The present paper reports an original, predominantly experimental study of the mass transfer efficiency in biofilters. Hydrodynamic and mass transfer parameters were investigated at two different scales. These parameters have been first considered (i) at a global scale in a three-phase fixed bed reactor with investigating the influence of the physic chemical properties of liquid phase and then (ii) from a local point of view by focusing at the bubble/packing contact. Experiments at global scale have been conducted in a semi-industrial scaled reactor (4.5 m height, 0.15 m in diameter) operating in batch mode, in co-current gasliquid upflow. Air was injected at the bottom of the reactor through porous disc diffusers. Two kinds of packing, Meteor and Biolite, which have been thoroughly studied by Garcia-Maldonado et al. (2008), have been tested and compared. The impact of the liquid phase was investigated for different solutions containing clear water and some additives (salts, sugar, suspended solids and varied pH) that can be encountered in industry. For each liquid phase tested, gas holdup, pressure drop, slip velocities and bubble sizes were estimated, as well as volumetric mass transfer coefficients under different superficial gas velocities (ranging from 2.3 x 10(-3) m s(-1) to 2.9 x 10(-2) m s(-1)). For all the tested cases, variations in the hydrodynamic behavior were observed with increasing superficial gas velocity and with all the compounds added to the liquid phase. Mass transfer coefficients decreased with all the tested compounds except for low concentrations of salts, acid and basic solution. Local-scale experiments were performed in a 2D cell made of PMMA, with a height of 200 mm, width of 100 mm and thickness of 2 mm, to investigate the visualization of mass transfer and hydrodynamics in the axial profile of bubbles rising through a fixed bed. A high speed camera was used with an oxygen sensitive dye to visualize oxygen transfer and Kalliroscope particles to visualize bubble hydrodynamics. A specific approach was proposed for estimating the mass transfer coefficient in such a configuration. It was found that the mass transfer coefficient k(L) depended on bubble behavior through the packing. Low porosity of packing, bubble size and velocity were the principal parameters influencing the hydrodynamics and mass transfer coefficients at this scale. This study considered a new approach to obtain precise data on biofilter systems, investigate the hydrodynamics and gasliquid mass transfer at two scales, enriching the database on biofilters and providing new insights that could improve this system in industry. (C) 2013 Elsevier Ltd. All rights reserved.