Recycle Fermentors

Combining the two equations, the cell concentration within the vessel is 117 

The cell concentration within the vessel of a cell recycle fermentor is then greater by a factor of 1/(1 + a - Ca ). 

The performance and capacity of an anaerobic reactor can be expressed in terms of two parameters the solid retention time (SRT), and the reactor volume loading (VL). 

The SRT is the average retention time of organisms in the reaction vessel. For a suspended growth system, SRT is usually defined as the ratio of volatile suspended solids (VSS) in the reactor to the VSS lost in the effluent or intentionally wasted per day.88 

The specific growth rate (SGR) is equal to the reciprocal of the SRT it can be expressed as a linear function of the specific substrate utilization rate, KS, to give 88 

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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. 

Tros, M T. N. P. Bosma, G. Schraa, and A. J. B. Zehnder, Measurement of minimum substrate concentration (Smin) in a recycling fermentor and its prediction from the kinetic parameters of Pseudomonas sp. strain B13 from batch and chemostat cultures , Appl. Environ. Microbiol., 62,3655-3661 (1996). 

Figure 13.18 Continuous recycle fermentor membrane reactor. An ultrafiltration module removes the liquid products of fermentation as a clean product. This system is being developed for production of ethanol, acetone and butanol by fermentation of food processing waste streams...

The final system, shown in Figure 30.4D, is the continuous system with a partial (PRF) or complete (RF) cell recycle. It is similar to the continuous system, but cells are returned to the fermentor by means of a biomass separation device. Cross-filtration units, centrifuges, and settling tanks have all been used for biomass separation.22 In the partial cell recycle fermentor, a steady state is achieved as in the continuous system. This process is typically used to increase the productivity of the system and is used commonly in wastewater treatment and ethanol production type applications. 

Biomass recycle as sketched in Figure 30.4D is frequently used in fermentors as a way of increasing the biomass productivity. The increased biomass productivity obtained with a recycle fermentor is a function of the recycle ratio, r, and the cell concentration factor, C = XJX, achieved in the concentrator. Equations expressing the recycle system behavior are derived from material balances around the reactor. For the cell biomass balance at steady state ... 

Because (1 + r — rC) < 1, it is possible to operate the system at dilution rates greater than the maximum growth rate. It is this stability imparted by the cell recycle fermentor system that makes it useful, especially in waste treatment applications. 

Cell recycle fermentors consist of two main units a vessel where the biomass is allowed to grow, and a membrane separation unit (as in Figure 7.40). Vessels are usually designed to insure a uniform concentration of nutrients and pH throughout the whole volume. Due to complete mixing, process control and stability of the microbial slurry are not difficult to achieve.88 After anaerobic stabilization, when the biomass is well developed, the reactor biomass is pumped to the UF unit where solid-liquid separation occurs. The sludge is flushed back to the reactor. In most cases, the flow rate of nutrient feed is kept equal to the permeate flow rate thus keeping a constant liquid level in the anaerobic reactor. 

Higher cell concentrations are usually achieved in cell recycle fermentors than in the usual fermentor. Steady state mass balances on viable cells and on the limiting growth substrates for a continuous fermentor (Figure 7.43) can be written as 117... 

A steady state mass balance on a continuous cell recycle fermentor, over the vessel alone is as follows (see Figure 7.40) ... 

Figure 7.44 Fermentation kinetics of a membrane recycle fermentor. Feed concentration = 150 g/C lactose. Cell concentration = 90 g/C.89...

With regard to the fermenters, the most common configurations are membrane recycle fermentor (MRF) and hollow fibre fermentor (HFF). In the MRF the membrane module forms a semi-closed loop with a conventional fermentation vessel the MRF gives much better performance than the HFF, where the microbial cells are loaded onto the shell-side and the feed is pumped through the lumen side. Further advantages of this system are a cell/particulate-free product stream and the reduction of capital costs. Furthermore, in these systems cell growth is a caitical point. 

FIGURE 12.6 Continuous fermentor with recycle of live cells. 

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... 

It may also be economical to remove the inhibitory product directly from the ongoing fermentation by extraction, membranes, or sorption. The use of sorption with simultaneous fermentation and separation for succinic acid has not been investigated. Separation has been used to enhance other organic acid fermentations through in situ separation or separation from a recycled side stream. Solid sorbents have been added directly to batch fermentations (18,19). Seevarantnam et al. (20) tested a sorbent in the solvent phase to enhance recovery of lactic acid from free cell batch culture. A sorption column was also used to remove lactate from a recycled side stream in a free-cell continuously stirred tank reactor (21). Continuous sorption for in situ separation in a biparticle fermentor was successful in enhancing the production of lactic acid (16,22). Recovery in this system was tested with hot water (16). 

A column fermentor is a fixed-bed reactor. Solid medium is put into the column with entries at both ends for aeration [58-60]. Figure 5 shows an experimental apparatus developed by Saucedo-Castaneda et al. [28]. Sterile air can be supplied either by a radial or axial gradient method. The water activity is maintained by humidified air. The temperature for the solid fermentation is monitored and controlled by recycling water in the jacket from an isothermal bath. In a fixed-bed column fermentor, the oxygen transfer and CO2 dissipation are im-... 

Damiano, D., Wang, S. S. (1985). Improvements in ethanol concentration and fermentor ethanol productivity in yeast fermentations using whole soy flour in batch, and continuous recycle systems. Biotechnology Letters 7 135-140. 

From the data and from the experimental procedure described in the patent the overall distribution coefficient based on total acetate is calculated as 0.004. This is an extremely small number, which, of course, is a consequence of the small amount of free acid available in solution at the high pFI of the solution. Other authors (Yates, 1981 Busche, Shimshick, and Yates, 1982) address the problem of incomplete extraction of organic acids from high-pH solutions using CO2. The authors suggest that the raffinate from the extractor can be recycled to the fermentor. 

Figure 7.44 shows the typical dependence at steady state of substrate and product concentrations, and of productivity on permeate flow rate, i.e., dilution rate, for lactose fermentation to ethanol by Kluyveromices fragilis in a cell recycle membrane fermentor.89 As dilution rate increases fermentor productivity increases, attains a maximum value and then decreases. Product and substrate concentrations in the permeate, instead, steadily decrease and increase, respectively, as dilution rate increases. A compromise generally has to be made between production rate and product concentration in the effluent. When the absence of substrate in the permeate is required, it obviously limits fermentor productivity, as in the case of wastewater treatment in the dairy industry. On the other hand, low substrate concentrations in the permeate keep recovery... 

Kluyveromices fragilis. The behavior of the fermentor is similar to that of a cell recycle reactor with the same fermentation system. Higher productivity and yields than with a batch fermentation are obtained (Tables 7.1, 7.2). The long term cell stability in an HFF (Figure 7.46) is also better than in a batch fermentor. However, even with HFF, reactor productivity is increased at the expense of low substrate conversions, at least at low dilution rates. 

What has been previously said about the advantages of continuous membrane fermentors also applies to such complex systems. UF membranes can in fact be used to recycle both enzymes and microbial cells thus increasing overall system productivity.98... 

A cross-flow nanofillration module (SEPA CFII, GE Osmonics, Miime lis, MN) was used for this process with a maximum operating pressure of 7.0 MPa. The sur ce area of the membrane is 140 cm. The holdup volume of the membrane unit is 70 mL. The fermentation broth was placed in a 5-L fermentation vessel to control the temperature, agitation, and pH. A bench-top pump (M03-S, Hydra cell, MinneapoUs, MN) was used to pump the fermentation broth through the cross-flow membrane separation unit and recycle back to the fermentor (Fig. 2). The permeate was collected on a digital balance attached to a laptop computer with a RS-COM version 2.40 system (A D, Milpitas, CA) that recorded the amount of permeate collected every 0.5 min. The fermentation brofli was kept at constant temperature (37 °C), pH (5.5), and agitation (200 rpm). Transmembrane pressures of 1.4, 2.1, and 2.8 MPa were used in the nanofiltration tests. Each condition was tested twice, and each test lasted for 2 h. Samples of the original broth (before separation), permeate, and letentate were collected for analysis. 

Fig. 24.9. Single-stage continuous fermentor with recycle.

For example, different fermentation schemes have been developed for the production of ethanol. Conventional batch, continuous, cell recycle and immobilized cell processes, as well as membrane, extraction and vacuum processes, which selectively remove ethanol from the fermentation medium as it is formed, were compared on identical bases using a consistent model for yeast metabolism (Maiorella et al., 1984). The continuous flow stirred tank reactor (CSTR) with cell recycle, tower and plug flow reactors all showed similar cost savings of about 10% compared to batch fermentation. Cell recycle increases cell density inside the fermentor, which is important in reducing fermentation cost. 

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