Seasonal Variations and Operational Influences on Microplastic Removal in a Municipal Sequencing Batch Reactor Wastewater Treatment System

Microplastic (MP) pollution in aquatic ecosystems is a critical global concern, with municipal wastewater treatment plants (WWTPs) serving as a primary discharge pathway. This study evaluates the abundance and removal efficiency of MPs within a municipal sequencing batch reactor (SBR) system in Lamphun, Thailand, across three distinct seasons. MP concentrations ranged from 0.11 ± 0.33 to 3.00 ± 2.60 particles/L, with the highest levels observed during summer (2.74 ± 2.49 particles/L)—representing an 85–93% increase compared to rainy and winter seasons. Seasonal operational dynamics significantly influenced SBR performance, yielding removal efficiencies of 33% in the rainy season, 0% in winter, and a negative efficiency (–29%) in summer. These findings suggest that seasonal wastewater characteristics, coupled with particle resuspension and fragmentation, profoundly impact MP retention. While larger MPs (351–1,000 µm) dominated the influent (62%), mechanical stress during aeration promoted fragmentation, increasing the proportion of smaller MPs (