Integrated fixed film activated sludge process (IFAS) consists in the addition of carriers into suspended culture of conventional activated sludge plants, to support the slow growing autotrophic biomass (Onnis-Hayden A. 2007). IFASs are increasingly implemented at domestic WWTPs due to their low footprint and consequently the low-cost application when upgrading existing activated sludge plants (Randall and Sen 1996). Robust know-how is already available to design IFAS for given nitrification performance. However very few information is available on the start-up phase dealing for instance with the development of the autotrophic biomass on the carriers. Thus, this step is always problematic for operators and done a bit arbitrarily. Therefore operating start up strategies of IFAS must be studied to 1) secure the development of autotropic biomass, 2) reduce the length of the start-up period, and 3) prevent from nitrites build-up. The objective of this research is to test an operating strategy aiming at developing rapidly autotrophic bacteria on the carriers on an IFAS plant filled with nitrifying sludge. In fact, the contribution of sludge (liquor) and biofilm (carriers) to the nitrification rate was studied, along with its evolution with time and loading rate.