Synergistic effects of trace metals on hydrogen and methane production from palm oil mill effluent using two-stage anaerobic digestion

Abstract

The two-stage anaerobic digestion of palm oil mill effluent (POME) was optimized through trace metal supplementation (Mo2+, Ni2+, Co2+, and Fe2+). Optimal concentrations were determined as 10 mg·L−1 Mo2+, 6 mg·L−1 Ni2+, 6 mg·L−1 Co2+, and 10 mg·L−1 Fe2+, resulting in significant biogas yield improvements on hydrogen yield increased by 86.4 % (29.5 ± 0.9 to 55.0 ± 2.1 mL-H2·g−1-VS) and methane yield by 84 % (from 173.8 ± 7.8 to 320.0 ± 8.4 mL-CH4·g−1-VS). Gas composition improved, with H2 content increasing from 18.5 % to 32.0 % and CH4 content from 58.2 % to 72.5 %. Maximum process efficiency was achieved at 4-day HRT for hydrogen production and 20-day HRT for methanogenesis with metal removal 0f 93.5–94.8 %. Statistical analysis revealed strong correlations between metal concentrations and enzyme activities (R2 = 0.94, p < 0.001) and enzyme activities with biogas yields (R2 = 0.92, p < 0.001). Metabolite profiles showed an 81 % increase in acetic acid (3,800 ± 120 mg·L−1) and a 93 % increase in butyric acid (2,900 ± 95 mg·L−1), while propionic acid decreased by 57 % in H2 stage. Thermoanaerobacterium thermosaccharolyticum was dominant in the H2 stage, while Methanobacter sp. and Methanosarcina sp. dominated in the CH4 stage, with their abundance influenced by specific trace metal supplementation. Process stability was maintained through precise control systems (temperature stability index of 0.95 ± 0.05 and pH stability index of 0.92 ± 0.05) with rapid response times (<5 min). COD removal efficiency increased from 65.3 % to 85.2 %, while metal removal efficiencies exceeded 90 % for all supplemented metals. These findings demonstrate significant enhancement in biogas production through optimized trace metal supplementation and precise process control strategies.