Catalytic wet air oxidation (CWAO) is a prospective method for the treatment of distillery effluents. In this work, the biodegradability of biomethanated distillery spent wash (biochemical oxygen demand (BODs) = 8100 and chemical oxygen demand (COD) = 40 000 mg/L) was improved by oxidation over a ferrous sulfate catalyst. The homogeneous CWAO process was optimized by investigating the effects of reaction variables, such as temperature (150-225 degrees C), oxygen partial pressure (0.69-2.07 MPa), and catalyst loading (16-48 mg/L). After oxidation at T = 175 degrees C for 1 h, adsorption over activated carbon (loading 5%) was performed at ambient temperature. As a result, a 73% reduction in COD and a substantial increase in the BODs/COD ratio (from 0.2 to 0.45) was observed. This ratio increased further to 0.52 when anaerobic digestion was performed, using 1% acclimatized biomass. For every cubic meter of wastewater, 1.1 N m(3) of biogas was formed (69% methane). After a final polishing step by aerobic treatment, BODs/COD = 0.58 and 91% COD was destroyed. Clearly, this work has provided a useful solution to the effective pretreatment and valorization of biomethanated distillery wastewaters. Lastly, we employed a two-step power-law model to describe the kinetics of COD conversion.