A Study Of Biofilters For Gaseous Volatile Organic Compounds Removal

The gaseous volatile organic compounds (VOCs) pollution and itscontrol are paid more and more attention nowadays. It is very important andurgent to develop cost-effective techniques for VOCs gas treatment. The aimof this study is to evaluate and optimize the performance of biofilters forgaseous toluene treatment. After improving the structure of the filter bed andscreening better packing media, the toluene removal performances andmicrobial growth characteristics of the biofilters under different conditionswere investigated. A theoretic model was successfully build to simulate andpredict the performance of the biofilters under different conditions.By comparing two buckwheat-hull biofilters' performances during more than300 days, the effects of adding inert spheres into the natural filter beds were evaluated.The results showed that adding inert spheres could prevent bed compactioneffectively and increase the void fraction of the filter beds. This made the biofiltersachieve more stable toluene elimination capacity and lower bed pressure drop. Withinert spheres in the filter beds, the performances of three biofilters packed withbuckwheat hulls, wood chips and granular activated carbons (GACs) respectively,were compared. The biofilter packed with wood chips showed no significantdifference with that packed with GACs. Both of them showed better performancethan the biofilter packed with buckwheat hulls.Two biofilters packed with wood chips and GACs separately run in parallel for270 days under different conditions. Under the inlet concentrations of lower than500 mg·m-3 and velocities of 25~48 m·h-1 (the corresponding empty bedretention times were 30~58 s), the emitted toluene concentration of the twobiofilters could meet the emission standard which was lower than 60 mg·m-3. Thebed pressures of the two biofilters were kept quite low between 50~70 Pa·m-1under the same conditions. It was also found that higher inlet concentrationsand higher velocities induced more significant elimination capacities dropand bed pressure increase caused by more biomass accumulation.The biomass changes in four biofilters packed with wood chips weremeasured and a microbial growth model was also established. The modelcould well simulate the change of the microbial concentration in the filterbeds. According to the simulation results of the model, the fraction of theactive microbes, as well as bed void fraction, specific surface area andeffective diameter would decrease with biomass accumulation.A theoretic model of biofilter treating simple VOCs gas was set up. Themodel successfully simulated biofilter's toluene removal performance andbed pressure drops simultaneously. Under different inlet concentrations andvelocities, the model predicted the minimum bed length and empty bedretention time which were necessary for wood-chip biofilter to meet theemission standard. The corresponding bed clogging time was also predictedby the model under the same conditions. These simulation results couldsupply theoretical guide for biofilter design and operation optimization.The metabolic characteristics of the microbial communities in the wood-chipbiofilter and GACs biofilter were monitored using Biolog microplates periodicallyduring the experiments. The metabolic activities of the microorganisms in bothbiofilters were observed to decrease during long-term operation. The metaboliccharacteristics of the microbial communities also changed throughout the filter bedsafter long-term operation.