Abstract

Wastewaters and agricultural sludges can contain high concentrations of phosphate and ammonium- or nitrate–nitrogen, which run off easily to waterways causing eutrophication in water bodies. However, it is possible to precipitate phosphate and ammonium as struvite and use it as recycled fertilizer. In this research, MgSO₄ solution obtained by treating fly ash (FA) or dolomite (DOL) with sulfuric acid was used as a precipitant for simultaneous phosphorus and nitrogen removal from synthetic (NH₄)₂HPO₄ solution. Precipitation experiments were performed at room temperature (20°C ± 2°C) using different molar ratios Mg:P:N (1.1–2:1–2:1–2) and precipitation times (4–24 h). The pH was adjusted to 9.0 and kept constant. In all cases, there were only minor changes in the removal of both ammonium and phosphate after 4 h of reaction time. Highest ammonium removal percentages after the 4 h test were 79.5% for MgSO₄ 1.3:1:1, DOL 1.1:1:1, and DOL 1.3:1:1. Highest ammonium removal percentages (75.5%) for FA based MgSO₄ solution were achieved using molar ratios Mg:P:N 2:1:1 and 1.1:2:2. Also, the highest phosphate removal percentages were achieved with these samples (97% for MgSO₄ 1.3:1:1, 93.3% for DOL 1.3:1:1, 89.8% for DOL 1.1:1:1, 84.5% for FA 2:1:1, and 82.5% for FA 1.1:2:2) Struvite was the only formed precipitation product in all cases as was confirmed by X-ray diffraction. Results indicate that fly ash and DOL based MgSO₄ solutions performed quite well compared to commercial MgSO₄ salt and they have great potential in the ammonium and phosphate precipitation.