Benzene, toluene and xylene (BTX) as hazardous volatile organic materials from various emission sources such as oil and gas refineries, petrochemical industries, shoe-making manufactures, printing and paint manufacturing industries, are considered as great threat to the public health and the environment. Biotreatment with the advantages of high efficiency, low-cost, and non-secondary pollution is suitable to purify waste gas in low concentrations [1–4].
In many biochemical processes the oxygen supply to the broths is not enough to meet the demand of the microorganisms. Oxygen transfer is often the limiting factor in the aerobic bioprocess due to the low solubility of oxygen in the medium; so aeration is a critical factor in industrial aerobic fermentations [5–9].
In stirred tank bioreactors the oxygen mass transfer is a function of many variables, such as the physical properties of the liquid (viscosity, surface tension, etc.), the geometry of the vessel and stirrer, the type of sparger and the operational conditions. Unfortunately, the available information in the literature about the effect of these variables on the mass transfer is sometimes confusing [8, 10].
Stirred tank bioreactors provide high values of mass and heat transfer rates and excellent mixing. In these systems, a high number of variables affect the mass transfer and mixing, but the most important among them are stirrer speed, type and number of stirrers and gas flow rate used [6, 11]. The most important role is played by the impeller, which accomplishes three major tasks, solids suspension, mixing and dissolution of the required atmospheric oxygen into the aqueous phase, and maximizing the interfacial area between the gaseous and aqueous phases [8, 12]. The most studied impellers have been the standard Rushton turbines, different pitched blade turbines and various propellers as well as combinations of two or three of them to optimize the power consumption [13–15].
Fujasova  studied the mass transfer rate of seven types of impellers in 29 triple configurations. They found that Rushton turbine impeller in triple configuration and combination of Rushton turbine with Pitched blade are the most efficient impeller combinations for the mass transfer performance in the triple-impeller vessel. Tomoa Moucha et al.  reported that the conclusions about the influence of impeller configuration on the mass transfer efficiency are ambiguous. They reported that this is partially caused by the improper methods used for volumetric oxygen transfer coefficient (KLa) data evaluation and usually all phenomena were not taken into account, which affects the results.
Extensive investigations on KLa have been conducted by previous researchers, especially for reactors using conventional impellers. Several studies are also available in the literature that have investigated different aspects of oxygen transport in different works [17, 18].
To optimize the impeller design for effective gas dispersion, it is essential to understand the mechanism of better oxygen mass transfer performance, so the present work on the bioreactor design was directed towards the study of oxygen transfer and its availability in the bioreactor. In this study, design and construction of a laboratory scale stirred tank bioreactor was followed by measurements of KLa in the aerated bioreactor in order to identify the optimal operational conditions of the oxygen mass transfer from gas into the aqueous phase. The independent variables were: type of impellers, number of impellers, aeration rate, agitation speed and types of sparger.