Linkages of volatile fatty acids and polyhexamethylene guanidine stress during sludge fermentation: Metagenomic insights of microbial metabolic traits and adaptation

Abstract

The massive use of polyhexamethylene guanidine (PHMG), as a typical bactericidal agent, raised environmental concerns to the public. This work comprehensively revealed the hormesis effects of PHMG occurred in waste activated sludge (WAS) on the generation of volatile fatty acids (VFAs) during anaerobic fermentation. The low level of PHMG (100 mg/g TSS) significantly promoted the VFAs generation (1283 mg COD/L, compared with 337 mg COD/L in the control) via synchronously facilitating the solubilization, hydrolysis, and acidification steps but inhibiting methanogenesis. Metagenomic analysis showed that the functional anaerobe (i.e., Bacteroides, Macellibacteroide and Parabacteroide) and corresponding genetic expressions responsible for extracellular hydrolysis (i.e., clpP), membrane transport (i.e., ffh and gspF), intracellular substrates metabolism (i.e., ald and paaF) and VFAs biosynthesis (i.e., ACACA and FASN) were enhanced in the optimal presence of PHMG. Moreover, the anaerobic species could respond and adapt to low PHMG stimuli via quorum sensing (i.e., cqsA, rpfC and rpfG), and thus maintain the high microbial metabolic activities. However, they were unable to tolerate the toxicity of excessive PHMG, resulting in the extremely low VFAs production. This work enlightened the effects of emerging pollutants on WAS fermentation at the genetic levels, and provided guidance on the WAS treatment and resource recovery.