Modeling of indoor air treatment of polychlorinated dibenzo-p-dioxins and dibenzofurans using high-efficiency particulate air-carbon filtration.

A high-efficiency particulate air (HEPA)-carbon filtration system was developed by the Access Business Group, LLC, to reduce the indoor levels of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). The HEPA filter removes the particle-bound PCDD/Fs, and the carbon filter removes the gaseous fraction. Because of the toxicity of PCDD/Fs, it is very difficult to handle them in the laboratory. In this study, mathematical modeling was performed to evaluate the performance of the HEPA-carbon filtration system for PCDD/Fs removal and to optimize its design and operation. The model was calibrated with experimental data conducted with toluene in a sealed room. Model simulations with four selected congeners demonstrated that it takes approximately 1 hr for the indoor air treatment system to reach the maximum removal efficiency and that the carbon air filter has a life time of 10(7) yr for dioxin removal. Given a zero emission from the HEPA filter, the overall removal efficiency is 78.7% for 2,3,7,8-tetrachloro dibenzo-p-dioxins, 89.8% for octa-chlorodibenzodioxin, 78% for tetra-chlorodibenzofuran, and 89.8% for octa-chlorodibenzofuran. The larger the mass emission from the HEPA filter, the lower the overall removal efficiency, and the larger the ratio of the filter flow rate (Q(f)) to the room flow rate (Q), the higher the overall removal efficiency. When the ratio of Q(f)/Q is 15, an overall removal efficiency of 90% can be reached for all four of the selected compounds. The removal of the four selected compounds does not change as the relative humidity increases < or = 90%.

Biogeochemical analysis of hydrogen sulfide removal by a lava-rock packed biofilter.

Although lava-rock-based biofilters have demonstrated their efficiencies for hydrogen sulfide (H2S) removal found in odorous air emissions, the biogeochemical basis for this removal is unclear. In this study, samples of lava rock and rinse water from biofilters at Cedar Rapids Water Pollution Control Facilities (Iowa) were used to study the structure and chemical composition of lava rock and to identify the predominant microorganism(s) present in lava-rock-based biofilters. It was found that iron, in the form of Fe2+ and Fe3+, was present in lava rock. Although literature suggests that Acidithiobacillus thiooxidans are primarily responsible for gaseous H2S removal in biofilters, our study showed that Acidithiobacillus ferrooxidans was the dominant microorganism in the lava-rock-based biofilters. A novel mechanism for H2S removal in a lava-rock-based biofilter is proposed based on the biogeochemical analysis of lava rock.

Optimization of biofiltration for odor control: model calibration, validation, and applications.

A dynamic model that describes the biofiltration process for hydrogen sulfide removal from wastewater treatment plant air emissions was calibrated and validated using pilot- and full-scale biofilter data obtained from the Cedar Rapids (Iowa) Water Pollution Control Facilities. After calibration, the model was found to predict the dynamic effluent concentrations of the pilot- and full-scale biofilters well, with the measured data falling within 58 to 80% of the model output values. In addition, the model predicted the trend of the field data, even under field conditions of changing input concentration and at effluent concentrations below 1 ppm by volume. The model demonstrated that increasing gas residence time and temperature and decreasing influent concentration decreases effluent concentration. In addition, model simulations showed that a longer residence time is required to treat dynamic loading increases, indicating that biofilter design should account for the maximum influent concentration. These results can be used to help design and operate biofilters for controlling odorous and hazardous air emissions.

Optimization of biofiltration for odor control: model development and parameter sensitivity.

A dynamic model that describes the mass transport and attenuation of odor-causing air emissions (i.e., hydrogen sulfide and other reduced sulfur compounds) in a biofiltration unit was developed and incorporated into a software package called Biofilter. Mechanisms included advective flow, mass transfer from the bulk phase to the biofilm, biofilm internal diffusion, and biological reaction in the biofilm. A dimensionless analysis revealed that the mass transport and attenuation of target compounds can be characterized by several dimensionless groups. Model equations were converted to ordinary differential equations using orthogonal collocation and the resulting ordinary differential equations were solved using the DGEAR algorithm. Numerical solutions were verified by comparing model simulations to analytical solutions. The model simulations showed that the existence of a water layer surrounding the biofilm in a biofiltration unit lowers the removal efficiency of hydrogen sulfide. A sensitivity analysis of model parameters (including the film transfer coefficient, biofilm diffusivity, biofilm thickness, maximum specific biomass growth rate, yield coefficient, half-saturation coefficient, and initial active biomass concentration) using data from two biofilters located at the Cedar Rapids (Iowa) Water Pollution Control Facilities, showed that biofilm internal diffusion and biofilm kinetics have a significant effect on hydrogen sulfide removal, while external mass transfer has little effect.