Endogenous denitrification: A sustainable strategy for residual sludge reduction and nitrogen removal

Residual sludge-rich in organic matter, microorganisms, heavy metals, and other contaminants-remains a major operational and environmental challenge for wastewater treatment plants, with direct implications for ecological safety, resource recovery, and long-term sustainability. This review provides a comprehensive assessment of existing physical, chemical, and biological sludge reduction technologies, with a particular focus on the emerging and innovative role of endogenous denitrification as the core mechanism underpinning in-situ biological sludge reduction. The biochemical pathways, regulatory factors, and process dynamics governing endogenous denitrification are systematically elucidated. Furthermore, the review highlights the potential of endogenous denitrification-based strategies to achieve simultaneous sludge reduction and advanced nitrogen removal while supporting energy-efficient, low-carbon, and resource-oriented wastewater treatment. By integrating current progress with future technological prospects, this study provides new insights and a robust theoretical foundation to guide the development of next-generation sustainable sludge management and circular resource utilization strategies.