The activated sludge process is one of the most frequently used processes for biological wastewater treat-ment. Conventional gravity sedimentation (CGS), which is widely used as a secondary clarifier in activated sludgeprocesses, has a routine problem due to floating tendency, called bulking, caused by filamentous microorganisms. Dis-solved air flotation (DAF) has been applied as potential alternative to CGS as a secondary clarifier. A series of ex-periments were performed to measure physico-chemical characteristics and removal efficiency of activated sludge flocs.The removal efficiency of flocs corresponding in lag and exponential growth phases was lower, while that of flocs bothin stationary and endogenous phases considerably increased. The rise velocity of floc/bubble agglomerates was cal-culated by using a population balance (PB) model explaining the distribution of floc/bubble agglomerates. The ex-perimental results of flotation efficiency showed a similar tendency with the results predicted by PB model for the risevelocity and distribution of floc/bubble agglomerates. It was found from our study that the DAF process was very effec-tive as a secondary clarifier in the activated sludge process.