Struvite crystallization versus amorphous magnesium and calcium phosphate precipitation during the treatment of a saline industrial wastewater

Struvite crystallization (MgNH₄PO₄·6H₂O, MAP) could be an alternative for the sustainable and economical recovery of phosphorus from concentrated wastewater streams. Struvite precipitation is recommended for those wastewaters which have high orthophosphate concentration. However the presence of a cheap magnesium source is required in order to make the process feasible. For those wastewater treatment plants (WWTP) located near the seashore magnesium could be economically obtained using seawater. However seawater contains calcium ions that could interfere in the process, by promoting the precipitation of amorphous magnesium and calcium phosphates. Precipitates composition was affected by the NH₄⁺/PO₄³⁻ molar ratio used. Struvite or magnesium and calcium phosphates were obtained when NH₄⁺/PO₄³⁻ was fixed at 4.7 or 1.0, respectively. This study demonstrates that by manipulating the NH₄⁺/PO₄³⁻ it is possible to obtain pure struvite crystals, instead of precipitates of amorphous magnesium and calcium phosphates. This was easily performed by using either raw or secondary treated wastewater with different ammonium concentrations.