Gas bubbles entrapped in biocatalyst particles subjected to hydrostatic pressure oscillations, e.g. during recirculation in loop reactors, will induce intraparticle liquid flows, and thereby enhance mass transfer in excess of diffusion. This ‘breathing particle’ mechanism was already demonstrated in methanogenic granules from an IC reactor, and led to an average macroscopic activity increase of 13%. The existence of the alternating convective liquid flow responsible for this higher activity has now been established independently with pulsed field gradient NMR, as the intraparticle water mobility during pressure oscillations was found 16.5% larger. Micro-electrode measurements of the internal pH of a granule revealed the occurrence of a fast liquid flow through a channel between a central cavity and the periphery during pressure cycling, and the subsequent diffusive relaxation under atmospheric conditions.

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