Can copper and zinc replace a commercial trace element mixture in enhancing methane production from agricultural digestate post-processing?

Post-treatment of digestate benefits biogas plants by reducing gas emissions and recovering residual methane potential, but the substrate often lacks nutrients for soil application. Adding trace elements (TEs) can enhance energy production and agricultural value. This study examines whether copper (Cu) and zinc (Zn) solutions can improve residual methane potential as effectively as a commercial TEs mixture, supplied at a higher cost. Digestate from a hybrid full-scale biogas plant underwent batch anaerobic digestion assays. Cu and Zn from reagents were tested at 0, 0.1, 1, 2, and 5 mg L−1 in laboratory-scale reactors (LBRs), while a commercial mixture also containing Mn, Co, Ni, Mo, Fe, and B was tested in both LBRs and bench-scale reactors (BBRs). Potential energy and net present value (NPV) were estimated across 5000 scenarios. Methane production rate was maximised with 0.1 mg L−1 Cu and Zn and additional TEs due to synergistic effects. Residual methane increased by 39 % and 25.5 % with 2 mg L−1 Cu and Zn and 1 mg L−1 Cu, respectively, compared to the commercial mixture. Adding 1 mg L−1 Cu resulted in a 30 % higher probability of achieving an NPV of €15,200-€38,000 compared to 2 mg L−1 Cu and Zn.