Surface-active substances such as antifoam agents reduce the oxygen transfer rate. In pure water, the bubble surface is constantly renewed through vibration and oscillation. As soon as surface-active substances are added, the renewal of the bubble surface by bubble movement ceases.
Microorganisms themselves have an effect on the oxygen transfer rate by acting as a
barrier, thus inhibiting the
The gas bubbles are replenished in locations of the bioreactor where there is negative pressure, such as behind the agitator blades. As the aeration rate increases, various conditions can be characterized. At low aeration rates, large gas bubbles form behind individual turbine blades and smaller bubbles are spun off centrifugally into the nutrient solution. As the aeration rate is increased, gass bubbles collect behind all turbine blades and continue to accumulate. The energy input is one-third less than that used in unaerated systems. In this intermediate stirring range, gas dispersion is the best. At very high aeration rates, many large gass bubbles adhere to each other and the impeller is flooded with gas, resulting in sharply lowered gas dispersion.
The critical oxygen concentration is the term used to indicate the value of the oxygen
uptake rate or oxygen absorption rate which permits respiration without hindrance.
Generally the critical oxygen concentrations