Environmental Engineering Reference
In-Depth Information
Complex organic matter
(carbohydrates, proteins, fats)
1
Soluble organic molecules
(sugars, aminoacids, fatty acids)
2
Volatile fatty
acids
3
3
H
2
, CO
2
Acetic acid
4
4
CH
4
, CO
2
FIGure 32.1
Anaerobic pathway of complex organic matter degradation.
Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other
molecules such as volatile fatty acids (VFAs) with a chain length that is greater than acetate must
first be catabolized into compounds that can be directly utilized by methanogens. The biological
process of acidogenesis is where there is further breakdown of the remaining components by
acidogenic (fermentative) bacteria. Here VFAs are created along with ammonia, carbon dioxide,
and hydrogen sulfide as well as other byproducts. The process of acidogenesis is similar to the way
that milk sours.
The third-stage anaerobic digestion is acetogenesis. Here simple molecules created through the
acidogenesis phase are further digested by acetogens to produce largely acetic acid as well as carbon
dioxide and hydrogen.
The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here
methanogens utilize the intermediate products of the preceding stages and convert them into
methane, carbon dioxide and water. It is these components that make up the majority of the biogas
emitted from the system. Methanogenesis is sensitive to both high and low pH values and occurs
between pH 6.5 and pH 8. The remaining nondigestible material that the microbes cannot feed
upon, along with any dead bacteria, constitutes the digestate.
A simplified generic chemical equation for the overall process outlined next is as follows:
a
bc de
3
42
CHONS
+−−+ +
H O
2
→
abcde
42
abc
3
84 284
de abcde
3
84
→−+
++
CO
++−− −
CH
+
d
NH
3
+
e
HS
2
4
2
284
If this equation is applied to basic components of organic materials—carbohydrates, fats, and
proteins—the following results will be obtained:
1. Carbohydrates, example glucoses
CH O
3→+ 3
CO
CH
6 26
2
4
50
50
(gasscomposition, vol. %)
