Optimization of CH₄ Removal from Diluted Emissions and Continuous PHB Production by Methylocystis hirsuta: Towards GHG Biorefineries

Abstract

This work aimed at optimizing the removal of diluted CH4 emissions (4 % v/v) by decoupling the actual gas residence time from the overall empty bed residence time (EBRT) in a continuous bubble column bioreactor (BCB) inoculated with Methylocystis hirsuta. Different EBRTs (120, 60, 30 and 15 min) were tested along with six internal gas-recycling ratios (QR/Q = 0, 2, 3, 6, 10 and 15, where QR is the recycling gas flow rate and Q the inlet gas flow rate treated). The results showed a CH4 elimination capacity of 35.2 ± 0.4 g m-3 h-1 (RE=72.9 ± 0.5 %) at an EBRT of 30 min and QR/Q of 15, which was identified as the optimum operating conditions in terms of CH4 abatement. The production of poly-3-hydroxybutyrate (PHB) was initially evaluated batchwise, in experiments limiting micro and macro nutrients such as K, Mn, N, and N with excess of Fe. Nitrogen starvation resulted in the highest PHB accumulation (28.0 ± 1.2 %) batchwise. Based on these results, nine sequential N feast-famine cycles were implemented in the BCB operating at an EBRT of 30 min and a QR/Q of 15. N starvation resulted in a gradual increase in the PHB content from 0.4 ± 0.0 % to 25.7 ± 0.1 % in the first cycle and up to 37.2 ± 2.0 % from the fifth cycle onwards. PHB productivities remained roughly constant during operation under N feast-famine cycles at 1.82-2.23 kg m-3 d-1, which corresponded to specific PHB productivities ranging from 15.9 to 21.6 mg PHB gX-1 h-1.