(456d) Detailed Analysis of Foam Bioreactor Using Escherichia coli Expressing Carbonic Anhydrase for Optimization of CO2 Capture

The foam bioreactor consisting of the recombinant E. coli expressing carbonic anhydrase (CA) in tiny aqueous foam bubbles has demonstrated high CO₂ capture at short gas retention time (up to 93% and 9570 g CO₂ per m³ _bioreactor h^-1, 24 s gas retention time, cell conc. of 4 g_dw L^-1 at 4% inlet CO₂). The present study showed the detailed analysis of alternative medium, mass transfer, biokinetic, characterization of carbonic anhydrase and operating strategies. The glucose-based auto inducible medium resulted in the specific growth rate of 0.082 h^-1 andCA activity of 1022 mU OD_600nm^-1 mL^-1 at 24 h which was four to fourteen times higher than those by  the conventional medium (LB-IPTG). The CA in the E. coil grown on the glucose-based auto inducible medium showed highest activity at pH 8.5 while it exhibited high stability up to 40 ^oC. The foam bioreactor provided high mass transfer (K_La of 80 - 210 h^-1 at 1.3 - 2.3 x 10^-5 m³/s) due to the large gas-liquid interfacial area in the foam bioreactor. The K_La in the bioreactor was also linked with the foam structure such as foam size, liquid film thickness of foam and gas holdup.  The biokinetic equation for the CO₂ capture followed the conventional Monod-type equation, but it also showed the drastic substrate inhibition with the gaseous CO₂ concentration above 12%. On the other hand, our study showed of significant inhibition of carbonic anhydrase by the produced bicarbonate in the medium which was 50% rate reduction at 40 mM bicarbonate. However, the periodic removal of bicarbonate from the culture medium was implemented and was shown to allow sustained high and stable CO₂ removal (~ 92%) with a high rate of mineralization. Besides, the long term operation of the foam bioreactor revealed limitation of glucose (for cell growth), lactose (for enzyme expression) and nitrogen source, thus feeding of glucose, lactose and ammonium chloride at regular interval (dilution rate of 2 d^-1) helped to sustain stable CO₂ removal ( > 90-92%) for 8 d. In short, the foam bioreactor using the E. coli displaying carbonic anhydrase would be a cost-effective process to effective CO₂ capture at short gas contact time and mild operating conditions.