A dynamical model describing the biological wastewater treatment process, taking into account the variability of the activated sludge biomass structure, is proposed. Where biomass turnover time is short, fast-growing microorganisms, consuming easily oxidizable substances, prevail in activated sludge. In such conditions, slowgrowing microorganisms, consuming poorly digestible substances, do not get enough time to become part of the biomass. As a result, such substances are discharged undecomposed. If the biomass turnover time is sufficiently large, then slow-growing microorganisms get established in the digester, augmenting the diversity of species. A method of dynamical decomposition developed by one of the authors is applied to model the above phenomenon. This method permits to represent the process as a sequence of relatively simple models which follow one another according to the conditions of process behavior. A method for stabilization of the biological treatment process has been developed.