1 Continuous flocculation airlift bioreactor with high cell loading – hydrodynamic and rheological aspects Jaroslav KLEIN a , João MAIA b , Martin JURAŠČÍK c , António A. VICENTE a , Lucília Domingues a and José A. TEIXEIRA a* a Centro de Engenharia Biológica, Department of Biological Engineering, University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal b Department of Polymer Engineering, University of Minho, Campus de Azurém, 4800 - 058 Guimarães, Portugal c Dept. of Chemical & Biochemical Engineering, Faculty of Chemical & Food Technology Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia e-mail: jateixeira@deb.uminho.pt Keywords: continuous ethanol fermentation, airlift bioreactor, high cell density system, flocculation, rheology INTRODUCTION Cheese whey, a by-product of dairy industry, represents a well-known significant environmental problem due to its very high BOD and COD values. Modern dairy industry, recently more concentrated into less but larger production plants, does not look at whey as a disposal problem any more; cheese whey is seen as a potential source of additional profit. Hence, bioutilization of the cheese whey has become a rapidly growing branch of dairy industry and science. One of the attractive possibilities for whey disposal is a coupled system consisting of ultrafiltration for protein recovery followed by the alcoholic fermentation of lactose from whey permeate. At later step of the process, a continuous high cell density system with flocculating yeast can be efficiently used. A recombinant flocculating strain of Saccharomyces cerevisiae containing genes (lactose permease and b-galactosidase) for lactose transport and hydrolysis has been constructed for this purpose (Domingues et al. 1999). In practice, such a system usually represents a three-phase (gas-liquid-solid) dispersion operating in a continuous mode. For this system, an airlift bioreactor (ALR) with an enlarged head zone seems to be a very attractive option due to a high retention of the solid phase and the advantageous combination of sufficient mixing and low shear stress. Due to the very high solids loading (up to 50-60 % vol.) the maintenance of the solid particles in suspension and circulating is of a particular importance requiring a suitable bioreactor design and a correct regulation of the sole energy input – gas flow rate. Three basic flow regime in three-phase airlift reactor were defined (Fan et al. 1984): the packed bed regime without suspension of particles and no liquid circulation, the fluidised bed regime with particle suspension exclusively in the riser zone and nonzero liquid circulation and the circulated bed regime with solids distribution throughout the reactor. For high cell density system, the circulated flow regime is highly desirable having the best even solid distribution throughout the ALR. In airlift bioreactors with a well-defined liquid circulation loop, the liquid velocity is the major hydrodynamic parameter, which considerably affects all physical phenomena (Chisti 1989). Despite an extensive number of velocity measuring methods applied to ALRs (Boyer et al. 2002), most of these techniques are not suitable for use in fermentation processes for several reasons (tagging of liquid elements with chemicals due to their interference with the strictly controlled composition of the fermentation medium and sterility problems, visual techniques as Laser Doppler Anemometry (LDA) or Particle Image Velocimetry (PIV) due to the opaqueness of the broth). The use of small flowfollowing particles with non-invasive detection of their movement can be efficiently used in such biosystems. Detection technique using inductive coils and magnetic particle is one of the cheapest and the simplest methods. The technique was already successfully tested to 31th International Conference of SSCHE May 24–28, 2004, Tatransk´ e Matliare, Slovakia Po-Tu-6, 105p.pdf 105–1