In order to enhance the nitrogen removal in the activated sludge process, a rectangular airlift bubble column where aerobic and anaerobic regions coexisted was used, and immobilizing carrier particles were added in the column. The effects of equipment and operation conditions on the liquid circulation flow rate and liquid-phase volumetric mass transfer coefficient were investigated. On the basis of oxygen balance, the distribution of dissolved oxygen concentration in the column was calculated by using these now characteristics. From the dissolved oxygen distribution the volume fraction of anaerobic region was estimated. The results indicated that the volume fraction of the anaerobic region and the concentration of the immobilizing particles strongly affected the nitrogen removal. The nitrogen removal had a maximum value when the volume fraction of the anaerobic region was about 50%.