Struvite crystallization: a feasible and reliable way to fix phosphorus in anaerobic supernatants

The possibility to fix phosphate in sludge handling liquors supplied from a carbon–nitrogen wastewater treatment plant through the crystallization of struvite was investigated in this paper. The anaerobic supernatant used for the experiments was obtained from the dewatering section (belt-press) of a 85000 equivalent inhabitants civil wastewater treatment plant, with a combined carbon oxidation and nitrogen nitrification-denitrification and anaerobic digestion of sludge. The experimental results showed that a natural aging with phosphate precipitation can be observed in a few days with a different percentage of struvite and hydroxyapatite in relation to the liquor composition; moreover supersaturation curves play a major role in the interpretation of the process. Finally, the FBR plant managed in batch mode assures a good phosphate removal through crystallization on quartz sand in about 100 min.