Influence of paraments on the transformation behaviors and directional adjustment strategies of phosphorus forms during different thermochemical treatments of sludge

A variety of sludge originating from wastewater treatment is increasingly accumulating worldwide, especially in the developed regions. Fortunately, phosphorus (P) in sludge can potentially be recovered via thermal treatment, and P forms are crucial to the quality of the recovered products. This review provides an overview related to the transformation of P and its regulation strategies in the different thermal treatment processes, highlighting the current knowledge regarding the influence factors and mechanisms of transformation. First, the current re-searches with P recovery by thermal treatments (i.e., incineration, pyrolysis, and hydrothermal carbonization) were illustrated and critically discussed, which indicates that the selection of appropriate method significantly affects the recovery effects, such as the proportion of Apatite Phosphorus (AP) or Ca-P (P combing with Ca). Furthermore, the effects of various factors (e.g., treatment techniques, temperature, additives, the composition of sludge, pH and metals ions) on the transformation or regulation of P forms were critically discussed, suggesting that temperature and Ca/Cl-based additives are the two crucial factors, which have the considerable effects on the emergence of Ca-P. Especially the Ca-based additives, which have the highest combining ability with P and the uppermost impact on hydroxyapatite formation. In addition, an overall comparison of incineration, pyrolysis, and hydrothermal carbonization technologies in practical applications is provided, focusing on the recovery efficiency, advantages, and disadvantages, demonstrating that HTC will be more predominant for P recovery from sludge in the future. Finally, challenges and potential development directions regarding the transformation mechanisms of P forms and regulate strategies are identified. This review will become the foundation for future research devoted to improving the quality of the recovered products.