Enhancing the destruction of hydrophobic VOCs in trickle bed air biofilters

Trickling bed air biofilters (TBABs) facilitate more consistent operation than traditional biofilters because of better control of overall pressure drop, nutrient concentration, and pH. While hydrophilic compounds are degraded easily in TBABs, hydrophobic compounds are retarded until biological cultures produce a sufficient RNA or enzyme/protein to utilize this compound. Furthermore, hydrophobic compounds are not readily bio-available which makes them reluctant to biodégradation if mass transfer between the gas and liquid phases is the rate limiting step. Two lab-scale controlled TBABs were operated for investigating the difference in performance for a hydrophobic volatile organic compound (VOC) with and without the addition of non-toxic surfactants (Triton X-100 and Tomadol® 25-7) that could serve dual purposes; increasing solubility and limiting excess biomass growth. n-Hexane was selected as a model hydrophobic VOC. Operating conditions were kept similar in both TBABs including empty bed retention time (120 seconds), bed material (diatomaceous earth pellets) and constant temperature of 20°C. Effects of changing loading rates from 0.45 to 0.88 Kg COD/m³.day (5.37 and 10.41 g/m³.hr respectively), acclimation period, removal profile along biofilter depth, nitrogen consumption, and CO₂ production were compared under continuous loading operation conditions. The optimum amount of surfactant added to the nutrient feed was determined by studying the effect of different surfactant concentrations on VOC water solubility with time by considering different VOC concentration sets within the TBAB loading rate range. Water volume/head space ratio on the solubility behavior was studied at chosen concentrations to further understand the fate of the contaminant within the biofilter bed and to determine the rate limiting step in the biodegradation of n-Hexane in TBAB.