2 H 5 OH þ 2NO 3 ! N 2 3 : 33 CO 2 þ H 2 O þ 2OH ;
ð 2 : 15 Þ
1 : 25 CH 3 COOH þ 2NO 3 ! N 2 2 : 5CO 2 þ 1 : 5H 2 O þ 2OH ;
ð 2 : 16 Þ
0 : 42 C 6 H 12 O 6 þ 2NO 3 ! N 2 2 : 5CO 2 þ 1 : 5H 2 O þ 2OH :
ð 2 : 17 Þ
These electron donors are very similar from stoichiometrical and economical
points of view (Table 2.3 ).
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