Biofiltration of hydrophilic and hydrophobic volatile organic compounds using wood-based media

The biological degradation of hydrophilic and hydrophobic volatile organic compounds (VOCs), discharged in pulp and paper and wood products air emissions, was examined in two separate biofiltration experiments. Two identical bench scale biofilters were operated in long term experiments in order to study the removal of alpha-pinene, a hydrophobic VOC, and methanol, a hydrophilic VOC, separately and as a mixture. The biofilter media consisted of a mixture of wood chips and spent mushroom compost (50:50 vol-%) that was further mixed with either perlite, granular activated carbon (GAC), or small-size wood chips. The wood chip/compost mixture was added to the activated carbon or perlite in a ratio of 75:25 vol-%. A steady state model was also developed that considers the biofilm as an organic matrix and uses Monod kinetics plus inhibition.; Under steady state conditions and at 40°C bed temperature, the biofilters performed similarly and treated up to 40--45 g alpha-pinene/m3 bed/h or 250 g methanol/m3 bed/h with percentage removals of more than 90--95% and gas retention times between 20 and 60 sec. For mixtures of methanol and alpha-pinene, the removal rate was measured based on the total carbon and was about 80--90 g carbon/m3 bed/h. The effect of GAC on the removal of alpha-pinene was significant only during the start-up period where the GAC biofilter achieved 100% removal due to adsorption. When the biofilters were subjected to step changes in the pinene loading rate, both biofilters experienced a period of lower performance and the GAC did not provide a buffering capacity to the biofilter. Unlike the initial start-up period, when the addition of alpha-pinene was restarted after three days of shutdown, no adsorption took place in the GAC biofilter because the GAC was fully covered by a thick layer of biofilm.; For the treatment of mixed VOCs, the presence of methanol in the air stream significantly influenced the removal of alpha-pinene. The removal capacity for alpha-pinene per unit volume of the bed decreased linearly with increasing loading rates of methanol. The presence of methanol, a hydrophilic and easily biodegradable compound, suppressed the growth of the alpha-pinene degrading microbial community, thereby decreasing alpha-pinene removal capacity of the biofilters. alpha-Pinene degradation increased along the biofilter length while the concentration of methanol decreased. Unlike alpha-pinene, methanol was not affected by the presence of alpha-pinene, and was removed equally from the air stream under different alpha-pinene loading rates.; The temperature of the biofilter significantly affected performance. The alpha-pinene removal rate decreased by 20--30% when the bed temperature decreased from 40°C to 30°C. For methanol, however, the temperature drop of 10 degrees did not change biofilter performance. The growth of the biofilm, coupled with bed compaction, increased the biofilter pressure drops from less than 200 Pa in the first few weeks to about 1500--3000 Pa after more than six months of biofilter operation.