This research investigated the utilization pattern of volatile fatty acids (VFAs) in the denitrification process. To be thorough, both synthetically produced and naturally generated VFAs were considered with the synthetic compounds being acetate, propionate and a mixture of these acids. The natural VFAs (acetate, propionate, butyrate and valerate) were produced from a laboratory-scale, completely mixed anaerobic digester. The wastewater used was a 1: 1 mixture of municipal sludge and a potato-processing wastewater diluted to 5000 mg L-1 of total solids (TS). Steady-state, acidogenic conditions produced net VFAs of approximately 780 mg L-1. In addition, 36 denitrification experiments were conducted in 0.6 L batch reactors (18 using natural VFAs (the N-series) and 18 using synthetic VFAs (the S-series)). Nitrates completely disappeared and the average specific denitrification rate (k(N)) for the N-series (0.0111 g NO3- -N g VSS-1 d(-1)) was close to the mixture of the S-series (0.0134 g NO3- -N g VSS-1 d(-1)). Within the S-series, the kN value for acetate was more than double that of propionate reflecting a preference for acetate. The average specific carbon consumption rate (k(C)) for the N-series (0.0252 g VFA-C g VSS-1 d(-1)) was similar to the kc for the S-series (0.0248 g VFA-C g VSS-1 d(-1)). A hierarchy (acetate followed by butyrate followed by propionate) was evident when the kc rate was calculated on the basis of individual acids within the natural mixture. This hierarchy was also supported by the carbon consumption profiles. (c) 2007 Elsevier Inc. All rights reserved.