Antimicrobial PCMX facilitates the volatile fatty acids production during sludge anaerobic fermentation: Insights of the interactive principles, microbial metabolic profiles and adaptation mechanisms

Para-chloro-meta-xylenol (PCMX), as a massively used antimicrobial agent, exhibited dose-dependent effects on the sludge fermentation for volatile fatty acids (VFAs). The presence of PMCX at environmental levels (10-20 mg/g TSS) promoted the VFAs production by 2.5-3.0 folds but caused evident inhibition at high level (100 mg/g TSS). PCMX synchronously promoted the sludge solubilization, hydrolysis, and acidification but inhibited methanogenesis. Mechanistic investigations revealed that PCMX could interact with the extracellular polymeric substances (EPS) in sludge and cause the release of biodegradable substrates (especially proteins) by altering the WAS structures. Moreover, the functional fermentative bacteria (i.e., Firmicutes and Chloroflexi) were enriched while those VFAs consumers (i.e., Methanothrix and Methanomassiliicoccus) were reduced with low level of PCMX exposure. The microbial enzymatic activities (i.e., protease) and critical genes relevant to the substrates transport (i.e., gluC and lhpN) and metabolisms (especially the proteins, i.e., pckA), and VFAs biosynthesis (i.e., fabG) were all upregulated with the low PCMX stimulus. The microbial response and adaptation to PCMX toxicity were mainly attributed to the regulation of two-component and quorum sensing systems. Overall, PCMX contributed to the VFAs accumulation during sludge fermentation by mainly interacting and disrupting the sludge structures, and improving the microbial metabolic function traits. This work provided new insights into exogenous pol-lutants' interactions, potential influences, and reaction mechanisms in the complicated sludge matrix.