BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 449

the organic substrate ( n ) and the N-content of the standard amino acid composition
of the protein chosen (N PR ):

n
N PR

η PR = : :

ð Eq 14 9 Þ

The fraction of LIP ( η LIP ) can be calculated from the different oxidation states of lipids

( γ LIP ) compared to the other organic substrates defined:

η LIP = h

− 2 o − 3 n

: :

ð Eq 14 10 Þ

γ LIP − 4

The fraction of carbohydrates subsequently follows from the total balance:

η CHO =1 − η PR − η LIP ð Eq : 14 : 11 Þ

One should realize that assumptions have been made while deriving these equations
and results should therefore be handled with care. The calculated fraction of lipids,
e.g., may be overestimated in the presence of other reduced organic compounds like
humic substances or alcohols. The estimation of the fraction of lipid compounds ver-
sus carbohydrates has furthermore shown to be strongly susceptible to measurement
errors in the COD/TOC ratio (Heijnen and Kleerebezem, 2010).
Figure 14.4 shows an example that demonstrates how a few simple waste measure-
ments allow for feedstock characterization and prediction of the biogas composition.

14.2.3 Factors Affecting the Process

14.2.3.1 Electron Acceptors The presence of electron acceptors in the anaerobic
digestion process may give rise to the generation of side products in the process. Due
to its poor solubility, dissolved oxygen

will hardly affect the anaer-
obic digestion process since the electron equivalent amount of organic carbon is many
orders of magnitude higher than the maximum oxygen solubility. Oxidized forms of
nitrogen (nitrite or nitrate) are only encountered in environments that have been
exposed to aerobic conditions. This normally means that most organic carbon is
degraded as well and therefore a mixture of organic carbon and strong inorganic elec-
tron acceptors is unlikely to be present in a single substrate.
Sulfate is often present in significant concentrations in substrates for anaerobic
digesters. Since sulfate is preferred as electron acceptor for organic carbon oxidation
in the anaerobic digestion process, it will normally be reduced to hydrogen sulfide,
which at approximately neutral pH will end up in the biogas. The hydrogen sulfide
concentration in the biogas can be estimated by including sulfate reduction in the
stoichiometric equations derived earlier (RX. 14.1). Typical hydrogen sulfide concen-
trations in biogas range from 100 ppm to 1 or 2 vol.%. A more detailed description
of the sulfate reduction process during anaerobic digestion can be found elsewhere

L −1

8mg O 2