A review of efficient nitrogen removal and phosphorus recovery by anammox-hydroxyapatite based processes: Challenges and opportunities

The anaerobic ammonia oxidation (anammox) induces calcium and phosphate precipitation to form hydroxyapatite (HAP) crystals, and the biomass grows synchronously with the crystals to form anammox-HAP (A-HAP) multi-layer granules. The enhanced granule has excellent mechanical strength and settleability, making its related processes the promising technologies for simultaneous nitrogen removal and phosphorus recovery. In order to comprehensively understand and digest the application of inorganic precipitation assisted microbial technology in wastewater treatment, the structure, granulation mechanism and advantages of such multi-layer granules are firstly introduced in this paper. Secondly, in this paper, the existing A-HAP based processes put into practice are summarized and their superiority in treating wastewater is systematically reviewed. Thirdly, the three main stable operating conditions of inorganic carbon (IC), phosphorus concentration and Ca/P ratio are presented for better application of the processes. Finally, the challenges and opportunities for future research and practical applications are provided.