Experimental data available in the literature for bed voidage and bed expansion index of biofilm-coated particles in a fluidized bed was extracted, sorted and used to study the validation of the proposed bed expansion index equations in the literature. Drag coefficients of biofilm-coated particles for all the aforementioned data were calculated using a new proposed equation [1] that describes drag forces as a function of Archimedes numbers. Archimedes number was proposed to be superior to Reynolds number for bed expansion definition. New equations based on Archimedes number and physical properties of biofilm-coated particles were proposed to calculate the bed expansion index of fluidized biofilm-coated particles as well as bed voidage in fluidized bed bioreactors. The new equations predicted the existing experimental bed expansion index and voidage with less standard average error than all other literature equations that related bed expansion to Reynolds number. (C) 2012 Elsevier B.V. All rights reserved.