BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 459

(a) (b)

Biogas

Biogas

Digestate

Digestate Substrate Substrate

FIGURE 14.10 Vertical Dranco dry anaerobic digestion bioreactor (a) and horizontal
Kompogas dry anaerobic digestion bioreactor (b).

content, and consequently, these bioreactors are two to three times larger than the
reactors used in dry anaerobic digestion for treatment of the same amount of substrate.

14.4.2.3 Substrate Properties A wide range of substrates are currently used in the
anaerobic digestion process, and they exhibit a big variation in biogas potential.
Table 14.3 provides indicative values for some feedstocks for the anaerobic digestion
process. The different substrates vary strongly in terms of biodegradability, hydrolysis
kinetics, and biogas potential. Pig manure, e.g., originates from a highly efficient
anaerobic digester and therefore, its biodegradability

is limited. Furthermore, due to the high nitrogen content of pig manure, ammonia inhi-
bition is potentially of importance.
A similar line of reasoning can be followed for sludge obtained from a sewage
treatment plant. Sewage sludge often consists of particulate organic matter in sewage
secondary sludge: biomass that is grown on water-soluble organic compounds during
secondary treatment. Due to the long residence times in the sewer as well as in the
sewage treatment plant, most of the readily degradable organic carbon is converted
to new biomass (poorly degradable organic carbon) and mineralized, and only poorly
degradable organic carbon remains. Evidently, this results in a limited biodegradabil-
ity of the remaining organic carbon that is used as substrate for the anaerobic digestion
process. Removal of particulate organic matter from sewage sludge prior to biological
secondary treatment (primary sludge) yields a stream containing more biodegradable
particulate organic matter and thus offers a higher biogas potential compared to sec-
ondary sludge.
The situation is different and typically more favorable for substrates that have not
been exposed to microbial degradation prior to the anaerobic digestion process, like
the OFMSW, and residues from bioenergy crops like maize silage. These substrates

— the intestinal tract of the pig —