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S. Radel, C. Hauser, M. Gröschl, E. Benes:
"Application of dual ultrasonic separation on suspensions containing yeast cells and oil";
in: "Proc. 2nd Congress of Alps-Adria Acoustics Association and 1st Congress of Acoustical Society of Croatia,", Acoustical Society of Croatia, Zagreb/Croatia, 2005, ISBN: 953-95097-0-x, S. 55 - 61.

The principle of ultrasonically enhanced settling is successfully applied in biotechnology for filtration
purposes. Particles (biological cells) are concentrated in certain regions of a sonicated volume by an ultrasonic
standing wave field (~2 MHz). Due to the dependence of the final settling velocity on the diameter of an object (Stokes'
law), the resulting agglomerates settle more quickly than single cells. This principle, which relies on a sufficiently large
difference in mass densities of the particles and the host liquid, is also applicable to dispersed material lighter than the
liquid. Up to now, no feasible solution was at hand when a suspension contained both heavier and lighter particles.
For this demand, a novel approach, the dual ultrasonic separation, was tested on suspensions of yeast cells and oil
droplets in water as a model system as well as dispersions using cell and oil free production broth as host liquid. The
standing wave field was applied to the suspension firstly in a modified acoustic settler with a bottom outlet for the
filtrate and secondly to the standard version with the outlet at the top. Experiments show, that the dual ultrasonic
separation was able to reduce the oil content significantly in the first step and subsequently deliver a final filtrate
containing some 80% less yeast cells than the original dispersion. One first optimisation step - a reduction of the oil
first systems size - led to a 93-94% reduction of yeast cells at a throughput of some 18 l/d.
Key words: ultrasound, filtration, ultrasonically enhanced settling, dispersions, yeast

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