Performance of alternative magnesium sources for phosphorus recovery by struvite precipitation¹

Desempenho de reagentes alternativos de magnésio para recuperação de fósforo por precipitação de estruvita

ABSTRACT

Struvite (NH₄MgPO₄∙6H₂O) is a slow-release fertilizer obtained through phosphorus and/or nitrogen recovery from wastewaters, usually by adding magnesium salts. In this sense, the aim was to evaluate the viability of replacing commercial magnesium sources with alternative ones in the struvite precipitation process regarding ammonium and phosphate removal and precipitated crystal quantification and characterization. Experiments were conducted on a bench scale in a completely randomized design at pH 9.5 by precipitation of synthetic wastewater solution with four magnesium sources: MgCl₂∙6H₂O, MgSO₄∙7H₂O, MgO, and magnesite (MgCO₃). After pre-acidification, derived solutions from low-cost alternative sources of magnesium removed above 55 and 90% of ammonium nitrogen and phosphate in solution, respectively, and reached around 10 g L^-1 of precipitated crystals. Results proximity obtained with solutions derived from commercial sources of magnesium (46ꟷ56% of ammonium nitrogen removal, 97% of phosphate removal, and more than 6 g L^-1 of precipitate formed) indicated that alternative reagents could easily substitute commercial ones if submitted to the previous digestion process, making magnesium available in solution. Qualitative analysis by X-ray diffraction confirmed the presence of both struvite and newberyite in most precipitates.

Key words:
ammonium; phosphate; magnesite

HIGHLIGHTS:

Phosphorus removal through struvite precipitation using alternative sources of magnesium can reach 97% efficiency.

The use of magnesium low-cost source implicates solution acidification to enable struvite chemical precipitation.

Struvite precipitation can recover more than 55% of nitrogen by using low-cost sources of magnesium.