Effect of composition of anaerobic supernatants from an anaerobic, anoxic and oxic (A(2)O) process on struvite and hydroxyapatite formation

Real anaerobic supernatant supplied from an anaerobic, anoxic, oxic (A(2)O) process is used to investigate the effect of composition on phosphate crys tallisation The first phase defines the possibility to precipitate phosphat e (77-80%) as struvite and hydroxyapatite mixture up to seven days of natur al ageing. The typical composition of supernatant does not require addition of chemicals to obtain these results. A different starting composition can only influence the salt but not the phosphate conversion. In the second ph ase, an air stripping procedure is adopted, which gives higher phosphate co nversions (85 - 91%) and a contact time less than 90 min attributed to a hi gher working pH. In the third phase, phosphate is partially crystallised on quartz sand (78-94%) and partially is lost (1-11%) as fine particles. Fina lly, preliminary runs on a fluidized bed reactor are conducted demonstratin g a feasible and reliable process to crystallise phosphate as struvite or h ydroxyapatite. atone or as a mixture in relation to the starting compositio n of supernatant. High performances (83-87%) and a negligible phosphate los s (less than or equal to 1%) are always observed. The different phenomena a re explained comparing the experimental runs with the relative supersaturat ion curves. The results obtained confirm the inhibition of carbonate and ma gnesium on hydroxyapatite formation and the fluidized bed reactor working f arthest from the supersaturation condition (metastable zone), assures the c rystallization process.