Ultrasonic waves, in various formats, have been used to induce aggregation of algae and speed up separation of algae. The concept is to use ultrasonic energy to con­centrate the algae in a specific location or node to facilitate rapid recovery. Ultra­sonic separation can be run continuously, has low stress (cells remain viable), is non-fouling, has few moving parts to break down, and occupies relatively little space. However, this process was considered less cost-effective than centrifugation for industrial harvesting due mainly to the requirement for a cooling system and the low concentration factor compared to centrifugation and microfiltration (Bosma et al. 2003). It might be useful in a system where the alga secretes a metabolite, and it would be advantageous to remove live cells for reuse in the production of more material while harvesting the metabolite from the medium.

One example of the application of ultrasonic separation of algae is a process deployed with Monodus subterraneus UTEX 151 that was followed by enhanced sedimentation as a harvest tool (Bosma et al. 2003). The algal cells were pumped through a chamber equipped with an ultrasonic transduction chamber fitted with a resonator and reflector that provided a standing wave in the chamber. Cells were forced to the nodes and agglomerated such that when the field was removed, they quickly settled due to gravity. High cell densities and slow flow rate were most favorable for ultrasonic separation providing a maximum recovery of 92 % of the cells and an 11-fold increase in concentration.

Another application of ultrasound is the acoustic focusing method recently analyzed within the NAABB consortium’s harvesting group. The use of ultrasound to both separate and lyse the cells was investigated such that both the harvesting and extraction methods could be combined. Using a low-frequency ultrasonic field, the separation of Nannochloropsis salina, Nannochloris oculata, and Auxeno — chlorella protothecoides was demonstrated. Their acoustic focusing harvester was scaled to 45-225 L h-1 and provided an 18-fold increase in biomass density (NAABB 2014).