Continuous anaerobic fermentations were performed
in a biofilm reactor packed with Poraver® beads.
Dilution rates (D) varied between 0.054 and 0.72 h−1, and
D-glucose and CO2 gas were used as carbon substrates.
Steady-state conditions were shown to be repeatable and
independent of the operational history. Production stability
was achieved over periods exceeding 80 h at values of D
below 0.32 h−1. In these situations, steady-state variation
(expressed as fluctuations in NaOH neutralisation flow
rates) exhibited a standard deviation of less than 5 % while
no indication of biofilm deactivation was detected. The
total biomass amount was found to be independent of the
dilution rate with an average dry concentration of 23.8±
2.9 g L−1 obtained for all runs. This suggests that the
attachment area controls the extent of biofilm accumulation.
Specific succinic acid (SA) productivities, based on the
total biomass amount, exhibited a substantial decrease with
decreasing D. An SA volumetric productivity of
10.8 g L−1 h−1 was obtained at D=0.7 h−1—the highest
value reported to date in Actinobacillus succinogenes fermentations.
SA yields on glucose increased with decreasing
D, with a yield of 0.90±0.01 g g−1 obtained at a D of
0.054 h−1. Production of formic acid approached zero with
decreasing D, while the succinic to acetic acid ratio increased
with decreasing D, resulting in an increasing SA
yield on glucose.