The aim of this work is to investigate a co-current air-liquid downward flow bubble column with air entrainment by liquid injection nozzle in order to use it as ail aerator in activated sludge treatment plants. The study concerns the determination of mass transfer efficiency by measuring the mass transfer coefficient, k(L)a, both in clean water and in activated sludge. In clean water, this parameter is determined by three methods, i.e., gassing out method, absorption with chemical reaction and off-gas method. In activated sludge medium, k(L)a values are measured by two methods, i.e., sludge reoxygenation and the hydrogen-peroxide method. The values of k(L)a obtained in clean water are compared to (those obtained in sludge, enabling the assessment of the a factor, i.e., ratio of oxygen transfer coefficient sludge/clean water. The results are in good agreement with those reported previously in the literature.