Integrated Biodigester, Fermenter, and Composter System for Biogas, Liquid Organic Fertilizer, and Compost Production from Palm Oil Mill Waste

Palm oil mill effluent (POME) is a high-strength organic wastewater that poses significant environmental challenges if not properly managed.Although anaerobic digestion, composting, and liquid fermentation have been widely applied, comprehensive performance evaluations of fully integrated biodigester-composter-fermenter systems remain limited, particularly regarding the role of hydraulic retention time (HRT) and process bottlenecks.This study evaluates the HRT-dependent performance and limitations of an integrated POME valorization system using a laboratory-scale continuous stirred tank reactor (CSTR) operated at HRTs of 10-250 days.System performance was assessed based on digester stability, solid compost quality, and nutrient and heavy metal characteristics of the liquid effluent.POME exhibited high organic strength (COD 45,000 mg L -1 ; BOD 35,000 mg L -1 ; VS/TS ratio 0.82), supporting effective anaerobic digestion (COD removal up to 92 %, VS destruction up to 88 %, biogas yield up to 0.60 m³ kg -1 VS, and methane content up to 71 % at an HRT of 250 days).The solid fraction was successfully valorized through composting of empty fruit bunches using biodigester effluent, producing compost that met Indonesian National Standards.The resulting compost had a moisture content of 33 %, organic carbon content of 20.14 %, nitrogen content of 1.34 %, phosphorus content of 0.79 %, potassium content of 2.48 %, and magnesium content of 2.07 %, thereby meeting six of seven SNI 19-7030-2004 parameters; only pH at 8.85 exceeded the standard range of 6.8-7.49.In contrast, the liquid fraction showed limited agronomic potential, characterized by low organic carbon and macronutrient contents, and elevated lead (Pb) concentration exceeding permissible limits.These results identify the liquid effluent as the primary performance bottleneck of the integrated system, driven by nutrient depletion and heavy metal accumulation, and highlight the need for targeted post-treatment strategies.Overall, the integrated biodigester-composter-fermenter system demonstrated strong potential for solid-phase resource recovery and circular POME management, while effective liquid-phase utilization requires further process optimization.