Cell Recycling

For the continuous operation of a PFF or CSTF, cells are discharged with the outlet stream which limits the productivity of fermenters. The productivity can be improved by recycling the cells from the outlet stream to the fermenter. 

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Secondary Lead. The emphasis in technological development for the lead industry in the 1990s is on secondary or recycled lead. Recovery from scrap is an important source for the lead demands of the United States and the test of the world. In the United States, over 70% of the lead requirements are satisfied by recycled lead products. The ratio of secondary to primary lead increases with increasing lead consumption for batteries. WeU-organized collecting channels are requited for a stable future for lead (see BATTERIES, SECONDARY CELLS Recycling NONFERROUS METALS). 

Stuckey, D.C., Caridis, K.A., Leak, D.J., Design of a novel bioreactor with cell recycle for continuous biotransformation and product extraction, Proc. 3rd Asia Pacific Biochemical Eng. Conf, Singapore, pp.315-317, 1994. 

Figure 2.5 Possible technological solutions to bioprocess problems a) Fed-batch culture b) Continuous product removal (eg dialysis, vacuum fermentation, solvent extraction, ion exchange etc) c) Two-phase system combined with extractive fermentation (liquid-impelled loop reactor) d) Continuous culture, internal multi-stage reactor e) Continuous culture, dual-stream multi-stage reactor f) Continuous culture with biomass feedback (cell recycling). (See text for further details).

With cell recycling, chemostat efficiency is improved. To maintain a high cell density the cells in the outlet stream are recycled back to the fermentation vessel. Figure 5.10 represents a chemostat unit with a cell harvesting system. The separation unit is used for harvesting the cells and recycling then to the culture vessel to increase the cell concentration. 

Downstream Processing Microfiltration plays a significant role in downstream processing of fermentation products in the pharmaceutical and bioprocessing industry. Examples are clarification of fermentation broths, sterile filtration, cell recycle in continuous fermentation, harvesting mammahan cells, cell washing, mycelia recovery, lysate recovery, enzyme purification, vaccines, and so forth. 

Y. L. Lee, H. N. Chang (1990) High cell density culture of a recombinant Escherichia coli producing penicillin acylase in a membrane cell recycle fermentor. Biotechnol. Bioeng., 36 330-337. 

D.P. Cemeus and A. Van der Ende. Apical and basolateral transferrin receptors in polarized BEWO cells recycle through separate endosomes. J Cell Biol. 114 1149-1158 (1991). 

More glucose can be converted to ethanol if cell recycle is used (Jackman, 1987) or by excluding a nitrogen source from the fermentation medium and using anaerobic conditions so that produced ethanol cannot be reconsumed. This principle was applied by Moebus and... 

Warren, R.K., Hill, G.A. and Macdonald, D.G., Continuous cell recycle fermentation to produce ethanol. Food Bioprod. Proc., 72 (1994) 149-157. 

The MIOX Corporation prepared cost estimates on the MIOX system based on bench-scale testing. They estimated that the active mixed oxidant solution produced by the process costs about 7 cents/gal to produce, including the costs of power, salt, and electrolytic cell recycling. At an injection ratio of 1 to 500, two gallons of mixed oxidants would be required to treat 1000 gal of water. The amount of mixed oxidants required varies with each individual waste stream, and with the treatment goals, so this estimate is by no means universal (D15848Z, p. 114). 

Battery type Materials recovered Fraction of cells recycled Economic advantages... 

Fig. A3.2 Flow sheet for the SAFT NIFE AB nickel-cadmium cell recycling process.

It appears that dCx/dt and rx are always the same, but this is not true. The former is the change of the cell concentration in a fermenter, which may include the effect of the input and output flow rates, cell recycling, and other operating conditions of a fermenter. The latter is the actual growth rate of the cells. The two quantities are the same only for batch operation. 

A PFF requires the initial presence of microorganisms in the inlet stream as a batch fermenter requires initial inoculum. The most economical way to provide cells in the inlet stream is to recycle the part of the outlet stream back to the inlet with or without a cell separation device. Figure 6.17 shows the schematic diagram of a PFF with cell recycling. Unlike the CSTF, the PFF does not require the cell separator in order to recycle, though its presence increases the productivity of the fermenter slightly as will be shown later. The performance equation of the PFF with Monod kintics can be written as ... 

Fig. 6.19 Schematic diagram of CSTF with cell recycling.

If all cells are recycled back into the fermenter, the cell concentration will increase continuously with time and a steady state will never be reached. Therefore, to operate a CSTF with recycling in a steady-state mode, we need to have a bleeding stream, as shown in Figure 6.19. The material balance for cells in the fermenter with a cell recycling unit is... 

It should be noted that actual flow rates of the streams going in and out of the filter unit do not matter as far as overall material balance is concerned. For a steady-state CSTF with cell recycling and a sterile feed,... 

A strain of yeast is being cultivated in a 30-L CSTF with a cell recycling system (cell settler) as shown in the following figure. The cell settler was designed so that the cell concentration of its outlet stream is 30 percent of that of its inlet stream, whereas the substrate concentrations of the two streams are the same. The growth rate of the cells can be represented by the Monod kinetics with the parameters Ks = 0.05 g/L, max = 0.3 h-1, and YX/s = 0.025. Calculate the steady-state substrate and cell concentrations in the fermenter. The inlet substrate concentration is 100 g/L and the flow rate is 20 L/hr. The feed stream is sterile. 

Batch fermentations result in high product concentration (120-150 kg/m3) but in low productivity (2 kg m Iff1). Dramatic improvements in productivity (20-80 kg m Iff1 in laboratory- and pilot-plant experiments) were obtained by using cell recycle via MF or immobilized cells, at the expense of lactate concentration in the effluent (usually lower than 50 kg/m3). 

A great deal of research work has been carried out to enhance bioreactor productivity (2 14 kg m 3h 1) using cell recycle via membrane processing (Boyaval and Corre, 1987 Boyaval et al., 1994) and recovering propionic acid by monopolar or bipolar ED (Boyaval et al., 1993 Weier et al., 1992 Zhang et al., 1993). 

Enzminger, J.D. and Asenjo, J.A. 1986. Use of cell recycle in the aerobic fermentative production of citric acid by yeast. Biotechnol. 8, 7-12. 

Representative applications Sterile filtration Clarification/sterilization of beer and Representative applications Continuous culture/cell recycle Filtration of oilfield produced water... 

The production of substances that preserve the food from contamination or from oxidation is another important field of membrane bioreactor. For example, the production of high amounts of propionic acid, commonly used as antifungal substance, was carried out by a continuous stirred-tank reactor associated with ultrafiltration cell recycle and a nanofiltration membrane [51] or the production of gluconic acid by the use of glucose oxidase in a bioreactor using P E S membranes [52]. Lactic acid is widely used as an acidulant, flavor additive, and preservative in the food, pharmaceutical, leather, and textile industries. As an intermediate product in mammalian metabolism, L( +) lactic acid is more important in the food industry than the D(—) isomer. The performance of an improved fermentation system, that is, a membrane cell-recycle bioreactors MCRB was studied [53, 54], the maximum productivity of 31.5 g/Lh was recorded, 10 times greater than the counterpart of the batch-fed fermentation [54]. 

The variables considered for optimization were the same as those used in the work of Costa et al. (5) inlet substrate concentration (S0), cell recycle rate (R), residence time (f,), and flash recycle rate (r). They were determined by Silva et al. (3) as the relevant variables. 

Optimizing the ABE fermentation process has long been the aspiration of more than a century of research. Conventionally, the profitability of fermentation is influenced by the type and concentration of substrate, dilution rate, pH, culture medium, and product recovery. Even using cell recycle, cell immobilization, or extractive fermentation to increase cell density and productivity, the yield of the combined ABE production never exceeded 0.44 g/g (13-15). 

The different operation modes used in microbial fermentations are employed also in animal cell cultivation. Although many different classifications can be adopted, the most general is the one that considers the following operation modes batch, fed-batch, continuous, and perfusion, which is a continuous mode with cell recycle/retention (Castilho and Medronho, 2002). 

Separating the yeast from the ethanol by centrifugation and recycling it can improve significantly the reactor productivity. For example, concentrations of yeast up to 83 g/1 can be achieved with productivities of 30 to 51 g/1 h, or tenfold over fermentation without cell recycle. 

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