Foam fractionation : an effective technology for harvesting microalgae biomass

Harvesting and dewatering can account for up to 30% of the overall cost of production of usable microalgae biomass for the biotechnology and bioenergy sectors. Harvesting is particularly challenging due to the small amount of algal biomass produced relative to water volume. This process exacts high energy and cost demands and therefore limits further expansion in the microalgae biomass industry. Foam fractionation has potential to deliver a low cost, low energy harvesting solution. Microalgae cells adsorb to the surface of a stream of fine air bubbles, which then rise up a closed column, discharging the concentrated product at the top. Foam fractionation significantly reduces construction, maintenance, and energy costs compared to other harvesting technologies. In this research, a fractional factorial design of experiments followed by a central composite design were used to determine the optimal levels of major variables influencing the harvest of the freshwater microalga Chlorella sp. The effects of bubble size within the liquid pool and foam phase of the harvesting unit were determined, a high concentration factor of 427 as achieved using fluidic oscillation for microbubble generation. The influence of microalgal growth phase on harvest efficiency was investigated to gain insight into the optimal time to harvest during cell cultivation. The effect of surfactant, used to induce foaming, on lipid recovery was examined through methods including total lipid recovery, gas chromatography, energy dispersive x-ray spectrometry and solid phase extraction. The results indicate that the surfactant had the additional benefit of significantly increasing the overall lipid recovery. These encouraging results suggest foam fractionation offers considerable potential as an efficient, low cost, and scalable microalgae biomass harvesting technology.