Environmental Engineering Reference
In-Depth Information
molecules are converted to the precursors for methane production by acetogenic
bacteria: acetates and molecular hydrogen.
Acetates and molecular hydrogen (with carbon dioxide) are the main substrates for
methanogenic archaea that are responsible for methane production. Archaea are a dis-
tinct group in the evolutionary tree of life and are positioned between prokaryotes
(bacteria) and eukaryotes (plants and animals). The range of substrates that can be
used by methanogenic archaea is limited to carbon compounds with no more than
two carbon atoms, explaining the sequence of reactions required to establish the anaer-
obic digestion process. It should be noted that actual removal of organic carbon from
the solid/liquid system is only achieved in the final step of the anaerobic digestion
process. The other steps in the process have no other function than to generate the
precursors for methane production.
14.2.2 Chemical Substrate Characterization
The overall stoichiometry of the anaerobic digestion can be estimated from the ele-
mental composition of the biodegradable organic substrate. Assuming an elemental
composition of the substrate of C
c
H
h
O
o
N
n
, stoichiometric coefficients can be esti-
mated for the end products of the anaerobic digestion process: methane (CH
4
), carbon
dioxide (CO
2
), water (H
2
O), and ammonium bicarbonate (NH
4
HCO
3
). A solution is
obtained by setting up elemental balances for C, H, O, and N:
0
1
0
1
2
+
h
c
3
n
8
−
o
4
c
2
−
h
8
−
5
n
8
+
o
@
A
CH
4
+
@
A
CO
2
C
c
H
h
O
o
N
n
!
8
−
4
0
1
h
4
−
c
−
7
n
4
+
o
@
A
H
2
O
+
n
NH
4
HCO
3
+
ð
RX
:
14
:
1
Þ
2
Evidently, more reduced substrates result in a higher CH
4
:CO
2
ratio in the biogas pro-
duced. A higher nitrogen content in the substrate also results in a higher production of
ammonium bicarbonate and, consequently, a (slightly) lower carbon dioxide content
in the biogas. More importantly, ammonium bicarbonate production results in the pro-
duction of pH buffer capacity. It should be noted that not only ammonium production
from organic carbon results in bicarbonate production but also other forms of alkalin-
ity result in the formation of bicarbonate, which is essential to maintain an approxi-
mately neutral pH value (pH ~7) as strictly required for a stable anaerobic digestion
process.
Alkalinity can be defined as the stoichiometric sum of the bases in solution. It is
typically expressed in base equivalents per unit volume (eq.L
−1
). In the anaerobic
digestion process, bicarbonate
-
carbonate alkalinity is the most important form of
alkalinity, and the bicarbonate
carbon dioxide chemical equilibrium typically deter-
mines the pH of the system. Neutralized organic acids such as common intermediates
in the anaerobic digestion process, e.g., acetate, propionate, and butyrate, also
-
Search WWH ::
Custom Search