In this study, the effluent of an alcohol distillery after passing through a chemical treatment is characterized in terms of its inert COD fractions and the changes likely to be induced by two-stage biological treatment on the magnitude of these fractions are observed. The experimentally obtained results indicated that the anaerobic treatment gives lower soluble inert COD fractions than a corresponding aerobic treatment for the investigated wastewater. Therefore, the preference of anaerobic treatment instead of an aerobic one as the first stage is ascertained. The anaerobic first stage has a COD removal efficiency of 96 % whereas with a subsequent aerobic treatment 79 % removal can be achieved on the basis of the stoichiometry of the inert GOD. The anaerobic fill-scale plant performance of 80 % can be improved up to 96 %.