Effects of single and combined microplastic and copper ion contamination on sequencing batch reactor performance and microbial community

Microplastics (MPs) are ubiquitous in wastewater treatment plants (WWTPs), but there is still much to learn about their role in WWTPs. In this study, polypropylene (PP) and polylactic acid (PLA) were added to a sequencing batch reactor (SBR), and the single and combined effects of MPs and copper ions on the wastewater treatment process were evaluated. The study revealed that single copper ions had a significant negative effect on SBR operational performance, inhibiting the removal efficiency of chemical oxygen demand (COD) and nitrogenous compounds. The removal efficiencies of COD and NH₄⁺-N decreased to 70.61% and 16.42%, respectively, at the end of the SBR. Single MPs had no effects on SBR performance. However, when MPs coexisted with copper ions, the MPs partially mitigated the toxicity of copper ions through adsorption. In addition, the coexistence of MPs with copper ions significantly promoted the secretion of extracellular polymers (EPS). Copper ions significantly affected the abundance of copper resistance genes (CRGs), while MPs in combination with copper ions could promote the abundance of CRGs. The combination of MPs and copper ions increased the abundance of antibiotics resistance genes (ARGs) and integrons (intl1), and PLA presented a more severe risk of transmission compared to PP. Furthermore, single copper ions significantly suppressed the abundance and diversity of microbial communities, while MPs and copper ions combined to relieve the inhibitory effect of single copper ions on microbial communities. Dechloromonas, OLB12, Saccharimonadales, and Blastocatellaceae were the dominant genera in SBR. pcoA, copA, and cusA were the primary CRGs influencing the copper resistance of Dechloromonas, while sul1, sul2, tetQ, and tetM were the main ARGs affecting Dechloromonas and Saccharimonadales. This study provides new insights into understanding the effects of heavy metal and MPs contamination on the performance of SBR.