Simultaneous Recovery of Ammonium Nitrogen and Phosphate from Anaerobic Digestate Liquor in Struvite Crystallisation with External Addition of Magnesium Hydroxide and Phosphoric Acid

To recover soluble nitrogen and phosphorus from the anaerobic digestate liquor in crystallisation, sets of batch experiments were conducted with varied dosing of liquid phosphoric acid and powdered Mg(OH)2. Both nutrients were recovered from the liquor by 90% within about 20 minutes when the initial molar ratios of Mg/N and P/N were set at 1.6 and 0.95, respectively. A high initial Mg/N ratio accelerated the reaction, whereas a low initial P/N ratio resulted in a lowered nitrogen recovery. The crystals obtained at the end of the experiment were composed of struvite (91 w/w%), Mg-PO4 species (1.0 w/w%), and remaining Mg(OH)2 (8.0 w/w%), where about 15 mmol L−1 of magnesium was present in a soluble form. Struvite was the species to capture the nitrogen and phosphate, whilst the formation of Mg-PO4 crystal minimised the soluble phosphate. To reuse the remaining Mg(OH)2, a sequential batch operation was examined without withdrawing the solids, where the Mg(OH)2 dosing was intentionally reduced in the consecutive runs. The experiment indicated that a total of 3–5 cycles of slurry reuse seemed to be of interest because of saving magnesium addition by 0.3 mol-Mg mol-N−1 in an acceptably short reaction time with about 40 min per cycle.