Wastewater treatment plant serves as a potentially controllable source of microplastic: Association of microplastic removal and operational parameters and water quality data

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Wastewater treatment plant (WWTP) has great potential to regulate microplastics (MPs) emissions, but few studies have focused on this subject. The present study performed a long-term investigation in a typically combined sewers WWTP. Compared to dry weather, rainfall contributed approximately 1.5 times more MPs to the inlet of the combined WWTP. The equalization tank served as a primary site for MPs' removal, with a 46.67-87.91% removal rate. Decreasing surface overflow rate (SOR) was an effective measure for improving MPs removal, and a suitable SOR was recommended at ~11 m 3 (d·m 2 ) -1 ) in the equalization tank. However, MPs' existing settling/rising velocity models failed to obtain a theoretical SOR. In cyclic activated sludge systems, reducing sludge wastage was proposed to facilitate the removal of MPs. As a result, outlet MPs could be reduced by an order of magnitude by adjusting operational parameters on rainy days. Our study highlighted the potential for WWTP to operate as a controlled source of MPs by altering the operational parameters. We also advocated exploiting the mechanisms of MPs' removal by operational parameters and finding an equilibrium between MPs' removal and activated sludge performance. • MPs in WWTP can be mitigated 90% by adjusting operational parameters on rainy days. • Decreasing the SOR (or raising the HRT) improve MPs' removal in equalization tank. • Models were incapable to predict MPs’ velocities, failing to obtain suitable SOR. • Reducing sludge wastage enhanced MPs' removal in the CASS. • Balance the MPs’ removal and the activated sludge’s performance was needed in CASS.