Dynamic transformation and mechanisms of volatile sulfur compound releasing during anaerobic digestion of sludge.

Volatile sulfur compounds (VSCs) are important nodes of sulfur metabolism; their emission, transformation and corresponding mechanisms during the anaerobic digestion are influenced by physical, chemical and biological components during the dynamic process. This study investigated the dynamic sulfur flows and distributions in terms of various forms and phases in sludge with different solid contents, and revealed the effects of extracellular polymeric substances (EPS) on VSCs and their interactions with microbial metabolites. The generation of H2S and other VSCs from sludge with low solid content (3 %) was fast initially, and the accumulated sulfur distribution into VSCs was 1.21 μg·g-1 TS while sulfate decreased by 11.7 % during the period. With higher solid contents (e.g., 10 %), the sulfur distribution in VSCs was less than 0.1 μg·g-1 TS, and sulfate increased by 10 %. Higher solid contents favored the growth of sulfur-oxidizing microbes (SOB) in the initial stage, while their abundances decreased significantly when sulfur metabolism was less active. In contrast, sulfur-reducing microbes play important roles in sulfate reduction and H2S generation in sludge with low solid contents. EPS compositions and characteristics are important for both sulfur-related microorganisms and VSCs releasing from sludge, and their physical barrier effect is considered as a key factor of reducing VSCs emission from sludge with high solid contents. This study provides a comprehensive understanding of the mechanism of VSC generation and release from dynamic sulfur transformation, contributing to the potential management of VSCs during sludge management.