Enhanced short-chain fatty acids production from waste activated sludge via synergistic pretreatment with free nitrous acid and peroxydisulfate

<b>ABSTRACT</b>Organic matter in waste activated sludge (WAS) is trapped within intact cell structures and stable extracellular polymeric substances (EPS), and its slow hydrolysis severely restricts short-chain fatty acids (SCFAs) production in anaerobic fermentation. Therefore, this study evaluated the synergistic effect of combined free nitrous acid (FNA) and peroxydisulfate (PDS) pretreatments to enhance SCFAs yield. Under optimal conditions (250 mg/L nitrite and 0.07 g/g TS PDS), the SCFAs yield peaked on day 8 at 426.5 ± 25.9 mg COD/g VS, which was 18.3 times higher than that of the control (22.1 ± 1.3 mg COD/g VS). This mechanism was systematically investigated. The results demonstrated that the combined pretreatment significantly enhanced WAS disintegration. This was evidenced by a remarkable increase in soluble chemical oxygen demand (SCOD) in the supernatant, along with significant increases in the protein and polysaccharide contents of EPS. The substantial release of nitrogen and phosphorus further indicated effective cell rupture and hydrolysis. Furthermore, pretreatment upregulated key hydrolytic enzymes (protease and α-glucosidase), while suppressing methanogenesis via coenzyme F420 inhibition, thereby facilitating SCFAs accumulation. Fluorescence spectroscopy confirmed the enhanced degradation of released organic matter. Microbial analysis also showed that pretreatment altered the microbial community and enriched acidogenic bacteria such as <i>Macellibacteroides</i> and <i>Fonticella</i>. This study demonstrated that combined FNA and PDS pretreatment is a highly effective strategy for boosting SCFAs production from WAS, offering a promising avenue for sustainable sludge resource recovery.