Long-term effect of total cycle time and aerobic/anoxic phase ratio on nitrogen removal in a sequencing batch reactor

A laboratory-scale sequencing batch reactor was used to study nitrogen removal from a synthetic wastewater with an ammonium-nitrogen concentration of 50 mg/L. The effect of two key parameters (i.e., the total cycle time and the aerobic/anoxic phase duration ratio) on the performance of the reactor was investigated. Four sets of operational parameters resulting from the combination of 6 and 8 hours of cycle time and 1:1 and 1:3 aerobic/anoxic ratios were studied. Denitrification was based mainly on endogenous carbon sources as long as organic carbon, provided in the form of acetate, was quickly removed from the mixed liquor during the aerobic phase of the reactor operation. In terms of nitrogen removal efficiency, the set of 8 hours of cycle time and 1:3 aerobic/anoxic phase ratio was found to be superior in that it consistently yielded an effluent total nitrogen concentration of less than 15 mg/L (consisting almost exclusively of nitrate-nitrogen) and exhibited a percentage nitrogen removal rate of 77 +/- 2.5%. In terms of wastewater throughput (i.e., wastewater volume treated per day), however, the implementation of 6 hours of cycle time and 1:3 aerobic/anoxic phase ratio was satisfactory for nitrogen removal efficiency (72 +/- 2%), although elevated amounts of ammonium- and nitrite-nitrogen were found in the effluent. It was also demonstrated that under certain operating conditions nitrogen removal via nitrite is observed, as nitratification (i.e., oxidation of nitrite-nitrogen to nitrate) and denitratification (i.e., reduction of nitrate-nitrogen to nitrite) are bypassed.