Seasonal variation in abundance and characteristics of microplastic in sewage sludge from major cities across the upper stretch of River Ganga, India.

Microplastic (MP) load in urban sewage sludge could vary annually in wastewater treatment plants (WWTPs) depending on seasonal precipitation and human activities. We investigated the seasonal dynamics in abundance and characterization of MP loading in WWTPs located in two cities across River Ganga, India's ecologically sensitive upper stretch. During a 12-month seasonal sampling (pre-monsoon, monsoon, and post-monsoon), sludge samples (n = 36) (primary sludge, PS; drying bed sludge, DBS) were collected and analyzed for load, polymer types, shape, colour, and size (20-1000 µm). Across the three seasons, MP concentrations (particles/kg) were found to be in the ranges of 93.4 ± 5.0 × 10³-189.4 ± 11 × 10³ in the PS and 39.6 ± 4.0 × 10³-152.0 ± 7 × 10³ in the DBS. The trend of MP loading was in the following order: monsoon > post-monsoon > pre-monsoon. The dominant MP size was 50-200 µm (36.22%) followed by 20-50 µm (27.65%), 200-500 µm (24.55%) and 500-1000 µm (11.58%). ATR-FTIR results revealed polypropylene, polyethylene terephthalate, polyvinyl chloride, and nylon dominating MP in sludge. This study highlights the importance of long-term monitoring of MP loading in sewage sludge to offer a more accurate estimate of MP contamination in sludge from WWTPs and develop a possible mechanism for its elimination to safeguard the environment.