Struvite precipitation potential for nutrient recovery from anaerobically treated wastes

Geochemical equilibrium speciation modeling was used to determine optimum conditions for precipitation of magnesium ammonium phosphate, or struvite, for the recovery of nutrients from anaerobically digested wastes. Despite a wide range of pH values with the potential to precipitate struvite, the optimum pH was determined to be 9.0. Bench experiments conducted on effluent from an anaerobic sequencing batch reactor (ASBR) treating swine wastes achieved a maximum of 88% ammonia removal at a pH of 9.5 with added magnesium and phosphate to achieve an ammonium: magnesium: phosphate molar ratio of 1:1.25:1. Struvite precipitation was performed on a continuous basis in a pilot-scale ASBR treating swine wastes. Through the addition of supplemental magnesium and phosphate, the ammonia concentration was reduced from 1500 mg/L as nitrogen to less than 10 mg/L. The supenatant from the struvite precipitation clarifier was recycled to the feed of the ASBR without adverse impact, simulating on-farm effluent reuse as flush water.