BACKGROUNDHydrodynamic shear plays a major role in the formation, maintenance, structure, and metabolism of self-immobilized microbial communities in reactors. The study attempts to correlate the performance and microbial community structure of granular sludge in a mesophilic hybrid anaerobic reactor under different upflow velocities to fluidize the bed. The reactor was started with an initial upflow velocity of 4mh(-1), which was increased gradually till system deterioration to observe its effect on granule size, reactor performance, and microbial community structure. Monte Carlo method was used to quantify hydrodynamic shear range at different upflow velocities using a modified Ergun equation for the respective ranges of granule size. RESULTSUp to 6mh(-1) performance was positively affected, but beyond this upflow velocity, the system started showing reduced performance and completely deteriorated at 10mh(-1). Significant reduction in the abundance and diversity of both Archaea and Bacteria was observed. Among all methanogenic groups, Methanosaetaceae was most severely affected due to breakage and wash out of granules. CONCLUSIONIt can be concluded that hydrodynamic shear can be employed as a sensitive control parameter to improve reactor performance, and a specified range should be determined experimentally to avoid negative influences associated with this. (c) 2013 Society of Chemical Industry