EVALUATION OF TRICKLE-BED AIR BIOFILTER PERFORMANCE AS A FUNCTION OF INLET VOC CONCENTRATION AND LOADING, AND BIOMASS CONTROL

The 1990 Amendments to the Clean Air Act have stimulated strong intere st in the use of biofiltration for the economical, engineered control of volatile organic compounds (VOCs) in effluent air streams. Trickle bed air biofilters (TBABs) are especially applicable for treating VOCs at high loadings. For long-term, stable operation of highly loaded TB ABs, removal of excess accumulated biomass is essential. Our previous research demonstrated that suitable biomass control for TBABs was achi evable by periodic backwashing of the biofilter medium. Backwashing wa s performed by fluidizing the pelletized biological attachment medium with warm water to about a 40% bed expansion. This paper presents an e valuation of the impact of backwashing on the performance of four such TBABs highly loaded with toluene. The inlet VOC concentrations studie d were 250 and 500 ppmv toluene, and the loadings were 4.1 and 6.2 kg COD/m(3) day (55 and 83 g toluene/m(3) hr). Loading is defined as kg o f chemical oxygen demand per cubic meter of medium per day. Performanc e deterioration at the higher loading was apparently due to a reductio n of the specific surface of the attached biofilm resulting from the a ccumulation of excess biomass. For a toluene loading of 4.1 kg COD/m3 day, it was demonstrated that the long-term performance of biofilters with either inlet concentration could be maintained at over 99.9% VOC removal by employing a backwashing strategy consisting of a frequency of every other day and a duration of 1 hr.