Use of Chlorella vulgaris for CO₂ mitigation in a photobioreactor

Carbon dioxide (CO₂) is a colorless gas that exists at a concentration of approximately 330 ppm in the atmosphere and is released in great quantities when fossil fuels are burned. The current flux of carbon out of fossil fuels is about 600 times greater than that into fossil fuels. With increased concerns about global warming and greenhouse gas emissions, there have been several approaches proposed for managing the levels of CO₂ emitted into the atmosphere. One of the most understudied methods for CO₂ mitigation is the use of biological processes in engineered systems such as photobioreactors. This research project describes the effectiveness of Chlorella vulgaris, used in a photobioreactor with a very short gas residence time, in sequestering CO₂ from an elevated CO₂ airstream. We evaluated a flow-through photobioreactor's operational parameters, as well as the growth characteristics of the C. vulgaris inoculum when exposed to an airstream with over 1850 ppm CO₂. When using dry weight, chlorophyll, and direct microscopic measurements, it was apparent that the photobioreactor's algal inoculum responded well to the elevated CO₂ levels and there was no build-up of CO₂ or carbonic acid in the photobioreactor. The photobioreactor, with a gas residence time of approximately 2 s, was able to remove up to 74% of the CO₂ in the airstream to ambient levels. This corresponded to a 63.9-g/m³/h bulk removal for the experimental photobioreactor. Consequently, this photobioreactor shows that biological processes may have some promise for treating point source emissions of CO₂ and deserve further study.