Characterization of phosphorus species distribution in waste activated sludge after anaerobic digestion and chemical precipitation with Fe3+ and Mg2+

Given the great concern about phosphorus (P) resource depletion, it's promising to recover P from waste activated sludge (WAS) especially for agricultural application. To this end, it's necessary to understand the P speciation and fractionation within the sludge before and after stabilization and precipitation. In this study, we systematically studied P species and distribution within WAS after anaerobic digestion and chemical precipitation using liquid state P-31 nuclear magnetic resonance (NMR) spectroscopy, sequential extraction method as well as XRD Technology. P transformation and bioavailability were also discussed with the obtained results. Inorganic P was found to be dominant in WAS before (67.6%) and after digestion (77.2%). Vivianite seems to dominate among the P-containing minerals both in the raw and digested sludge from XRD spectra. In the NMR spectra, the orthophosphate (Ortho-P) and polyphosphate (Poly-P) were two dominant species in the raw sludge. However, the Poly-P peak vanished after anaerobic digestion, indicating Poly-P was released and degraded under anaerobic environment. Fe3+ addition and Mg2+ addition with pH adjustment displayed comparable P removal efficiencies. No significant differences on P distribution by sequential extraction test was observed after P precipitation. Considering that no pH adjustment was needed, it might be more cost efficient to recover P by iron addition.