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Exogenous contaminants often co-occur in waste-activated sludge (WAS), but their potential mutual interactions and joint effects on the WAS treatment are less concerning. This work revealed that surfactants induced and aggravated the toxicity of Fe 2 O 3 nanoparticles (NPs) in volatile fatty acids (VFAs)' biosynthesis during WAS anaerobic fermentation. The VFA production exhibited a 40.1 and 20.3% inhibition compared to the individual LAS and CTAB reactors, respectively. A mechanistic exploration implied that the WAS solubilization, hydrolysis, and acidification steps during VFA generation were restrained. Moreover, the Fe 2 O 3 NPs' and surfactants' coexistence suppressed the acid-forming bacteria (i.e., Saprospiraceae_uncultured, Bacteroidetes vadinHA17_norank) and the homologous metabolic functions responsible for substrate hydrolysis (e.g., malZ and pepD), intracellular metabolism (e.g., pf kA and gdhA), VFAs' biosynthesis (e.g., bkdA and ackA), and ATP generation (e.g., atpE and atpH), resulting ultimately in VFAs' reduction. Furthermore, the surfactants contributed to the dispersibility of coexisting Fe 2 O 3 NPs to maintain the high reactivity of NPs. Besides, the WAS disintegration triggered by surfactants increased the interactions between Fe 2 O 3 NPs and microorganisms, thereby aggravating toxicity. Our study provides novel insights into environmental behaviors of co-occurring contaminants in the anaerobic environment and provides in-depth guidance for practical WAS treatment.

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