In-situ disinfection and a new downstream processing scheme from algal harvesting to lipid extraction using ozone-rich microbubbles for biofuel production
The scaling up and downstream processing costs of biofuels from microalgae are major concerns. This study focuses on reducing the cost by using energy efficient methods in the production of microalgae biomass and the downstream processes (biomass harvesting and lipid extraction). Ozonation of Dunaliella salina (green alga) and Halomonas (Gram-negative bacterium) mixed cultures for 10 min at 8 mg/L resulted in a reduction in the bacterial contaminant without harming the microalga. Harvesting of D. salina cells through microflotation resulted in a 93.4% recovery efficiency. Ozonation of the harvested microalgal cells for 60 min produced three main saturated hydrocarbon compounds (2-pentadecanone, 6, 10, 14-trimethyl; hexadecanoic acid; octadecanoic acid) consisting of 16 to 18 carbons. By systematically switching the carrier gas from CO2 to O3, the microbubble-driven airlift loop bioreactor (ALB) delivers nutrient to the culture and in-situ disinfection respectively. Further, modulating the bubble size to match particle size ensures recovery of the cells after culture. All three key operations (disinfection, harvesting and lipid extraction) are assembled in a scalable, relatively energy efficient process.