In this study, the influence of metal nanoparticle size on treatment efficiency and bacterial community dynamics in a sequencing batch reactor was investigated. The experiments revealed that while the presence of metal nanoparticles did not significantly alter the removal rates of chemical oxygen demand or total phosphorus, the size of the metal nanoparticles clearly affected the nitrogen removal efficiency and bacterial community structure. Smaller metal nanoparticles, such as 20–40 nm copper oxide and < 50 nm zinc oxide, led to reductions in mixed liquor suspended solids, which correlated with decreased removal efficiencies for total nitrogen and ammonium nitrogen. This adverse impact was linked to significant alterations in bacterial community composition, where the dominant genera included Pseudomonas sp., Perlucidibaca sp. and Acinetobacter sp. Bacterial morphological analysis revealed damage to bacterial membranes with relatively small metal nanoparticles. In contrast, larger copper oxides (60–80 nm) increased the mixed liquor suspended solids, improving the nitrogen removal efficiency because microbes secreted extracellular polymeric substances, which reduced the toxicity of metal nanoparticles. The dominant bacteria in this case were Pseudomonas sp., Perlucidibaca sp., and Azospirillum sp. The study concluded that the size of metal nanoparticles critically influences microbial community dynamics and treatment efficiency, emphasizing the need for strategies to reduce the toxicity of metal nanoparticles in wastewater treatment plants. These findings provide valuable insights into environmental management and the design of sustainable wastewater treatment processes.
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