Biofiltration of volatile organic compounds using fungal-based bioreactors
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Stricter regulations for volatile organic compounds have increased the demand for efficient abatement technologies. Biofiltration, a process in which contaminated air is passed through a biologically active bed, can be used to remove these pollutants from air streams. In this study, the black fungi Exophiala lecanii-corni and Cladophialophora sp. were evaluated for use in vapor-phase bioreactors. Batch tests indicated that E. lecanii-corni can tolerate low pH and minimal nutrient conditions and degrade a wide variety of contaminants including toluene and methyl propyl ketone. A series of bench-scale bioreactor studies were then conducted to determine how nitrogen supply, mite predation and surfactant washing affected pollutant removal in fungal biofilters. In experiments to evaluate the influence of nitrogen supply on bioreactor performance, it was determined that a nitrogen mass loading of approximately 2% of the carbon mass loading was needed to achieve near-complete removal of the pollutant. When the nitrogen supply was discontinued, the bioreactors continued to degrade pollutant; however, clogging of the bioreactor occurred due to more extensive filamentation and conidiophore formation by the fungi in the biofilm. Additional bench-scale studies using Cladophialophora sp. indicated that clogging can be controlled using the fungal-grazing mite, Tyrophagus putrescentiae. Clogging also was minimized by packing the bioreactors with an open-structured foam medium, which was found to favor hyphal growth in the internal pores of the packing. To reduce the 7- to 10-day start-up period typically observed in fungal bioreactors, surfactants were evaluated as fungal spore activators. Results showed that Tween 20, a nonionic surfactant, enhanced inoculum development by shortening the lag period prior to spore germination. However, when bioreactors were presoaked in medium containing Tween 20, washout of the cells occurred during inoculation. Finally, results from the bench-scale experiments were used to evaluate Ottengraf’s bioreactor model developed for bacterial biofilters in an attempt to predict toluene removal profiles in fungal systems. The model reasonably predicted toluene removal profiles for inlet concentrations of less than 250 ppm , but it did not account for nutrient limitations nor the complex morphology of fungal biofilms. Overall, the fungal biofilters evaluated in this study efficiently removed gas-phase pollutants and tolerated harsh environmental conditions, indicating that they are a viable alternative to physical-chemical treatment options.