Four anaerobic reactors were studied for the purposes of this work : two anaerobic fluidized bed reactors (AFBR) using diatomaceous earth and granular activated carbon as immobilization media (R1 and R2, respectively), a packed-bed reactor (R3), and a suspended growth reactor (R4). A nutrient-supplemented wastestream with glucose as the main carbon source was treated. Successful reactor start-up was achieved for all four anaerobic reactors. These reactors were able to handle organic loading rates of more than 12000, 7500, 6000 and 650 mg dm(-3) day(-1) for R1-R4, respectively. Anaerobic fluidized bed reactors were less affected by interruptions and adverse operating conditions than were packed-bed and suspended growth reactors. Immobilized cell reactors and, specifically, AFBRs were clearly superior to conventional high-rate digesters. This enhanced performance is primarily due to the very high cell retention ability of such reactors. High total organic carbon (TOC) removal efficiencies were achievable under pseudo-steady state operation. Removal efficiencies above 98% were observed for all reactors, Specific biogas production rates of 1.5-1.7, 1.4-1.7, 1.1-1.5 and 0.9-1.3 dm(3) of methane per gram of TOC removed for R1-R4, respectively, were attained. A consistent biogas methane content of 52.5-55.9% was observed. Biomass concentrations of 84, 91, 21 and 1.9 g VS dm(-3) were measured for R1-R4, respectively. Extremely high biomass concentrations in AFBRs were possible due to the high available specific surface area.