Phosphorus solubilization from sewage sludge via pH-controlled microwave treatment for vivianite recovery

Sewage sludge is a promising secondary source of phosphorus (P), yet effective P solubilization is required for its recovery as well-defined products. This study combined pH adjustment and microwave heating to solubilize P from waste activated sludge for vivianite recovery. The effect of pH and microwave heating on the transformation of P speciation was examined using sequential P extraction and ³¹P nuclear magnetic resonance spectroscopy. The dominant P in the raw sludge were Fe-bound P (Fe-P, 70 % of TP) and non-reactive P (nrP, 19 % of TP) which mainly included organic phosphate and polyphosphate. Acidic and neutral treatment (pH 2-8) caused negligible P solubilization, while alkaline treatment (pH 12) led to significant P release primarily from Fe-P and nrP. Combination with 5 min microwave heating increased the P release efficiency from 0.1-0.9 % to 2.6-8.2 % at pH 2-8 due to enhanced hydrolysis of nrP, but no enhancement at pH 12. Microwave heating prolonged to 15 min increased P solubilization from 43 % to 52 % at pH 12, mainly due to hydrolysis of nrP (e.g., pyrophosphate) in the solid phase; it also improved the sludge dewaterability thus favoring generation of P-rich supernatant for vivianite crystallization. Following hybrid treatment at pH 12 with 15 min microwave heating, 90 % of orthophosphate in the supernatant was recovered as vivianite with positive economic analysis. This study provides a cost-effective strategy for P recovery from sludge, especially those from wastewater treatment plants employing Fe dosing and activated sludge process without anaerobic digestion. Further studies are warranted for optimization of P solubilization and vivianite crystallization, e.g., complete hydrolysis of nrP and higher vivianite purity.