Leftover sludge from landfill treatment is typically rich in ferrous chemical compounds, whose recovery can often be used to enhance the performance of anaerobic bioreactors. This investigation examines the biosynthesis of iron nanoparticles (Fe-NPs) from the sludge residue derived from landfill leachate treatment, and then assesses their performance as micronutrient supplements for fermentation, alone and in combination with other additives, specifically FeCl3 with sludge. The preparation of Fe-containing sludge through landfill leachate treatment involved reaching optimal recovery of Fe and its biosynthesis via black tea waste extract. The bio-synthesized Fe-NPs were then fully characterized using a vast range of analytical techniques such as ultraviolet-visible (UV–vis) spectroscopy, X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), and energy dispersive X-ray (EDX) analysis. The bio-synthesized NPs presented a chain and spherical structure with a diameter of 13–150 nm, and the alignment and adjustment of the biosynthesis process were close to those resulting from our predictive model. The anaerobic digestion trials led to COD removal of 73.64 % and increased biogas production by 69.85 % compared to the control bioreactor, under the optimal conditions of Fe-NPs, sludge, and fresh FeCl3 at the concentrations of 17.06, 0.28 and 1.46 mg/L, respectively. In conclusion, the addition of trace elements not only reduced the anaerobic digestion’s lag phase, but it also minimized the time to reach the peak of biogas production. Using a combined physicochemical-biological process is a cost-effective and eco-friendly technique for sludge disposal and energy production from treated leachate.

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