THE FINE ART OF CONTROLLING AERATION IN FERMENTOR

aer
Aeration is very costly but a necessary component in operating aerobic bioreactors, You need to supply oxygen continuously to maintain the supply of oxygen to the aerobic microorganisms due to the limited solubility of air under normal temperature and pressure. The question is:

1) how are we going to supply enough oxygen on demand?
2)And how to supply oxygen efficiently and economically?

There are usually four basic methods used in controlling the supply of air to fermentors:
1) Controlling the air flow rate
2) Controlling the speed of the stirrer
3) Controlling the air pressure
4) Controlling the composition of the air


CONTROLLING THE AIR FLOW
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Is basically controlling the volume and supply of the air to the fermentation broth. Bigger flow would simply mean more air for the microorganisms

CONTROLLING THE SPEED OF STIRRER
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More speed of impeller would mean more mixing, dispersion of the existing supp;y of air. More turbulence would be generated resulting in increasing shear forces and increase in rate of mass transfer of oxygen from environment to microorganisms

There is the danger that increasing the turbulence would increase the damage to the microbial cells or microbial aggregates which can be detrimental to the physiology of the microorganisms thus affecting the fermentation process itself.

INCREASING THE PRESSURE
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Is tantamount to increasing the number of air molecules per volume of air. This increases the concentration of oxygen molecules thus increasing the diffusion process. Increasing the pressure however would lead to increasing turbulence in the broth and nore physical damages if not regulated properly

CHANGING COMPOSITION
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In this method pure oxygen or highly enriched air would increase the composition of oxygen thus increasing the rate of oxygen diffusion into the broth


These steps are only possible theoretically and significantly in an ideal and theoretical system when the system is in the need of oxygen and capable of accepting the oxygen. If we can picture it out in our mind that the rate of oxygen demand and accommodation is in the form of sigmoid curve, where there is a steady state tolerance or saturation value of dissolved oxygen, there is very little which we can do to significantly improve the rate of oxygen supply to the system.

Despite these impending setbacks and limitations of improving the supply of oxygen, it is still necessary to keep on supplying the oxygen to the fermentation, even though knowing most of the invested air will be wasted and returned to air

There are many factors which control the rate of oxygen transfer and utilization by the fermentation process, among which are:
1) Continuously changing rheology of the fermentation broth with time
2) The number and physiology of the microorganisms with time
3) Environmental parameters under which the fermentation system is operating.

Another key point is the sharp increase in cost to sustain a slight increase in oxygen supply to the microorganisms. Is it worth the effort?

METHODS OF CONTROL RECOMMENDED
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Traditionally the better combination in increasing the or controlling the supply pf air is maintaining the lower level of air flow rate into the fermentor but increasing the rate of oxygen supply by varying the speed of the stirrer. This will lead to fine tuning and control of the oxygen supply to the microorganisms

For sudden transient needs of high oxygen such as to accommodate the high oxygen demand during the log phase is to supply additional air through pure oxygen injection through another independent outlet into the fermentor