Alternative route for the recovery of nitrogen as ammonium phosphate crystals from high strength waste streams

This study suggests a method for the recovery of nitrogen in waste or wastewater as ammonium phosphate crystals. Recovered crystals were a mixture of mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP), which are valuable fertilizers. A lab-scale system was constructed with an ammonia stripper and absorber with phosphoric acid solution. Formation of ammonium phosphate crystals was dependent on the concentration and volume of phosphoric acid in the absorber. The absorption efficiency of ammonia in the absorber increased as the concentration and volume of phosphoric acid increased. However, the crystals formed better in a smaller volume of absorbing solution. When the N/P ratio in the ammonia absorbed solution reached 0.38 in a continuous stripping condition, crystals could not be formed in the absorber, but could be formed in a crystallizer maintained at 10 °C, the optimal temperature for crystallization. The N/P ratio of the recovered crystals was 1.47, indicating that the crystals were a mixture of MAP and DAP. Analyses of SEM and XRD revealed that the recovered crystals were composed of MAP and DAP.