Aeration energy requirements for lipid production by Scenedesmus sp. in airlift bioreactors

•CO2 availability is dependent on power input for aeration and CO2 content of gas.•Critical minimum CO2 supply and transfer rates exist for algal growth.•Above the critical values, aeration energy is expended with no increased production.•By considering these critical values, improved net energy ratios (NER) were obtained.•The positive NERs in the airlift reactor facilitate algal bioenergy production.

Microalgae have potential to yield various bioenergy products, including algal biodiesel. For algal energy production, the process energy input must be substantially lower than the product energy. Airlift photobioreactors provide controlled environments with good mixing and mass transfer; however, previous work reports a net energy ratio (NER; energy produced divided by energy consumed) less than 1. Here, the energy consumption in these reactors was improved by combined optimisation of superficial gas velocity and its CO2 concentration. Increasing CO2 concentration resulted in increased tolerance to lower superficial gas velocities, down to a critical minimum value. A 75% reduction in aeration power input was obtained by reducing superficial gas velocity from 0.0210 to 0.0052 m s− 1 at 5 400 ppm CO2, without substantial reduction in biomass concentration (2.27 to 1.93 g L− 1, respectively) or productivity (0.189 to 0.173 g L− 1 d− 1, respectively). The NER under these conditions was 5.47 for biomass plus lipid and 1.01 for lipid only. The CO2 supply rate, product of superficial gas velocity and CO2 concentration, correlated with the CO2 transfer rate which influenced algal productivity. The range of NERs measured across the superficial gas velocities studied indicates the ability to optimise algal cultivation in photobioreactors for the improved feasibility of algal bioenergy.