A mathematical model of the biological wastewater treatment process, based on simple balance equations, is proposed. The model of the process, a nonlinear system of ordinary differential equations, has a general nature: the functions describing oxidation and the input flow are not specified. The invariant set is found and its asymptotic global stability is proved. On the basis of the proposed model, a technique has been developed to stabilize the wastewater treatment process when the pollutant inflow is non-stationary, piecewise-constant.