From Sanitation to Soil: Evaluating Faecal Sludge-Based Amendments for Nutrient Circularity and Sustainable Agriculture

Abstract

Globally, especially across developing regions, cities face a dual challenge: managing faecal sludge (FS) from on-site sanitation systems in a safe and sustainable manner, while simultaneously restoring soils degraded by prolonged reliance on synthetic fertilisers. Synthetic fertilisers, though widely used, contribute significantly to greenhouse gas emissions—particularly nitrous oxide (N₂O)—and further degrade soil health over the long term. Meanwhile, the availability of traditional organic amendments such as farmyard manure (FYM) is declining due to reduced livestock ownership and changing rural livelihoods, creating resource constraints for nutrient recycling. With over 3.4 billion people lacking access to safely managed sanitation and climate-smart agriculture seeking alternatives to energy-intensive fertilisers, the reuse of FS has emerged as a promising circular solution.

This intersection is particularly pronounced in India, where more than 60% of urban households depend on on-site sanitation systems for wastewater management, while large areas of agricultural land face nutrient depletion and declining organic carbon levels. Despite the nutrient richness and organic matter content of treated FS, its reuse remains largely informal, unregulated, and underexplored. Concerns over treatment variability, microbial safety, and the absence of national reuse standards continue to hinder its integration into mainstream nutrient recycling frameworks.

Despite growing interest in FS reuse, key knowledge and implementation gaps persist. Most of the existing studies have overlooked the evaluation of FS quality specifically in relation to soil application and its suitability for agriculture. Comprehensive assessments of agronomic performance and microbial safety under field conditions—particularly in comparison with widely used organic amendments and synthetic fertilisers—remain scarce. There is also limited evidence on how treatment variability influences immediate and residual nutrient availability and microbial contamination risks across diverse cropping systems. Furthermore, the absence of national regulatory benchmarks for FS use in India makes it challenging to assess risks and develop safe, context-specific reuse guidelines.

This thesis addresses the interconnected scientific, policy, and agronomic gaps by providing a comprehensive assessment of the characteristics, agronomic performance, and safety of FS-based amendments derived from nature-based treatment systems—widely used in India and other developing regions. Drawing on faecal sludge characterisation, two-season field trials across diverse cropping systems, and a global regulatory review, the study provides new insights into the suitability of FS products for safe agricultural use. The research evaluates their comparative performance with conventional organic and synthetic fertilisers and develops comprehensive quality criteria tailored to FS reuse in agriculture. The findings culminate in evidence-based recommendations to support the safe, context-specific application of FS within sustainable nutrient management strategies.

The first component of the study characterises FS products from three widely used nature-based treatment processes in India: unplanted drying beds, planted drying beds, and co-composting units, with a focus on their soil application potential. Thirty samples collected from full-scale treatment sites were analysed for physicochemical parameters, plant essential nutrients (macronutrients, micronutrients), and heavy metals. The treatment processes significantly influenced the faecal sludge properties, revealing significant differences in conductivity, total organic carbon, total nitrogen, and total potassium content. Co-composting resulted in higher conductivity (4.9 dS/m) and potassium content (1.09%) but lower total nitrogen (2.15%) and organic carbon contents (28%). The results revealed a balanced nutrient profile with a wide range of micronutrients and high variability. All products met the Indian Fertiliser Control Order (FCO) standards for composts, including thresholds for heavy metals, confirming their suitability for agricultural use. These findings underscore the need to consider treatment processes in shaping FS quality and inform soil application rate and recommendations tailored to sludge source and crop type.

The second and third components of this research assessed the agronomic performance and microbial safety of FS-based amendments through two field experiments involving diverse cropping systems—maize–cowpea and fenugreek–radish—over two growing seasons. The study compared three FS-based products (unplanted drying bed sludge, planted drying bed sludge, and co-compost) against conventional inputs including farmyard manure (FYM), city compost, synthetic fertilisers, and a control. FS-based amendments achieved 85–90% of the yields of maize and fenugreek (first-season crops) relative to synthetic fertilisers. In the second-season crops (cowpea and radish), FS treatments outperformed synthetic fertilisers and matched the performance of FYM. FS products consistently enhanced yields, as well as crop nitrogen and protein content, with results comparable to those of conventional fertilisers. Soil nutrient assessments indicated that FS amendments improved nitrogen and organic carbon levels, sustained nutrient availability across seasons, and outperformed synthetic fertilisers in secondary macronutrients.

Microbial safety—a major barrier to FS reuse—was assessed by analysing edible plant parts (cobs, seeds, roots, and leaves) for E. coli and faecal coliforms. Across both cropping systems, produce from FS-amended plots consistently showed microbial contamination levels within acceptable limits set by global safety standards, particularly when appropriate waiting periods and postharvest hygiene practices were followed. Maize cobs, cowpea plants, and seeds exhibited minimal contamination, meeting global safety benchmarks. In contrast, fenugreek displayed elevated contamination levels, largely attributed to early harvest and surface exposure, whereas radish—harvested five months after amendment application and analysed post-washing—demonstrated consistently low contamination. Importantly, indicator organisms were also detected in plots treated with FYM and city compost, underscoring that microbial risk is not unique to FS products. Contamination was additionally observed in control and synthetic treatments, suggesting environmental exposure as a contributing factor. Among the FS products, co-composted FS emerged as the safest treatment, yielding produce with the lowest contamination levels. These findings provide evidence that, when properly treated and managed, FS-based amendments can perform on par with conventional organic inputs in terms of microbial safety. They underscore the importance of treatment quality, crop selection, harvest timing, and hygiene practices in mitigating microbial risks across all organic fertilisers.

Overall, the study demonstrates that stabilised FS-based amendments, when properly treated and managed, can safely enhance crop productivity and soil health. They offer a viable alternative to traditional organic inputs and serve as a complementary strategy to synthetic fertilisers within regulated, sustainable nutrient recycling frameworks.

The fourth component of this research presents a comparative review of international regulations governing sludge reuse across 35 countries. It traces the evolution of quality standards—from early emphasis on heavy metal thresholds to more comprehensive frameworks encompassing pathogens, emerging contaminants, and physicochemical criteria. However, most global regulations are designed for sewage sludge from centralized systems, with limited attention to faecal sludge (FS) from on-site sanitation systems, which predominate in developing regions. Due to fundamental differences in composition, sewage sludge guidelines cannot be directly applied to FS. Addressing this regulatory void, the study proposes a draft set of quality criteria tailored to Indian conditions. Drawing from international benchmarks and national guidelines, the proposed framework outlines limits for nutrients, heavy metals, pathogen indicators, and key physicochemical parameters relevant to agricultural application. These recommendations offer a foundational step toward developing national standards that enable safe, regulated FS reuse—balancing agronomic benefits with public health protections.

Together, the thesis presents one of the first integrated studies in India that combines sludge characterisation, comprehensive field-level agronomic trials across diverse cropping systems, and regulatory policy analysis to holistically evaluate FS reuse potential. It advances the science of sludge-to-soil recycling by showing that FS amendments—when properly treated, timed, and managed—can deliver agronomic benefits comparable to traditional fertilisers. The work also contributes a policy-ready, evidence-based framework to support the broader goals of resource circularity and sustainable agriculture.

Several novel contributions emerge from this research. First, it establishes the critical role of treatment processes in determining the agronomic suitability of FS products. Second, it provides robust, field-level validation of FS reuse across diverse cropping systems under real-world conditions, benchmarked against conventional fertilisers. Third, it introduces a context-specific regulatory framework—addressing a long-standing policy vacuum around FS reuse in India. Finally, the thesis outlines key directions for future research, including the long-term cumulative impacts of FS application, tracking of specific pathogenic strains, socio-economic factors affecting adoption, and comparative climate impacts of sludge reuse versus conventional fertilisers.

By linking science, agriculture, and sanitation policy, this thesis seeks to shift the conversation on faecal sludge from a waste disposal problem to a resource opportunity. In doing so, it contributes to advancing sustainable agriculture, closing nutrient loops, and promoting climate-resilient development—both in India and beyond.