Enhanced nitrification at short hydraulic retention time using a 3-stage biological aerated filter system incorporating an organic polishing reactor

•An organic polishing reactor was assessed in 3-stage BAF for enhancing nitrification.•The organic polishing reactor contributed to effective NH4-N removal at the short HRT of 1h.•Also, it prevented nitrite accumulation.•It ultimately resulted in the enhanced T-N removal of 3-stage BAF system. In this work, a 3-stage biological aerated filter (BAF) system was proposed for the enhancement of nitrification at short hydraulic retention time (HRT). This was done by incorporating an intermittent aeration reactor, as an organic polishing reactor, into a conventional 2-stage pre-denitrification/nitrification BAF system. Laboratory experiments were performed at three different HRTs (i.e., 3, 2 and 1h). Experimental results clearly showed that NH4-N removal of a conventional BAF system severely deteriorated in the presence of even a very small amount of organic matter at the short HRT of 1h. Moreover, nitrite accumulation was observed at this HRT. This means that organic matter could be a limiting factor in nitrification at short HRTs. These problems were effectively controlled in the 3-stage BAF system. When the HRT of a conventional BAF system was reduced from 2 to 1h, NH4-N removal decreased in from 96.6% to 74.0%. In contrast, for the experimental 3-stage BAF system, NH4-N removal remained high over 94% at the HRT of 1h. This was because the polishing reactor eliminated organic matter before it could enter the nitrification reactor. Finally, this organic polishing reactor contributed to the higher removal of T-N and T-P.