PROCESS MONITORING AND CONTROL

The integral part of a high-quality bioreactor is a process controller. Such a controller is commonly specially formed for a definite bioreactor brand. This is rather connected with the fact that microorganism cultivation processes have relatively high requirements in respect to precision and sophistication. All this is despite the fact that almost all bioreactors monitor and regulate the same values actually invariably.

The monitoring and control scheme of a typical fermentation process looks like that:

Usually, the following parameters are monitored and controlled in bioreactors:

Temperature

Temperature is an important parameter of fermentation, since, in the cultivation of many microorganisms, the temperature deviation by a couple of degrees can diminish dramatically the growth and biosynthesis productivity. The cultivation temperature is commonly monitored with an accuracy not less than ± 0.5°C. For temperature measurements, stainless steel Pt100 sensors are normally used. The temperature in laboratory bioreactors is controlled by one of the following ways:

1. A heater is located inside the bioreactor vessel, and cooling is ensured by thin-wall pipes located in the upper cover, which are connected with an electromagnetic valve with the cooling water.
2. Heating and cooling proceed in a thermostat, and this thermostatted water, with the help of a pump, circulates through the bioreactor jacket.

Variant 1 is less complicated, and it ensures a more economic constructive solution. This variant works very well for small bioreactors with the volume up to about 5 litres. Variant 2 ensures a more even distribution of heat throughout the bioreactor volume, which is essential in microorganisms' cultivation.

In the temperature regulation process, the main reason for the regulation inaccuracy are the incorrectly chosen PID parameters. This manifests itself as temperature oscillations.

To regulate the temperature precisely, the main obstacle is often the too high minimal portion of the cooling water. In this connection, the valves in the cooling water supply line should be adjusted correspondingly. Another factor for the regulation accuracy is the area and density of the heat transfer surface, since the higher is inertia, the more difficult is to reach a higher accuracy.

pH

pH control is based on the comparison of the adjusted "set point" and pH real values. For pH measurement, practically only sterilisable electrodes (most often, "Mettler-Tolede" electrodes) are used. The control of pH values is ensured with the help of peristaltic pumps (silicone tubes are commonly used), correspondingly metering out the acid and the alkali. Normally, the "set point" adjustment consists of the lower pHmin and higher pHmax values. If pH is between these values, then no influence occurs. Such an adjustment of the pH "set point" is applied to prevent the overdose of the titration solution. On the other hand, the "narrow" regulation limits of pH are not necessary for the successful course of the cultivation process. It should be mentioned that pH measurements should be accurate (± 0.02 pH units), since the dynamics of pH values' changes provides valuable information on the process kinetics.

pO2 (partial pressure of dissolved oxygen)

One of the most specific aspects of the fermentation monitoring is pO₂ measurement and control. pO₂ control is characteristic only for fermentation processes. There are different pO₂ control principles:

1. Varying the mixer's rotational speed n, assuming that pO₂ ~ n.
2. Combining the change of the mixer's rotation speed n and the amount of the inlet compressed air Q. It is assumed that pO₂ ~ n, pO₂ ~ Q. First of all, n is usually regulated until it reaches one of the limiting values - nmin or nmax, and its regulation is realised by varying Q.
   If n and Q have reached the limiting values, but pO2 is not within the necessary limits, then the regulating effect does not occur.
3. Feeding up the substrate or its any component. It is assumed that pO₂ is proportional to the feeding up intensity. Feeding up is normally realised with controlled peristaltic pumps. This way is sometimes combined with the regulation of the mixer's rotational speed n and the oxygen or air supply flow Q.

In pO₂ regulation, when adjusting the parameters, the following should be taken into account:

1. pO₂ is commonly adjusted in % from the fixed one. The adjusted pO₂ value has a lower and upper limit. The difference between both these limits is usually 10% - 20%.
2. Important parameters in pO₂ control are the control limits of the mixer's rotational speed n: n_min and n_max. It means that, when controlling pO₂, n will vary only within this range. These limits are determined in connection with eliminating of different undesirable phenomena:
1. n_min choice is determined:
   1. to secure the minimal partly turbulent mixing level;
   2. by the guaranteed bubble dispersion;
   3. by the prevented sedimentation.
2. n_max choice is determined by:
   1. setting in of the intensive foaming regime;
   2. irreversible mechanical damages of cells;
   3. liquid surface fluctuation and evaporation.

Foam

The appearance of foam is a very undesirable phenomenon, since, in the course of its appearance, there is a risk to loose an essential part of the fermentation broth. During the foaming, it is not possible to perform high-quality analyses and measurements. For elimination of foam, 2 methods or their combinations are commonly used:

1. Additional metering of an antifoam, based on the information provided by the foam sensor. The given impulses are relatively low, with long pauses and a limited metering time. This additional control is necessary to avoid the possible overdose, since, in this case, the mass exchange parameters can decrease dramatically.
2. Mechanical metering of foam. For this purpose, an upper drive with a special disk-type or other type of the mechanical foam breaking mixer is installed in the bioreactor's upper cover. If an intensive foaming begins, then the mechanical breaking of foam will not help any more.

An optimal solution is the combination of both the parameters. The application of Variant 1 is more widely used in laboratory bioreactors.