Comparative evaluation of phosphorus recovery from sewage sludge thermal products via magnesium ammonium phosphate and hydroxyapatite methods

Shortage of phosphorus resource has become a global concern. Due to the high phosphorus content in sewage sludge, phosphorus recovery can be realized from thermal products of sewage sludge. Phosphorus recovery performance of smoldering ash (SA), incineration ash (IA) and pyrolysis char (PC) was investigated. The precipitate rate of phosphorus in acid and alkali leaching solutions is over 94 % by magnesium ammonium phosphate (MAP) and hydroxyapatite (HAP) methods. For MAP method, the recovered P contents in the precipitations of SA, IA and PC are 23.25 f 0.35, 31.71 f 0.79 and 23.76 f 0.24 mg/g, respectively. For HAP recovery, the phosphorus contents per unit mass of precipitated products are lower than that by MAP, ranging from 13.67 f 0.10 to 22.89 f 0.34 mg/g. The purity of the recovered products was evaluated based on the contents of major elements and heavy metals in recovered products. Most of major elements and heavy metals can coprecipitate with phosphorus in the recovery products by acid leaching-MAP method. Due to the low impurity content in the alkali leaching solution and insolubility of most heavy metals in it, the products recovered by alkali leaching-HAP shows higher purity than that by acid leaching-MAP method. The phosphorus recovery performance, reagent consumption and purity of recovered products of the two methods were compared. Acid leaching-MAP recovery is optimal for IA due to its highest P recovery and purity, with lower reagent consumption compared to alkali leaching-HAP. For SA and PC, alkali leaching-HAP recovery is preferable due to its higher P recovery purity and market price of hydroxyapatite products.