BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 458

Figure 14.9. The left graph in Figure 14.9 depicts the effluent composition of a PFR
with recirculation as a function of the recirculation ratio ( R ) imposed. R is defined as
the ratio of the recirculation flow rate to the influent flow rate. The influent of the
reactor is assumed to be composed of 100 g P-COD L − 1 , and the retention time in

the system is set to 20 days.
From Figure 14.9, it can readily be seen that at low values of R ( R < 0.3), only
hydrolysis occurs, and P is converted to S . This is due to the fact that a PFR with
no recirculation cannot sustain a microbial growth process because all microorgan-
isms in the system are washed out if the influent does not contain the required micro-
organism. For batch processes, this is probably more easy to imagine: if in a batch
process no microorganisms are supplied (no recirculation), no microbial growth will
occur, so no methane-containing biogas will be produced.
The extent of particulate substrate conversion is higher in a PFR than in a CSTR.
This is due to the particulate substrate (P) gradient in a PFR, resulting in a higher aver-
age substrate concentration and thus a higher average hydrolysis rate compared to that
in a CSTR. Consequently, the extent of particulate substrate (P) conversion in a PFR
decreases with increasing recirculation.
Evidently, the theoretically optimal reactor configuration is a PFR reactor with a
recirculation rate that is adequate to maintain the microorganisms that are catalyzing
the second step in the anaerobic digestion model proposed. In the specific case
described here, this implies a recirculation ratio of approximately 0.5.

14.4.2.2 Wet versus Dry Digestion Commercial forms of anaerobic digestion are
available for treatment of both substrates with a high solid content (30 40 wt% TDM,

dry anaerobic digestion) and substrates with a lower solid content (<30 wt%, wet anaer-
obic digestion). Wet anaerobic digestion is also implemented for dry substrates by
recycling of the water after filtering of the digestate. An evident advantage of wet
anaerobic digestion is easier handling (i.e., pumping and mixing) of the slurry, but
a major disadvantage is the requirement for dewatering after anaerobic digestion
and treatment of the liquid residue.
Dry anaerobic digestion typically is conducted in plug flow bioreactors with recir-
culation. The orientation of the reactor can be vertical like in the Dranco process or
horizontal like in the Kompogas process. Schematics of both types of dry anaerobic
digesters are shown in Figure 14.10. Both the Dranco and Kompogas processes are
operated at thermophilic temperatures (~60 C) and at a retention time of approxi-
mately 20

−

25 days, resulting in a volumetric loading rate of approximately 10 kg
ODM.m −3 .d −1 . Reported biogas yields in the process are 100

−

200 m 3 per tonne of −

waste processed.
Wet anaerobic digestion typically is conducted in CSTRs. Sometimes, a limited
degree of plug flow is established by placement of two bioreactors in series. Commer-
cial forms of the wet anaerobic digestion process are the Citec process and the Biogen
Greenfinch process. Both processes are operated at mesophilic temperatures.
Reported retention times range from 20 to 100 days. Evidently, the organic loading
rates at comparable retention times are two to three times lower for wet anaerobic
digestion processes compared to dry anaerobic digesters due to the higher water