The Chemostat

The ehemostat is an arrangement for eontinuous fermentation in whieh the properties of the system are regulated by a eontrolled supply of some limiting nutrient. It is a powerful researeh tool in mierobial physiology and for evaluating proeess parameters. The ehemostat 

BIOCHEMICAL UNIT MAIN PROCESS DATA LINE PHYSIOLOGICAL 

Evaluation of the consistency of relevant experimental results. Stoichiometric constant. Mass and energy balances. 

Expression of the mass balances around the bioreactor (process). 

Determining tlie nature of the physiological term in the model. 

Graphical evaluation of relationships between the selected key process, rates and parameters. 

Mathematical expression of the culture physiological state using the marker . 

The term chemostat refers to a tank fermentation which is operated continuously. This bioreactor mode of operation normally involves sterile feed (Xo=0), constant volume and steady state conditions, meaning that dV/dt=0, d(VSd/ dt=0, d VX1)/dt=0. 

For constant density the total mass balance simplifies to 

Inserting the Monod-type rate expressions gives For the cell balance 

Here D is the dilution rate and is equal to 1/t, where r = V/F and is equal to the mean residence time of the tank. 

the specific growth rate in a chemostat is controlled by the feed flow rate, since // is equal to D at steady state conditions. Since ft, the specific growth rate, is a function of the substrate concentration, and since fi is also determined by dilution rate, then the flow rate F also determines the outlet substrate concentration S. The last equation is, of course, simply a statement that the quantity of cells produced is proportional to the quantity of substrate consumed, as related by the yield factor Yx/s- 

0 = — F X + rx V Inserting the Monod-type rate expressions, gives For the cell balance  

the specific growth rate in a chemostat is controlled by the feed flow rate, since p is equal to D at steady state conditions. Since p, the specific growth rate, is a function of the substrate concentration, and since p is also determined by 

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The ICR flow rate was five to eight times faster than the CSTR. The overall conversion of sugars in the ICR at a 12 hour retention time was 60%, At this retention time, the ICR was eight times faster than CSTR, but in the CSTR an overall conversion rate of 89% was obtained. At the washout rate for the chemostat, the ICR resulted in a 38% conversion of total sugars. Also, the organic acid production rate in the ICR was about four times that of the CSTR. At a higher retention time of 28 hours, the conversion of glucose in the ICR and CSTR are about the same, but the conversion of xylose reached 75% in the ICR and 86% in the CSTR. 

We wish to compare the performance of two reactor types plug flow versus CSTR with a substrate concentration of Csf = 60g-m 3 and a biomass yield of Y = 0.1. In a plug flow bioreactor with volume of 1 m3 and volumetric flow rate of 2.5 m -li what would be the recycle ratio for maximum qx compared with corresponding results and rate models proposed for the chemostat ... 

The performance data for plug versus mix reactor were obtained. The data were collected as the inverse of qx vs inverse of substrate concentration. Table E.1.1 shows the data based on obtained kinetic data. From the data plotted in Figure E.1.1, we can minimise the volume of the chemostat. A CSTR works better than a plug flow reactor for the production of biomass. Maximum qx is obtained with a substrate concentration in the leaving stream of 12g m 3. 

Continuous Stirred Tanks Without Biomass Recycle. The chemostat without biomass recycle is a classic CSTR. The reactor is started in the batch mode. 

In the above ODEs, X] and x2 represent the biomass and substrate concentration in the chemostat, cF is the substrate concentration in the feed stream (g/L) and D is the dilution factor (h 1) defined as the feed flowrate over the volume of the liquid phase in the chemostat. It is assumed that both state variables, xt and x2 are observed. 

It was specifically stated that the proposed metabolic pathway, which suggest C—N bond cleaving and so, a N-specific mechanism, was found when PTA-806 was employed as the biocatalyst. However, when the quinoline-adapted microorganisms, initially isolated from the chemostats (the native P. ayucida), were tested, they were found to fully degrade quinoline, utilizing it as both, a carbon as well as a nitrogen source. 

Hellio, G.B., Bremer A.M., Pons, G., Cottenceau, Y. and Le Gal, N. (2000). Antibacterial and antifungal activities of extracts of marine algae from Brittany France. Use as antifouling agents. Applied Microbiology Biotechnology 54 543-549. Hsu, S.B. and Waltman, P. (1998). Competition in the chemostat when one competitor produces a toxin. Japan Journal of Industrial and Applied Mathematics 15 471-490. 

Novicik, A. and Szilard, L. (1950). Experiments with the chemostat on spontaneous mutations of bacteria. Proceedings National Academy of Sciences USA 36 708-719. 

What is the effect of operating the chemostat at a dilution rate of D>(um ... 

Operate the chemostat initially as a batch reactor with D = 0, and then switch to chemostat operation with D [Pg.541]

Many reviews and several books [61,62] have appeared on the theoretical and experimental aspects of the continuous, stirred tank reactor - the so-called chemostat. Properties of the chemostat are not discussed here. The concentrations of the reagents and products can not be calculated by the algebraic equations obtained for steady-state conditions, when ji = D (the left-hand sides of Eqs. 27-29 are equal to zero), because of the double-substrate-limitation model (Eq. 26) used. These values were obtained from the time course of the concentrations obtained by simulation of the fermentation. It was assumed that the dispersed organic phase remains in the reactor and the dispersed phase holdup does not change during the process. The inlet liquid phase does not contain either organic phase or biomass. 

There are two different ways of operating continuous stirred-tank fermentors (CSTFs), namely chemostat and turbidostal. In the chemostat, the flow rate of the... 

In the chemostat, the dilution rate is set at a fixed value, and the rate of cell growth then adjusts itself to the set dilution rate. This type of operation is relatively easy to carry out, but becomes unstable in the region near the washout point. 

There are certain circumstances when it may be desirable to operate a series of stirred-tanks in cascade, with the effluent of one forming the feed to the next in the chain. In the case of two such fermenters the first will behave in the manner of a simple chemostat and the performance equations of the chemostat will apply. The second tank, however, will not have a sterile feed so that some of the simplifications which led to those relationships will not be valid. 

Moser, H. Carnegie Institute Publ No. 614 (1958). The dynamics of bacterial populations in the chemostat. 

Wilson, G., "Continuous Culture of Plant Cells Using the Chemostat Principle," in Advances in Biochemical Engineering, Vol. 16, Ed. A. Fiechter, New York Springer-Verlag, 1980, pp. 1-25. 

Furthermore, since most large-scale fermentations are carried out in batch mode, the kinetic parameters determined by the chemostat study should be able to predict the growth in a batch fermenter. However, due to the significantly different environments of batch and continuous fermenters, the kinetic model developed from the CSTF runs may fail to predict the growth behavior of a batch fermenter. Nevertheless, the verification of a kinetic model and the evaluation of kinetic parameters by running chemostat is the most reliable method because of its constant medium environment. 

Aiba et al. (1968) reported the results of a chemostat study on the growth of a specific strain of baker s yeast as shown in the following table. The inlet stream of the chemostat did not contain any cells or products. 

The chemostat is a biological CSTR where the substrate concentration in the tank is maintained constant. The tur-bidostat is similar to the chemostat except that the cell mass in the reactor is kept constant. The primary distinction between the two reactors is the control mechanism used to maintain continuous operation. A unique feature of a biological CSTR is the washout point. When the flow rate is increased so that the microbes can no longer reproduce fast enough to maintain a population, the microbes wash out of the tank, and the reaction ceases. This washout point represents the limits of maximum flow rate for operation. 

The chemostat is a biological heterogeneous CSTR. The microbes are considered a solid phase, and for aerobic... 

A consequence of applying Equations 11-68 and 11-69 respectively in describing the chemostat, is that the maximum dilution rate is limited to a value that is slightly less than the maximum specific growth rate iniax. This is referred to as the critical dilution rate Dc. 

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