Intra-day microplastic variations in wastewater: A case study of a sewage treatment plant in Hong Kong

Wastewater treatment plants (WWTPs) in coastal region play a primary role in transferring microplastics into the marine environment. Wastewater is closely related to anthropogenic activities, thus the intra-day variation of abundance of microplastics in the influent should be large and could have significant impact on their estimation of the daily mass load. In this study, a 2–hour interval sampling campaign was conducted at a secondary WWTP in Hong Kong to investigate the intra-day variations and daily loads of microplastics in influent. Results show that the average microplastic abundances increased from 7.1 ± 6.0 to 12.8 ± 5.8 particles/L over time, with predominant particle sizes ranging 1–5 mm. Approximately 80% of the microplastics in samples collected from 9:30–15:00 were polyethylene and polyester, while most samples collected at 17:00 were polypropylene and polyurethane. Microplastic loads exhibited large intra-day variations ranging 6.60 × 10⁸–1.16 × 10⁹ particles/day, indicating that calculated daily microplastic loads based on a specific sampling period may inaccurately estimate the actual daily load.