Toxic effects of polycyclic aromatic hydrocarbons on the two-sludge system: Combined functional gene, resistance gene and microbial community assessment

Polycyclic aromatic hydrocarbons (PAHs) were a class of persistent organic matter, and contained PAHs wastewater was discharged into wastewater treatment plants (WWTPs) finally, which could greatly affect their operational efficiency. The toxic effects of PAHs on the two-sludge system were assessed from functional genes, resistance genes, and microbial community. Three types of PAHs including Naphthalene (NAP), Anthracene (ANT), and Phenanthrene (PHE) were individually introduced into three separate two-sludge systems. Results showed that three types of PAHs did not adversely affect on nitrification performance, but exhibited a significant toxic effect on denitrification and phosphorus (PO43--P) removal. The effluent NH4+-N was almost 0 mg/L in all three systems, the highest total nitrogen (TN) removal rate was achieved at 90.88% in the PHE system, whereas the lowest rate was 72.46% in the ANT system. Concurrently, the PO43--P removal rate decreased from 93.10% to 51.22% in the ANT system. Functional genes, such as NORB and NIRS, and resistance genes such as PAH-RHD GNF/R and NAH-F/NH-R, were enriched in the PHE and NAP system, contributing to improved pollutant removal. However, the NIRK and PAH-RHD GPF/R were more abundant in the ANT system, resulting in varied pollutant removal capabilities. The composition of the microbial community was also influenced by PAH exposure with some specific functional bacteria being identified across the different systems. Among the PAHs tested, ANT was found to exert the highest toxicity towards the two-sludge system, surpassing the toxic effects of NAP and PHE. This study highlights the importance of considering PAH toxicity in the context of WWTPs operations and the potential need for tailored strategies to mitigate their impact on biological processes.