Miniaturized Biotrickling Filters and Capillary Microbioreactors for Process Intensification of VOC Treatment with Intended Application to Indoor Air.

Typical biofilters and biotrickling filters used for volatile organic compounds (VOCs) control have treatment rates limited to 30-200 g m-3 h-1, mostly because they are exposed to dilute VOC streams, have moderate biomass density and activity, and moderate mass transfer coefficients. For these reasons and the concern over releasing bioaerosols and humidity, traditional biofilters and biotrickling filters are not ideal for the treatment of indoor air. Here we report on the development and evaluation of microbioreactors for the intensive treatment of VOCs that could be used for indoor air quality control, when coupled with a VOC microconcentrator (developed separately). The microconcentrator will function to adsorb VOCs from indoor air and release them to the microbioreactor at a higher concentration. The miniaturized bioreactors, with maximized surface area-to-volume ratios, allow for increased mixing and mass transfer of pollutants to the biofilm, resulting in a greater degradation rate of the VOCs. Three different microbioreactors were designed, constructed and their performance for removing vapors of toluene and methanol was assessed. Results showed that they were able to achieve maximum elimination capacities (ECs) for methanol around 1000 g m-3 h-1, 780 g m-3 h-1, and 12 600 g m-3 h-1 for the glass beads packed bed, polyurethane (PU) foam biotrickling filters and capillary microbioreactor, respectively, and around 120 g m-3 h-1, 250 g m-3 h-1 and 3050 g m-3 h-1, respectively, when treating toluene vapors. These values, especially for the capillary microbioreactor, are 40-80 times greater than the rates generally obtained in conventional biofilters and biotrickling filters. The interphase mass transfer coefficient (KLa) was determined. The capillary microbioreactor had values 13-17 times greater than the other two bioreactors, suggesting that improved mass transfer could have contributed to the very high performance observed in the capillary microbioreactor. The results demonstrate that microbioreactors are promising novel technologies for controlling small amounts of organic pollutants.

Capillary microbioreactors for VOC vapor treatment: Impacts of operating conditions.

Micro-capillary bioreactors (1 mm ID, 10 cm long) were investigated for the biodegradation of toluene vapors as a model volatile organic compound (VOC). The intended application is the removal of VOCs from indoor air, when such microbioreactor is coupled with a microconcentrator that intermittently delivers high concentrations of VOCs to the bioreactor for effective treatment. The effects of key operating conditions were investigated. Specifically, gas film and liquid film mass transfer coefficients were determined for different gas and liquid velocities. Both mass transfer coefficients increased with gas or liquid velocity, respectively, and the overall gas-liquid mass transfer was dominated by the liquid-side resistance. Experiments with the microbioreactors focused on the effects of gas velocity, liquid velocity and mineral medium renewal rate on the treatment of toluene vapors at different inlet concentrations. The best performance in terms of toluene removal and mineralization to CO2 was obtained when the gas and liquid velocity ratio was close to one and achieving Taylor or slug flow pattern. Sustained treatment over extended periods of time with toluene elimination capacities ranging from 4000 to over 9000 g m-3 h-1 were obtained, which is orders of magnitude greater than conventional biofilters and biotrickling filters. Biological limitations generally played a more important role than mass transfer limitation. Continuous mineral medium supply at a high rate (10 h liquid retention time) enabled pH control and provided ample nutrient supply and therefore resulted in better toluene elimination and mineralization. Overall, these studies helped select the most suitable conditions for high performance and sustained operation.