Acid treatment enhances phosphorus release and recovery from waste activated sludge: Performances and related mechanisms

Highlights: • Optimal acid pH of 3.0 was proposed for satisfied phosphate recovery. • Total dissolved phosphate was 37.3-flod higher than that at natural pH. • Phosphate was primarily obtained from degradation of short chain polyP. • WAS dewatering performances were improved after acid treatment. • Released P was recovered in form of FePO₄ crystal via Fe³⁺ addition. Waste activated sludge (WAS) has attracted considerable attention as an excellent material for P recovery from sewage. This study took concentrated phosphorus removal sludge as objective, and aimed at providing an effective route to promote the transformation of polyphosphate in sludge pellets to dissolved phosphate. After acid pH adjustment, total dissolved phosphate at pH 3.0 was 37.3-fold higher than that at natural pH. The P distribution results illustrated that acid pH accelerated the degradation of short chain polyphosphate into orthophosphate. Furthermore, cell staining results confirmed that the polyphosphate was sourced from the leakage of intracellular matters. Low field NMR and rheology properties analysis were adopted to illustrate that acid pH treatment further improved WAS dewatering performances. Accompanied with the reduction of heavy metals in WAS pellets, the acid pH treatment was also beneficial for WAS subsequent treatment or final disposal. Lastly, Fe³⁺ addition was proposed as favorable P recovery method, and spherical nanometric materials of FePO₄ crystal was obtained accordingly.