The mass transfer of ethanol and toluene from air stream to liquid phase, and bioremediation of contaminated air streams containing either ethanol or toluene have been investigated using a stirred tank bioreactor. This investigation was conducted in six phases:
1) mass transfer experiments involving the transport of toluene and ethanol from contaminated air streams into the liquid phase,
2) study of air stripping effects of ethanol and toluene out of the liquid phase,
3) batch growth experiments to determine growth kinetic models and model parameters,
4) bioremediation of ethanol or toluene as the sole substrate to determine the capacity of Pseudomonas putida (P. putida) (ATCC 23973) growth on these substrates,
5) toluene removal from contaminated air streams using ethanol and benzyl alcohol as co-substrates, and
6) modelling the above studies using metabolic pathways to better understand the bioremediation process.
Preliminary oxygen mass transfer studies showed that the presence of ethanol in the liquid phase enhances the overall oxygen mass transfer coefficients. Increasing the ethanol concentration from 0 to 8 g/L caused the oxygen mass transfer coefficients to increase from 0.015 to 0.049 s-1, and from 0.017 to 0.076 s-1, for impeller speeds of 450 and 600 rpm, respectively. Mass transfer studies using ethanol vapor in the air stream demonstrated complete absorption into the aqueous phase of the bioreactor at all operating conditions investigated (air flowrates up to 2.0 L/min and inlet concentrations up to 95.0 mg/L) and therefore mass transfer coefficients for ethanol absorption could not be determined. On the other hand, toluene mass transfer coefficients could be measured and were found to be 8.3x10-4, 8.8x10-4 and 1.0x10-3 s-1 at agitation speeds of 300, 450 and 600 rpm, respectively. The ethanol air stripping parameters (b values) were determined (at initial ethanol liquid concentration of 8.6 g/L) to be 0.002 and 0.007 h-1 for air flow rates of 0.4 L/min (0.3 vvm) and 1.4 L/min (1 vvm), respectively. The toluene air stripping rates, at initial liquid toluene concentration of 440 mg/L, were found to be 1.9, 5.3, 10.4, and 12.6 h-1 for air flow rates of 0.4, 0.9, 1.4, 2.1 L/min, respectively, which is much higher than those of ethanol at the same air flow rates and stirring speed of 450 rpm. It was also observed that benzyl alcohol was not stripped to any detectable level at any of the operating conditions used in this study.
The growth of P. putida using toluene as sole substrate was carried out at several operating conditions by varying the dilution rates (D) from 0.01 to 0.1 h-1, the toluene air inlet concentration from 4.5 to 23.0 mg/L and air flow rates of 0.25 to 0.37 L/min (resulting in inlet toluene loadings from 70 to 386 mg/L-h). Steady state operation could not be achieved with toluene as the sole substrate. Ethanol and benzyl alcohol were therefore used as co-substrates for the toluene removal process. In order to understand the kinetics of P. putida growing on ethanol or benzyl alcohol, batch growth experiments were carried out at different initial substrate concentrations. The specific growth rates determined from the batch runs showed that ethanol had no inhibition effect on the growth of P. putida. The growth on ethanol followed the Monod equation with the maximum growth rate of 0.56 h-1 and yield of 0.59. The results from the batch growth experiments on benzyl alcohol showed that benzyl alcohol inhibits the growth of P. putida when the initial concentration of benzyl alcohol in the growth media is increased. The maximum growth rate was 0.42 h-1 in the inhibition model and the yield value was 0.45. By operating the bioreactor in continuous mode using a pure strain of P. putida, it was possible to continuously convert ethanol into biomass without any losses to the gas phase or accumulation in the bioreactor at inlet ethanol concentrations of 15.9 and 19.5 mg/L. With ethanol as a co-substrate, toluene was efficiently captured in the bioreactor and readily degraded by the same strain of P. putida. A toluene removal efficiency of 89% was achieved with an ethanol inlet concentration of 15.9 mg/L and a toluene inlet concentration of 4.5 mg/L. With the introduction of benzyl alcohol as co-substrate at a feed rate of 0.12 g/h, the toluene removal efficiency reached 97% at toluene inlet concentrations up to 5.7 mg/L. All the experimental results at steady state were obtained when the bioreactor operated in a continuous mode at a dilution rate of 0.1 h-1, an air flowrate of 0.4 L/min, an agitation speed of 450 rpm and a reactor temperature of 25.0oC. The results of this study indicate that the well-mixed bioreactor is a suitable technology for the removal of VOCs with both high and low water solubility from polluted air streams. The results were achieved at higher inlet pollutant concentrations compared to existing biofilter treatments.A metabolic model has been developed to simulate the bioremediation of ethanol, benzyl alcohol and toluene. For continuous steady state operations, ethanol as a sole substrate required less maintenance for biomass growth (0.010 C-mol/C-mol-h) than bioremediations in the presence of toluene, as seen with the ethanol/toluene mixture (0.027 C-mol/C-mol-h), and the benzyl alcohol/toluene mixture (0.069 C-mol/C-mol-h)
