Effect of hydrodynamic shear on biogas production and granule characteristics in a continuous stirred tank reactor

• The Metzner–Otto method is appropriate to estimate mean shear rate in a CSTR.
• Suitable hydrodynamic conditions contribute favorably to the biogas production.
• The methane content decreases continuously with increasing shear rate.
• The deformation of granule is observed with unchanged density after experiments.

Hydrodynamic shear plays an essential role in the formation, structure, mass transfer, metabolism of microbial community, and consequently the performance of a bioreactor. The present work focuses on an original investigation to study the effect of hydrodynamic shear on the anaerobic biodegradation process in a continuous stirred tank reactor (CSTR) equipped with a helical ribbon. The classical Metzner–Otto method was adopted to estimate the mean shear rate and the particle image velocimetry (PIV) was applied to visualize the instantaneous velocity field in the reactor. A constant prefactor of 17.1 was obtained to compute the mean shear rate from the stirring speed of the helical ribbon. An optimal shear rate of 6.8 s−1 is obtained for a maximal biogas production rate. It is due to two antagonist mechanisms, namely the enhanced mass transfer and the structural change of granules. Intense hydrodynamic shear diminishes the methane content as a result of the accumulation of volatile fatty acids (VFAs). In addition, the noticeable deformation of granules is mainly due to the abrasion rather than compaction of granules. The density of granules remains unchanged while mechanical strength is slightly enhanced after the experiments.

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