Characterisation of Faecal Sludge from Different Nature-Based Treatment Processes for Agricultural Application

Nature-based systems predominantly treat faecal sludge in developing regions due to their cost-effectiveness and operational simplicity. These systems, including solid–liquid separation, anaerobic digestion, dewatering, phytofiltration, and composting produce, treated sludge with variable characteristics. However, application-specific characterisation of treated sludge from these systems remains limited, hindering evidence-based agricultural application. This study investigated thirty treated faecal sludge samples from unplanted drying beds, planted drying beds, and co-composting, with a focus on their soil application potential. Nonparametric statistical analysis revealed that treatment processes significantly influenced the key properties, including electrical conductivity, total organic carbon, total nitrogen, and potassium content. The co-compost yielded comparatively higher conductivity (4.9 dS/m) and potassium levels (1.09%) but lower total nitrogen (2.15%) and organic carbon contents (28%). Additionally, co-composted sludge exhibited a balanced nutrient profile with a wide range of micronutrients and high variability. Despite this variability, all samples met the Indian compost quality guidelines for heavy metals. The findings underscore the importance of treatment-specific characterisation to inform appropriate soil application rates and ensure safe use. This study contributes to the development of quality criteria and guidelines for use of faecal sludge in agriculture, particularly in regions such as India, where no regulatory framework currently exists for faecal sludge application.