Fe(II) and Cu(II)-Mediated Enhancement of Biogas Yield and BioMethane Content in Cow Dung - Chicken Manure Co-Digestion: A Kinetic Study

Trace metal supplementation is an exciting, low-cost approach to enhancing anaerobic digestion (AD) performance, yet the concentration-dependent effects of individual metals on co-digestion kinetics and biogas quality has remained incompletely resolved for heterogeneous livestock-waste mixtures. This study investigated the impacts of Fe(II) and Cu(II) supplementation at concentration of 0.02, 0.05, and 0.10 mM as FeSO₄·7H₂O and CuCl₂·2H₂O) on the anaerobic co-digestion of cow dung (CD) and chicken manure (CM) at a 50:50 ratio, in terms of both digestion kinetics and biogas compositional quality over a 50-day hydraulic retention time. Physicochemical characterization of the CD and CM was conducted by standard protocols. The result proved that compositionally, CM was richer than CD in nitrogen, protein, and macro-minerals, whereas CD contributed majorly a carbon-dominant, fibrous fraction. Across all bioreactors, cumulative biogas yield followed a clear biphasic (diauxic) trajectory, which was accurately described by a two-phase logistic model (R² = 0.992–0.999).This pattern reveals sequential use of readily degradable soluble organics, followed by the refractory lignocellulosic materials. Fe(II) displayed a typical hormetic dose - response, and 0.05 mM demonstrated the best performance (29.90 dm³ cumulative biogas, which 21.16% above control; 75.34% CH₄) while 0.10 mM turned out to be inhibitory, with only 23.51 dm³ (with 63.27% CH₄, lower than control). In contrast, Cu(II) produced a monotonically positive response in all the tested range, with 0.10 mM producing the overall highest cumulative yield (30.25 dm³, 22.56% above control) and biomethane content (76.79%), alongside near-complete suppression of sulfurous off-gas, consistent with copper-mediated sulfide precipitation. Furthermore, gas chromatographic analysis revealed a concurrent increase in residual hydrogen under high-dose Cu(II), suggesting possible a partial uncoupling of hydrogenotrophic and acetoclastic methanogenic pathways. These findings demonstrate that micro-dosing of Fe(II) (0.05 mM) or Cu(II) (0.10 mM) can enhance both the yield and quality of biogas substantially from CD/CM co-digestion, while highlighting that effective trace-metal concentrations are strongly substrate- and matrix-dependent, underscoring the value of two-phase kinetic modelling for optimizing metal-amended anaerobic digestion systems.