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

ABSTRACT 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.