In a biological aerated filter (BAF) packed with ceramic media (void fraction of BAF=0.32), nitrite accu-mulation was studied with the variation of hydraulic retention time (HRT) and superficial air velocity. Synthetic ammo-nium wastewater and petrochemical wastewater were fed at a constant load of 1.6 kgNH4+-N/m3·d. Ammonium removalrate was mainly affected by the superficial air velocity in BAF, but nitrite ratio(NO2-N/NOx-N) in the effluent wasdependent on both HRT and superficial air velocity. For a fixed HRT of 0.23 hr (corresponding to the empty bed contacttime of 0.7 hr) ammonium removal rate was 73/90/92％ and nitrite ratio was 0.92/0.82/0.48 at the superficial air velocityof 0.23/0.45/0.56 cm/s, respectively. When HRT is increased to 0.9 hr with superficial air velocity ranging from 0.34 to0.45 cm/s, the ammonium removal rate was 89％ on average. However nitrite ratio decreased significantly down to0.13. When HRT was further increased to 1.4 hr, ammonium removal rate decreased, thereby resulting in the free ammo-nia (NH3-N, FA) build-up and nitrite ratio gradually increased (>0.95). Although aeration rate and FA concentration atHRT of 0.23 hr were unfavorable for nitrite accumulation compared with those at HRT of 0.9 hr, nitrite ratio at HRT of 0.23hr was higher. Taken together, HRT and nitrogen load were found to be critical, in addition to FA concentration and aer-ation condition, for nitrite accumulation in the BAF tested in the present study.