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

• Optimal metal addition enhanced 86.4% of H 2 yield and 84% of CH 4 yield. • 10 mg·L −1 Mo 2+ , 6 mg·L −1 Ni 2+ , 6 mg·L −1 Co 2+ , 10 mg·L −1 Fe 2+ was optimized. • Ni 2+ and Fe 2+ had significant individual effects on H 2 and CH 4 production. • Mo 2+ and (Co 2+ and Fe 2+ ) had significant interactive effects. • Enhanced process stability and metal removal efficiency >90 %. The two-stage anaerobic digestion of palm oil mill effluent (POME) was optimized through trace metal supplementation (Mo 2+ , Ni 2+ , Co 2+ , and Fe 2+ ). Optimal concentrations were determined as 10 mg·L −1 Mo 2+ , 6 mg·L −1 Ni 2+ , 6 mg·L −1 Co 2+ , and 10 mg·L −1 Fe 2+ , resulting in significant biogas yield improvements on hydrogen yield increased by 86.4 % (29.5 ± 0.9 to 55.0 ± 2.1 mL-H 2 ·g −1 -VS) and methane yield by 84 % (from 173.8 ± 7.8 to 320.0 ± 8.4 mL-CH 4 ·g −1 -VS). Gas composition improved, with H 2 content increasing from 18.5 % to 32.0 % and CH 4 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 (R 2 = 0.94, p < 0.001) and enzyme activities with biogas yields (R 2 = 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 H 2 stage. Thermoanaerobacterium thermosaccharolyticum was dominant in the H 2 stage, while Methanobacter sp. and Methanosarcina sp. dominated in the CH 4 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.