Two laboratory-scale sequencing batch reactors were used to study biological treatment of carbon and nitrogen in a new combined anaerobic-aerobic system. Piggery wastewater was used as a model effluent. The anaerobic reactor, fed with raw wastewater and nitrite/nitrate recycling from the aerobic reactor, carried out denitrification and anaerobic digestion of organic carbon. In the aerobic reactor, organic carbon removal and nitrification of ammonia to mainly nitrite occurred. Three recycle-to-influent ratios from1 to 3 were tested. The higher the recycle-to-influent ratio, the lower the concentrations of nitrogen oxides in the final effluent. Carbon removal efficiency was quite stable, whatever the ratio. However, the effect of this ratio was attenuated because of denitrification in the aerobic reactor, which increased the performances of the process. The use of sequencing batch reactors was essential to apply the configuration proposed, in order to combine denitrification and methanogenesis that require antagonistic conditions.

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