By maintaining the same operational conditions of one conventional fluidized-bed bioreactor (CFB) and two tapered fluidized-bed bioreactors (TFBs), the performance of the TFBs with taper angles of 5 degrees and 2.5 degrees were found to be superior to that of the CFB with a taper angle of 0 degrees. Experimental results together with statistical analyses showed that the bioparticle and hydrodynamic characteristics of the TFBs were significantly different from those of the CFB. Also, bioparticle stratification occurred in the three bioreactors. The biofilm thickness (6) and the specific biomass (beta) of the three bioreactors varied in the following decreasing order 5 degrees > 2.5 degrees > 0 degrees under the same volumetric loading. Meanwhile, the specific energy dissipation rate (w) and the bioparticle washout rates (W = 0.214 +/- 0.219; 0.537 +/- 0.493 g BAC dm(-3) day(-1)) of the two TFBs were considerably lower than that of the CFB (W = 1.086 +/- 0.916g BAC dm(-3) day(-1)). A lower w value results in increases in delta and beta, and a lower dry density of the biofilm (rho(d)). Accordingly, the performance enhancement with TFBs should be related to their lower w and W, thicker 6 and larger beta values.