Recovery of nutrients from anaerobic digester supernatant using magnesium-rich waste material

Formation of magnesium ammonium phosphate (struvite, MgNH₄PO₄.6H₂O), which has commercial value as fertiliser, is a sustainable technology for ammonia and phosphate removal and recovery from anaerobic digester supernatant. Considering that magnesium concentration relative to ammonia and phosphate concentrations in the supernatant is low, magnesium dosage is usually required to force struvite formation. This research was conducted to investigate the optimum reaction conditions and the feasibility of Mg-rich waste material as a magnesium source for ammonia and phosphate removal and recovery as struvite from anaerobic digester supernatant.<br><br>Struvite formation was carried out by adding Na₂HPO₄ and MgCl₂ or magnesium-rich waste material at different conditions: (i) pH; (ii) reaction time; (iii) molar ratios (Mg²⁺:NH4⁺:PO₄^3-). Confirmation of struvite formation and measurement of struvite precipitation’s particles size were analysed by XRD and ESEM, respectively. Economic analysis was conducted to estimate the cost of using Mg-rich waste material in the process. A chemical equilibrium software Visual MINTEQ was used for predicting struvite formation using high range of the pH and molar ratio. <br><br>Increasing the mixing rate and the reaction time had little effect on ammonia and phosphate removal. Crystal growth had only a minor relationship to the reaction time. pH change with time indicated that induction time was extremely short. The optimum pH was between 9 and 9.5. The reaction time of 10 minutes was adequate for struvite formation due to a high reaction rate. The Mg²⁺:NH₄⁺:PO₄^3- ratio of 1.2:1.0:1.0 was found to be the optimum molar ratio in order to use Mg²⁺ and PO₄^3- dosage efficiently. The chemical equilibrium software Visual MINTEQ successfully predicted struvite formation. The prediction indicated that struvite formation was at a wide pH range from 6.5 to 11.5. MgHPO₄.3H₂O, Mg₃(PO₄)₂ and Brucite (Mg(OH)₂) formed with struvite, but struvite was still the main product. The removal of ammonia and phosphate was achieved over 98.00% using Mg-rich waste material as a magnesium source at pH 9.5 and the Mg²⁺:NH4⁺:PO₄^3- ratio of 1.2:1.0:1.0. Precipitates analysis using XRD showed that nearly pure struvite formed. Economic evaluation indicated that Mg-rich waste material as the magnesium source can reduce the total cost compared with MgCl₂.<br>