New insights of NaClO induced polyvinyl chloride microplastics affect sewage treatment in activated sludge

The microplastics (MPs) are often exist as cumulative state in the bioreactor (especially MBR) of wastewater treatment plants (WWTPs). NaClO is a common chemical cleaning agent which will inevitably enter the MBR membrane pool and contact with the accumulated MPs in the membrane pool during membrane cleaning process. Therefore, the focus of this study is the toxicity mechanism of NaClO and MPs co-exposure to activated sludge. In this study, the COD, TN, and TP removal efficiencies decreased at first several cycles, and then gradually recovered without NaClO stress. While the nutrient removal efficiency showed a continuous declination trend with NaClO stress and have no recovery trend. Meanwhile, the key enzymes (such as AMO, NXR, NIR and NAR), which could echo the trend of nutrient metabolism. The dominate bacteria related to nitrogen removal including nitrification (Saccharimonadales) and denitrification (Tessaracoccus, Mobilicoccus, vadinHA17, Mesorhizobium and Hyphomicrobium) were both inhibited with NaClO and PVC-MPs co-exposed. This study also confirmed that NaClO stress could change the types of MPs leachate and enhance the release of main additives (BPA) from MPs. The NaClO and MPs co-exposure could enhance the level of oxidative stress, which led to the broken of cell membrane. The toxicity mechanism of NaClO and MPs co-exposure to activated sludge is as follows: NaClO can increase the oxidative stress level of microplastics, which has nothing to do with the release of trace additives.