A laboratory study using two tapered anaerobic fluidized bed reactors (AFBRs) with taper angles (theta) of 2.5 degrees and 5 was undertaken to explore the hydrodynamic behavior of AFBRs during metabolic gas production. Predictive models were formulated to describe the hydrodynamic behavior of tapered AFBRs in a way that accounted for wake theory, hydrostatic pressure, biofilm thickness distribution, and ambient temperature. When the two tapered AFBRs were maintained in their respective fully fluidized states, both the measured and simulated three-phase bed heights, H-b,H-GLS, were higher than the corresponding two-phase bed heights, H-b,H-LS-a bed-expansion effect. In contrast, the measured and simulated three-phase bed-pressure gradient (-Delta P-t/V-b)(GLS), was lower than the corresponding two-phase value (-Delta P-t/V-b)(LS), especially at higher superficial liquid (u(l)) and gas velocities (u(g)). This helped to thicken the biofilm. Increasing theta from 2.5 degrees to 5 degrees, decreased H-b,H-GLS sufficiently to offset increases in (1 - epsilon(f)). Consequently, the values of both (Delta P-flu)(GLS) and (-Delta P-t/V-b)(GLS) decreased. The simulated values for H-b,H-GLS and (-Delta P-t/V-b)(GLS) were in fairly good agreement with experimental results. Compared to conventional AFBRs, the effect of metabolic gas production on the expansion behavior of tapered AFBRs was smaller. Thus, tapering of AFBRs facilitates the development of thick biofilms. (C) 2008 Elsevier B.V. All rights reserved.