Constructing and Testing a Photobioreactor Designed for Precise Characterizations of Growth of Microalgae for Biofuel Applications

Algae biofuels are a renewable alternative to fossil fuels for energy production and associated carbon recycle. A common current practice for growing algae in a laboratory setting involves delivering a mixture of air and CO₂ through the top of a gyrating Erlenmeyer flask. This setup can lead to slow growth due to light penetration limitations. In this study, we designed, constructed, and tested a flat-panel channel-flow photobioreactor (named the Alginator) capable of facilitating rapid growth of high-density algae with a particular focus on light availability and defined circulation patterns to avoid CO₂ depletion or oxygen accumulation. The Alginator design provides a high surface area to volume ratio, allowing for consistent light penetration for the defined pathlength of ~1cm for the entire flat flow path. By sparging the gas mixture with small bubbles at the bottom of a riser for a loop-type air-lift configuration, the design increases CO₂ (and O₂) mass transfer while driving circulation of the bioreactor at a minimum gas flow rate. This minimization of flow and a defined exit at the top should permit off-gas analysis for measurements of CO₂ consumption and oxygen evolution.

Initial experiments were conducted to test the Alginator using Chlorella vulgaris in WFAMC media where growth of 6.7 mg/L was more than 10-times greater than achieved in a control shake flask culture (0.342 mg/L) due to extended exponential growth beyond an optical density of OD-550nm = 10. Towards evaluating energy and carbon capture efficiencies, Chlorella was grown on replete and â?? nitrogen-limited WFAMC media. Biomass was characterized by dry weight, hydrothermal liquefaction (HTL), bomb calorimetry, and optical density. As expected, N-limited growth was reduced considerably to 4.71mg/mL. This result clearly indicates that the Alginator provides for rapid growth of algae to high cell densities as a result of its design which will be the basis of comparing numerous other algae for energy capture efficiency. Using HTL, the crude oil yield from the Alginator was 24 ± 1.5% in complete media and 18.3 ± 4% in nitrogen limited media. The lower yield in this preliminary test was unexpected, as we hypothesized that higher fatty acid content during nitrogen limitation would improve HTL yield. Further work is needed including our recent reconstruction of multiple Alginators where we will be able to better control temperature and more rapidly test different algae and growth conditions. A major goal of this work will be to provide a well-defined energy balance to assess energy and CO₂ capture efficiencies.