Membrane bioreactors fermentation broths

Fermentation is typically conducted in dilute suspension culture. The low concentration in such systems limits reaction efficiency, and the presence of particulate and colloidal solids poses problems for product recovery and purification. By circulating the fermentation broth through an ultrafiltration system, it is possible to recover product continuously as they are generated while minimizing loss of enzyme or cells and keeping product concentration in the bioreactor below the self-inhibition level for the biocatalyst. This process is referred to as perfusion. As the ultrafiltration unit is part of the production process, the entire system is often considered a membrane reactor. 

An example of this configuration is filtration of the methane fermentation broth from a sewage sludge liquor [Kayawake et al., 1991]. The liquor is u eated anaerobically in a fermentor. The broth is pumped to a ceramic membrane module which is contained in the fermentor. The retentate is returned to the fermentor while the permeate is discharged to the environment This is schematically shown in Figure 8.2. Although the membrane module is enclosed in the bioreactor for compactness and process simplification, the membrane step in essence follows the fermentation step. 

Gryta [150] conducted integration of fermentation process with membrane distillation for the production of ethanol. The removal of by products, which tends to inhibit the yeast productivity, from the fermenting broth by MD process increased the efficiency and productivity of the membrane bioreactor. The ethanol concentration in permeate was 2-6 times higher than that in the fermenting broth. The enrichment coefficient was found to increase with decrease of ethanol concentration in the broth. 

Mori and Inaba (1990) applied a PV technique to attain both high productivity and efficient recovery of EtOH from a fermentation broth. The manbrane bioreactor consisted of a jar fermenter and a PV systan for the direct production of EtOH from uncooked starch with a thermophilic anaerobic bacterium, Clostridium thermohydrosulfuricum. From the four types of EtOH-selective monbranes tested, microporous PTFE membrane, the pores of which were impregnated with silicone rubber, was chosen for its large flux, high EtOH selectivity, and high stability. During... 

Penicillin in fermentation broth were measured after sample withdrawal from the bioreactor. The samples were filtered by an in situ membrane module with a 0.22 pm nominal pore diameter. 

Interestingly, membrane bioreactors produce lactic acid that can be manufactured either by chemical synthesis or by fermentative processes. In recent years, the amount of lactic acid obtained by biotechnological methods has increased. The highest cost in the traditional process of lactic acid production by carbohydrate fermentation lies in the separation steps that are needed to recover and purify the product from the fermentation broth. 

In the production of bioethanol, sugar is fermented, yielding low concentrated alcohol solutions that are recovered from the broth by ultrafiltration or pervaporation membranes. Ultrafiltration and pervaporation of bioethanol from fermentation broth are IG biofuel processes. As in all bioreactor-coupled membrane processes, membrane fouling and drop in permeate fluxes during continuous operation are the main concerns. 

The aeration in the bioreactor differs significantly from that in the shake flask. Aeration in shake flasks was accomplished by gas-exchange at the fermentation-broth-air surface area that was generated by die vigorous shaking motion. The bioreactor was aerated by a pump that delivered the moistened gas air bubble-free via a membrane to the reactor. All transport processes in the bioreactor were based on diffusion. 

Gryta et al. (2000) combined batch fermentation with the removal of ethanol from the broth by means of the MD process. To separate volatile compounds from the feed (broth), formed as a result of fermentation, they used a porous capillary polypropylene membrane with the following characteristics inner diameter (id) 1.8 mm and outer diameter (od) 2.6 mm, pore sizes with a nominal and maximum diameter of 0.22 and 0.6 mm, respectively, porosity about 73 %, and effective membrane area 490 cml The distillate temperature was 293 K, while the fermentation was performed at 303 (conventional process) and 309 K (MBR). The bioreactor was connected through a pump with a module for MD.The best results correspond to the following factors ... 

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