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 gm(-3) h(-1)) were recorded under process operation at an empty bed residence time of 30 min and 0.50 m(gas)(3) m-(-3)(reactor) 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 m(gas)(3) m-(-3)(reactor) min-1 supported a PHB content of up to 34.6 +/- 2.5%, with a volumetric PHB productivity of 1.4 +/- 0.4 kgm(-3) d(-1) and elimination capacities of 16.2 +/- 9.5 gm(-3) h(-1).