Effect of nanoscale zero-valent iron (nZVI) on performance and microbial succession in anaerobic sludge under prolonged exposure to chloroxylenol

With the expanding use of antibacterial and disinfection products, chloroxylenol (para-chloro-meta-xylenol, PCMX) has been detected in various environments, especially in sewage treatment plants. However, the influence of PCMX has received limited attention, with only sporadic studies available. Over a period of 110 days, efforts were undertaken to restore the performance of PCMX-affected anaerobic sludge through the addition of nanoscale zero-valent iron (nZVI) and the reduction of PCMX concentration. nZVI addition promoted COD removal efficiency under a high PCMX concentration (50.0 mg L⁻¹, GH) and accelerated the anaerobic digestion process under a low PCMX concentration (0.5 mg L⁻¹, GL). The slow recovery in GH highlighted the difficulty in restoring the functionality of municipal anaerobic sludge under the prolonged influence of high PCMX concentrations. Microbial communities exhibited distinct succession patterns under different treatments. nZVI demonstrated efficacy in mitigating the decline in microbial richness induced by PCMX. Shannon and Pielou evenness and niche breadth increased as the PCMX concentration decreased, suggesting an enhanced restorative capacity of the microbial community when alleviating the stress imposed by PCMX. Bacteroidetes, Proteobacteria, Actinobacteria, Firmicutes, and Chloroflexi were predominant functional phyla in the anaerobic digestion process. Olsenella, Rectinema, Desulfovibrio, Azonexus, and Methanobacterium were key genera responding to nZVI and PCMX. PCMX markedly diminished community resistance and resilience, while nZVI mitigated the damage of PCMX on community stability. Altogether, this study contributes to a better understanding of the performance and microbial succession in anaerobic sludge through the addition of nZVI and variations in PCMX concentration.