The cylindrical anaerobic digester has a diameter of 13.5 m and height of 15.3 m, resulting in an effective volume 2,100 m3. The reactor comprises of gear motor, torque tube, scrapper set and draft tube for mixing. The system provides good contact between microbes and substrates, reduces resistance to mass transfer, minimizes buildup of inhibitory intermediates and stabilizes environmental conditions [4]. Palm oil mill effluent was pre-treated in an oil recovery tank and primary ponds before being fed to the anaerobic digester by a centrifugal pump. Seed inoculums of 600 m3 were taken from existing anaerobic ponds and fed to the digester. The total volatile solid concentration of seed was 18,319 mg/L, pH 7.17 and alkalinity 7,500 m/L as CaCO3. The characteristics of palm oil mill wastewater after being pre-treated by oil recovery and primary ponds before feeding to anaerobic digester are shown in Table 1. The main purpose of pre-treatment was to reduce the temperature of wastewater. The pH of influent was in the range of 4.72 - 5.38. The BOD5/COD ratio for wastewater from palm oil production was in the range of 0.63 – 0.85. This wastewater was considered to be a treated biological process because the BOD5/COD ratio for untreated wastewater was 0.5 or greater [5]. The pH of wastewater was closely monitored and maintained at 7.0-7.2 and Volatile Fatty Acid (VFA) content was maintained at less than 400 mg/l as CH3COOH to ensure an efficient anaerobic digestion process.