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Anaerobic technology is an efficient and sustainable approach for disposing waste activated sludge (WAS), but the performance is easily affected by environmental conditions. In this study, the potential impacts of chlorhexidine (CHD) and hexadecyltrimethylammonium bromide (HTAB), two highly detected biocides in WAS, on the short-chain fatty acids (SCFAs) generation during anaerobic fermentation were explored. The experimental results indicated that the biocides promoted SCFAs generation unexpectedly, particularly for acetic acid. The accumulation of SCFAs increased from 567.3 mg COD/L in the control to 2144.5 and 3400.0 mg COD/L with low CHD and HTAB exposure (50 mg/L), respectively. However, the enhancing effects were weakened in reactors with high loads of biocides or CHD and HTAB coexistence. Mechanisms exploration revealed that the CHD and HTAB first enhanced the extracellular polymeric substances disruption and then simultaneously accelerated both the hydrolysis and acidification processes while inhi...