This study was conducted to examine the feasibility of recovering ferric coagulant from drinking water treatment sludge (DWTS) for green synthesis of iron nanoparticles (NPs). Black tea extract was utilized for the bio-reduction of ferric chloride. The as-synthesized product was characterized and confirmed as iron NPs, using UVevis spectrometry, XRD, FT-IR, SEM, and EDX analyses. The synthesized NPs were sphere-like with diameter in the range of 20e40 nm. The performance of the iron NPs as micronutrients supplements in anaerobic digestion of slaughterhouse wastewater was studied at three different concentrations. The results showed that the addition of iron NPs in all concentrations improved the biogas production and shortened the lag phase. The highest biogas yield was obtained from 9 mg L
1 of additive which corresponds to up to 37.6% enhancement over the control reactor. Moreover, iron NPs improved COD reduction efficiency to 42%

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