Effect of temperature on the anaerobic digestion of palm oil mill effluent

Two continuous stirred tank reactors (CSTRs) each fed with palm oil mill effluent (POME), operated at 37 degrees C and 55 degrees C, respectively, were investigated for their performance under varies organic loading rates (OLRs). The 37 degrees C reactor operated successfully at a maximum OLR of 12.25 g[COD]/L/day and a hydraulic retention time (HRT) of 7 days. The 55 degrees C reactor operated successfully at the higher loading rate of 17.01 g[COD]/L/day and had a HRT of 5 days. The 37 degrees C reactor achieved a 71.10% reduction of chemical oxygen demand ( COD), a biogas production rate of 3.73 L of gas/L[reactor]/day containing 71.04% methane, whereas the 55 C reactor achieved a 70.32% reduction of COD, a biogas production rate of 4.66 L of gas/L[reactor]/day containing 69.53% methane. An OLR of 9.68 g[COD]/L/day, at a HRT of 7 days, was used to study the effects of changing the temperature by 3 degrees C increments. The reactor processes were reasonably stable during the increase from 37 degrees C to 43 degrees C and the decrease from 55 degrees C to 43 degrees C. When the temperature was increased from 37 degrees C to 46 degrees C, the total volatile fatty acid (TVFA) concentration and biogas production was 2,059 mg as acetic acid/L and 1.49 L of gas/L[reactor]/day at day 56, respectively. When the temperature was reduced from 55 degrees C to 40 degrees C, the TVFA concentration and biogas production was 2,368 mg as acetic acid/L and 2.01 L of gas/L[reactor]/day at day 102, respectively. By first reducing the OLR to 4.20 g[COD]/L/day then slowly increasing the OLR back to 9.68 g[COD]/L/day, both reactors were restored to stable conditions at 49 degrees C and 37 degrees C respectively. The initial 37 degrees C reactor became fully acclimatized at 55 degrees C with an efficiency similar to that when operated at the initial 37 degrees C whereas the 55 degrees C reactor also achieved stability at 37 degrees C but with a lower efficiency.