This work deals with the anaerobic digestion using upflow anaerobic sludge blanket (UASB) reactor to treat a mixture of food waste and domestic wastewater to generate renewable energy (biogas). The reactor was operated under the conditions of mesophilic temperature (35 °C), pH 6.5 (controlled by the addition of 3,500 mg/L calcium carbonate), 10 days of hydraulic retention time (HRT), and the influent C/N ratio of 29. The chemical oxygen demand (COD) removal efficiency and the methane content were 80% and 56%, respectively, achieved at the organic loading rate of 2 g COD/L/day. The methane content dropped to 37% due to the accumulation of long chain fatty acids (LCFAs), subsequently inhibiting the methanogenic activity.