Precipitation of struvite using MgSO₄ solution prepared from sidestream dolomite or fly ash

Struvite (NH₄MgPO₄∗6H₂O) is a slow-release fertilizer produced from phosphorus and nitrogen-containing wastewater in the presence of Mg salts. Commercial Mg salts are the single most significant cost of struvite precipitation. In this study, H₂SO₄ formed as an industrial sidestream was used to prepare MgSO₄ solution from waste dolomite (DOL) and fly ash (FA). MgSO₄ solution was then used to precipitate struvite from a synthetic (NH₄)₂HPO₄ solution and from actual industrial process waters. The best results were obtained with real process waters where over 99% of phosphate and about 80% ammonium removals were achieved with both MgSO4 solutions after 30 min of reaction time. A higher molar ratio between Mg and P improved the phosphate removal efficiency, especially with DOL-based MgSO4 solutions; however, it had no practical effect on ammonium removal. The struvite content of precipitates was 75.49% with an FA-based chemical and 60.93% with a DOL-based chemical; other valuable nutrients (Ca, K, S, Fe, Mn, and Cl) were captured in the precipitates. The results indicate that both sidestream-based reagents perform well in struvite precipitation and that the formed precipitates could be used as fertilizers.