Long-term operation for biohydrogen production with an efficient carrier-induced granular sludge bed (CIGSB) bioreactor had encountered problems with poor biomass retention at a low hydraulic retention (HRT) as well as poor mass-transfer efficiency at a high HRT or under a prolonged operation period. This work was undertaken to develop strategies enabling better biomass retention and mass-transfer efficiency of the CIGSB reactors. Supplementation of calcium ion was found to enhance mechanical strength of the granular sludge. Addition of 5.4-27.2 mg/l of Ca2+ also led to an over three-fold increase in biomass concentration and a nearly five-fold increase in the H-2 production rate (up to 5.11 H-2/h/l). Two reflux strategies were utilized to enhance the mass-transfer efficiency of the CIGSB system. The liquid reflux (LR) strategy enhanced the H2 production rate by 2.2-fold at an optimal liquid upflow velocity of 1.09 m/h, which also gave a maximal biomass concentration of ca. 22 g VSS/1. Similar optimal H2 production rate was also obtained with the gas reflux (GR) strategy at a rate of 1.0-1.49 m/h, whereas the biomass concentration decreased to 2-7g VSS/l and thereby the specific H2 production rate was higher than that with LR. The operation strategies applied in this work were effective to allow stable and efficient H2 production for nearly 100 days. (C) 2004 Elsevier Inc. All rights reserved.