10. Microplastics in Industrial Wastewater Treatment Plants: Dynamic Distribution, Seasonal Variation, and Removal Efficiencies

Wastewater treatment plants (WWTPs) are one of the significant pathways of microplastics (MPs) entering the environment. Thus, information regarding this issue is still requisite in Thailand. In this study, wastewater and sludge samples were collected from an activated sludge operation of an industrial WWTP in different seasons, that is, dry season and wet season. The MP particles were identified by size, shape, and polymer types under stereomicroscope and μFT-IR. From the wastewater, MPs were detected in the highest amounts after the aeration unit (134.35 ± 20.79 particles/L) during dry season and from influent (103.13 ± 59.48 particles/L) during wet season. In addition, sludge contained relatively low MP concentrations (2.27 ± 0.08 in the dry and 1.86 ± 0.28 particles/g dw. in the wet seasons). The size fraction in wastewater varied, but in sludge was > 300 μm commonly in both seasons. Fragments and pellets were identified as the most common shape of wastewater and sludge in both seasons. The treatment units and seasonal variation had a significant effect on MP abundance. The suspected particle was confirmed to be 71.13% plastic polymer, mainly polypropylene (PP) and polyethylene (PE). The overall removal efficiency was 93.86%, indicating that there are many MPs still discharged to the river due to effluent MPs, which was approx. 1.8 × 10⁸ particles per day. Consequently, this study suggests that, to define the effect of a season, prolonged monitoring is required. Moreover, the attention must be on WWTPs in Thailand, as they act as the greatest source of MP contamination in the environment.