Airborne toluene removal for minimizing occupational health exposure by means of a trickle-bed biofilter

The paper presents the experimental results on a biotrickling pilot plant, with a water scrubber as pre-treatment, finalised to the treatment of an airborne toluene stream in a working place. The air stream was characterized by a very high variability of the inlet concentrations of toluene (range: 4.35-68.20 mg Nm-3) with an average concentrations of 16.41 mg Nm-3. The pilot plant has proved its effectiveness in toluene removal, along a 90 days experimentation period, in steady-state conditions. The scrubbing pre-treatment has achieved an average removal efficiency of 69.9%, but in particular it has proven its suitability in the rough removal of the toluene peak concentrations, allowing a great stability to the following biological process. The biotrickling stage has achieved an additional average removal efficiency of 75.6%, confirming the good biodegradability of toluene. The biofilm observation by a scanning confocal laser microscope has evidenced a biofilm thickness of 650 µm fully penetrated by toluene degrading bacteria. Among the micro-population Pseudomonas putida resulted the dominant specie. This bacterium can therefore be considered the responsible for most of the toluene degradation. The whole experimented process has determined an average 92.7% for toluene removal efficiency. This result allows to meet the most stringent limits and recommendations for occupational safety, given by authoritative organizations in USA and EU; it also meets the odorous threshold concentration of 11.1 mg Nm-3.