the release of biogas. To avoid such problem, continuous feeding is advisable, since
the fresh feed will disrupt the scum and will also provide continuous mixing.
According to the type of digester and TS content, the performance of the stirring
equipment varies. The most promising AD technologies in developing countries
contain no stirring equipment. Recirculation of removed digestate into the inlet of
the digester achieves a passive mixing process, which tends to flush the inlet pipe
and helps blending of new feed with digestate rich in microbes (Deublein and
Steinhauser 2008).
3.9.8 Toxicants
Antibiotics and the disinfection agents are also the important factors affecting the
production process. These are used in cattle to treat the sick animals and also to keep
the farm and the milk parlours clean. Low concentration doesn’t have any negative
impact. Continuous feeding leads to inhibition. Other substances like heavy metals,
salts, oxygen, hydrogen, sulphide, organic acids, free ammonia, tannins, herbicides,
insecticides, and micronutrients are also inhibiting the biogas production process
(Table 3.10). Ammonia nitrogen is the most common inhibiting agent of AD
process. In optimum pH condition, the total inorganic nitrogen share is in the form
of ammonium, but at higher pH and temperature, ammonia concentration increases
(Chen et al. 2008). The increased ammonia diffuses the cell membrane and disrupts
the proton and potassium balance leading to cell function damage. So, the intermediate digestion products are accumulated which results in acidification process, and
the biogas production could be ceased (Kayhanian 1999). Low concentrations of
heavy metals, minerals, ions, and detergents are important for bacterial growth but in
higher rate inhibit the bacterial growth.
Table 3.9 The C/N ratio
of different types of raw
materials
Sample raw materials
C/N ratio
Bird excreta
8–10
Human excreta
8
Animal dung
12–43
Water hyacinth
25
Agricultural waste
60–90
Table 3.10 The inhibitory and toxic concentration of heavy metals
Metal
Inhibition start (mg/l)
Toxicity to adopted microorganism (mg/l)
Cr
3+
130
260
Cr
6+
110
420
Cu
40
170
Ni
10
30
Cd
70
600
Pb
340
340
Zn
400
600
3 Biogas: An Effective and Common Energy Tool – Part I
93
the fresh feed will disrupt the scum and will also provide continuous mixing.
According to the type of digester and TS content, the performance of the stirring
equipment varies. The most promising AD technologies in developing countries
contain no stirring equipment. Recirculation of removed digestate into the inlet of
the digester achieves a passive mixing process, which tends to flush the inlet pipe
and helps blending of new feed with digestate rich in microbes (Deublein and
Steinhauser 2008).
3.9.8 Toxicants
Antibiotics and the disinfection agents are also the important factors affecting the
production process. These are used in cattle to treat the sick animals and also to keep
the farm and the milk parlours clean. Low concentration doesn’t have any negative
impact. Continuous feeding leads to inhibition. Other substances like heavy metals,
salts, oxygen, hydrogen, sulphide, organic acids, free ammonia, tannins, herbicides,
insecticides, and micronutrients are also inhibiting the biogas production process
(Table 3.10). Ammonia nitrogen is the most common inhibiting agent of AD
process. In optimum pH condition, the total inorganic nitrogen share is in the form
of ammonium, but at higher pH and temperature, ammonia concentration increases
(Chen et al. 2008). The increased ammonia diffuses the cell membrane and disrupts
the proton and potassium balance leading to cell function damage. So, the intermediate digestion products are accumulated which results in acidification process, and
the biogas production could be ceased (Kayhanian 1999). Low concentrations of
heavy metals, minerals, ions, and detergents are important for bacterial growth but in
higher rate inhibit the bacterial growth.
Table 3.9 The C/N ratio
of different types of raw
materials
Sample raw materials
C/N ratio
Bird excreta
8–10
Human excreta
8
Animal dung
12–43
Water hyacinth
25
Agricultural waste
60–90
Table 3.10 The inhibitory and toxic concentration of heavy metals
Metal
Inhibition start (mg/l)
Toxicity to adopted microorganism (mg/l)
Cr
3+
130
260
Cr
6+
110
420
Cu
40
170
Ni
10
30
Cd
70
600
Pb
340
340
Zn
400
600
3 Biogas: An Effective and Common Energy Tool – Part I
93