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Intra-day microplastic variations in wastewater: A case study of a sewage treatment plant in Hong Kong
Summary
This study measured intra-day variation in microplastic concentrations in the influent wastewater of a Hong Kong sewage treatment plant at two-hour intervals, finding that concentrations peaked during morning and evening rush hours. Daily activity patterns drive significant within-day fluctuations in microplastic loads entering treatment plants, affecting estimates of daily microplastic discharge to coastal waters.
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 × 10-1.16 × 10 particles/day, indicating that calculated daily microplastic loads based on a specific sampling period may inaccurately estimate the actual daily load.