Micro(nano)plastic size and concentration co-differentiate the treatment performance and toxicity mechanism in aerobic granular sludge systems

Microplastics (MPs) and nanoplastics (NPs) released into wastewater may pose threats to the biological treatment process. Aerobic granular sludge (AGS), a prospective biotechnology for wastewater treatment, can effectively intercept MPs and NPs, but the effect of these emerging contaminants on the AGS system lacks investigation. This study showed that low concentrations of MPs and NPs (1 mg/L) posed negligible inhibitory effects on the characteristics and performance of the AGS system. Yet, increasing MPs concentrations to higher levels (20 and 100 mg/L) impaired the granular integrity and diminished the total nitrogen (TN) removal efficiency by 3.8 %–17.8 %, especially for the denitrification process. Compared to MPs, prolonged exposure to NPs at the same levels showed greater adverse effects on the AGS, as evidenced by overgrowth of filamentous bacteria and lower TN removal efficiency (decreased by 11.4 %–22.8 %). This was possibly due to biofilm formation on the surface of MPs and excessive secretion of extracellular polymeric substances served as barriers to mitigate the toxic effects of MPs. In contrast, microorganisms were directly exposed to the toxicity of NPs, resulting in greater oxidative stress, impaired cytomembrane structure, deteriorated cell viability, and decreased proteins production. Metagenomic analysis further revealed that the populations of key functional bacteria (e.g. denitrifiers Acidovorax and Flavobacterium) decreased after long–term exposure to MPs and NPs. The key enzymes and genes (e.g. nirK, napAB, nirS, norBC and nosZ) related to nitrogen transformation processes, electron donor and energy production (e.g. mdh) were declined accordingly, unravelling the inherent mechanism for the decreased nitrogen removal. This study indicated that NPs have more detrimental impacts on AGS system, which needs to be taken into account when treating plastic containing wastewaters.