Polyvinyl Chloride Microplastics Facilitate Nitrous Oxide Production in Partial Nitritation Systems

Partial nitritation (PN) is an important partner with anammox in the sidestream line treating high-strength wastewater and primarily contributes to nitrous oxide (N 2 O) emissions in such a hybrid system, which also suffers from ubiquitous microplastics because of the growing usage and disposal levels of plastics. In this study, the influences of polyvinyl chloride microplastics (PVC-MPs) on N 2 O-contributing pathways were experimentally revealed to fill the knowledge gap on N 2 O emission from the PN system under microplastics stress. The long-term results showed that the overall PN performance was hardly affected by the low-dose PVC-MPs (0.5 mg/L) while obviously deteriorated by the high dose (5 mg/L). According to the batch tests, PVC-MPs reduced biomass-specific ammonia oxidation rates (AORs) by 5.78–21.94% and stimulated aerobic N 2 O production by 9.22–88.36%. Further, upon increasing dissolved oxygen concentrations from 0.3 to 0.9 mg O 2 /L, the degree of AOR inhibition increased but that of N 2 O stimulation was lightened. Site preference analysis in combination with metabolic inhibitors demonstrated that the contributions of hydroxylamine oxidation and heterotrophic denitrification to N 2 O production at 0.3 mg O 2 /L were enhanced by 18.84 and 10.34%, respectively, accompanied by a corresponding decreased contribution of nitrifier denitrification. Finally, the underlying mechanisms proposed for negative influences of PVC-MPs were bisphenol A leaching and reactive oxygen species production, which led to more cell death, altered sludge properties, and reshaped microbial communities, further resulting in enhanced N 2 O emission. Overall, this work implied that the ubiquitous microplastics are a hidden danger that cannot be ignored in the PN system.