A systematic review on nitrogen control during hydrothermal carbonization of sewage sludge under PRISMA guidelines

The rising global production of sewage sludge (SS), driven by urbanization, presents substantial challenges for effective management due to its high moisture content and disposal costs. Hydrothermal carbonization (HTC) is an effective thermochemical method for converting SS into valuable products while minimizing energy use. In addition, HTC can also remove nitrogen (N) from hydrochar (HC) and process water. The current review examined the removal of N in SS using HTC and analyzes the impact of additives (such as oxides, acids and inorganics) on the behavior of nutrients and heavy metals. For this purpose, a systematic literature review was conducted using the PRISMA guidelines on N control during HTC. The key findings revealed that HTC significantly influences N recovery, with optimal conditions for N removal identified at temperatures around 280°C. Moreover, HTC was identified as an effective method for N recovery by achieving approximately 80% recovery, whereas the upflow anaerobic sludge blanket reactor (UASB) achieves only 5.49% N recovery. By advancing our understanding of N recovery, this study recommends more effective SS management and resource recovery. •A systematic review of nitrogen transformation pathways and recovery strategies during sewage sludge hydrothermal carbonization (HTC).•Temperature and additives are critical parameters affecting nitrogen recovery efficiency and heavy metal behavior.•HTC significantly enhances the resource utilization of sewage sludge while mitigating environmental risks from heavy metals and nitrogen emissions.•Focus on optimizing additive usage and operational conditions to advance the practical application of HTC technology in sludge treatment.