Steady state kinetics chemostats

Microbial kinetics can be quite complex. Multiple steady states are always possible, and oscillatory behavior is common, particularly when there are two or more microbial species in competition. The term chemostat can be quite misleading for a system that oscillates in the absence of a control system. 

Fig. 2 Steady states A (stable) and B (unstable) for a chemostat with inhibition kinetics.

Several special terms are used to describe traditional reaction engineering concepts. Examples include yield coefficients for the generally fermentation environment-dependent stoichiometric coefficients, metabolic network for reaction network, substrate for feed, metabolite for secreted bioreaction products, biomass for cells, broth for the fermenter medium, aeration rate for the rate of air addition, vvm for volumetric airflow rate per broth volume, OUR for 02 uptake rate per broth volume, and CER for C02 evolution rate per broth volume. For continuous fermentation, dilution rate stands for feed or effluent rate (equal at steady state), washout for a condition where the feed rate exceeds the cell growth rate, resulting in washout of cells from the reactor. Section 7 discusses a simple model of a CSTR reactor (called a chemostat) using empirical kinetics. 

The global stoichiometry changes with operating conditions and feed composition kinetics and stoichiometry obtained from steady-state (chemostat) data cannot be used reliably over a wide range of conditions, unless fundamental models are employed. 

A second type of culture described by Monod kinetics is the continuous culture, in which a chemical is constantly fed into a vessel and both microbial cells and the chemical are constantly lost from the vessel at a given rate. This culture is often called a chemostat when operated under steady-state conditions. Like the batch culture, a continuous culture may be a useful model of certain environmental systems, such as lakes receiving continuous discharges of pollutants. Continuous cultures are common in industrial processes as... 

Harrison, P.J., Conway, H.L. and Dugdale, R.C. (1976) Marine diatoms grown in chemostats under silicate or ammonium limitation. I. Cellular chemical composition and steady-state growth kinetics of Skeletonema costatum. Marine Biology, 35, 177-186. 

Assume that you obtained the Cg versus t curve you calculated in part (a) experimentally. Estimate K/ and by plotting the (Cg - Cs)/ln(Cg /Cg) versus f/ln(Cgj,/Cg) curve according to Eq. (2.38). Is tnis approach reliable Chemostat (continuously stirred-tank reactor) runs with various flow rates were carried out. If the inlet substrate concentration is 300 mol/m and the flow rate is 100 cm / min, what is the steady-state substrate concentration of the outlet The reactor volume is 300 cm. Assume that the enzyme concentration in the reactor is constant so that the same kinetic parameters can be used. 

The most import consideration is the design of the reactor that is used to collect kinetic data. A CSTR (called a chemostat in the biological field) is ideal because a significant volume is at a uniform composition and temperature and gives a significant extent of reaction so that chemical analysis is easy. Operation at steady state allows measurements to be repeated, further improving experimental accuracy. 

Figure 6 compares the experimentally measured metabolite profiles resulting from the oxidation of a pulse of [ C]-indene by steady state chemostat cells with the kinetic profiles predicted by Eqs. (7)-(12) using flux values independently determined for the same steady state. The excellent agreement between the actual tracer data and the predicted oxidation profiles provides an additional validation of the fluxes calculated for the KYI network. 

Fig. 6. Comparison of kinetic model predictions with experimental measurements of C-in-dene metabolites for Rhodococcus KYI cells obtained from a chemostat at steady state obtained with a dilution rate of 0.065 h and 100 ppm indene air feed concentration. Reaction rate constants used in the kinetic model were determined from flux estimates as described in the text...

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