Influence of the flow velocity on membrane-aerated biofilm reactors: Application of a rotating disk for local flow control

•A novel rotating MABR for examining the effect of the flow velocity was developed.•The COD removal was almost constant at 0.7−7 cm/s of the aclimated flow velocities.•Lower flow velocities (< 1 cm/s) can be a dominant limiting factor for the removal.•The biofilm was classified into aerobic (compact) and anaerobic (ragged) layers.•The flow conditions mainly affected the ratio of aerobic and anaerobic bacteria. Membrane aerated biofilm reactors (MABRs) are energy-efficient wastewater treatment technologies that utilize gas-permeable membranes for aeration and biofilm attachment. In this study, a rotating flat-sheet MABR was developed as a novel experimental facility to control the local flow velocities by considering the distance from the rotational center in addition to the rotational speed. The MABRs were acclimated under different flow velocities to examine their impacts on the overall performance and the microbial community. The COD removal rates increased with the flow velocity when the latter was less than approximately 1 cm/s, and then plateaued (30–35 g·m−2·d-1) after that, implying a threshold of the two types of rate-limiting mass transfer resistances: biofilm-liquid boundary layer (< 1 cm/s) and biofilm itself (> 1 cm/s). Although the boundary layer resulted in the lowest COD removal rate when interrupting the rotation, it remained more than 70 % of the plateaued value and acceptable as a practical treatment. The microbial population suggests that the local flow velocities mainly affected the fate of anaerobic bacteria that proliferated in the outer (bulk-side) layer via the shear stress, whereas no significant impact was observed for major aerobic bacteria except for a few genera in the inner layer.