As shown in Fig. 2(a), the influent COD concentration fluctuatedgreatly during the entire operation period. Effluent COD concen-tration was stable after a start-up period of 75 days. The CODtremoval rate was steadily maintained in the range of 61–86 %from day 75 to day 285. More specifically, when HRT graduallyreduced from 45 h to 22 h, both CODtand CODsolremoval effi-ciencies increased approximately 10% on average. With furtherreduction of HRT to 18 h, CODtand CODsolremoval efficienciesdecreased by 8% and 6%, respectively