BACKGROUND The potential of methane production from high solid anaerobic digestion (AD) of oil palm decanter cake (OPDC) in a laboratory-scale modified solid inclined (MSI) reactor was investigated. The OPDC was pretreated with liquid hot water (LHW) before being fed to the MSI reactor which operated under high solid content (21% total solids, TS) with leachate recirculation once a day at ambient temperature. The result was compared to the non-pretreated OPDC. The performance of the MSI reactor was evaluated in term of the percentages of solid reduction, methane production rate and yield including removal of organic solids (lignocelluloses and oil and grease). The process stability was also monitored. RESULTS Pretreatment of OPDC with LHW (OPDC-P) influenced lignocellulosic materials by increasing decrystalline cellulose from 50.6 to 28.9% and partial solubilization by reduction of hemicellulose and lignin from 7.6 to 4.9 and 10.6 to 9.5%, respectively. It was found that the chemical oxygen demand (COD) in OPDC-P was increased from 28.5.9 to 62.95 g.L-1. The methane yield and rate of OPDC-P from the anaerobic MSI reactor were as high as 411 L kg(-1)VS(added) and 23.8 L d(-1)(respectively) within a short time of only 15 days compared to corresponding values for nonpretreated OPDC of 245 L kg(-1)VS(added) and 4.9 L d(-1) within 50 days. The MSI reactor had high stability within a pH range of 7-8 and a TVA:alkalinity ratio of 0.1-0.2. CONCLUSION The anaerobic MSI reactor was able to handle high solid content at 21% of OPDC with high performance and stability within a short incubation time. This reactor could be implemented and promoted to utilize solid waste in the palm oil industry. (c) 2019 Society of Chemical Industry