Conventional laboratory scale annular reactors were employed to investigate the growth dynamics of both heterotrophic and autotrophic biofilms. Based on the experimental observations and physiological aspects, which consist of defining two types of biomass: active biomass (Ma) responsible for substrate removal, and non-active biomass (Md), which plays no role in the biological substrate removal process but is responsible for the observed additional accumulation of biofilms. The experimental results showed that the biological constants were strongly dependent on the influent substrate concentration (So). It was found that the same was true for the volumic substrate removal rate (kov), which shows a surface reaction independent of the film thickness, and that substrate removal in both heterotrophic and autotrophic biofilm reactors remains reactive. The results demonstrated that thinner biofilms ranging from 20 to 30μm have a higher specific removal rate. It is preferable to use thin biofilms for attached culture industrial processes such as three-phase fluidized-bed and turbulent reactors. The results showed that it is possible to eliminate very high carbon loadings up to 10 to 15 kgTOD/m3/d in a three-phase fluidised-bed reactor, and promising results were obtained for simultaneous removal of carbon and nitrogen in the turbulent bed.