Coexistence of Microplastics and Heavy Metals in Wastewater Treatment Plants of Delhi, India: Occurrence, Transport, Potential Interactions, and Associated Environmental Implications

This study investigated five wastewater treatment plants operating on the activated sludge process in Delhi to assess their microplastic (MP) removal efficiencies and to investigate possible environmental implications of MP co-existence with potential toxic elements (PTEs). Results revealed that MPs decreased through successive treatment stages from 2173.3 ± 172.04 n/L to 613.33 ± 47.84 n/L by number (71.51% removal) and 0.24 ± 0.03 mg/L to 0.06 ± 0.01 mg/L by mass (75.47% removal); however, a significant fraction of MPs is still discharged via treated effluent. Elongated MPs increased in effluent proportionately (from 15% to 25%) due to their small size and higher L/D ratio. Larger MPs (>500 μm) constituted only 1.23% in raw WW, while smaller MPs (<500 μm) dominated (>98%) across all WWTPs. μ-FTIR imaging identified eight types of polymers, with polyolefins dominating in all samples, posing a greater risk due to their high interaction potential with co-contaminants. Most MPs get accumulated in sewage sludge [(52.38 ± 8.11) × 103 n/kg of d.w. SS; 6.22 ± 1.21 mg/kg] with diverse sizes and shapes, posing a considerable risk via soil amendment. SEM-EDS analysis confirmed micro-morphological changes, adsorption of PTEs, and microbial colonization. Correlation analysis highlighted MPs’ role as vector for PTEs transport; however, not all MPs are equally effective in adsorbing metals, highlighting need for further research. These findings underscore need for a multifaceted approach, combining improved WWTPs’ performance, source control and responsible consumption, policy and regulatory interventions, behavioral changes, and continuous monitoring and research.