| Summary: | Direct addition of sulfur-reducing agents during anaerobic digestion (AD) is very effective in controlling hydrogen sulfide (H<sub>2</sub>S) content in biogas, although one major problem is the high operational cost due to the large amount of chemicals used. The objective of this study was to remove H<sub>2</sub>S using a waste mill scale (MS) as a sulfur-reducing agent. To evaluate its feasibility, MS was added to AD fed with food waste (FW) at concentrations between 0 and 160 g MS/kg total chemical oxygen demand (TCOD) during the batch test, and the experimental results were compared to those of the batch test with the addition of iron chloride (FeCl<sub>3</sub>). Both FeCl<sub>3</sub> and MS played an important role as electro-conductive materials in improving methane productivity by promoting direct interspecies electron transfer. An increase in H<sub>2</sub>S removal efficiency was observed with increases in both materials. In total, 30%, 60%, and 90% of H<sub>2</sub>S production based on the maximum sulfur in the form of H<sub>2</sub>S (control) was 3.7, 9.4, and 23.8 g FeCl<sub>3</sub>/kg TCOD and 13.3, 34.1, and 86.2 g MS/kg TCOD, respectively. This finding indicates that MS can be used as a sulfur-reducing agent substitute for H<sub>2</sub>S removal in AD fed with FW.
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