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Tackling microplastic contamination in sewage sludge: Optimizing organic matter degradation, quantifying microplastic presence, and evaluating ecological risks for sustainable agriculture
Summary
Researchers optimized a Fenton reagent-based method to remove 86.6% of organic matter from sewage sludge, enabling more accurate quantification of microplastics in the samples. Analysis of sludge from 14 treatment plants in Ahmedabad, India revealed microplastic concentrations ranging from 2,430 to 227,200 particles per kilogram, with small fibers and fragments being the most abundant. Ecological risk assessments indicated extreme hazard levels, raising concerns about using contaminated sewage sludge as agricultural fertilizer.
The omnipresence of Microplastics (MPs) is a growing global concern. Using sewage sludge as fertilizer for soil amendment can be a potential source of MPs in agricultural soil if sludge contains MPs. Sludge is a complex matrix rich in organic matter, which hinders MPs separation. For maximal organic matter degradation, in this study, the application of Fenton reagents optimized for (Fe/HO) molar ratios, i.e., 1/2, 1/4, 1/6, 1/8, and 1/10. The results show that a molar ratio of 1/2 of Fe/HO can remove 86.6 % of the organic matter in the sewage sludge. The greenness of the optimized method was assessed and compared to available methods using AGREEprep software. The method achieved a greenness score of 0.61, significantly higher than the highest score of 0.45 among the previously reported optimized methods. This optimized method was used in the analysis of MPs in sewage sludge from 14 sewage treatment plants in Ahmedabad. Also, the ecological risks due to the application of such sludge in agriculture were assessed. MPs analysis reveals variability in MPs contamination ranging from 2.43 to 22.72 × 10 units/kg of sludge. Small-sized MPs (0.05-0.25 mm) constitute the highest proportion (65 %), predominantly comprising fibers and fragments. From a chemical composition point of view, six different types of MPs are identified, among which PU, Nylon, HDPE, and PP are the most abundant. Ecological risk assessment indicated extreme hazards in terms of the potential ecological risk index being higher than 1200 for all the sludge samples due to the abundance of MPs, specifically of PU and Nylon.