BACKGROUND: Simultaneous removal of sulfur, nitrogen and carbon compounds from wastewaters is a commercially important biological process. The objective was to evaluate the influence of the CH3COO-/NO3- molar ratio on the sulfide oxidation process using an inverse fluidized bed reactor (IFBR). RESULTS: Three molar ratios of CH3COO-/NO3- (0.85, 0.72 and 0.62) with a constant S2-/NO3- molar ratio of 0.13 were evaluated. At a CH3COO-/NO3- molar ratio of 0.85, the nitrate, acetate and sulfide removal efficiencies were approximately 100%. The N-2 yield (g N-2 g(-1) NO3--N consumed) was 0.81. Acetate was mineralized, resulting in a yield of 0.65 g inorganic-C g(-1) CH3COO--C Consumed. Sulfide was partially oxidized to S-0, and 71% of the S2- consumed was recovered as elemental sulfur by a settler installed in the IFBR. At a CH3COO-/NO3-molar ratio of 0.72, the efficiencies of nitrate, acetate and sulfide consumption were of 100%, with N-2 and inorganic-C yields of 0.84 and 0.69, respectively. The sulfide was recovered as sulfate instead of S-0, with a yield of 0.92 g SO42--S g(-1) S2- consumed. CONCLUSIONS: The CH3COO-/NO3- molar ratio was shown to be an important parameter that can be used to control the fate of sulfide oxidation to either S-0 or sulfate. In this study, the potential of denitrification for the simultaneous removal of organic matter, sulfide and nitrate from wastewaters was demonstrated, obtaining CO2, S-0 and N-2 as the major end products. (C) 2008 Society of Chemical Industry.