Simulation and experimental validation of multiple phosphate precipitates in a saline industrial wastewater

• Visual Minteq was a reliable tool for predicting P precipitation in saline wastewater.
• The use of seawater, as Mg source, may produces the formation of different mineral species.
• Visual Minteq may be used to optimize the seawater dose in struvite precipitation.
• Precipitation of amorphous calcium magnesium phosphate was a reversible reaction.

Struvite precipitation represents an economical alternative to recover phosphorus from concentrated phosphate wastewater streams. This process is not only used for removing phosphate from wastewaters but also generates a product with commercial value as a fertiliser. Nevertheless, the economic viability of the process depends on the availability of magnesium in the wastewaters or the use of a low cost magnesium source. Seawater may be used as a cheap magnesium source in order to obtain efficient precipitation. However, seawater overdosing may cause the formation of different phosphate magnesium or calcium precipitates. The aim of this study was to determine whether thermodynamics or kinetics rule the precipitation of multiple phosphates that were observed in a precipitation reactor. A chemical equilibrium model, Visual Minteq, was used in order to verify which of the two indicated processes governed phosphate precipitation in a saline wastewater. A good correspondence between the observed and predicted multiple phosphate precipitates and phosphate concentration in the reactor was found. The nature of the phosphate precipitates depended on the characteristics of the wastewater and the physico-chemical reactor operational conditions. This study demonstrates that phosphorus precipitation may be predicted, making it possible to obtain information about the conditions in which struvite precipitation can be promoted.