Long-term potassium ferrate exposure on primary sludge acidogenic fermentation: the interplay of Fe and alkaline conditions

Potassium ferrate (PF), a green and environmentally friendly water treatment disinfectant, has been widely used in sludge dewatering. To expand its application in sludge treatment, this study conducted an in-depth investigation into the effectiveness of PF in long-term maintenance of short chain fatty acids (SCFAs) production from primary sludge (PS) and simultaneous control of nitrogen and phosphorus to address persistent challenges of low carbon recovery efficiency. Results demonstrated that PF exerted dual chemical-biological enhancements: PF could disintegrate sludge flocs, reducing protein and polysaccharide contents in tightly bound extracellular polymeric substances (TB-EPS) by 25.00% and 34.00% and elevating bioavailable organic matter by 1.4–2.6 times. Analysis of microbial metabolic pathways revealed that PF effectively enriched alkali-tolerant hydrolytic and denitrifying bacteria, stimulated glycolytic and pyruvate metabolic pathways, and inhibited the carbon chain elongation reaction. This synergy boosted SCFAs accumulation by 91.00%, increased the C/N ratio of fermentation liquid by 88.00–100.00% and achieved 35.95–96.33% phosphorus removal via iron-phosphate co-precipitation. Concurrently, alkaline conditions and Fe³⁺/Fe²⁺ cycling inhibited methanogens while redirecting carbon flux toward SCFAs. Mechanistically, PF established a self-sufficient system, in which the transformation of iron forms drove the reorganization of microbial communities, thereby enabling the occurrence of Fe-mediated acidogenesis and denitrification processes.