Enhanced phosphorus release from waste activated sludge using ascorbic acid reduction and acid dissolution

Due to the widespread application of various iron (Fe)-derived substances used in phosphorus (P) removal during wastewater treatment, Fe-P species generated in this process constitute an important part of P speciation in non-digested sludge. SEM-EDS and sequential extraction methods were utilized to analyze the speciation, distribution, and spatial variation of P contained in the sludge. Inorganic P accounted for 91.3% of the total P, and Fe(III)-P represented the greatest percentage (68.5%) in the inorganic P fraction. Ascorbic acid, also known as vitamin C (VC), performed well in releasing P from sludge, especially in combination with subsequent pH adjustment to 3.0 using HCl. Fe(III)-P in sludge was first reduced to Fe(II)-P by VC, then dissolved in acidic conditions to release Fe²⁺ and PO₄^3-. Other metal-P compounds were also partially dissolved and released. VC disrupted the sludge floc structure, releasing organic P via organic efflux. There was a positive correlation (R²>0.97, p<0.05) between the amount of released P and the amount of reductant (VC). There was a synergistic effect between 120 mmol/L VC and acidity, producing the greatest P release of 67.1% of total sludge P. The P release efficiency achieved in this study was higher than other reported methods. Additionally, VC provides a more sustainable option due to its natural biodegradability. Released P and Fe²⁺ can be recovered as vivianite with recovery rates of 88% and 99%, respectively. This finding provides a new direction for effective, sustainable sludge P recovery and utilization.