VOCs removal from waste gases: gas-phase bioreactor for the abatement of hexane by Aspergillus niger

In this study a biofilter reactor was successfully applied to remove hexane (a VOC compound) from contaminated air streams. Since hexane is very poorly water soluble and hardly metabolized by most bacteria, because of its short hydrocarbon chain, a gas phase bioreactor inoculated by Aspergillus niger was adopted. In fact, filamentous fungi include many paraffins degrading species and develop aerial structures which give a large superficial area and allow for a direct pollutant mass transfer from the gas phase to the biological one without the presence of a liquid phase which is a limiting factor for hydrophobic compounds. Soil near gasoline stations was chosen as an adequate source for isolation of suitable fungi strains which were, then, selected, principally on the basis of hexane tolerance. A lab-scale bioreactor of 1.7710-3 m3 was assembled with media consisting of expanded clay and inoculated with the selected strain. After a first trial to assess the feasibility of the process, many experimental runs were carried out in order to investigate the stability of the system in the long period together with the trend of the elimination capacity and removal efficiency depending on the organic load. The flow rate was always kept constant (410-3 m3/h) while the hexane concentration in the inlet stream varied from 2 to 20 g/m3/h. At a certain point of the experimentation a second identical reactor was connected in series to the first one. The system revealed to be efficient and stable during a two months trial. The average elimination capacity was 150 g/m3reactor/h and it increased with the organic load until a maximum level after a load of 300 g/m3reattor/h. On the opposite, the removal efficiency was over 70% for the lowest hexane concentrations. Considering the plant as the sum of the two biofiltration columns, the RE was almost always over the 80%. The fungal development onto the support was also monitored in terms of weight increase and visual assessment by SEM observations of expanded clay particles from the biofilters. A good aerial mycelial growth was observed together with a large amount of spores over almost all the support surface. A steady state model was tested to describe the process but many parameters are still lacking since very hard to determine. In particular, some preliminary kinetic tests showed the great variability of the fungi growth inside the bioreactor