Abstract:In this study, micro-oxygen was integrated into a rusty scrap iron (RSI)-loaded anaerobic digester. Under an optimal RSI dosage of 20 g/L, increasing O 2 levels were added stepwise in seven stages in a semi-continuous experiment. Results showed the average methane yield was 306 mL/g COD (chemical oxygen demand), and the hydrogen sulphide (H 2 S) concentration was 1933 ppmv with RSI addition. O 2 addition induced the microbial oxidation of sulphide by stimulating sulfur-oxidizing bacteria and chemical corrosion of iron, which promoted the generation of FeS and Fe 2 S 3 . In the 6th phase of the semi-continuous test, deep desulfurization was achieved without negatively impacting system performance. Average methane yield was 301.1 mL/g COD, and H 2 S concentration was 75 ppmv. Sulfur mass balance was described, with 84.0%, 11.90% and 0.21% of sulfur present in solid, liquid and gaseous phases, respectively. The Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis (PCR-DGGE) analysis revealed that RSI addition could enrich the diversity of hydrogenotrophic methanogens and iron-reducing bacteria to benefit methanogenesis and organic mineralization, and impoverish the methanotroph (Methylocella silvestris) to reduce the consumption of methane. Micro-oxygen supplementation could enhance the diversity of iron-oxidizing bacteria arising from the improvement of Fe(II) release rate and enrich the sulphur-oxidising bacteria to achieved desulfurization. These results demonstrated that RSI addition in combination with micro-oxygenation represents a promising method for simultaneously controlling biogas H 2 S concentration and improving digestion performance.

a b s t r ac tIn this study, microaerobic was integrated into rusty scrap iron (RSI)-loaded anaerobic digester, which has been experimentally demonstrated to improve digestion performance and to decrease biogas H 2 S content during waste-activated sludge digestion. A series of batch experiments were carried out, determining that an RSI dosage of 20 g/L was optimal for anaerobic digestion, average methane yield increased by 39.9% and H 2 S content decreased by 84.6%. Under optimal RSI dosage conditions, increasing O 2 levels were added stepwise seven stages in a semi-continuous experiment. O 2 addition induced the microbial oxidation of sulfide by stimulating sulfur-oxidizing bacteria and chemical corrosion of iron, which promoted generation of Fe(II) and Fe(III), which subsequently precipitated as FeS and Fe 2 S 3 . In the sixth period of semi-continuous experiment, deep desulfurization was reached without negatively impacting system performance. Average methane yield was 301.1 mL/g chemical oxygen demand, H 2 S concentration was 75 parts per million by volume (ppmv) and desulfurization efficiency reached 99.40%. Sulfur mass balance was described for the system, with 84.0%, 11.90% and 0.21% of sulfur present in solid, liquid and gaseous phases, respectively. These results demonstrated that RSI addition in combination with microaeration represents a promising method for simultaneous controlling biogas H 2 S concentration and improving digestion performance.

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