A lab-scale SBR was used for the study of nitrogen removal from a synthetic wastewater with an ammonium-nitrogen concentration of 50 mg/L. The react phase of the reactor operation was divided into three sets of consecutive aerobic and anoxic periods with a duration ratio of 1:3 (20 min aerobic and 1 h anoxic phase). Under these operating conditions, nitrogen removal was achieved via nitrite i.e. no nitratification (oxidation of nitrite to nitrate) and hence no denitratification (reduction of nitrate to nitrite) was taking place in the aerobic and anoxic phase, respectively. This was attributed to the suppression of the nitrite-oxidizers activity due to the short aerobic phase duration. This presumption was supported by the ever decreasing amount of nitrate-nitrogen generated in the react phase during the transient, even when the activated sludge of the reactor was supplemented with additional nitrite-oxidizers. On the other hand, denitrification was mainly based on stored carbon sources, as long as the organic carbon (provided in the form of acetate) was never accumulated during the anoxic/anaerobic fill phase of the reactor operation.

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