Effects of external carbon source and empty bed contact time on simultaneous heterotrophic and sulfur-utilizing autotrophic denitrification

The effects of external carbon source (both type and concentration) and emp ty bed contact time (EBCT) on denitrification efficiency during simultaneou s heterotrophic and sulfur-utilizing autotrophic denitrification were evalu ated. Continuous experiments were conducted with up-flow mode sulfur packed bed reactors (SPBRs) fed with nitrified leachate containing 700-900 mg/l N O3--N. The fraction of NO3--N removed by heterotrophic denitrification (HDN Rfraction) for alkalinity production to balance the alkalinity consumption by autotrophic denitrification varied with the type of external carbon sour ce. 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 comple te denitrification and denitrification was not complete when molasses was u sed 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 gr owth 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. (C) 20 01 Elsevier Science Ltd. All rights reserved.