Comprehensive characterization and classification of sludge from sewage treatment plants in Tamil Nadu, India:...

Rapid urbanization in Chennai has increased sewage sludge generation, while challenges such as land scarcity and limited treatment infrastructure continue to affect its management. Furthermore, there is a lack of comprehensive region-specific data on physicochemical, organics, nutrients, metals, and microbiological properties of digested and dewatered sludge from sewage treatment plants (STPs), limiting decision-making for its safe disposal and reuse. This study provides a comprehensive characterization of sewage sludge from the STPs in Chennai. Key indicators like the fertilizer index, cleanliness index, and pathogen load were used to assess sludge quality and classify it into Classes A, B, C, D and restricted-access categories. The fertilizing potential was calculated based on the total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK), which ranged from 1 to 2.2. The cleanliness index was determined to have a value of 5. Based on the fertilizer and cleanliness indices, the sludge was classified into Class D, indicating limited suitability for direct agricultural application. The nutrient concentrations were lower than the minimum requirements specified in the Fertilizer Control Order standards for compost. Despite high pathogen levels, including fecal coliforms (33-28,000 MPN/g TS), helminth eggs, and Salmonella species, the sludge met United States Environmental Protection Agency (USEPA) Class B biosolid standards. Pearson correlation analysis showed a strong connection between heavy metals and nutrient content, indicating a similar sludge composition across the STPs. The findings highlight the need for advanced treatment methods, such as pasteurization or thermo-chemical processes, to achieve Class A quality biosolids and enable safe reuse. This study provides critical region-specific insights to support improved sludge management and recycling strategies in India.