A comprehensive review of sewage sludge valorization for resource recovery and sustainable agriculture use

Abstract

The swift intensification of urbanization and industrialization has resulted in the generation of over 53 million tonnes (dry solids) of sewage sludge/year globally, posing major environmental and management challenges. Sewage sludge (SS), a solid or semi-solid output from sewage treatment plants, may mostly be utilized in agriculture as organic fertilizer and enhancing nutrient turnover and soil organic matter through reuse. SS commonly contains 10.00–74.30% organic carbon, total nitrogen; 4.7–98.00 g/kg, total phosphorus; 2.12–36.00 g/kg, and appreciable micronutrients, making it a valuable soil amendment. This review summarizes sludge characteristics, agronomic impacts across dosage gradients, and the biochemical processes underpinning nutrient mineralization and heavy metals stabilization. Despite its considerable benefits, SS application faces challenges, primarily due to potential heavy metal contamination. Most studies report improved soil fertility and crop productivity at application rates of 5–40 t/ha, while substantially higher rates have been used in selected fodder, plantation, and reclamation systems. Nevertheless, excessive applications may promote heavy metal accumulation, necessitating standardized application rates and long-term monitoring to ensure environmental safety. This review discuss the complementary potential of anaerobic digestion, composting, co-composting, and thermal transformations (pyrolysis/hydrothermal carbonization), emphasizing how their strategic integration could maximize resource recovery, enhance heavy metals stabilization, reduce processing time, and lower the overall carbon footprint. Thermal conversion can reduce sludge volume by 60–90% while producing value-added biochar. When viewed through a circular bio-economy lens, the combined outputs (biogas, biochar, organic amendments, biofuel and bio-adsorbents) diversify revenue for farmers and municipalities, lower dependence on synthetic fertilizers, and deliver a scalable, low-impact model. Overall, this review bridges current knowledge and emerging valorization strategies by integrating agronomic, environmental, and technological perspectives into a unified framework, offering a roadmap for sustainable SS management, resource recovery, and evidence-based policymaking.