Sequential distribution, potential sources, and health risk assessment of persistent toxic substances in sewage sludge used as organic fertilizer in Indo-Gangetic region.

This study evaluates the environmental and human health impact of sewage sludge generated in the Indo-Gangetic region (Uttarakhand and Uttar Pradesh) used as organic fertilizer and landfill disposal. The research conducts a comprehensive risk assessment, including physicochemical and heavy metals analysis, on triplicate sludge samples obtained from 30 sewage treatment plants. The study provides both qualitative and quantitative insights into potential hazards associated with sewage sludge. The results indicate varying concentrations (mg/kg) of heavy metals in sewage sludge (expressed in mean ± SD) as determined by inductively coupled plasma mass spectrometry (ICP-MS) with the following order, zinc (966.15 mg/kg ± 279) > chromium (851.23 mg/kg ± 3079) > cadmium (150.07 mg/kg ± 307) > copper (186.09 mg/kg ± 56.25) > arsenic (5.24 mg/kg ± 3.54) > nickel (21.97 mg/kg ± 2.13) > mercury (1.05 mg/kg ± 0.12). The metal pollution indices underscore greater non-compliance in samples from STPs of Uttar Pradesh compared to those from Uttarakhand, with 40% of samples falling into poor to particularly poor categories. Multivariate analysis of samples reveals potential pollution sources, implicating industrial effluents and agricultural runoff, with identified controlling factors being Cu (0.948, p < 0.05); Zn (0.941 p < 0.05); Pb (0.921 p < 0.05); Ni (0.806 p < 0.05); Cd (0.717, p < 0.05); and electrical conductivity (0.620, p < 0.05). Monte Carlo-based uncertainty analysis emphasizes sludge-based chromium (Cd) as the highest risk at 62.86% (p < 0.001), trailed by chromium (59.29%, p < 0.001) for target cancer risk. The study also suggests potential management options, including the application of AI-based sensors for heavy metal monitoring, exploration of improved trapping or diluting technologies, and raising public awareness about stringent rules regarding sewage sludge remediation for effective risk mitigation strategies.