In order to apply a spout-fluid bed reactor with a draft tube for the nitrification process of wastewater treatment, the influence of fluid-mechanical parameters on volumetric mass transfer coefficient was analyzed. Experiments were carried out in a 1000 mm high 2D semi-column with a draft tube. The draft tube had a cross section of 50 x 50 mm(2) and the annular region of the reactor had a cross section of 50 x 140 mm(2). The work described in this paper reports the influence of several fluid-mechanical parameters including liquid and gas flow rates, particle diameters and particle circulation on volumetric mass transfer coefficient, k(L)a. The liquid flow ranged from 3.5-4.5 m(3)/h and gas flow ranged from 300 to 800 L/h. The particles used were glass spheres with diameters of 3, 4, 5 and 6 mm. Tap water and air were used as the liquid and gas phase, respectively. The experimental results have shown that particle circulation through the draft tube contributes to k(L)a increase as well as the increase in the particle diameter, the gas and liquid flow rates. (C) 2015 Elsevier Ltd. All rights reserved.