The motion of air and water is characterised in a downflow co-current column reactor. The velocity of the bubble swarm relative to that of the bulk water is determined using a new approach for the computation of the slip velocity in a CFD simulation using a modified drift flux model and incorporating the data obtained from the CFD modeling. The validity of the numerical model is confirmed by comparing the simulation results with those of experimental. The average slip velocity is found to be approximately 3.3 cm/s in the down-corner co-current column, which is lower than those prevailing in countercurrent columns. It is concluded that in a bubbly flow regime lower slip velocity improves the bubble particle collection efficiency. (C) 2016 Elsevier B.V. All rights reserved.