revealed that a derivative of magnesium pyrophosphate (Mg2P2O7) was formed during MAP pyrolysis, and that magnesium phosphate (Mg3(PO4)2) was a possible candidate when there was continuous addition of Mg(OH)2 to MAP during struvite precipitation. However, Mg2P2O7 and Mg3(PO4)2 has only a weak effect on ammonia nitrogen removal, which are the main cause of the steady decline in the ammonia nitrogen recovery ratio with repeated recycling. Effective methods have not yet been found to resolve this issue.