Optimizing CO2 transfer in algal open ponds

The objective of this study was to optimize the CO2 injection system of algal raceway open ponds to minimize CO2 losses to the atmosphere and therefore reduce operational costs. CO2 transfers can be optimized by increasing the bubble-to-culture CO2 transfer rate and by decreasing the culture-to-atmosphere CO2 transfer rate. CO2 transfer coefficients were first experimentally determined on a pilot-scale algal pond for different designs of the injection system. The impact of five design parameters was tested: the CO2 diffuser length, the CO2 diffuser position, the CO2 flow rate, the paddle wheel rotation speed and the water level. Culture-to-atmosphere transfer coefficient was only significantly impacted by the paddle wheel rotation speed (range: 3.42-48.7 10(-3) min(-1); base-case value: 17.1 10(-3) min(-1)). Bubble-to-culture transfer coefficient, determined for a mix of fine and coarse CO2 bubbles, varied significantly with the CO2 injection flow rate (range: 1.47-18.3 10(-4) min(-1); base-case value: 9.11 10(-4) min(-1)) and water level (range: 7.46-14.4 10(-4) min(-1)). The efficiencies of the various system designs at full-scale were then extrapolated through simulation by coupling an algal productivity model to the CO2 transfer model developed in this study. Reducing the water level from 0.2 m down to 0.1 m reduced CO2 supply by 14%. Reducing the paddle wheel rotation speed from 21 rpm to 13 rpm decreased CO2 supply by 40%. Increasing the pH set point from 7.2 to 8 reduced CO2 supply by 38% but also decreased Dunaliella sauna biomass productivity by 17%. A trade-off must be achieved between high biomass productivity and lower environmental footprint.