71
bacterial cells due to osmotic pressure (Alhraishawi and Alani 2018). Salt toxicity
is determined mostly by the type of cation the salt has.
Feedstock inflow to anaerobic digesters usually contains light metal ions, namely,
sodium, potassium, calcium, and magnesium. These cations may also be liberated
during the AD process (Chen et al. 2007). A study by Albraishawi and Alani (2018)
on codigestion of food waste demonstrated that increasing salt concentrations (0,
16, 30, and 60 g NaCl L
−1
) has a negative effect on the volume of biogas produced
(45, 21, 5, and 2 ml d
−1
). In terms of methane yield, a study by Lee et al. (2009) on
anaerobic digestion of leachate from a food waste recycling facility showed that
low-salt concentrations (0.5 and 2 g NaCl L
−1
) increase methane yield, but higher
salt concentrations (5 and 10 g NaCl L
−1
) resulted in a decrease in methane yield (36
and 41% reduction).
Anwar et al. (2016) showed that methane yield inhibition in anaerobic digestion
of food waste is negligible at salt concentrations of 8 g NaCl L
−1
, but salt concentrations greater than 8 g Nacl L
−1
resulted in a sharp decline in methane yield. The
cubic regression model y = 0.508 + 2.401x – 0.369x
2
+ 0.033x
3
was derived from
this experiment to describe sodium salt inhibition, where y is the methane yield and
x is the sodium salt concentration. This model predicted experimental results with a
small discrepancy of 10%. A study by Ogata et al. (2016) on the effect of salt on
biogas production of leachate in a waste landfill showed that methane production
decreased while carbon dioxide production was unchanged at a salt concentration of
35 ms cm
−1
(approximately 19 mg L
−1
). A salt content of 80 ms cm
−1
(approximately 44 mg L
−1
) decreased production of both methane and carbon dioxide. Based
on these studies on salt inhibition of the anaerobic digestion process, it can be
inferred that low salt concentrations in the AD reactant mixture (up to 2 g L
−1
NaCl)
increase methane yield, but higher salt concentrations (greater than 5 g L
−1
NaCl)
decrease methane yield.
5.1.3.5 Carbon to Nitrogen (C/N) Ratio
The quality of biogas produced by anaerobic digestion is determined by the growth
of the community of bacteria in the digester. The optimal carbon to nitrogen (C/N)
ratio for bacteria to grow is in the range of 20–30 because the bacteria use up carbon
20 to 30 times quicker than nitrogen (Bardiya and Gaur 1997; Malik et al. 1987). If
the C/N ratio is higher than optimal, the decomposition rate will be slower. When
the C/N ratio is low, the accumulation of ammonia can occur, which can inhibit the
activity of bacteria. Some substrates for AD with different C/N ratios are shown in
Table 5.4.
The difference in C/N ratios show that plant materials have a high C/N ratio and
animal manures have a low one. To achieve the optimum C/N ratio of 20/1–30/1,
plant material and animal manures are codigested.
The C/N ratio is a critical factor in the anaerobic digestion process, which shows
the balance of nutrients of input materials. Depending on the type of paddy rice,
untreated rice straw has a low concentration of total N content, even less than 1% of
5 Anaerobic Digestion of Rice Straw for Biogas Production
bacterial cells due to osmotic pressure (Alhraishawi and Alani 2018). Salt toxicity
is determined mostly by the type of cation the salt has.
Feedstock inflow to anaerobic digesters usually contains light metal ions, namely,
sodium, potassium, calcium, and magnesium. These cations may also be liberated
during the AD process (Chen et al. 2007). A study by Albraishawi and Alani (2018)
on codigestion of food waste demonstrated that increasing salt concentrations (0,
16, 30, and 60 g NaCl L
−1
) has a negative effect on the volume of biogas produced
(45, 21, 5, and 2 ml d
−1
). In terms of methane yield, a study by Lee et al. (2009) on
anaerobic digestion of leachate from a food waste recycling facility showed that
low-salt concentrations (0.5 and 2 g NaCl L
−1
) increase methane yield, but higher
salt concentrations (5 and 10 g NaCl L
−1
) resulted in a decrease in methane yield (36
and 41% reduction).
Anwar et al. (2016) showed that methane yield inhibition in anaerobic digestion
of food waste is negligible at salt concentrations of 8 g NaCl L
−1
, but salt concentrations greater than 8 g Nacl L
−1
resulted in a sharp decline in methane yield. The
cubic regression model y = 0.508 + 2.401x – 0.369x
2
+ 0.033x
3
was derived from
this experiment to describe sodium salt inhibition, where y is the methane yield and
x is the sodium salt concentration. This model predicted experimental results with a
small discrepancy of 10%. A study by Ogata et al. (2016) on the effect of salt on
biogas production of leachate in a waste landfill showed that methane production
decreased while carbon dioxide production was unchanged at a salt concentration of
35 ms cm
−1
(approximately 19 mg L
−1
). A salt content of 80 ms cm
−1
(approximately 44 mg L
−1
) decreased production of both methane and carbon dioxide. Based
on these studies on salt inhibition of the anaerobic digestion process, it can be
inferred that low salt concentrations in the AD reactant mixture (up to 2 g L
−1
NaCl)
increase methane yield, but higher salt concentrations (greater than 5 g L
−1
NaCl)
decrease methane yield.
5.1.3.5 Carbon to Nitrogen (C/N) Ratio
The quality of biogas produced by anaerobic digestion is determined by the growth
of the community of bacteria in the digester. The optimal carbon to nitrogen (C/N)
ratio for bacteria to grow is in the range of 20–30 because the bacteria use up carbon
20 to 30 times quicker than nitrogen (Bardiya and Gaur 1997; Malik et al. 1987). If
the C/N ratio is higher than optimal, the decomposition rate will be slower. When
the C/N ratio is low, the accumulation of ammonia can occur, which can inhibit the
activity of bacteria. Some substrates for AD with different C/N ratios are shown in
Table 5.4.
The difference in C/N ratios show that plant materials have a high C/N ratio and
animal manures have a low one. To achieve the optimum C/N ratio of 20/1–30/1,
plant material and animal manures are codigested.
The C/N ratio is a critical factor in the anaerobic digestion process, which shows
the balance of nutrients of input materials. Depending on the type of paddy rice,
untreated rice straw has a low concentration of total N content, even less than 1% of
5 Anaerobic Digestion of Rice Straw for Biogas Production