Removal of ammonium and phosphate from the supernatant of anaerobically digested waste activated sludge by chemical precipitation

The removal of ammonium (NH4+) and phosphate (PO43-) from supernatant of anaerobic digestion by magnesium ammonium phosphate (MAP) formation was studied. To be able to find out the loss of NH4+ by its transformation to NH3, rates of NH4+ removal by aeration and by MAP formation combined with aeration were compared. NH4+ removal by aeration was found to be insignificant. The kinetics of MAP formation was fast. The removal rate of NH4+ and Mg2+ was the smallest and the largest, respectively based on rate constants calculated. The removal of NH4+ showed sensitivity to Mg2+ and PO43- concentrations and controlled the purity of MAP precipitate. The removal of PO43- was found to be sensitive to mainly Mg2+ ion. These results also indicated that as the supernatant solution became more saturated in terms of MAP forming ions, the effect of pH increase became insignificant in the removal of ions from the solution.