The DAF-system, new technology of controlling the combined sewer overflows(CSOs) combining settler and dissolved air flotation process was studied and tested the possibility to replace the first clarifier which was required 3 to 5 hours of hydraulic retention time(HRT). In order to remove suspended solid in CSOs using the DAF-system, the optimized operating condition is as follows. To achieve SS removal efficiency over 90%, the DAF-system should be operated under 15min of HRT and it showed that the DAF-system in contrast with the 1st settling basin was more effective and compact process because it requires 15min of HRT to gain 90% of SS removal efficiency stably in comparison with 1st clarifier of 3 to 5 hours achieving 70% of SS removal efficiency.