Boosting nitrogen/phosphorus removal and VFAs production in sludge fermentation system with MgSO4 addition

MgSO 4 resolved the acid-production/phosphorus-release conflict in alkaline WAS fermentation. • Boosts VFAs to 347 mgCOD/gVS and acetate to 57.2% while controlling PO 4 3 ˉ-P at 12.4 mg/L. • A ternary synergy (Mg 2+ bridging, enzymatic activation, SRBs metabolism) drives acetate production. • This low-cost strategy enables carbon self-sufficiency & simultaneous P recovery in WWTPs. Alkaline fermentation of waste activated sludge (WAS) was a promising technology for producing volatile fatty acids (VFAs) to supplement carbon sources for denitrification and phosphorus removal in wastewater treatment. However, the accompanying intense phosphate release limited its engineering applicability. To resolve the inherent conflict between acid production and phosphorus control, this study established a novel MgSO 4 -enhanced alkaline fermentation strategy. Batch experiments (pH 9.0, 35 °C) systematically revealed its chemical-biological synergistic mechanism. The results indicated that MgSO 4 addition significantly overcame the hydrolysis rate limitation in sludge, enabling peak VFAs production to reach 347 mgCOD/gVS at 18 days. This represented increases of 37.2% and 18.4% compared to the Na 2 SO 4 and FeSO 4 groups, respectively. Moreover, driven by SRB-mediated metabolic diversion, this system achieved directed conversion of metabolites toward acetate (198.4 mgCOD/gVS, accounting for 57.2%). More critically, unlike the high phosphorus release risk associated with traditional alkaline fermentation, this system utilized Mg 2+ -induced in situ struvite crystallization to stably control soluble phosphates in the fermentation broth at lowest 12.4 mg/L. Meanwhile, the concentration of ammonia nitrogen was significantly lower than that in the control group. Mechanistic analysis further confirmed that Mg 2+ promoted deep desorption of EPS at the physicochemical level through cation bridging; at the enzymatic level, it significantly upregulated the catalytic activity of pyruvate kinase and acetate kinase as a cofactor; while at the ecological level, it drove the targeted succession of microbial communities toward functional bacterial genera such as Thiopseudomonas (17.23%), establishing an efficient coupled network of sulfate reduction to acid production and effectively suppressing methanogenesis. This study demonstrated the dual advantages of the MgSO 4 strategy in achieving high-quality carbon source self-sufficiency and in situ phosphorus resource recovery, providing a viable technical paradigm for low-carbon operation in wastewater treatment plants. Download: Download high-res image (143KB) Download: Download full-size image