Enhanced phosphorus release from waste activated sludge triggered by ferrate/sulfite oxidation process: The overlooked role of high valent iron

Waste activated sludge is considered a promising resource for phosphorus recovery. Therefore, it's of great significance to release phosphorus from sludge. Potassium ferrate (PF) is deemed to be a green oxidant, which has great potential in the field of sludge treatment. In this study, sulfite (Na2SO3) coupled with the PF oxidation process was proposed to improve phosphorus release from sludge. The analysis of extracellular polymeric substances (EPS) of sludge indicated that sludge flocs were destroyed and nutrients were released, the total phosphorus and organic phosphorus increased significantly to 33.26 mg/L and 20.16 mg/L (40 min), respectively. Proteins, polysaccharides, and total organic carbon were transformed from tightly bound EPS to soluble EPS. In addition, the dissolved organic matter in tightly bound EPS decreased sharply, while the tyrosine-like protein and soluble microbial by-product substances in loosely bound EPS and soluble EPS increased significantly. Ethanol quenching experiments suggested that free radicals have little effects on phosphorus release, while the existence of high valent iron was validated by a chemical probe method applying methyl phenyl sulfoxide (PMSO) as the probe molecule. Thus, it was speculated that high valent iron may be the active substance for phosphorus release. The coupled PF/Na2SO3 process produced multiple active species (reactive oxygen species and high valent iron) to destruct sludge, among which high-valent iron species were mainly responsible for phosphorus release. This study provides new insights into the improvement of phosphorus release from sludge by the PF/Na2SO3 coupled oxidation process.