Granule floatation may occur at higher organic loadings, resulting in a loss of active granules and a decrease in the treatment capacity of UASB reactors. Granular sludge sampled from full-scale UASB reactors was studied to investigate the mechanism of granule floatation. It was found that granules float due to the buoyant force created by the gas bubbles entrapped in the hollow spaces of granules. Two hypotheses on the mechanisms of hollowing were presented, and in order to prevent granule floatation, the protocol, i.e., grinding of floating granules by a mechanical device and dosing of Fe into the reactors, was proposed. The protocol was evaluated in a series of lab-scale experiments and proven to be effective and practical for use in full-scale UASB reactors.