Journal of Environmental Engineering

Biofiltration of a gaseous stream contaminated with benzene, toluene, ethylbenzene, and the three xylene (BTEX) compounds was evaluated in this study. Experimental investigations were conducted on a pilot-scale biofilter containing randomly packed 6-mm R-635 Celite pellets as biological attachment media. The main focus of the study was to expand biofiltration technology to treat high volatile organic compounds (VOC) concentrations while maintaining consistently high removal efficiencies. Operational parameters investigated were BTEX loading, empty bed residence time (EBRT), backwashing frequency and duration, recovery of biofilter removal efficiency after backwashing, development of removal rate constants, and nutrient-phosphorous (nutrient-P) limitation as a biomass control. Periodic backwashing of the biofilter with medium fluidization was necessary for removing excess biomass and attaining stable long-term high removal efficiencies. Removal efficiencies above 99% were achieved for all the BTEX components for loadings up to 1.8 kg chemical oxygen demand (COD)/m³/d. Stable overall removal efficiencies of 88% were achieved for a BTEX loading of 6.2 kg COD/m³/d and an EBRT of 0.67 min. Evaluations of pseudo–steady-state performance data, one day following backwashing, yielded first-order removal rate constants for all the BTEX components. Removal efficiencies above 99% at BTEX loading of 2.1 kg COD/m³/d were attained for a period of 14 d without biofilter backwashing in the absence of nutrient-P.

Biofiltration is a biological air pollution control technology for volatile organic compounds (VOCs). This paper summarizes the fundamentals, design and operation, and application of the process. Biofiltration has been demonstrated to be an effective technology for VOCs from many industries. Large and full-scale systems are in use in Europe and the United States. With proper design and operation, VOC removal efficiencies of 95–99% have been achieved. Important parameters for design and performance are empty-bed contact time, gas surface loading, mass loading, elimination capacity, and removal efficiency. Key design and operation factors include chemical and media properties, moisture, pH, temperature, nutrient availability, gas pretreatment, and variations in loading.

A pilot-scale compost biofiltration system was operated at a gasoline soil vapor extraction site in Hayward, Calif. for one year. The media was composed of equal volumes of compost and perlite, a bulking agent. Supplements added included nitrogen (as KNO₃), a gasoline degrading microbial inoculum, buffer (crushed oyster shell), and water. The biofiltration system was composed of four identical units with outside dimensions of 1.2 × 1.2 × 1.2 m (4 × 4 × 4 ft) operated in an up-flow mode. The units were configured in parallel during the first eight months and then reconfigured to two parallel systems of two units in series. Air flux values ranged from 0.29 to 1.0 m³/m² per min. Inlet total petroleum hydrocarbon (TPH_gas) concentrations ranged from 310 to 2,700 mg/m³. The average empty bed contact time was 2.2 min. Following start-up, performance of the individual biofilters varied considerably for a seven-month period. The principal factor affecting performance appeared to be bed moisture content. Overall TPH_gas removals reached 90% for short periods in one unit, and BTEX removals were typically above 90%. Drying resulted in channeling and loss of bed activity. Management of bed moisture content improved over the study period, and recovery of system performance was achieved without replacement of bed media. Overall TPH_gas removals exceeded 90% during the final 50 days of the study.

A field study on the treatment of volatile petroleum hydrocarbons (PHC) by biofiltration was conducted by the Petroleum Environmental Research Forum (PERF) between 1992 and 1994. Its objective was to assess the technical, regulatory, and economic feasibility of biofiltration for off-gases from petroleum processing and soil remediation equipment. It involved three small-scale biofilters provided by U.S. vendors, treating off-gas from two soil vapor extraction sites and a source of refinery wastewater. Comprehensive monitoring of biofilter operating parameters and performance was conducted. The results suggest that biofilters remove major petroleum hydrocarbon classes to strongly varying degrees. Typically more than 95% of aromatic compounds such as benzene, and odorous reduced sulfur compounds can be removed at residence times of one minute or less, while removal of more than 70% of light aliphatics will require residence times of several minutes, and will thus require correspondingly large filter volumes and higher capital expenditure. This indicates that off-gas composition and regulatory control objective will largely determine whether biofiltration is economically feasible for a given off-gas stream. The high percentage removal of aromatic hazardous air pollutants (HAP) and odors can generally be accomplished at comparatively short residence times (one minute or less) while the need for high-percentage removal, particularly of light aliphatics (<C₅) will generally render biofiltration noncompetitive. Results from the field tests also showed that biofilter performance and operational reliability depend on the selection of filter material, the reliability of the moisture control system, and the level of fluctuation in PHC concentrations. This agrees with the findings from other full-scale biofilter projects. Finally, a preliminary economic comparison of biofilters and established control technologies suggested that biofiltration may, for a representative design case involving off-gas from a refinery wastewater treatment operation, offer considerable savings in total cost of air pollution control for petroleum hydrocarbon streams if regulations require the control of aromatic HAP or odors.

The work describes the aerobic biodegradation of volatile organic compound (VOC) mixtures from effluent air streams in laboratory-scale compost based biofilters. A rapid start-up of the system (3–5 d) was observed and 82% of the carbon in the influent pollutant (methyl ethyl ketone) was recovered as carbon dioxide in the exhaust air after this period. Biofilter performance and carbon dioxide production were also investigated during and after 0.5–1-h step inputs of 2–10 g m⁻³ of hexane, acetone, 1-propanol, and/or methyl isobutyl ketone (MIBK) during steady methyl ethyl ketone (MEK) treatment. Carbon dioxide patterns suggest that pulsed pollutants were first sorbed onto the packing material and subsequently degraded within 2–5 h. Hexane was not sorbed, and thus not degraded to a significant extent. Little effect was observed of the step inputs on the overall MEK removal process, mainly because the reactors were operated well below MEK breakthrough loading. However, the analysis of MEK profiles within the biofilter bed showed that significant inhibition did indeed occur. An even more important inhibition was observed between 1-propanol, MIBK, and acetone, when such a mixture was injected into the reactor. Practical design of biofilters should consider the possibility of reduced performances due to the presence of multiple VOCs in the waste air stream.

This paper reports the result of studies using two biotrickling filters (BTFs) with wood and plastic packings, respectively, for treatment of methylethylketone (MEK) in an air stream. Effects of packing material, specific surface area of the packings, volumetric loading of MEK, nutrient addition, and the recirculation rate of liquid on the MEK elimination capacity were tested. Results show that the elimination capacity of MEK in the BTF system can be evaluated on the basis of unit surface area of packing materials. Surface properties may not be a major concern in choosing a packing. Nutrient addition is essential for obtaining an efficient elimination of the influent contaminants to BTFs. For treating water-soluble contaminants, the liquid recirculation rate is a minor operation factor in the case of minute residual contaminants in the liquid. A high liquid recirculation rate should be avoided at such high level of contaminant loadings, since some contaminants remain in the liquid.

The determination of transfer rate constants (k) and concentration partition coefficients (k_h) is required for biofilter mathematical models. A bench-scale technique was developed and used to evaluate k and k_h values for four different packing materials: (1) compost; (2) a mixture of compost and diatomaceous earth (DE); (3) Calgon granular activated carbon (GAC); and (4) Westates GAC. Two vapor phase contaminants were used: ethanol and carbon dioxide. Experimental results showed that this technique produced estimated k_h values that were similar to both “batch” and “spike test” results, indicating the accuracy of the procedure. Calculated k_h values for carbon dioxide were two orders of magnitude less than those for ethanol. The water content of the packing material strongly affects k_h values. High water content increased k_h values for ethanol and decreased k_h values for carbon dioxide.

A predictive model has been developed to describe degradation of toluene in a flat-plate vapor phase bioreactor (VPBR). The VPBR model incorporates kinetic, stoichiometric, injury, and irreversible loss coefficients from suspended culture studies for toluene degradation by P. putida 54G and measured values of Henry's law constant and boundary layer thickness at the gas-liquid and liquid-biofilm interface. The model is used to estimate the performance of the reactor with respect to toluene degradation and to predict profiles of toluene concentration and bacterial physiological state within the biofilm. These results have been compared with experimentally determined values from a flat plate VPBR under electron acceptor and electron donor limiting conditions. The model accurately predicts toluene concentrations in the vapor phase and toluene degradation rate by adjusting only three parameters: biomass density and rates of death and endogenous decay. Qualitatively, the model also predicts gradients in the physiological state cells in the biofilm. This model provides a rational design for predicting an upper limit of toluene degradation capability in a VPBR and is currently being tested to assess applications for predicting performance of bench and pilot-scale column reactors.

A membrane bioreactor system that overcomes many of the limitations of conventional compost biofilters is described. The system utilizes microporous hydrophobic hollow fiber membranes for mass transfer of volatile organic compounds (VOCs) from the gas phase to a microbially active liquid phase. The reactor design provides a high biomass concentration, a method for wasting biomass, and a method for addition of pH buffers, nutrients, cometabolites, and/or other amendments. A theoretical model is developed, describing mass transfer and biodegradation in the membrane bioreactor. Reactor performance was determined in a laboratory scale membrane bioreactor over a range of gas loading rates using toluene as a model VOC. Toluene removal efficiency was greater than 98% at an inlet concentration of 100 ppm_v and a gas residence time of less than 2 s. Factors controlling bioreactor performance were determined through both experiments and theoretical modeling to include: compound Henry's law constant, membrane specific surface area, gas and VOC loading rates, liquid phase turbulence, and biomass substrate utilization rate.

Treatment of air streams contaminated with binary volatile organic compound (VOC) mixtures in classical biofilters under steady-state conditions of operation was described with a general mathematical model. The model accounts for potential kinetic interactions among the pollutants, effects of oxygen availability on biodegradation, and biomass diversification in the filter bed. While the effects of oxygen were always taken into account, two distinct cases were considered for the experimental model validation. The first involves kinetic interactions, but no biomass differentiation, used for describing data from biofiltration of benzene/toluene mixtures. The second case assumes that each pollutant is treated by a different type of biomass. Each biomass type is assumed to form separate patches of biofilm on the solid packing material, thus kinetic interference does not occur. This model was used for describing biofiltration of ethanol/butanol mixtures. Experiments were performed with classical biofilters packed with mixtures of peat moss and perlite (2:3, volume:volume). The model equations were solved through the use of computer codes based on the fourth-order Runge-Kutta technique for the gas-phase mass balances and the method of orthogonal collocation for the concentration profiles in the biofilms. Good agreement between model predictions and experimental data was found in almost all cases. Oxygen was found to be extremely important in the case of polar VOCs (ethanol/butanol).

A steady-state mathematical model is presented to describe the kinetics of volatile organic compound (VOC) removal in biofilters that consist of a mixed compost and granular activated carbon (GAC) medium. The model describes the basic transport of VOCs from the gas phase into the liquid phase of the compost biofilm and into the carbon particles, using the assumptions of diffusion as characterized by Fick's law. The kinetics of biological degradation of substrate (pollutant) in the compost biofilm are assumed to follow a Monod-like relationship. Experimental data were compared with model predictions under steady-state conditions for treatment of a mixture of benzene, toluene, ethylbenzene, and o-xylene (BTEX) vapors. Best results were obtained when model applications were divided according to first-order biodegradation kinetics for relatively low influent concentrations (<50 ppm) of pollutants and zero-order reaction for higher (235–440 ppm) influent concentrations. In both instances, the model produced suitable approximation of experimental bed depth versus concentration profiles at steady state for individual compounds in biofilters containing small but varying amounts of GAC. The presence of GAC improved BTEX removal efficiencies over a biofilter containing only compost.

This paper describes a phenomenological model for simulating the removal of toluene, ethylbenzene and o-xylene (TEX) from contaminated air streams by a biofilter. The phenomena incorporated into the model are interphase mass transfer between the gas and the aqueous biofilm with equilibrium partition, advection, diffusion, and biological reactions. The reaction rate for each TEX component is quantified using a more generalized Monod equation to allow for inhibitive effects in the presence of multiple substrates. Solutions to the system of coupled nonlinear partial differential equations representing component mass conservation are obtained by a fully implicit finite difference method with first-order accuracy in time and second-order accuracy in space. A parametric study is performed to evaluate the sensitivity of the Péclet number and the normalized mass transfer coefficient, both of which show strong influence on the removal rates. Comparison of the concentration distribution along the biofilter against available measured data and the exact closed-form solution indicates a good agreement with discrepancies being within experimental uncertainties.