Botryococcus braunii cells: Ultrasound-intensified outdoor cultivation integrated with in situ magnetic separation

•Outdoor cultivation of algal cells with in situ magnetic harvesting was developed.•Ultrasonic treatments improved algal cell biomass and hydrocarbon productivity.•Algal cells were harvested efficiently using a magnetic separator. An integrated system combining ultrasound-intensified outdoor cultivation of Botryococcus braunii with in situ magnetic harvesting of the algal cells was developed. The algal cells were cultivated in 200L plastic bag reactors, and seven five-minute ultrasonic treatments at a four-day interval using a fixed frequency of 40kHz and a total power of 300W improved algal cell biomass and hydrocarbon productivity. The algal cells were harvested using functional magnetic particles and a magnetic separator, and a recovery efficiency of 90% was obtained under continuous operation at a flow rate of 100mL/min using the in situ magnetic separation system. The overall production cost using the integrated system was US$ 25.14 per kilogram of B. braunii dry biomass. The system developed in this study provides a base for the industrial production of B. braunii.