BACKGROUND: Anaerobic co-digestion of refractory liquid organic wastes is an alternative environmental management strategy with economic benefits arising out of biogas production. Laboratory-scale experimental investigations were carried out on the anaerobic co-digestion of two liquid organic wastes, food waste leachate (FWL) and piggery wastewater (PWW). Three important parameters affecting methane yield were chosen for this study, namely, mixing ratio, alkalinity and salinity, which were optimized using response surface methodology. RESULTS: The results were analyzed statistically and the optimum conditions identified as: mixing ratio (FWL: PWW) 33 (in terms of volatile solid, w/w) (2 on v/v), alkalinity 2850 mg CaCO 3 L -1, and salinity 3.4 g NaCl L -1. Under the optimum conditions, a cumulative methane yield (CMY) of 310 mL CH 4 g -1 VS added and VS reduction (VSR) of 54% were predicted. Mixing ratio and alkalinity showed the greatest individual and interactive effects on CMY and VSR (P < 0.05). A confirmation experiment under optimum conditions showed a CMY and VSR of 323 mL CH 4 g -1 VS added and 50%, respectively. This was only 1.04% and 1.1%, respectively, different from the predicted values. CONCLUSION: Anaerobic co-digestion of FWL and PWW carried out under the optimum condition may be a feasible and efficient treatment option for methane production. © 2012 Society of Chemical Industry.