Optimization of methane gas-liquid mass transfer during biogas-based ectoine production in bubble column bioreactors

Abstract

Nowadays, the utilization of biogas for energy generation is hindered by the declining production costs of solar and wind power. A shift towards the valorization of biogas into ectoine, a highly valuable bioproduct priced at 1000 €⸱kg^-1, offers a novel approach to fostering a more competitive biogas market while contributing to carbon neutrality. This study evaluated the optimization of CH₄ gas-liquid mass transfer in 10 L bubble column bioreactors for CH₄ conversion into ectoine and hydroxyectoine using a mixed methanotrophic culture. The influence of the empty bed residence time (EBRTs of 27, 54, and 104 min) at different membrane diffuser pore sizes (0.3 and 0.6 mm) was investigated. Despite achieving CH₄ elimination capacities (CH₄-ECs) of 10-12 g⸱m^-3⸱h^-1, an EBRT of 104 min mediated CH₄ limitation within the cultivation broth, resulting in a negligible biomass growth. Reducing the EBRT to 54 min entailed CH₄-ECs of 21-24 g⸱m^-3⸱h^-1, concomitant to a significant increase in biomass growth (up to 0.17 g⸱L⸱d^-1) and reaching maximum ectoine and hydroxyectoine accumulation of 79 and 13 mg⸱gVSS^-1, respectively. Conversely, process operation at an EBRT of 27 min lead to microbial inhibition, resulting in a reduced biomass growth of 0.09 g⸱L⸱d^-1 and an ectoine content of 47 mg⸱gVSS^-1. While the influence of diffuser pore size was less pronounced compared to EBRT, the optimal process performance was observed with a diffuser pore size of 0.6 mm.