Ultrasound, a new separation
technique to harvest microalgae
R. Bosma, W.A. van Spronsen, J. Tramper, R.H. Wijffels-2003
Journal of Applied Phycology, 15(2):
143-153
Abstract:
In this article it is proven that ultrasound can be
used to harvest microalgae. The separation process is based on gentle
acoustically induced aggregation followed by enhanced sedimentation. In this
paper, the efficiency of harvesting and the concentration factor of the
ingoing biomass concentration are optimized and the relevance of this
process compared to other harvesting processes is determined. For the
optimisation, five parameters were modeled simultaneously by the use of an
experimental design. An experimental design was chosen, because of possible
interaction effects between the different parameters. The efficiency of the
process was modeled with a R-squared of 0.88. The ingoing flow rate and the
biomass concentration had a lot of influence on the efficiency of the
process. Efficiencies higher than 90% were reached at high biomass
concentrations and flow rates of 4–6 L day-1.
At most, 92% of the organisms could be harvested and a concentration factor
of 11 could be achieved at these settings. It was not possible to harvest
this microalga with higher efficiencies due to its small size and its small
density difference with water. The concentration factor of the process was
modeled with a R-squared of 0.75. The ingoing flow rate, biomass
concentration and ratio between harvest flow and ingoing flow rate had a
significant effect on the concentration factor. Highest concentration
factors, up to 20, could be reached at low biomass concentrations and low
harvest flows. On industrial scale, centrifuges can better be used to
harvest microalgae, because of lower power consumption, better efficiencies
and higher concentration factors. On lab- or pilot-plant scale, an
ultrasonic harvesting process has the advantages that it can be operated
continuously, it evokes no shear stress and the occupation space is very
small. Also, when the algae excrete a soluble high valued product this
system can be used as a biofilter.
(Food and Bioprocess Engineering
Group, Department of Agrotechnology and Food Sciences, Wageningen
University, Wageningen, 6700 EV, The Netherlands, e-mail: rouke.bosma@wur.nl)