SHOULD WE GO "BATCH" "FED BATCH" OR "CONTINUOUS"???

The mode of feeding the bioreactors determine whether a bioreactor or fermentor is classified as batch, fed batch or continuous bioreactor. In a way it is really difficult to define strictly if the bioreactor is 'batch' 'fed batch' or 'continuous' bioreactor just based on the feeding mode

BATCH REACTOR
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If the bioreactor is only fed once, and no more feeding after that we say that the bioreactor is operating on batch mode. The bioreactor will be allowed to run till completion. At the end of the rinthe fermentation is terminated before a new fermentation is initiated again.

In the real situation this is difficult to achieve as a true batch reactor since in a batch reactor there must not be any input or withdrawal from the bioreactor until the fermentation is completed. In most aerobic batch reactor air or oxygen is continually sparged in or the addition of some other 'minor nutrients'. If we do not supply oxygen into the aerobic bioreactor then the bioreactor will turn anaerobic and that will be the end of the aerobic batch bioreactor

There should not also be any removals of samples from the bioreactor throughout the fermentation run as in the case of off line monitoring and sampling

CONTINUOUS BIOREACTOR
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On the other hand, if the bioreactor is fed continuously then the bioreactor is termed a continuous bioreactor. Under such situation if the continuous bioreactor is a fixed volume bioreactor, the amount of feed introduced into the bioreactor is equivalent to the spent volume removed from the bioreactor.

Despite the ideal characteristics of the continuous bioreactor, the process itself is sensitive and subjected to influence from various factors such as risks of contaminations, cell or biomass washouts and changes in the biotic phase of the bioreactor


FED BATCH BIOREACTOR
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Fed Batch is the intermediary model of bioreactor operation. This fed batch mode is characterised by predetermined or controlled addition of nutrients into the bioreactor at certain times of fermentor operations. This mode of feeding nutrients into the fed batch bioreactor will allow temporal variations in the supply of nutrients. The beautiful thing about fed batch operation of a bioreactor it allows us a degree of control on the process and operations of the fed batch bioreactor. We can control the rate of growth of the microorganisms or the concentration of the biomass by controlling parameters such as frequency of feeds, hydraulic loadings and concentrations of feed.

It is used in industry to maintain high concentration of microorganisms and reduce feedback inhibition especially catabolite repression. Fed batch is the most popular mode in fermentation industries



THE RETENTION OF MICROORGANISMS IN BIOREACTORS
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In all three types of bioreactors discussed above, what is important and not discussed yet is the retention of the microorganisms in the bioreactors. The retention of the microorganisms in the bioreactors is the crucial factor in determining how successful the bioreactors will be.

In Batch bioreactor, it is very conducive to the bioreactors because:
1 The microorganisms are not lost throughout the fermentation
2 The microorganisms have time to adapt and multiply to its maximum rate in the batch bioreactor
3 The microorganisms will give a complete biotransformations of the substrate to products provided other nutrient and environmental parameters are sustained

The only negative thing about the microorganisms is that sooner or later it will enter the stationary phase either due to limiting nutrients or accumulation of toxic products

In Continuous bioreactor where there is continuous input of nutrients and withdrawal of spent liquor, there is always the problem of:

1 Continuous wash aways of microorganisms, raw feed and spent liquor.
2 Microorganisms need always to continually adapt to the input of nutrients

The only way to eliminate this if the microorganisms are either immobilized or recycled or returned

Fed Batch bioreactors still faces the intermittent input of raw feed and wastage of cells, nutrients and fermentation products. However the advantage is that the microorganisms are always maintained in a high and healthy state

FEED DOSING OF THE BIOREACTORS
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Many forget that the stability of the composition of the microbial population depends on:
1 The impact of the changes
2 The rapidity of the microorganisms to adjust to the changes forced upon the microorganisms

In this sense, the concentration, stable composition of the feed and how the feed is introduced will affect the performance of the bioreactor. This issue will become critical if we are dealing with:
1 Very sensitive and fastidious microorganisms
2 Very slow growers
3 Very small bioreactors
4 Bioreactors operated on very short retention time