Environmental Engineering Reference
In-Depth Information
metal ions such as Na þ ,NH 4 þ ,K þ and Ca þ are also found to be inhibitory.
Heavy metal toxicity only seems to be concerned with free metal ions. The
presence of H 2 S in the biogas indicates a sufficient number of sulphur
compounds to remove the toxic effect of heavy metals, especially Cu þþ (Van
Velsen and Lettinga, 1979). Chlorinated solvents, biocides, pesticides and
antibiotics are also inhibitory to the anaerobic process. Only materials in
solution can be toxic to biological life (Pfeffer, 1979).
The serial processes of an anaerobic system, whereby the substrate for one
group may be the end product of another group, are theoretically much
more vulnerable to toxicities than aerobic processes, which are dominated
by parallel biochemical pathways (the initial substrate is available to all
bacterial groups). Methanogens are the most sensitive to inhibiting agents
but have a very forgiving nature (Vochten et al., 1988). Toxicity is reversible
in many cases and a significant acclimation potential is observed if sufficient
time is allowed.
5.4
Gas production and feedstocks
The main digestible components of solid waste are carbohydrates, proteins
and fats. The carbon to nitrogen ratio to yield maximum methane
production is 25:1 (Hobson, 1990). With knowledge of the chemical
composition of a waste outlined in equation 5.1, the quantity of methane can
be predicted from the stoichiometric formula (Buswell and Hatfield, 1936)
H 2 O
CH 4 þ
CO 2
a
4
b
2
n
2 þ
a
8
b
4
n
2
a
8 þ
b
4
C n H a O b þ
n
!
½
5
:
1
The following details an example based on the organic fraction of municipal
solid waste (OFMSW). The methodology is described by Tchobanoglous
et al. (1993). An ultimate analysis of the dry solids (DS) of OFMSW is
outlined in Table 5.1 (48.3% C, 7.6% H 2 , 35.8% O 2 , 3.3% N 2 ). The
remaining 5% of DS is inert (or ash). Knowledge of the atomic weights of
the elements allows stoichiometric description of the DS of OFMSW as
C 17 H 32 O 9 N.
￿ ￿ ￿ ￿ ￿ ￿
Table 5.1 Chemical composition of OFMSW
Component
Number of atoms
per mole
Atomic
weight
Weight contribution of
each element
%
Nitrogen
2.36 (1)
14
33
3.3
Carbon
40.25 (17)
12
483
48.3
Hydrogen
76.00 (32)
1
76
7.6
Oxygen
22.38 (9)
16
358
35.8
Total
950
95
 
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