In this study, dissolved ozone flotation and conventional mechanical diffuser ozonation systems have been applied to treat the downstream municipal wastewater. The process efficiency has been evaluated in the lab bench scale on the basis of the removal rate of some wastewater quality parameters, like turbidity, SS, color, UV254, COD, BOD, T-N, T-P and bacteria. DOF is found to be an efficient ozonation technique in comparison to the mechanical diffuser system under identical conditions, though the removal efficiencies of coliform bacteria are found to be the same in both cases. The optimum ozone dose of 6.1 mg/L is found in the DOF system. As for the result of this investigation, a DOF pilot plant with the capacity of 200 m(3)/d was designed and set up to treat the effluence of secondary wastewater, and detailed assessment of performance was carried out during one month operation. The results indicate a good removal rate of 86.9% turbidity, 81% TSS, 72.6% color, 82.4% BOD and 92% total phosphorus. However, lower removal efficiencies of 42.9% and 33.4% were obtained for COD and T-N respectively. Almost 100% disinfection efficiency was achieved by removing heterotrophic and coliform bacteria. DOF technology is very effective and economically viable for municipal wastewater treatment in the present day context.