Microplastic removal efficiency in a megacity water treatment plant and dynamics in the distribution system

Microplastic (MP) contamination in drinking water has emerged as a pressing environmental and public health concern. However, its behavior across treatment and urban distribution systems remains poorly understood, particularly in rapidly growing megacities. This study investigates MPs' prevalence, removal efficiency in a major water treatment plant (WTP) in Bangkok, Thailand, and through the urban water distribution network. Seasonal samples were collected at four treatment stages, clarification, filtration, and chlorination, in a conventional WTP, and at five household taps located 3–11 km downstream in the distribution system. MP concentrations in raw water reached 114 ± 46 items/L in the dry season and 56 ± 11 items/L in the rainy season. The WTP achieved an overall MP removal efficiency of 75–81 %, with filtration being the most effective stage. However, residual MPs persisted in treated water, with concentrations ranging from 14 ± 8 items/L during the rainy season to 22 ± 18 items/L in the dry season. Notably, MP levels increased significantly with distance from the WTP, peaking at 66 ± 27 items/L at the farthest household, suggesting secondary contamination within the distribution system. Fragments were the most abundant MP morphology, while PP, PE, and PET were the dominant polymer types. Small-sized MPs (<100 µm) were prevalent, raising concerns about human exposure and treatment limitations. This study highlights the need for integrated water quality management addressing both treatment efficiency and distribution infrastructure. The findings serve as a reference for improving water safety in Bangkok and other rapidly urbanizing cities worldwide. • First study on the treatment and distribution of MPs in Bangkok tap water. • Raw water had up to 114 ± 46 MPs/L; treated water had 14–22 ± 18 MPs/L. • Small MPs (<100 µm) dominated, revealing the limits of conventional MP treatment. • MP levels rose with distance from WTP, reaching 66 ± 27 MPs/L in tap water. • Aging pipes release MPs via wear, biofilm detachment, and pressure surges.