Postdigestion struvite precipitation using a fluidized bed reactor

Controlled precipitation of struvite from postdigestion, sludge lagoon supernatant was studied using a pilot scale fluidized bed reactor (FBR). The process effectively and rapidly removed struvite constituents from the supernatant and produced effluent significantly undersaturated for struvite. Recently published information on struvite thermodynamic and kinetic properties was incorporated into the FBR system design and selection of components. Struvite crystals were selected as seed media, which enabled precipitation of nearly pure crystals and allowed nucleation, with its associated lag time, to be bypassed. Mixing energy in the FBR was demonstrated to be sufficient to overcome transport limitations to struvite growth, allowing growth kinetics to be governed by the surface integration rate of constituent ions into the growing crystal lattice structure. Struvite removal was optimized by incorporating pH elevation into the FBR system. Process fluid was circulated in the pH adjustment tank/FBR system, which was configured as a continuous feed stirred-tank reactor. Struvite removal efficiency exceeded 80% in continuous feed experiments where the hydraulic detention time exceeded 1 h.