Fig. 10.6. Schematic flow diagram for foam fractionation (Wane and Sinskey, 1970).
Microbial cells and other insoluble materials are normally separated from the harvested broth by filtration or centrifugation. Because of the small size of many microbial cells it will be necessary to consider the use of filter aids to improve filtration rates, while heat and fiocculation treatments are employed as techniques for increasing sedimentation rates in centrifugation. The methods of cell and cell debris separation described in the following sections have been practised for many years. Bowden et al. (1987) review some potential developments in cell recovery. These include the use of electrophoresis and dielectrophoresis to exploit the charged properties of microbial cells, ultrasonic treatment to improve fiocculation characteristics and magnetic separations. All these techniques suffer from high cost and scale-up difficulties and currently are not appropriate technologies. Of more current interest is the use of two-phase liquid extraction. Though still most appropriately used for separation of selected soluble components, it is easy to scale up and uses conditions which are gentle on the product.
of surfactants such as long-chain fatty acids, amines and quaternary ammonium compounds. Materials made surface active and collected are termed colligends whereas the surfactants are termed collectors. When developing this method of separation, the important variables which may need experimental investigation are pH, air-flow rates, surfactants and colligend-collec-tor ratios.
Rubin et al. (1966) investigated foam separation of E. coli starting with an initial cell concentration of 7.2 X 108 cells cm"3. Using lauric acid, stearyl amine or i-octyl amine as surfactants, it was shown that up to 90% of the cells were removed in 1 minute and 99% in 10 minutes. The technique also proved successful with Chlorella sp. and Chlamydomonas sp. In other work with E. coli, Grieves and Wang (1966) were able to achieve cell enrichment ratios of between 10 and 1 X 106 using ethyl-hexadecyl-dimethyl ammonium bromide.
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