Ultrasound-assisted steps for producing biodiesel from microalgae

Biodiesel production from microalgae is considered one of the main candidates to replace conventional fuels. In addition, the use of ultrasound can be crucial to enhance different steps in the industrial production of this biofuel from this type of microorganisms. This review focuses on the potential of ultrasound technology to increase lipid content in microalgae and improve biomass harvesting and lipid extraction, as well as its potential use in oil transesterification. Specifically, the use of ultrasound pulses in the stationary phase of microalgae growth can act as a stimulus to improve lipid content and can oxidise cell walls, improving lipid extraction and subsequent harvesting. Furthermore, if assisted with ultrasound, the reaction time, alcohol/oil molar ratio, separation process, and energy consumption of transesterification can be reduced compared to conventional methods due to the reduction of the interfacial area. Finally, ultrasound technology can be used if some of the previous processes (i.e., in situ transesterification) are coupled to decrease the number of steps in an industrial process. Regarding scale-up, although some ultrasonic reactors working in continuous operation mode have already been proposed, there are still some drawbacks, mainly related to the knowledge of bubble behaviour in different media and their effect on reactions (enzymatic or in situ transesterifications) as well as the energy consumption if ultrasound technology is used in more than one process simultaneously. These facts need to be studied in more detail to introduce this technology in a large-scale process.