Phosphate recovery in vivianite proved to have great potential for phosphorus recovery from digester supernatants. However, the size of the recovered vivianite was found to be very small (< 20 μm), impeding further processing. A process was developed to increase the crystal size of vivianite by controlled Fe²⁺ dosing and seeding effect, which can be a promising way for vivianite growth.

Abstract

Phosphorus (P) recovery by crystallization of vivianite in digester supernatant (DS) was investigated. A high recovery yield (> 90 %) was obtained with vivianite even for DS with low P concentration (74 mg L⁻¹), as opposed to the formation of struvite and hydroxyapatite where the yield was lower than 50–60 %. Various strategies were tested to produce vivianite with large sizes and narrow size distribution, such as a controlled reagent dosing profile, self-seeded crystallization, and multistage cascade-seeded crystallization. The obtained results demonstrated that the main challenge in the development of vivianite P recovery is to promote crystal growth and to suppress secondary nucleation of vivianite during the crystallization process.