| Abstract | Microplastic (MP) pollution has emerged as a critical environmental issue, particularly in rapidly urbanizing regions. This study investigated the quantity and characteristics of MPs from three major urban sources dry air deposition, domestic wastewater, and industrial wastewater and developed a Source-Pathway-Receptor (SPR) framework to quantify their influx into aquatic systems in the Bangkok Metropolitan Region (BMR) under dry weather conditions. MPs were detected in all sources. In air deposition, concentrations reached 639 ± 200 particles/m²/day and 3.67 ± 1.70 mg/m²/day. Domestic wastewater at the discharge point contained 4.61 ± 1.00 particles/L and 3.12 ± 0.85 mg/L, while treated domestic effluent showed reduced levels of 0.55 ± 0.19 particles/L and 0.28 ± 0.10 mg/L. Industrial effluent contained 0.31 ± 0.15 particles/L and 0.15 ± 0.07 mg/L. Across all sources, MPs were predominantly small particles (10–500 µm), particularly in air samples (93%), and fragments were the most common shape across all sources. Blue-colored MPs were most prevalent in domestic wastewater, while industrial sources exhibited greater color diversity. The study estimated an annual MP release to the environment of 4,875.81 ± 1,123.14 tons/year and 352.86 ± 109.33 trillion particles/year, with domestic wastewater contributing the most by mass (62.10%), air deposition (37.17%) and industrial wastewater (0.73%). In contrast, air deposition dominated by particle count (98.71%), while domestic wastewater and industrial wastewater contributed only 1.27% and 0.02%, respectively. Using the SPR framework, the estimated annual MP influx into aquatic systems was 1,849.28 ± 458.70 tons/year and 13.69 ± 3.59 trillion particles/year. By mass, the direct discharge of untreated domestic wastewater was the primary pathway (90.05%), treated domestic effluent (6.71%), air deposition onto water surfaces (3.09%), and treated industrial effluent (0.15%). By particle number, air deposition contributed the most 80.20%, untreated domestic wastewater 18%, treated domestic effluent 1.76%, and treated industrial effluent 0.04%. Wastewater treatment plants played a key role in reducing MP discharge to surface waters, yet a large proportion of MPs was retained in sewage sludge, totaling 1,271.33 ± 889.97 tons/year and 1.83 ± 0.90 trillion particles/year. Of the retained MP mass, 66.68% was found in domestic WWTP sludge, 30.73% settling in sewer system, and 2.59% in industrial sludge, with particle retention showing a similar distribution trend. These findings highlight the need to enhance wastewater treatment coverage and carefully manage sludge disposal. |