Effect of dissolved oxygen and carbon-nitrogen loads on denitrification byan aerobic consortium

Four samples of natural ecosystems and one sample from an activated sludge treatment plant were mixed together and progressively adapted to alternatin g aerobic/anoxic phases in the presence of nitrate in order to enrich the m icroflora in aerobic denitrifiers. Aerobic denitrifying performances of thi s mixed ecosystem at various dissolved oxygen concentrations and various ca rbon-nitrogen loads were evaluated and compared to those obtained with the aerobic denitrifier Microvirgula aerodenitrificans. The consortium and the pure strain exhibited an aerobic denitrifying activity at air saturation co nditions (7 mg dissolved oxygen l(-1)), i.e. there was co-respiration of th e two electron accepters with significant specific nitrate reduction rates. Dissolved oxygen concentrations had no influence on denitrifying performan ces above a defined threshold: 0.35 mg l(-1) for the consortium and 4.5 mg l(-1) for M. aerodenitrificans respectively. Under these thresholds, decrea sing the dissolved oxygen concentrations enhanced the denitrifying activity of each culture. The higher the carbon and nitrogen loads, the higher the performance of the aerobic denitrifying ecosystem. However, for M. aerodeni trificans, the nitrate reduction percentage was affected more by variations in nitrogen load than in carbon load.