Effect of Microwave and Ultrasonic Pretreatments on Biogas Production from Anaerobic Digestion of Palm Oil Mill Effleunt

Problem Statement: Oil palm production is a major agricultural industry in Malaysia. In 2006, palm oil mills in Malaysia produced more than 58 million tonnes of Palm Oil Mill Effluent (POME). Existing treatment in a series of open lagoons at high ambient temperatures, results in the uncontrolled production of methane and carbon dioxide, which are both green house gases (GHGs). With the increased worldwide concern on environmentally friendly production processes particularly the emission of methane, it is important to develop an alternative concept for POME treatment. This study elucidates the effects of pre-treatment of palm oil mill effluent by microwave irradiation and ultrasonic on anaerobic digestion. Approach: Effects of pre-treatment on sludge characterisation parameters were monitored. The Soluble Chemical Oxygen Demand (SCOD)/total COD ratio and biodegradability of soluble organic matter increased significantly after both the pre-treatments which indicated an increase in disintegration of the floc structure of the sludge. Three identical bioreactors with working volume of 5 litres were used as anaerobic digesters at 32-35°C. The reactors were separately fed with pre-treated sludge (microwave, ultrasonic and combination of microwave and ultrasonic) and control sludge at different Hydraulic Retention Times (HRT) to check for the production of methane. Results: The maximum SCOD/TCOD ratio reached almost 29% after 30 min of ultrasonic treatment, while it was 45% after 7 min of microwave irradiation. The BOD₅/SCOD ratio also increased after the pre-treatments suggesting the biodegradability of the soluble organic material increased during the treatment. It was observed that TVFA released was increased after both the treatments, with microwave treatment showing a higher yield of TVFA. Greatest enhancement in methane production was shown by the 3 min microwave plus 10 min ultrasonic treatment. Conclusion: The microwave in combination with ultrasonic would be a rapid and economical method for sludge pre-treatment for enhancement of biogas production.