Based on the essential role of nitrifiers in nitrification process, bio-augmentation of nitrifiers in activated sludge process via two configurations is proposed to improve nutrient removal under overloaded conditions. In the first configuration a bio-augmentation batch enhanced reactor was applied to activated sludge. To improve nitrogen removal in overloaded plant, side-stream bio-augmented sequential batch reactor was examined as the second configuration. The simulation of overloaded plant without additional aeration tanks as base-case was performed by ASM1 model. The operation of bio-augmentation configurations and overloaded plant with additional aeration tanks were also simulated. The results reveal that nitrification efficiency can be improved up to 88% under 30% overloading condition compared to the base-case by applying the second configuration to the overloaded plant. Second configuration shows a significant potential for nitrification improvement as a result of N-load reduction compared to the main process as well as augmentation of the endogenous nitrifiers. (C) 2017 Elsevier Ltd. All rights reserved.