Dose‐dependent membrane perturbation by didecyldimethylammonium chloride reconfigures acidogenic carbon flow during alkaline fermentation of waste activated sludge

Didecyldimethylammonium chloride (DDAC) can accumulate in waste activated sludge (WAS) during wastewater treatment, but its effects on WAS anaerobic fermentation remain unclear. This study investigated DDAC as an emerging contaminant affecting volatile fatty acid (VFAs) production from alkaline WAS anaerobic fermentation. Batch fermentation, semi‐continuous validation, extracellular polymeric substance (EPS) fractionation, enzyme activities, microbial community analysis and functional gene quantification were integrated to elucidate the underlying mechanisms. DDAC showed a clear dose‐dependent effect on WAS acidogenesis. At 0.10 mg/g volatile suspended solids (VSS), the maximum VFAs concentration reached 3221 mg chemical oxygen demand (COD)/L, 43.3% higher than the control. This dosage promoted EPS disintegration through charge neutralization and moderate membrane permeability perturbation, increasing soluble chemical oxygen demand (SCOD) to 5284.6 mg/L and enhancing soluble protein and polysaccharide release while maintaining high VFA/SCOD conversion efficiency. Protease, α‐glucosidase, acetate kinase and butyrate kinase activities increased, hydrolytic and acidogenic genes were enriched, and the methanogenic gene mcrA decreased, indicating carbon‐flow redistribution from methanogenic consumption to VFA accumulation. In contrast, 2.00 mg/g VSS DDAC caused severe membrane damage and oxidative stress, evidenced by increased lactate dehydrogenase (LDH), reactive oxygen species (ROS) and extracellular DNA, reduced live‐cell ratio, and a VFA/SCOD decrease to 34.77%. High‐dose DDAC enriched stress‐tolerant bacteria and resistance‐related genes, shifting the system from acidogenic metabolism to stress adaptation. This study provides a mechanistic basis for safe carbon recovery from DDAC‐contaminated sludge.