Effects of different microplastics levels combined with NaClO on activated sludge in MBRs: toxicity, key enzymes, extracellular polymers, and communities

NaClO, as a common membrane cleaning agent were introduced from the permeate side across membrane, which could diffuse into MBR membrane pool. Microplastics would also accumulate in MBR membrane pool during wastewater treatment, thus, NaClO and MPs are co-exposed to activated sludge. This study investigated the effects of PET-MPs (0, 50, 100 and 300 μg/g-ss) on activated sludge with or without NaClO stress. The presence of PET-MPs had no significant impact on nitrification and denitrification rates, but when exposed to NaClO, the increasing level of PET-MPs (i.e., from 0 to 300 μg/g-ss) inhibited the nitrification (94.85 % to 65.90 %) and denitrification (88.68 % to 60.09 %) rates. For microbial metabolism (key enzymes) evaluated, the key enzymes related to organic matter biodegradation, nitrification as well as denitrification were both inhibited with PET-MPs level rise under NaClO stress. The generation of reactive oxygen species (ROS) and the release of lactate dehydrogenase (LDH) increased significantly indicating NaClO caused the toxicity of MPs to activated sludge by enhancing the oxidative stress level. Correspondingly, a variation in abundance related to membrane pollution, nitrification, and denitrification after exposure to PET-MPs with NaClO stress accompanied these adverse effects.