Design and performance analysis of three phase semifluidized biofilm reactors for aerobic treatment of industrial effluents have been attempted. Bioconversion processes that follow Monod-type kinetics and those following Haldane-Andrews kinetics (involving substrate inhibition to microbial growth) have been considered as typical cases. The bioreactor has been modelled mathematically by solving performance equations separately for the packed section and for the fluidized section, considering dispersed flow in both of these sections, the degree of axial dispersion being of larger magnitude in the fluidized section. The developed CAD package has been verified by comparing with typical pilot plant data. The attractive features of semifluidized bioreactors of this kind (operation at high capacity, high degree of BOD removal/pollutant degradation, which increases with increase in feed flow rate, low reactor volume requirement as compared to fluidized bed bioreactors operating on the same feedstock) have been adequately highlighted.