From multiphase redistribution to emission control: deciphering iron-mediated interactions in sulfur dynamics during sludge anaerobic digestion.

Volatile sulfur compounds (VSCs) are malodorous and hazardous gases, which are generated during the anaerobic digestion (AD) of waste activated sludge (WAS). Iron can mediate sulfur transformation during AD process. The evolution and distribution of sulfur across the gas-liquid-solid phases in the presence of ferric chloride (FeCl3) remain poorly understood. In this study, semicontinuous anaerobic digesters were employed to decipher iron-mediated interactions in sulfur dynamics during the AD of WAS. The results showed that FeCl3 significantly decreased the generation of all kinds of VSCs (a total removal of 69%), shifting chemical equilibrium from VSCs to solid inorganic sulfur. It also enhanced the degradation of both solid organic sulfur and soluble organic sulfur, but reduced the production of soluble sulfide by forming more acid volatile sulfide and S0 in the solid phase. The generated soluble sulfide was immediately removed in two ways: 85% precipitated as FeS by Fe(II) ions derived from dissimilatory iron reduction, and 15% oxidized to S⁰ by Fe(III) or sulfur-oxidizing bacteria (SOBs). Microbial community analysis confirmed that the presence of FeCl3 increased the abundance of hydrolysis and acidification bacteria, iron-reducing bacteria and SOBs but reduced the relative abundance of sulfate-reducing bacteria. This resulted in efficient hydrolysis of organic sulfur, increased oxidation of soluble sulfide, and weakened reduction of soluble sulfate. By deciphering the multiphase fate of sulfur during AD, this study provides critical insights for controlling VSCs generation in sludge management.