Pii: S0043-1354(99)00170-0

ÐBiological 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, denitri®cation followed by anaerobic digestion of organic carbon was observed. In the aerobic reactor, more organic carbon removal and ammonia oxidation to mainly nitrite occurred. Denitri®cation was also observed in the aerobic reactor during the ®lling period, when mixed liquor dissolved oxygen concentration was very low. Three recycle-to-in ̄uent ratios from 1 to 3 were tested. Average performances of the overall process, in the di€erent 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 NNOx because of the low recycle-to-in ̄uent ratios used. The higher the recycle-to-in ̄uent ratio, the lower the concentrations of nitrogen oxides in the ®nal e‚uent. However, the e€ect of this ratio was attenuated by the phenomenon of denitri®cation in the aerobic reactor which increased the performances of the process. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐanaerobic digestion, denitri®cation, nitri®cation, piggery wastewater, sequencing batch