The flow pattern and longitudinal dispersion of liquid in upflow non-biological and anaerobic packed and fluidized bed reactors were studied. The data used for analysing the longitudinal dispersion in the packed and fluidized bed reactors were examined and the tracer experiments in a pilot-scale fluidized bed reactor were performed. The effects of biofilm growth and biogas production on the hydrodynamic behavior of these reactors were evaluated. A relationship between hydraulic retention time and Pe/Re ratio were obtained for both non-biological and anaerobic biofilm reactors. Some generalized correlations applicable to these reactors were found and the effect of the dispersion on reactor performance was discussed. An increase in the ratio of Pe/Re decreased the difference between the retention times of non-biological and anaerobic biofilm reactors. It was found that the Peclet number in the APBRs and AFBRs is higher than the non-biological ones.