Improve acid production of excess sludge anaerobic digestion using potassium ferrate or ferrate anode solution

Potassium ferrate (K₂FeO₄) has been widely applied as a pretreatment agent for residual sludge digestion, but the complete synthesis of its solid form is costly. In this study, the use of ferrate anode solution (FAS) was examined as a more economical alternative. FAS is generated as an intermediate byproduct during the electrolytic production of solid K₂FeO₄. The performance of FAS was compared with that of conventional solid K₂FeO₄ to assess their respective effects on anaerobic acid production and sludge dewatering. The results indicate that sludge soluble chemical oxygen demand (SCOD) was increased to 3807 mg/L at the low dosage of 10 mL/L, and the production of short-chain fatty acids (SCFAs) reached 2142 mg COD/L. At this dosage, higher efficiencies in protein and polysaccharide release were achieved compared with high-dose K₂FeO₄ (0.5 g/g TSS). Sludge settling and dewatering properties were also improved, and reductions of 9.1% in sedimentation ratio and 9.7% in cake moisture content were achieved during pretreatment. These findings suggest that FAS, as an intermediate byproduct, can replace solid K₂FeO₄ for sludge pretreatment because of its high efficiency at low dosages and its distinct enhancement of dewatering performance. FAS may therefore serve as a more economical and effective option.