This research was conducted to evaluate the capacity and stability of the Activated Sludge (AS) process retrofitted to the Integrated Fixed Film Activated Sludge (IFAS) process. Hydraulic retention time (HRT) and solids retention time (SRT) were used as independent variables in this investigation. The IFAS and AS processes were operated in parallel for carbon removal and nitrification at 6, 8, and 10 hours HRTs at which 4, 6, and 8 days SRTs were maintained. The AS system failed to attain steady state conditions at 10 hours HRT with 4 days SRT, 8 hours HRT with 4 and 6 days SRTs, and 6 hours HRT with 4, 6, and 8 days SRTs, whereas the IFAS system was stabilized until the SRT and HRT were at 4 days and 6 hours, respectively. Excessive filamentous microorganisms were observed in the IFAS and AS systems as the results of completely-mixed condition and high readily biodegradable organic content in the wastewater. The filamentous bulking was apparently the cause of system failure and the reduction of nitrification in the AS system. As the HRTs and SRTs were decreased or the system loadings increased, it was clearly demonstrated that the IFAS system was higher in capacity and stability than the AS system. The attached biomass in the IFAS system suppressed the growth of filamentous microorganisms by reducing the amount of substrates in contact with the filamentous microorganisms providing the system stability. Nitrification was completed in the IFAS system and could be independent of the suspended SRT. Both AS and IFAS systems could provide the same performance for COD removal at the experimental conditions.