ABSTRACT
Systems using flocculating cultures (Abate et al., 1996; Kida et al., 1992; Kuriyama et al., 1993; Roca et al., 1995; Teixeira et al., 1990; Wieczorek and Michalski, 1994) which take advantage of cell recycling by natural sedimentation of highly flocculating strains
are a very interesting technique due to their low operational costs (Sousa et al., 1994a) and simplicity-no complex mechanical devices (e.g. centrifuges) are needed. In fact, construction costs are low and energy input is not very significant. Moreover, it allows a natural selection of the flocculating organisms in continuous fermentations, as the nonflocculating ones will be dragged out of the system together with the effluent. Further, the use of flocculating cells may provide important contributions for the improvement of separation processes in fermentation; besides being less aggressive than other separation techniques, a reduction in production costs is obtained since the amount of cells to be separated by centrifugation or filtration is significantly reduced (Teixeira and Mota, 1992). An additional advantage is obtained when using bioreactors operating with genetically modified flocculent microorganisms. In fact, such reactors are able to retain the genetically modified biomass, thus minimising the danger of spreading it into the environment, while exerting a permanent selective stress in the organism by expelling all the individuals that have lost flocculation ability.
