Pilot-scale study of phosphorus recovery through struvite crystallization – II: Applying in-reactor supersaturation ratio as a process control parameter

A pilot-scale MAP (magnesium ammonium phosphate) crystallizer, which was used to remove/recover phosphorus through struvite formation, achieved ortho-P removal rates of over 90%, for a tested high phosphorus concentration (~220 mg/L). The desired degree of P-removal was achieved by either varying the operating pH or the supersaturation ratio at the inlet. The high P-removals (~90%) were achieved even at a pH of 7.3, which is contrary to the information found in the literature, where generally higher pH values (8.2~9) are recommended. This indicates that alkaline pH is not the only factor that can cause the process fluid to be supersaturated. Using solubility criteria, the in-reactor supersaturation ratio was used to define the metastable zone boundaries. The system performance, both in terms of process efficiency and the quality of the harvested product, was at its best when the in-reactor supersaturation ratio was between 1 and 5. The results showed that there was a narrow working zone for optimized crystallization process and a deviation from the optimal metastable zone always resulted in the plugging of the reactor. Key words: conditional solubility product, crystallization, phosphorus removal, struvite.