Free ammonia-driven sewage sludge valorization: from in-situ pretreatment to energy and resource recovery

Sewage sludge generated from wastewater treatment plants (WWTPs) represents both a significant environmental burden and a valuable secondary resource, yet its efficient treatment, particularly energy and resource recovery, remains constrained by slow sludge solubilization and hydrolysis. Free ammonia (FA, NH₃), with its membrane-permeable bactericidal activity and natural presence in sludge treatment processes, has emerged as an economical, efficient, and in-situ available pretreatment agent. However, a comprehensive and systematic review of the role of FA pretreatment across different sludge treatment scenarios is currently lacking. To fill this gap, this paper provides a critical and comprehensive review of FA pretreatment as a multifunctional regulator in sludge treatment. Specifically, this paper systematically explores the effectiveness and underlying mechanisms of FA pretreatment in the following aspects: (i) enhancement of sludge dewatering and reduction, (ii) promotion of methane production from anaerobic digestion, (iii) improvement of short-chain fatty acids and hydrogen recovery from anaerobic fermentation, (iv) recovery of high-value products (e.g., phosphorus and polyhydroxyalkanoates), and (v) removal of pollutants (e.g., antibiotic resistance genes and volatile sulfur compounds). Furthermore, the key operational parameters governing FA efficacy and the synergistic interactions of FA with physical, chemical, and biological co-pretreatment strategies are critically evaluated. Finally, current engineering challenges and future research priorities, especially pilot-/full-scale validation and multi-omics investigations, are outlined. This review is expected to provide a robust theoretical foundation and practical guidance for advancing FA pretreatment-based sludge management strategies, ultimately contributing to the transformation of WWTPs into energy-positive biorefineries with greater sustainability and alignment with circular economy principles.