Environmental Engineering Reference
In-Depth Information
2. Fats, example palmitine—fatty acid
CHOHO
+
7
→
4.5
CO
+
11.5
CH
16
32
2
2
2
4
28
72
(gas composition, vol. %)
3. Proteins. For proteins there is no general formula. An approximate empirical formula for
amino acids is C
13
H
25
O
7
N
3
S and would yield:
CHONSHO
6+ → +
6.5
CO
6.5
CH HH
+
3
+
S
13
25 73
2
2
4
3
2
46.5
46.5
0
77 gascomposition, vol.%)
In this case NH
3
shows 0% in gas composition because the reactor conditions are usually such
that NH
3
stays completely dissolved (as NH
4
+
ion).
If we summarize the examples above and presume that equal parts of groups (proteins,
carbohydrates and lipids) are present in the substrate, biogas has following composition:
Methane (CH
4
)
56%
Carbon dioxide (CO
2
)
42%
Hydrogen sulfide (H
2
S)
2%
On the basic relations described above, we can conclude that lipids (fat-soluble molecules) give
the highest production of biogas with highest percentage of methane, proteins give little less biogas
production with relatively low percentage of methane, and carbohydrates give the lowest amounts
of biogas also with relatively low percentage of methane.
In general, most authors state the amount of methane between 55 and 70% vol. percent, rarely
more. A second component is carbon dioxide with 27-44%. There are also other trace gases usually
present, such as hydrogen H
2
(1-2%), hydrogen sulfide (up to 3%) and gases usually below the limit
of detection (NH
3
—ammonia, CO—carbon monoxide, and N
2
—nitrogen).
32.3.2.2 Factors that affect anaerobic digestion
As with all biological processes the optimal environmental conditions are essential for successful
operation of anaerobic digestion (Table 32.3). The microbial metabolism processes depend on many
parameters; therefore these parameters must be considered and carefully controlled. Furthermore,
the environmental requirements of acidogenic bacteria differ from environmental requirements
of methanogenic bacteria. In aspiration for providing optimum conditions for each group of
microorganisms a two-stage process of waste degradation was developed, containing a separate
reactor for each stage. The first stage is for hydrolysis/acidification and the second for acetogenesis/
methanogenesis. Provided that all of the degradation process has to take place in one single reactor
(one-stage process), usually methanogenic bacteria requirements must be fulfilled with priority.
Namely, methanogenic bacteria have much longer regeneration time, much slower growth, and
are more sensitive to environmental conditions then other bacteria present in the mixed culture
(Table 32.4). However, there are some exceptions to the case:
• With cellulose containing substrates (which are slowly degradable), the hydrolysis stage is
the limiting one and needs prior attention.
• With protein-rich substrates, the pH optimum is equal in all anaerobic process stages.
Therefore a single digester is sufficient for good performance.
• With fat-rich substrates, the hydrolysis rate is increasing with better emulsiication, so that
acetogenesis is limiting. Therefore, a thermophilic process is advised.
