The present work is aimed to use the "relaxation time approach" in order to ascertain the main parameters affecting the steady state behaviour of a Rotating Biological Contactor (RBC). A large concentration range of surfactant was experimentally tested at different rotational speed and substrate load in aerobic conditions. Hydraulic residence time and time constants (relaxation time) relative to the following mechanisms were estimated : substrates and oxygen uptake and mass transfer; mixing effects as a consequence of discs rotation. Substrate uptake rate and biodegradation efficiency are considered as probe parameters for the RBC performances. Time constants of different phenomena are compared and discussed considering operational parameters such as rotational Reynolds number and specific substrate load. Biomass characteristics are taken into account by considering two morphological classes for biofilm. The mapping of bottleneck phenomena on operational parameters is set-up, analysed and commented.