Applicability of various pretreatment techniques to enhance the anaerobic digestion of Palm oil Mill effluent (POME): A review

•Various pre-treatment techniques undertaken on Palm Oil Mill Effluent (POME) was assessed.•The performance of the pre-treatment methods on other types of sludges were investigated.•Thermal pre-treatment technique is a viable option for the treatment of POME. Palm Oil Mill Effluent (POME) is a potential source of renewable energy. With the intensified energy demands in various industries, if the biogas production from POME treatment can be sustained, the palm oil industry can be made more environmentally and economically sustainable. The current POME treatment process is ineffective in removing residual oil and creates a large amount of greenhouse gas emissions. Furthermore, the limitation of the treatment process is the instability of the anaerobic digestion stage, which is affected by various operating parameters. Subsequently, the chemical and physical characteristics of POME vary depending on different factors, such as the harvest season and the efficiency of the palm oil extraction process. Therefore, having an effective pretreatment system can radically reduce the load on the anaerobic digesters, sustain biogas production, and enhance the treated effluent quality so that it can conform to the stringent environmental standards. The focus of this paper is to review the pretreatment methods (treatments undertaken prior to anaerobic digestion, which contributes to enhancing the hydrolysis step). A comparison of various types of sludge will be carried out to allow a thorough understanding of the impacts of these pretreatment methods and the suitability of their implementation for POME treatment. It was established that thermal pretreatment has bene proven to enhance solubilization, biogas production and the quality of the treated effluent. The increase in biogas production can easily compensate for the energy added to the system.