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Pilot-scale study of phosphorus recovery through struvite crystallization Adnan, Ali

Abstract

The workability of a new pilot-scale reactor, based on the same process principle as that of the previously tested bench-scale reactor, was examined. This work extended the long-term research program, and bridged the gap between phosphorus removal and recovery. The problem of the fines encountered during the bench-scale study was overcome. The pilot-scale UBC MAP Crystallizer, which was used to remove / recover phosphorus through struvite formation, achieved ortho-P removal rates of over 90%, for a tested range (47 mg/L ~ 220 m/L) of influent P concentrations. The desired degree of P-removal was achieved by controlling the reactor by varying operating pH and the supersaturation ratio at the inlet. The high P-removals rates (~ 90%) were achieved even at a pH 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 which can cause the process fluid to be supersaturated. Limited results showed that process fluid can also be supersaturated by an excessive dosage of the magnesium ions, thereby indicating that magnesium can also be used as a controlling parameter. About 80% of phosphate removed was recovered as harvestable struvite crystals. In general, there was no problem of fines production during the pilot-scale study. The average mean size of the harvested crystals remained over 2 mm, for all the experiments conducted. The inreactor supersaturation ratio and the crystal retention time (CRT) were identified as the major factors affecting mean crystal size. 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 2 and 3. 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. An equilibrium model (developed earlier) which predicts the effluent concentrations of struvite constituent ions, was validated using pilot-scale study results. The results predicted by an equilibrium model closely matched the actual pilot-scale results. With an expected knowledge of the effluent concentrations, a process engineer / operator can use an appropriate degree of recycle ratio, thereby ensuring the process conditions in the metastable zone of crystallization. Thus, the use of an equilibrium model is recommended in future related studies.

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