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

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 <euro>center dot kg- 1 , offers a novel approach to fostering a more competitive biogas market while contributing to carbon neutrality. This study evaluated the optimization of CH 4 gas-liquid mass transfer in 10 L bubble column bioreactors for CH 4 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 CH 4 elimination capacities (CH 4-ECs) of 10 -12 g center dot m- 3 center dot h-1 , an EBRT of 104 min mediated CH 4 limitation within the cultivation broth, resulting in a negligible biomass growth. Reducing the EBRT to 54 min entailed CH 4-ECs of 21 -24 g center dot m- 3 center dot h-1 , concomitant to a significant increase in biomass growth (up to 0.17 g center dot L center dot d-1 ) and reaching maximum ectoine and hydroxyectoine accumulation of 79 and 13 mg center dot 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 center dot L center dot d-1 and an ectoine content of 47 mg center dot 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.