A method of biologically treating wastewater with an integrated fixed film activated sludge process. The integrated fixed film activated sludge process includes biomass suspended in mixed liquor and biomass disposed on carriers. Under certain conditions the dissolved oxygen concentration in a reactor that includes the mixed liquor, biomass suspended in the mixed liquor, and the biomass on the carriers, biological treatment is performed primarily by the biomass in the mixed liquor. This is achieved by controlling or maintaining the dissolved oxygen concentration in the reactor at a relatively low concentration. When the biomass suspended in the mixed liquor is unable to adequately biologically treat the mixed liquor, the dissolved oxygen concentration in the reactor is controlled or maintained at a relatively high concentration. This enables biomass on the carriers to contribute more to the biological treatment of the mixed liquor than when the dissolved oxygen concentration was maintained relatively low.