Potential and Ecological Risk Evaluation of Sewage Sludge from Different Urban Sources in Kerala, India

Sewage sludge generated from wastewater treatment plants possesses significant resource potential but also raises environmental concerns due to its organic-rich composition and associated contaminants. This study collected raw sewage sludge from various urban sources in the state of Kerala, India, to evaluate its resource potential and assess ecological risks, with a focus on preliminary chemical characterization and risk screening. The average nutrient composition of the sludge samples revealed Total Organic Carbon (TOC) of 43.7%, Total Nitrogen (TN) of 4.86%, Total Phosphorus (TP) of 1.88%, and Total Potassium (TK) of 0.345%, with a Carbon-to-Nitrogen (C/N) ratio of 9.1. These values indicate agronomic potential of sewage sludge after appropriate stabilization method. However, ecological risk screening using pollution assessment indices indicate potential environmental risks upon land application. Hg exceeded the FCO (2009) limit in all sources, a nine-fold increase is reported in source SC4 and SC5. Cu exceeded the limit in source SC2 and SC5. Among the heavy metals assessed, Cd and Hg pose major ecological concerns based on the contamination and risk indices. Although, the sludge samples exhibited similar organic characteristics across different urban sources, metal contamination levels and associated risk indices varied significantly, reflecting source dependent inputs. These results indicate that sewage sludge reuse should be considered only after appropriate stabilization techniques, followed by continuous monitoring and site specific risk assessment before land application. The study also highlights the need to update regulatory frameworks to facilitate the safe, controlled and economically viable sewage sludge reuse, in line with the principles of a circular economy.