Anaerobic co-digestions with fat, oil and grease (FOG) were investigated in two-stage thermophilic (55 degrees C) semi-continuous flow co-digestion systems. One two-stage co-digestion system (System I) was modified to incorporate a thermo-chemical pre-treatment of pH = 10 at 55 degrees C, which was the best pre-treatment Condition for FOG co-digestion identified during laboratory-scale biochemical methane potential (BMP) testing. The other two-stage co-digestion system (System II) was operated without a pretreatment process. The anaerobic digester of each digestion system had a hydraulic retention time (HRT) of 24 days. An organic loading rate (OLR) of 1.83 +/- 0.09 g TVS/L.d was applied to each digestion system. It was found that System I effectively enhanced biogas production as the thermo-chemical pre-treatment improved the substrate hydrolysis including increased COD solubilization and VFA concentrations. Overall, the modified System I yielded a 25.14 +/- 2.14 L/d biogas production rate, which was substantially higher than the 18.73 +/- 1.11 L/d obtained in the System II. (c) 2015 Elsevier Ltd. All rights reserved.