Treatment of Odors and Pollutants using a Convective Biofilter

Biofiltration involves the use of biofilms to aerobically oxidize air-borne pollutants using either a packed bed of natural or synthetic media. In these cases the air-borne pollutants diffuse into the biofilm established on the biomedia surface and biodegrade, eventually forming carbon dioxide, nitrates, nitrites, sulfates, etc. The overall biological treatment rate is usually affected by this diffusion rate into the biofilm, which is part of the mass transfer process. When the air-borne concentration of the pollutant is low, as in odor treatment, this diffusive flux into the biofilm becomes the rate controlling step. This also occurs when the molecular weight of the pollutant is high, which decreases its diffusivity into the biofilm. A convective biofilter is designed to allow the exhaust gas with its air-borne pollutants to flow through the biofilm, rather than past it, and this allows the pollutant to flow through the biofilm rather than diffusing through it. This eliminates the dependance of the overall treatment rate on the diffusive flux through the biofilm while providing oxygen, carbon and nutrients into the biofilm. In this paper, experimental data on the treatment of toluene through a convective biofilter will be presented. The convective biofilter was constructed using a highly porous, ceramic, activated-carbon coated, rectangular block which had holes from one end to the other end. Exhaust gas with toluene flowed into the porous block at one end, and we closed the other end of the holes, which forced the gas through the porous walls between the holes. When the biofilms were established on these porous walls, the contaminated air was forced to flow through the biofilms adhering to the porous walls, while the media was kept moist through humidification of air using aerosolized water. The control was a Diffusive Flow Biofilter Media (DFBM) in which the other end of the straight holes were not plugged, and this allowed the contaminated air to flow through the holes while the biofilms were established on the porous walls inside each of the straight passage through the block. By using a similar size media block and the same flow of contaminated air with identical influent concentration of toluene, the CFBM filter could be compared side-by-side with a DFBM biofilter. The CFBM performed consistently better than the DFBM at a toluene concentration level below 100 ppmv, indicated by higher removal efficiency and more carbon dioxide production. Mathematical models for the CFBM were developed to better understand the processes within the CFBM biofilter and the results of these simulations for both the CFBM and DFBM biofilters agreed well with the experimental data. Results of this study will be presented in this paper.