Coupling effects of pH and Mg/P ratio on P recovery from anaerobic digester supernatant by struvite formation

Abstract

Abstract Phosphorus (P) in wastewater may be recovered by the formation of struvite as a potential fertilizer. The pH of the solution and the dosage of magnesium (Mg) are two important control parameters for struvite crystallization and, in general, the addition of alkaline solutions to lower the pH values required for crystal formation usually represents a large percentage of the cost. Batch experiments showed that the initial pH of the solution had a greater effect on P recovery, pH change, and crystal size than the Mg dose. The largest average crystal size was observed at a pH of 8.4. A higher Mg/P ratio had a greater effect on the elimination of P when operation at lower pH, but these conditions resulted in higher purity of the struvite crystals. Moreover, a continuous experiment was carried out to demonstrate the synergistic effects of pH and Mg/P ratio in an air-agitated reactor. A comparative efficiency of P recovery (90.2–90.7%) for ranges of pH = 8.6 and Mg/P = 1.5 and pH = 8.4 and Mg/P = 2.0 was observed after 15 hydraulic retention times of operation. This result indicates that a satisfactory P recovery efficiency could be obtained at lower pH accompanied by a higher Mg/P ratio, which would lead to a decrease in the chemical cost of P recovery.