Biological treatment represents low-cost and environmental friendly option for air pollution as compared to incineration, catalytic oxidation and adsorption. It has several advantages like minimal power consumption, few byproducts and cost effectiveness. However, several challenges face biological treatment processes such as variability of flow rate and composition of contaminants in waste streams. Furthermore, hydrophobic compounds are not readily available for the microorganisms, creating a deficiency for the use of biological treatment in the industry. Biofiltration for VOCs control is best operated at steady loads of hydrophilic VOCs. In practice the existence of hydrophobic compounds in waste streams is inevitable. In addition, variations in contaminant loads are common in real applications.
The objectives of this study are to introduce biological treatment as an effective technique for non-methane hydrocarbon removal from air under stressed operating conditions. Emphasis is being placed on hydrophobic compounds which are known to be recalcitrant to biological treatment. Two hydrophobic compounds; n-hexane and benzene, which are carcinogenic and toxic, were utilized as model compounds. Both compounds were studied separately under different operating conditions. Loading rates up to 48 and 77 g/(m3 h) for n-hexane and benzene were applied at the inlet prividing elimination capacities of 39 and 61 g/(m3 h) , respectively. These elevated elimination capacities have not been achieved by any reported research for hydrophobic compounds at practical removal efficiencies as obtained in this study (minimum 78%). Surfactants were introduced in the biofiltration system as means for enhancing solubility. The effect of two different surfactants; Triton X-100 and Tomadol 25-7, were investigated at different loading rates for the biodegradation of n-hexane operating under neutral pH.
Other means of increasing the bioavailability of hydrophobic compounds was the introduction of acidic environment, which supports the growth of fungi and suppresses bacteria. The shift in the working consortium enables the aerial mycelia of fungi to form larger surface area in the gas phase. This facilitates the uptake of hydrophobic volatile compounds overtaking the rate limiting step. Acidic operation at pH 4 was investigated in Trickle Bed Air Biofilters (TBABs) for both VOCs and compared to operation at neutral pH at the same loading rates. Operating with fungi microorganism for n-hexane enhanced the performance significantly, while very similar performance was observed for benzene. In addition, different mixing ratios of n-hexane and benzene were applied to TBABs to study the effect of mixture on the performance behavior at neutral and acidic pH to gain insight about their effect on each other, since in several industries both VOCs are emitted together.
Finally, in order to overcome the challenges of practical erratic loadings to biofilter bed an integrated cyclic adsorption/desorption beds followed by biofilter was evaluated. The system is designed to incorporate simultaneous adsorption and desorption processes, in order to dampen VOC concentration pulses in waste streams. The effectiveness of the integrated process scheme under four dynamic contaminant loadings was compared to a stand-alone biofilter system. The integrated unit was proven to achieve the goal of effectively treating fluctuating VOC loading with high removal efficiency; and attain consistent emission compliance and economical design of biofiltration facilities.