Modeling of a Fungal Biofilter for the Abatement of Hydrophobic Vocs

Alberto Vergara-Fernández and Sergio Revah. Process Engineering, Universidad Autónoma Metropolitana-Iztapalapa, Av. Sn. Rafael Atlixco 186 Col. Vicentina, CP. 09340, Iztapalapa, Mexico, Mexico

Biofiltration is nowadays one of the leading Air Pollution Control (APC) techniques for low VOCs concentrations. In these systems, the VOCs are mineralized by immobilized microorganisms in a solid support. Biofiltration performance is satisfactory for most VOCs, but it has shown to be less efficient to treat hydrophobic compounds which are sparsely soluble in the biofilms, which usually contain high water content to allow metabolic activities. Furthermore, the transfer of these contaminants to the biologically active phase is low considering that the flat bacterial biofilms offer low exchange surface. Filamentous fungi have been studied to improve these limitations. The transfer of the hydrophobic pollutant from the gas phase has been shown to be improved in fungi by the increased transfer surface, due to the formation of the aerial mycelium, and the more favorable phase equilibrium with the hydrophobic nature of the fungi. Furthermore, fungi are more resistant to acid and low- humidity conditions that may be found in biofilters. The objective of this work is to describe the growth of filamentous fungi in biofilters for the degradation of hydrophobic VOCs. The study system was hexane, as a model substrate and the fungus Fusarium solani B1. The system is mathematically described and the main physical (mass transfer, partition and transport area), kinetic data (substrate inhibition and affinity, growth and degradation rates and maintenance coefficient), and morphological parameters of aerial hyphae (hyphal diameter, average length of segments, critical length, maximum length of distal individual hyphae, branching number in individual hyphae, colony rate of radial extension) were obtained by independent experiments for model verification. The model proposed in this study describes the increase in the transport area by the growth of the filamentous cylindrical mycelia and its relation with hexane elimination in quasi -stationary state in a biofilter. To mathematically describe the system, we considered four processes: (1) mass transfer of VOCs in the bulk gas, (2) mass transfer of VOCs into the gas layer around the mycelium and considering the experimentally obtained gas-biomass partition coefficient, (3) mass transfer and reaction of the nitrogen source through the elongating mycelia, (4) and the kinetic of mycelial growth. Processes (2) and (3) include movable boundary conditions to account for the mycelial growth. The model describing fungal growth includes Monod-Haldane kinetic and hyphal elongation and ramification. The reduction in the permeability caused by mycelial growth was further related to pressure drop by Darcy′s equation. Partition coefficients and morphology parameters of Fusarium solani were determined in independent experiments in closed batch systems and microculture. The affinity constants for hexane and nitrogen were determined by respirometry and found to be 1.9 g.m-3 and 500 g.m-3, respectively. The maximum growth rate, obtained by determining the CO2 production at different hexane concentrations in the gas phase, was 0.052 h-1 with an inhibition constant of 30.1 g.m-3. Growth inhibition was observed at gas phase concentrations beyond 7 g.m-3. The maintenance coefficient was determined by direct measurement of hexane consumption in resting cells and found to be 1.51x10-4 h-1. The non dimensional partition coefficients obtained for the fungus grown in a biofilter fed with hexane was 0.20,±0.03 which contrasts with the water partition coefficient of 42.4. Morphology parameters obtained by image analysis were hyphal diameter, 2.99(±0.29) [μm]; average length of segments, 603.8(±48.3) [μm]; critical length, 510.8(±42.7) [μm]; maximum length of distal individual hyphae, 965.5(±93.8) [μm]; and branching number in individual hyphae, 6.0(±2.0) Radial growth, measured as the length increment of the colony every 24 h was 183.6(±1.38) [μm∙h-1]. The model was verified with biofiltration experiments using perlite as support and gaseous hexane as substrate. Data included hexane elimination capacity and axial variation, CO2 production and pressure drop. Biomass measurements were done at the end of the experiment.