Control and operation of a hybrid granular sludge reactor are mainly related to biological behavior of the granules under different process conditions. In this study, a 1-l digester was used to examine biological activity of the granules grown in a hybrid anaerobic reactor (AHR). The biogas production process was modeled and analyzed with three-process variables viz., influent COD (3000, 6500 and 10,000 mg COD/l), biomass concentration (2000, 4000 and 6000 mg VSS/l) and initial alkalinity (200, 1100 and 2000 mg CaCO3/l). Experiments were conducted based on a central composite face-centered design (CCFD) and analyzed using response surface methodology (RSM). In order to carry out a comprehensive analysis of the biogas production process, the indicative parameters viz., specific methanogenic activity (SMA), bicarbonate alkalinity produced-to-COD removed ratio, and COD removal were studied as the response. The maximum SMA was modeled to be 0.99 g CH4-COD/g VSS d under CODin, initial bicarbonate alkalinity (BA) and biomass concentrations of 10,000 mg COD/l, 2000 mg CaCO3/l and 2000 mg VSS/l, respectively, while the observed value was 1.039 g CH4-COD/g VSS d. The maximum COD removal was achieved at the condition when the influent COD was within the range of 4250-5250 mg/l and initial BA was more than 1100 mg CaCO3/l. Initial BA and CODin played an important role in the production of bicarbonate alkalinity during the digestion process. © 2008 Elsevier Ltd. All rights reserved.