Have you tried to precipitate ...
Published by Gheysa Pires, Environmental Researcher at Lactec - Institute of Technology for Development
Have you tried to precipitate it into struvite (magnesium-amonium-phosphate or calcium-amonium-phosphate)? You have to balance the three elements and slightly elevate the pH. I wrote my master thesis in Germany in this topic, you can free download it from my profile. If you can't feel free to contact me. Best regards from Brazil