Sustainable strategies for anaerobic digestion of oil palm empty fruit bunches in Indonesia: a review

Indonesia has abundant oil palm empty fruit bunches (OPEFBs). However, high lignin content in OPEFBs may hinder their valorisation to bioenergy resources, as its degradation by microorganisms is rate-limiting. This paper aims to review OPEFB as feedstock in anaerobic digestion (AD) systems and sustainable strategies to enhance methane yield. This paper provides a comprehensive review of the availability of OPEFB, the prospective of AD technology, and compares various biological pre-treatment and co-digestion strategies for superior AD process. Improving the value-added benefits from OPEFBs and simultaneously making pre-treatment processes more sustainable is critical to wider application. Several scenarios were proposed by combining multi-stage pre-treatment of physical and mushroom cultivation pre-treatment prior to AD. This paper presents an original approach to establishing a commercially viable and sustainable AD of OPEFBs in Indonesia by integrating multi-product biorefinery and a circular economy. Further investigation is required to ensure cost-effective and eco-friendly configurations. Abbreviations: ABR: Anaerobic Baffled Reactor; AD: Anaerobic Digestion; AF: Anaerobic Filter; AFEX: Ammonia Fibre Expansion; AI: Artificial Intelligence; AnMBR: Anaerobic Membrane Bioreactor; ANN: Artificial Neural Network; ASP: Activated Sludge Process; CMM: Cattle Manure and Maize Silage; COD: Chemical Oxygen Demand; CPO: Crude Palm Oil; CPW: Cocoa Pods Waste; CSTR: Continuous Stirred-Tank Reactor; DH: Dry Husk; DM: Dry Matter; DMF: 2,5-dimethylfuran; EM: Effective Microorganisms; EGSB: Expanded Granular Sludge Bed; FBR: Fluidised Bed Reactors; FFB: Fresh Fruit Bunch; FTIR: Fourier-Transform Infrared Spectroscopy; GHG: Greenhouse Gasses; HRT: Hydraulic Retention Time; IC: Internal Circulation Reactor; IoT: Internet of Things; Lac: Laccase Enzyme; L-AD: Liquid-AD; LiP: Lignin Peroxidase; LRAD: Low-Rate AD; MARS: Multivariate Adaptive Regression Splines; MCC: Microcrystalline Cellulose; MDF: Medium-Density Fibreboard; MEMR: Ministry of Energy and Mineral Resources, Republic of Indonesia; ML: Machine Learning; MnP: Manganese Peroxidase; MPA: Marine Predators Algorithm; MSW: Municipal Solid Waste; OFMSW: Organic Fraction of Municipal Solid Waste; OLR: Organic Loading Rates; OPEFBs: Oil Palm Empty Fruit Bunches; OPF: Oil Palm Fibre; OPKS: Oil Palm Kernel Shell; OPMF: Oil Palm Mesocarp Fibre; OPT: Oil Palm Trunk; P(3HB): Poly(3-Hydroxybutyrate); PHA: Polyhydroxyalkanoate; PLTBg: Biogas Power Plants; POM: Palm Oil Mill; POME: Palm Oil Mill Effluent; SCG: Spent Coffee Grounds; SRF: Solid Refused Fuel; SRT: Solids Retention Time; SS-AD: Solid-State AD; TS: Total Solid; TSI: Torrefaction Severity Index; UASB: Up-flow Anaerobic Sludge Blanket; VFA: Volatile Fatty Acids; VP: Versatile Peroxide; VS: Volatile Solid; W2E: Waste-to-Energy.