Pilot-scale demonstration of nitrogen recovery in the form of ammonium phosphate (AP) from anaerobic digestate

Abstract Nitrogen in anaerobic digestate was recovered as ammonium phosphates (APs), a valuable fertilizer, in a pilot-scale system consisting of ammonia stripper, absorber and crystallizer. The dissolved ammonium concentration in the anaerobic digestate was stripped and then absorbed into the phosphoric acid (42.5 wt%) with 95 % absorbing efficiency. As the NH3 stripping continued, both the N/P ratio and pH, key operating parameters in the absorber are optimized N/P = 0.6 and pH = 1.7. The residual AP solution after crystallization was reused to enhance the crystallization efficiency up to 88 %. The overall recovery efficiency of APs was estimated at 72 % of the input nitrogen mass. Analyses of SEM and XRD revealed that the recovered AP crystals were mainly composed of mono-ammonium phosphate (MAP). This pilot-scale study demonstrated that nitrogen load in anaerobic digestates could be effectively recovered as a valuable fertilizer source of AP crystals.