NaClO changes metabolic pathway by regulating oxidative stress and virulence of microplastics to activated sludge in MBR

Microplastics (MPs) and NaClO were both prevalent in MBRs, and their coexistence poses a potential threat to nitrogen biotransformation in wastewater treatment systems. However, investigations on how MPs combined with NaClO affect the microbial nitrogen conversion and metabolism of the activated sludge are still scanty. In this study, the MPs or NaClO exposure alone had marginal impacts on the nitrogen removal in MBRs, while the two pollutants' coexistence in MBR, the total nitrogen removal efficiency decreased from 83.59 % to 74.46 % by the suppression of nitrogen transformation processes. Metagenomic analysis revealed that the abundance of functional genes encoded by key enzymes related to nitrogen transformation, including nirK, norB, nosZ and nirS, were also decreased with MPs and NaClO co-exposure. The reason for the above phenomenon may be the biotoxicity of MPs to activated sludge caused by NaClO, the ROS production increased to 128.57 % with MPs and NaClO co-exposure. The increase of intracellular oxidative stress level and virulence factors synthesis confirmed that NaClO can aggravate the toxicity of MPs to activated sludge. The increased toxicity of MPs to activated sludge caused by NaClO may be attributed to the oxidation of MPs, that is, the change of functional groups. This study provides vital insights into the responses of microbial community structure and nitrogen conversion processes to interference MPs combined with NaClO backwash agent residual in MBRs.