Biological Trickling Filter (BTF) can be used as a measure of VOCs removal. Biofilm of microorganisms is formed on the surface of biofilter media, and VOCs passed through the BTF can be removed by the attached microorganisms. This method of microorganism fixation is usually highly efficient in the removal of VOCs. However, when microorganisms grow excessively on the surface of media, the efficiency will be decreased mainly due to the high pressure loss, leading to the decrease of flowrate and the increase of cost. Therefore, the understanding of the change in the amount of biofilm, the mechanism of growth and desorption of microorganism, and the efficient method of excess biomass removal is required.
In this study, BTF using porous ceramic media was used to remove toluene as a model VOC, and experiments for the effective removal of excess biomass, which is one of the critical problems in BTF, were performed. 3-phase behavior of air bubble, backwash water and media was simplified by modeling shear forces generated on media surface by water and air bubbles in the point of hydro-dynamics. Finally, the applicability of BTF was evaluated from field-scale pilot plat experiments.
When the high concentrations of VOCs were continuously supplied to BTF column, 460~520 P/m of pressure loss was developed in the lab-scale experiments due to the accumulation of excess biomass, which led to the decline of removal efficiency to 30% or less. From the experiments to overcome this pressure loss, it was found that the removal efficiency of excess biomass was enhanced as compared to the case using only water. When dissolved air (mean diameter of air bubble; 40 ㎛) was used as air bubble, removal efficiency of turbidity and solids was enhanced as compared to the case using only water and decreased as compared to the case using bigger air bubbles. Air bubble with the size of 0.2 cm has higher efficiency than that with 1 cm.
A model for the velocity fields on the media surface at various air-bubble sizes was developed by assuming the shape of all air bubbles and media as spherical form. An equation was established for the velocity field generated when the volume of air bubble was occupied by liquid. From the model analysis, as air-bubble size becomes smaller, the velocity field produced on media surface becomes greater, and this velocity field was related to the removal of biomass from the media surface.
The differences in backwash efficiency with air bubble size are resulted from the differences in the velocity field generated on the media surface by collapse-pulsing effect of air bubble. When air bubble is passed through BTF column evenly, smaller air bubble generates greater turbulence and, as a result, greater shear force than bigger air bubble does.
Field-scale pilot plant experiment was performed with 379∼520 ppm of total VOCs concentration and space velocity of 117∼262 $h^{-1}$ at room temperature. In this field test, it was found that about two weeks were required for the microbial adaptation to attain about 80% of removal, and, after three weeks, relatively stable efficiency was maintained. Without removing excess biomass, biological activity was also significantly lowered, leading to the system failure. This operational problem due to the excessive biomass growth, however, could be removed by periodic backwashing with air scour.
