Different kinds of volatile fatty acids (VFAs) were used to feed denitrifying processes for comparison in this study. The acetic and propionic acid-fed processes attained the highest nitrate removal rates. The nitrite accumulation mainly occurred in the short-chain VFAs (formic, acetic and propionic acid, rather than the other three) which exhibited two denitrifying stages (fast and slow) by reaction kinetic analysis. According to parallel analysis of microbial community, the cooperation of denitrifiers and long-chain VFAs (>3C) degrading microorganisms were probably responsible for the different denitrification rates from previous study. The secondary use of degraded long chained VFAs by denitrifies would explain the interesting results that similar community structures of denitrifying bacteria responded to the similar VFAs molecular structures (based on parallel DGGE fingerprint). Real time PCR showed that the abundance of nirK genes was lower in acetic acid-fed denitrifying process than other VFAs, which agreed with its highest nitrite accumulation, indicating the acetic acid appeared to be inadequate for nirIC-type denitrifiers enrichment, even though it achieved the highest nitrate reduction rate. The comparison of N2O accumulation and emission was further analyzed and discussed for the six reactors. (C) 2015 Elsevier Ltd. All rights reserved.