The effects of external carbon source (both type and concentration) and empty bed contact time (EBCT) on denitrification efficiency during simultaneous heterotrophic and sulfur-utilizing autotrophic denitrification were evaluated. Continuous experiments were conducted with up-flow mode sulfur packed bed reactors (SPBRs) fed with nitrified leachate containing 700-900 mg/l NO3--N. The fraction of NO3--N removed by heterotrophic denitrification (HDNRfraction) for alkalinity production to balance the alkalinity consumption by autotrophic denitrification varied with the type of external carbon source. When methanol or sodium acetate was added at HDNRfraction values of 60 and 44%, respectively, 100% denitrification was achieved without alkalinity addition. However, glucose required a HDNRfraction value of 70% for complete denitrification and denitrification was not complete when molasses was used at a HDNRfraction value of 70%. The EBCT and volumetric loading rate at which 100% denitrification efficiency could be achieved were 6.76 h and 2.84 kg NO3--N/m(3) day, respectively. The maximum nitrogen removal rate was 5.05 kg NO3--N/m(3) day observed with 89% removal efficiency. At short HRT, a clogging problem was observed near the bottom of the SPBR with excess growth of heterotrophic denitrifiers and gas accumulation within the pores of the SPBR. This problem may be eliminated by back-washing or by separating heterotrophic denitrification from sulfur-utilizing denitrification.