Abstract:
Actinobacillus succinogenes cells were grown on Poraver® support particles in a
packed-bed reactor. Dilution rates (D) of 0.054–0.72 h-1 were investigated. Glucose
was used as substrate. CO2 (g) was bubbled into a complex medium to satisfy the
fixation requirements and maintain anaerobic conditions. At D ≥ 0.31 h-1, an initial glucose concentration of 35 g.L-1 was used; at lower dilution rates, this was
increased to 60 g.L-1 in order to avoid substrate limitations. By-product formation
included acetic and formic acids. A maximum productivity of 10.7 g.L-1 was obtained
at D = 0.7 h-1.
It was found that the system provided repeatable results at a given D. The longest
steady state period was maintained for about 97 h at D = 0.31 h-1. Steady state
stability was maintained for > 72 h at D < 0.31 h-1. For periods longer than 75 h,
however, inhibitory acid titres resulted in a gradual decline in productivity. At higher
dilution rates, long-term stability could not be maintained. The low acid titres
produced significant biofilm sloughing following aggressive biofilm growth, resulting
in oscillatory system behaviour.
For fermentation times < 115 h, the dilution rate was secondary to the attachment
area in determining the total biomass at steady state. Total biomass values were
then used to determine specific rates. A clear trend was observed, with the specific
glucose consumption rate, and specific acid production rates, increasing with
increasing D. This was explained by assuming a maintenance-driven system at all
Ds studied.
A product analysis indicated that at ΔS < 15 g.L-1, pyruvate formate lyase was the
preferred oxidative route. A shift to the pyruvate dehydrogenase pathway occurred at
higher ΔS values, so that the highest YSS values obtained exceeded 0.85 g.g-1.
A decrease in C3 by-product formation resulted in high YSS values being maintained,
indicating an additional, unknown source of nicotinamide adenine dinucleotide
(NADH). It is recommended that any process utilising immobilised A. succinogenes cells
should operate at an intermediate D, in order to maintain long-term reactor stability,
high productivities and good yields.
