Bio-augmentation can be used to obtain nitrification in activated sludge processes that operate at sub-optimal solid retention times. In this study, we evaluated the potential of augmenting the endogenous nitrifying bacteria, by implementing a nitrification reactor in the sludge return line. This reactor can be fed with an internal N-rich flow (e.g. effluent from the sludge treatment) or with an external ammonium source. A mathematical model based on ASM1 was developed and used to evaluate the potential of this technique. The bio-augmentation studied here aimed to enhance the nitrification process of highly loaded activated sludge systems. A calibrated simulation model of a high loaded wastewater treatment plant in The Netherlands was used for this study. A side stream process (the named BABE process) was included in the simulation. This process was fed with the ammonia-rich water generated by sludge digestion and subsequent thickening by centrifugation (the so-called rejectwater). An external source (artificial) of ammonium was also considered to evaluate the differences between the two origins of ammonium. The results showed that with the augmentation process, high loaded activated sludge systems can achieve nitrification even at low winter temperatures. The best effect is obtained for systems operating at approximately 50% of the minimal SRT without augmentation. The use of an internal ammonia source is more effective than an external source. The results of this study give a quantitative basis for the design of process internal bio-augmentation processes and the effect on the N-removal capacity of the treatment plant.