Occurrence, quantification, and characterization of microplastics in sewage sludge from a university wastewater...

Sewage sludge (SeS) produced in wastewater treatment plants (WWTPs) is increasingly recognized as a significant reservoir for microplastics (MPs). Despite this, comprehensive data on MP occurrence in advanced institutional treatment systems, particularly in developing countries, remain limited. This study provides a quantitative evaluation of MPs in SeS from an academic institution in India operating a hybrid Moving Bed Biofilm Reactor-Zero Liquid Discharge (MBBR-ZLD) system. Samples were collected fortnightly over a seven-month period (January-July) and analyzed using wet peroxide oxidation, density separation, stereomicroscopy, ATR-FTIR spectroscopy, SEM-EDX, and Nile Red fluorescence microscopy. Microplastics were detected in all samples, with concentrations ranging from 120 to 1145 particles per kilogram dry weight (n/kg dw). Particle sizes ranged from 7.02 to 4592 µm, with the 500-1000 µm size fraction being the most prevalent. Fibres dominated the morphology (43.3%), followed by fragments (39.5%) and films (11.5%). Polymer characterization identified eight types, with Nylon-PA6 (37.7%) and LDPE (16.0%) as the most abundant. Method validation using co-spiked reference materials yielded an overall recovery efficiency of 81.66 ± 2.51%, though recovery varied depending on particle type and size. Accordingly, correction factors were applied to improve data accuracy. A strong positive Spearman rank correlation was observed between campus occupancy and monthly mean MP concentration (ρ = 0.756, p = 0.049), suggesting that human activity significantly influences MP loading. This study provides a comprehensive evaluation of the temporal relationships between academic occupancy patterns and MP concentrations in SeS, along with their polymer profiles, in an Indian institutional MBBR-ZLD system. These findings highlight the role of institutional usage patterns in MP accumulation within MBBR-ZLD systems.