Simultaneous methane abatement and PHB production by Methylocystis hirsuta in a novel gas-recycling bubble column bioreactor

The limited gas-liquid mass transfer represents the main challenge in the operation of cost-effective bioreactors devoted to the treatment of poorly soluble gas pollutants such as methane (CH4). This study evaluates the influence of internal gas-recycling strategies on the enhancement of CH4 abatement in a bubble column bioreactor inoculated with the methanotroph Methylocystis hirsuta. Maximum CH4 removal efficiencies of 72.9 ± 0.5% (corresponding to elimination capacities of 35.2 ± 0.4 g m−3 h−1) were recorded under process operation at an empty bed residence time of 30 min and 0.50 m3gas m−3reactor min−1 of internal gas-recycling rate. The accumulation of poly-3-hydroxybutyrate (PHB) in M. hirsuta was evaluated batchwise under limitations of potassium, manganese, nitrogen, and nitrogen with excess of iron. Nitrogen starvation resulted in the highest PHB content (28 ± 1%). Likewise, the implementation of sequential N starvation cycles in a continuous bubble column reactor operated at a gas residence time of 30 min and an internal gas-recycling rate of 0.50 m3gas m−3reactor min−1 supported a PHB content of up to 34.6 ± 2.5%, with a volumetric PHB productivity of 1.4 ± 0.4 kg m−3 d−1 and elimination capacities of 16.2 ± 9.5 g m−3 h−1.