Postdigestion struvite precipitation using a fluidized bed reactor

Controlled precipitation of struvite from postdigestion, sludge lagoon supe rnatant was studied using a pilot scale fluidized bed reactor (FBR). The pr ocess effectively and rapidly removed struvite constituents from the supern atant and produced effluent significantly undersaturated for struvite. Rece ntly published information on struvite thermodynamic and kinetic properties was incorporated into the FBR system design and selection of components. S truvite crystals were selected as seed media, which enabled precipitation o f nearly pure crystals and allowed nucleation, with its associated lag time , to be bypassed. Mixing energy in the FBR was demonstrated to be sufficien t to overcome transport limitations to struvite growth, allowing growth kin etics to be governed by the surface integration rate of constituent ions in to the growing crystal lattice structure. Struvite removal was optimized by incorporating pH elevation into the FBR system. Process fluid was circulat ed in the pH adjustment tank/FBR system, which was configured as a continuo us feed stirred-tank reactor. Struvite removal efficiency exceeded 80% in c ontinuous feed experiments where the hydraulic detention time exceeded 1 h.