Biofiltration of volatile organic compounds using fungal-based bioreactors
Abstract
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.
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