Struvite Pellet Crystallization for Phosphorus Recovery from Sludge Liquor and Characterization of Pellet Quality

Abstract:

In order to explore the recovery of phosphorous from sludge liquor using struvite(MAP) pellet crystallization, the synthetic sludge fermentation liquid was first used to obtain the optimal conditions for struvite pellet formation, and then the recovery of phosphorous and growth of struvite fed with sludge dewatering liquor was investigated. When fed with synthetic fermentation liquid, the optimal conditions for struvite pellet formation were as follows: pH was 8.2; hydraulic retention time (HRT) was 41 min; inflow rising velocity was 400 cm·min-1, and phosphorus to nitrogen molar ratio was 1∶6. The maximum struvite particle size reached 4~5 mm; the purity was all above 97.5%; the particle hardness was (43.8±1) kg·mm-2. Under the above optimal conditions, real sludge dewatering liquor was used to recover phosphorus. The highest removal efficiency of PO3-4P could reach 90.5%. High concentration of suspended solid (SS) went against the formation of struvite, and the extension of incubation time could not effectively increase the removal efficiencies of ions and struvite particle size. The maximum particle size was 2.0~3.2 mm while the purity of harvested pellets was more than 80% (ρSS<150 mg·L-1). The concentrations of heavy metals were below the limits of heavy metal contents for fertilizer standards in China.

Key words:struvite pellet crystallization; phosphorus recovery; optimal conditions; pellets quality; sludge dewatering liquor