(604aa) Carbon Dioxide Capture and Utilization from Aerobic and Anaerobic Bioprocesses Using Microalgae

Authors: 
Praveen, P., National University of Singapore
Loh, K. C., National University of Singapore

The most common by-product in the whole cell-catalyzed bioprocesses, both aerobic and anaerobic, is CO2. The amount of CO2 produced in bioprocesses is small and the contribution of bioprocessing industries in total global greenhouse gas emission and global warming in negligible. However, many bioprocesses can be a steady source of high quality CO2 streams which can be captured and utilized in the production of other valuable chemicals. While the CO2 conversion to other compound of commercial value can have its economic advantages, this will also benefit the environment by resulting in a negative CO2 emission from the integrated CO2 production and utilization process.

One example of a bioprocess resulting in the emission of relatively large amounts of CO2 is fermentation of sugars into ethanol by yeast. For each mole of glucose broken into ethanol, two moles of CO2 is produced. The CO2 stream generated in the anaerobic fermentation is a saturated gas at high purity and low pressure which can be readily converted into useful compounds. In this research, green microalgae, Chlorella vulgaris, was used to capture CO2 from a chemostat operating with Zymomonas mobilis for the fermentation of glucose into ethanol. The gas vent from the chemostat was connected to a photobioreactor using polypropylene hollow fiber membranes. In the resulting membrane photobioreactor, a continuous flow of CO2 was obtained from the chemostat through the tube side of the shell and tube membrane contactor, whereas the microalgae were circulated on the shell side for growth and for light exposure. The bioreactor operating parameters were optimized to maximize microalgae growth as well as bioethanol production.

The efficacy of microalgae in capturing and utilizing CO2  from bioprocesses was also demonstrated by combining a photobioreactor with a bacterial wastewater treatment system. In conventional aerobic wastewater system, microbial degradation of organic compounds produces CO2 which can be utilized for microalgae cultivation. On the other hand, O2 produced by the microalgae during photosynthesis can be used by the bacteria for growth and metabolism. Thus by combining a bacterial wastewater system with a microalgal photobioreactor, a symbiotic relationship can be established between the two microorganisms. In this research, microalgae immobilized in alginate beads were dispersed in a tank treating synthetic wastewater using Pseudomonas putida. The effects of symbiosis on microalgae growth and wastewater treatment were examined.

The results from these studies demonstrate the efficacy of microalgae in capturing and utilizing CO2 from bioprocesses. The results indicate that by innovative bioreactor engineering approach, a photobioreactor can be integrated with other bioprocesses producing CO2, improving the effectiveness, economy and sustainability of the integrated system.