VFA production from excess sludge (ES) was greatly enhanced by a low-cost and high-efficient treatment: 0.67 mg/L free nitrous acid (FNA) pretreatment combined with 0.04 g/g TSS rhamnolipid (RL) addition (FNA+ RL), which significantly shortened fermentation time to 3 days and increased VFA production to 352.26 mg COD/g VSS (5.42 times higher than raw ES). Propionic and acetic acids were the two leading components (71.86% of the total VFA). Mechanism investigation manifested FNA+ RL improved the biodegradability of ES, achieved positive synergetic effect on solubilization, hydrolysis and acidification efficiencies, and inhibited methanation. Microbial community distribution further explained the above phenomena. The bacteria related to polysaccharides/protein utilization and VFA generation, including Clostridium, Megasphaera and Proteiniborus, were mainly observed in FNA+ RL, whereas gas-forming bacteria Anaerolineae and acid-consuming bacteria Proteobacteria were assuredly suppressed. Besides, Propionibacterineae associated with propionic acid generation was exclusively enriched in sole RL and FNA+ RL. (C) 2016 Published by Elsevier Ltd.