Bio-oxidation of a model VOC in air

This study was performed to find a satisfactory regime of operation for the vapor phase bio-oxidation of ethyl alcohol, a model volatile organic compound (VOC), in a specially designed bioreactor. Ethanol was selected as a model compound representing bakery oven gas pollution. A spirally wound bioreactor module was used within which was immobilized a mixed bacterial culture from aerobic sludge. The activated sludge from municipal wastewater treatment plants readily attached with no pretreatment on the surface of the spiral biosupport which is a polymeric sheet. The parameters studied were air flow rate and inlet concentrations of ethanol. Ethanol was injected, along with air, into a water reservoir prior to being fed into the bioreactor. The vapor and liquid concentrations in the reservoir were measured continuously and reached an equilibrium state. The reaction rates for all runs were determined. An optimal vapor temperature was observed for the environment of the microorganisms. As expected, the reaction rate varied with air flow rate and vapor temperature. An optimal air flow rate which was used for the action of microorganisms with gaseous ethyl alcohol and oxygen was 20 L/min (retention time:1.45 min). At this flow rate, the desirable vapor temperature in the reservoir was between 27 °C and 30 °C. At 20 L/min of air flow, a local maximum reaction rate was maintained at about 44 to 50 mg of ethanol per minute for this 28 hour run at this feed injection level. The vapor concentration at the inlet in typical runs from this series at this flow rate reached equilibrium levels ranging from about 1,000 ppmv to 1,700 ppmv within the first four hours of the 28 hour runs. The air flow rates for this series ranged from 7.52 to 40 L/min, while the total amount of ethanol fed to the system per minute was kept constant. At higher inlet feed concentrations, the reaction rates increased. For this series, at 2.34 L/min of air flow (retention time: 12.35 min), the maximum inlet vapor concentration reached about 7,000 ppmv within 6 hours. The removal efficiency was 99 percent, equivalent to about 30 mg of ethanol/min due to the low air flow rate. Also, after a 6 hour run at 7.52 L/min of air flow (retention time: 3.84 min), about 6,000 ppmv at the inlet was converted to 24 ppmv at the outlet. The removal efficiency was 99 percent, equivalent to about 87 mg of ethanol/min. This is about 3 times the corresponding rate at 2.34 L/min of air flow