Sustainable reclamation of phosphorus from incinerated sewage sludge ash as value-added struvite by chemical extraction, purification and crystallization

A novel procedure for phosphorus (P) recovery as struvite from incinerated sewage sludge ash (ISSA) was proposed and investigated. The procedure is divided into three steps: transform P in the ISSA to amorphous iron phosphate (Fe–P) and aluminum phosphate (Al–P) by acid washing followed by alkali precipitation, re-dissolve Fe–P and Al–P via acid washing and adsorption of Fe and Al ions by a cation exchange resin (CER), and struvite crystallization. Results showed that about 80.4% P in the ISSA was successfully transformed to Fe–P and Al–P accompanied by achieving the removal of most undesirable metals. Subsequently, Fe and Al ions in the leachate of completely re-dissolution of Fe–P and Al–P in 2 N HCl were absorbed by CER to obtain a purified P solution. Over 99.0% of P in the purified P solution was recovered as struvite with a high purity (76.0%) and low heavy metal contents (Zn, Cu, As and Pb＜50 mg/kg) at a 1:1:1 M ratio of Mg: N: P and pH of 9.5. Overall, 79.7% P in the ISSA could be recovered as struvite with negligible environmental risk.