Targeted clean extraction of phosphorus from waste activated sludge: From a new perspective of phosphorus occurrence states to an innovative approach through acidic cation exchange resin

Waste activated sludge (WAS) is an important source of non-renewable phosphorus (P) recovery. Given the factor that the occurrence states of phosphorus in WAS determines its recovery efficiency, the spatial distribution and chemical speciation of phosphorus were comprehensively and simultaneously analyzed by in-situ and step-bystep extraction methods for the first time. It was confirmed that the phosphorus in solid phase of WAS could be mainly divided into three parts: polyphosphate in cells, extracellular polymeric substances (EPS)-bound P, and phosphate precipitated with metals (P-precipitates) in extracellular inorganic minerals. Among these forms, EPSbound P (mainly orthophosphate, Ortho-P) and P-precipitates (mainly Ca-P, Fe-P, Al-P, and Mg-P) were the major forms of phosphorus in WAS, accounting for 65%-82% of total phosphorus (TP). Owing to the acid solubility of P-precipitates, acid extraction could be a potentially effective means for phosphorus recovery. However, the co-solution of metals may hinder the phosphorus recovery and the EPS-bound P cannot be recovered by acid extraction. To enhance phosphorus release from EPS and reduce metal interference, a targeted clean extraction technology using acidic cation exchange resin (ACER) was also developed. The results showed that a low dosage ACER could effectively extract EPS-bound P and P-precipitates, and the content of phosphorus in the extract exceeded 50% of TP. Compared with acid extraction, the release efficiency of TP increased by 13%-23%, and the dissolved metal content decreased by more than 90% in the extract by ACER. This was attributed to the acidification and metal capture by ACER. Finally, more than 90% of Ortho-P in the extract was recovered as calcium phosphate, which alleviated the depletion of phosphorus resources.