Membrane fermentors

Lactic acid is one of the major organic acids produced by fermentation. AimuaUy about 35 000 tonnes are produced this way [56]. The work on NF-based membrane fermentors for the production of organic acids has been started with the observation that lactic acid retention of RO membranes shows a strong pH-depen-dence [57-59]. For selective removal of lactic acid a low pFI is necessary, however, this reduces the lactic acid productivity to about 1 g Compared with UF-... 

Rautenbach and MeUis [75] describe a process in which a UF-membrane fermentor and a subsequent NF-treatment of the UF-permeate are integrated. The retentate of the NF-step is recycled to the feed of the UF-membrane reactor (Fig. 13.8). This process has been commercialised by Wehrle-Werk AG as the Biomembrat -plus system [76] and is well suited for the treatment of effluents with recalcitrant components. The process also allows for an additional treatment process, like adsorption or chemical oxidation of the NF-retentate, before returning the NF-retentate to the feed of the UF-membrane fermentor. Usually, the efficiency of these treatment processes is increased as the NF-retentate contains higher concentrations of these components. Pilot tests with landfiU leachates [75] and wastewater from cotton textile and tannery industry have been reported [77]. An overview of chemical oxygen demand (COD) reduction and COD concentrations in the permeate are shown in... 

A.E. Abasaeed, S. Elnashaie, A novel configuration for packed bed membrane fermentors for maximizing ethanol productivity and concentration, J. Membrane Science, 82(1-2), 75-82, 1993... 

A. Mahecha-Botero, P. Garhyan, S. Elnashaie, Bifurcation, stabilization, and ethanol productivity enhancement for a membrane fermentor, Mathematical and Computer Mod-ellmg, 41 (2005), 391-406... 

Drioli E., lorio G. and Catapano G., Enzyme membrane reactors and membrane fermentors, in Handbook of Industrial Membrane Technology, M.C. Porter, ed., Noyes, Park Ridge, NJ, (1990), p. 401. 

Figure 2.63 Flow schematic of continuous membrane fermentor.

Membrane Fermentors. Many of the advantages of enzyme membrane reactors are applicable to fermentors. In some cases, a fermentor is simply an enzyme reactor using intracellular enzymes rather than extracellular enzymes. Enzymes are usually more stable within the cell wall and the living cell can regenerate itself along with the enzyme. A possible disadvantage is that the enzyme activity per unit reactor volume may be less in the cells than that of a pure enzyme preparation. 

There are two primary types of membrane fermentors. These are discussed more thoroughly in Chapter 7. 

Mobile Cells. Figure 3.10275 shows one of the earliest concepts of a continuous membrane fermentor. The membrane retains the biomass while products of the fermentation are continuously withdrawn through the membrane. In addition, metabolic waste products, which inhibit cell growth, are removed continuously through the membrane, thereby improving cell growth and fermentor productivity. 

In the stationary phase of the batch fermentation, enzyme activity begins to decrease due to an increasingly unfavorable environment. In the membrane fermentor, protease (the enzyme) continued to be excreted by the growing biomass for 50 hours. The ratio of enzyme excreted per unit mass of cell produced in the membrane system (70 units/mg) was almost twice that from the batch system (45 units/mg). This would indicate that under the more favorable environment in the membrane fermentor, the organism is more efficient. 

Other continuous membrane fermentors show similar results. In the continuous production of ethanol from Zymomonas mobilis, the production rate was 120 g/hr/E compared to less than 8 g/hr/E for the batch system.77... 

Last but not least, viable cells are grown and used in membrane fermentors for a wide range of applications.84-98... 

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