Point-source microplastic input to the river and coastal zone via wastewater treatment facilities: a case study from a tropical mega-city

Little is known about the contribution of wastewater treatment plants (WWTPs) on microplastics (MPs) contamination in Thailand's rivers and their transport into the Gulf of Thailand (GoT). This study investigated the abundance, characteristics, polymer composition and removal efficiency of MPs in urban WWTPs and MPs contaminants in the river flowing through Bangkok to the GoT. Eight WWTPs in Bangkok and twelve surface water samples from Chao Phraya River were examined. Moreover, surface water samples were collected over a complete tidal cycle at a fixed station near the river mouth. Approximately a quarter of MPs abundance in WWTPs was reduced during the activated sludge treatment process in WWTPs. Larger MP particles (>1000 μm to 5000 μm) and high-density polymers (e.g., polyamide) tended to be removed, whereas most smaller MPs (<500 μm) escape treatment and were released into the riverine environment via effluent. It was estimated that around 2.04 billion MPs are released daily into the Chao Phraya River from these eight WWTPs. In the river, MP concentrations were higher in the urban center compared to upstream areas, with characteristics such as shape, size, and color mirroring those found in wastewater effluent. Polymer analysis revealed polypropylene (PP) as the most common polymer in both wastewater effluent and river samples, indicating that WWTPs significantly contribute to MP pollution in the receiving river systems. The daily MP discharge from WWTPs ranged from less than 20 % during the dry season to exceeding the total MPs discharged from the Chao Phraya River to the GoT during the wet season, amounting to approximately 1.75 and 0.33 billion items per day, respectively.