A comprehensive study on the physicochemical characteristics of faecal sludge from septic tank and single pit latrine facilities in a typical semi-urban Indian town: a case study of Rajasthan, India

Faecal sludge (FS) generated from onsite sanitation (OSS) systems has become a significant pollutant that negatively impacts the environment. Environmental contamination results from the disposal of untreated FS. In semi-urban areas where numerous toilets are linked to OSS systems, such as septic tanks and single pits, faecal sludge management (FSM) becomes crucial to ensure a safe sanitation service chain. Integral to the faecal sludge management framework, treating FS is imperative, ensuring safe disposal and resource recovery. FS characterization plays a significant role in designing FS treatment plants. This case study characterized FS samples of OSS collected from Pilani, Rajasthan, India. The pH, temperature, electrical conductivity, total solids, chemical oxygen demand, faecal coliforms, total nitrogen, total phosphorus, and capillary suction time varied from 4.64 to 7.93, 20.6 to 27.5 °C, 1.857 to 6.315 mS cm⁻¹, 3430 to 95 393.33 mg l⁻¹, 4406 to 160 000 mg l⁻¹, 10³ to 10⁹ CFU ml⁻¹, 81.7 to 709.2 mg l⁻¹, 285 to 4471 mg l⁻¹, and 149 to 1256.8 seconds, respectively. The significant factors influencing the key FS characteristic parameter COD are found to be the FS age (p < 0.001) and type of OSS (p = 0.044), and for total solids, the factors affecting are identified as the FS age (p < 0.001), type of OSS (p = 0.002) and greywater dilution (p = 0.011). This case study can assist FSM stakeholders in designing FS treatment plants in Indian semi-urban towns and other developing nations with infrastructure, geographical and demographic factors, sanitation types, and FSM models similar to those in Pilani.