BACKGROUNDBiotrickling filters (BTFs) are among the most widely used biological technologies for air pollution control. The pollutant removal rate in BTFs relies to a large extent on its gas-liquid mass transfer performance. Therefore, knowledge of the design parameters affecting the mass transfer and mixing performance of full-scale BTFs is of paramount importance. RESULTSThis work showed that the height-to-diameter (H/D) ratio is an important parameter in the design of BTFs devoted to air pollution control. The H/D ratio significantly affected the mass transfer and mixing performance of the BTF with and without additional stirring in the holding tank. It was observed that under the same liquid velocity, the mass transfer coefficient (k(L)a) and the mixing time (t(mix)) can be optimized only by selecting an adequate H/D ratio. Additional stirring in the holding tank also affected k(L)a and t(mix), this effect being different for each H/D ratio tested. CONCLUSIONSThe H/D ratio should be considered in the design of full-scale BTFs since this parameter will significantly affect the mass transfer and mixing performance, regardless of the agitation conditions prevailing in the holding tank. As far as the authors know this is first report addressing the combined effect of the H/D ratio, liquid velocity and holding tank stirring on k(L)a and t(mix) in BTFs devoted to air pollution control. (c) 2017 Society of Chemical Industry