Mitigation of CO₂ in industrial off-gasses by sparging the gas through photosynthetic microalgae bioreactors is an attractive concept. The goal is for the CO₂ to be consumed by the microalgae as a nutrient, which in turn produces lipids suitable for conversion into biodiesel, as well as other value-added bioproducts such as Omega-3 fatty acids and antioxidants.
Open systems are considered the most economic outdoor, large-scale cultivation option but have large land space requirements due to their shallow depths (15-35 cm). Consequently, finding sufficient space to locate them close to off-gas sources on industrial sites can be a significant challenge. Shallow depths are also likely to result in low uptake of CO₂ and consequently reduced biomass productivity due to short gas residence times in the culture medium.
In order to obtain longer gas/liquid transfer times, as well as greater per area productivity, the tanks through which the off-gas is sparged must be as deep as possible. However, to make the tanks deeper and avoid the costs associated with sub-surface artificial lighting, the issue is how to ensure the microalgae receive adequate light exposure. We have, therefore, looked for a novel method for increasing the depth of the tanks through which the off-gas is sparged. To achieve this, we have investigated the use of a gas-lift circulating system in a deep top-lit open bioreactor. In addition to providing CO₂, the sparged gas also provides continual vertical circulation of the microalgae to ensure good mixing and an adequate light/dark cycle. Compared to existing shallow open systems, the results obtained showed comparable biomass productivity per unit volume, but importantly around three-times higher biomass productivity per unit area occupied by the bioreactor. The lipid productivity was also increased due to light and hydrodynamic stresses.
In order to enhance further light utilization efficiency in the deep cultivation bioreactor, the use of a novel non-energy-consuming light column was also evaluated. The results of using the light column showed a 33% increase in areal biomass productivity and a 16% increase in areal lipid production.
The proposed design and developed models can be easily translated into larger scale, onsite production facilities in industrial sectors emitting off-gas. The carbon capturing properties of microalgae can, therefore, help reduce industrial carbon dioxide emissions, whilst at the same time producing biodiesel from the resulting lipids.