The objective of this work is to study the treatment of oily wastewater containing anionic surfactant at Critical Micelle Concentration (CMC) by the Modified Induced Air Flotation (MIAF) process. MIAF, which is the combined process between the IAF process and the coagulation process, was applied and also compared with the Induced Air Flotation (IAF) and coagulation processes. The study has shown that the removal efficiency, considered in terms of COD, was related to the alum dosage, pH value and gas flow rate. Moreover, the interfacial area (a) obtained experimentally from the bubble hydrodynamic parameters (bubble size, bubble rising velocity, bubble formation frequency) and the velocity gradient (G) have been proven to be the important parameter for controlling the flotation process efficiency and operation cost. The simple proposed correlation, based on the a/G ratio, provides relatively a good coincidence between the experimental and predicted values of treatment efficiencies in this study. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.