Chemostat, control

Fig. 5.3. Schematic diagram of continuous culture with control units in a constant volume chemostat.

Fig. 5.7. Chemostat with inlet and outlet control loops, feed and product pump with cell loading and recycling.

Frost, R. W. and Franzen, N. C. (1992). Grazing and iron limitation in the control of phytoplankton stock and nutrient concentration A chemostat analogue... 

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. 

II. Chemostat In a chemostat, the medium is delivered at a constant rate, to keep constant growth rate. The nutrient medium is supplied to the culture vessel at a constant rate by a peristaltic pump used to control the washout rate. The rate of media flow is often set at approximately 20% of culture volume per day. Air is pumped into the algal culture vessel through an air compressor controlled by a flow meter and carried in two flasks of sterile water. This bubbling air has three effects (i) it supplies C02 and 02 to the culture, (ii) allows circulation and agitation of the cultures and (iii)... 

Thus, 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-... 

In the turbidostat, P and F are kept equal but the dilution rate D is automatically adjusted to a preset cell concentration in the product by continuously measuring its turbidity. Compared to chemostat, turbidostat operation can be more stable in the region near the washout point, but requires more expensive instruments and automatic control systems. 

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 an arrangement for continuous fermentation in which the properties of the system are regulated by a controlled supply of some limiting nutrient. It is a powerful research tool in microbial physiology and for evaluating process parameters. The chemostat... 

Professor Fiechter succeeded to show using the improved chemostat technique that glucose and oxygen influence various yeast stains differently. Beside the catabolite repression (glucose effect) a second regulation type exists which is controlled by the dynamic substrate flux (glucose). This causes different types... 

Most modem pumps are microprocessor controlled, allowing accurate resetting of pump speed from experiment to experiment. However, if adjustment of the flow rate at the start of a chemostat ran is required, it is done by measuring the time (/, sec/drop) between drops coming into the chemostat. Before the experiment, the chemostat volume (V, ml) and the amount of liquid in each drop (d, ml/drop) from the inflow spout are measured. The estimated generation time, g (hr-1), is... 

Chemostat cultures are highly dynamic systems which can be misleading, causing incorrect conclusions without adequate controls and repetition. 

Data from [WiJ - particularly figures 3, 6, 8, 10, 18, 19, and 21 - leave no doubt that, at least for certain populations of algae, individual cell volume varies significantly during the course of experiments in the chemo-stat. These data also suggest that steady-state size distributions are reached which have remarkably stable shapes with respect to changes in the control parameters for the chemostat (flow rates, temperature, CO2). 

Chemosiats are essentially CSTRs that contain microorganisms, A typi- S cal chemostat is shown in Figure 7-16, along with the associated monUoring S equipment and pH controller. One of the most important features of the chemostat is that it allows the operator to control the cell growtli rate. This control of the growth rate is achieved by adjusting the volumetric feetl rate is (dilution rate). 

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