Biological treatment of a piggery wastewater for organic carbon and nitrogen removal in a combined anaerobic–aerobic system was investigated using two laboratory-scale sequencing batch reactors. The cycle length was 24 h. In the anaerobic reactor, fed with raw wastewater and recycling from the aerobic reactor, denitrification followed by anaerobic digestion of organic carbon was observed. In the aerobic reactor, more organic carbon removal and ammonia oxidation to mainly nitrite occurred. Denitrification was also observed in the aerobic reactor during the filling period, when mixed liquor dissolved oxygen concentration was very low. Three recycle-to-influent ratios from 1 to 3 were tested. Average performances of the overall process, in the different conditions tested, were a TOC removal of 81 to 91% and 85 to 91% of TKN. 10 to 28% of the initial TKN was discharged as N-NOx because of the low recycle-to-influent ratios used. The higher the recycle-to-influent ratio, the lower the concentrations of nitrogen oxides in the final effluent. However, the effect of this ratio was attenuated by the phenomenon of denitrification in the aerobic reactor which increased the performances of the process.