IMPORTANCE OF FERMENTOR DESIGN IN MINIMIZING ENERGY CONSUMPTION AND INCREASING YIELD

The main function of the fermentor tank is to hold the volume fermentation broth. In fact any vessels or container can be used to carry out the fermentation process. Fermentors differed from any container by allowing optimal fermentation process to occur through design and control

It is almost unbelievable that the concept of CSTR still dominates the day, years after its inception during penicillin fermentation. Only lately have attempts been made to come up with fermentors with novel or new designs.

The change towards new design fermentors is more to:
1 New fermentation products
2 Shift from high volume low value products to low volume and high value products

Any changes in the fermentor design must be with the purpose of improving the efficiency of the fermentation process in terms of yield and productivity or increasing its mass transfers and lowering energy consumption

In the old days it seems that almost any product fermentation is just forced into the CSTR. This was the mental block that hover over most engineers and fermentation technologists that one fermentor can be used for all types of fermentation!. It just doesn’t make sense!

Nowadays the design of the fermentor is more to accommodate the demands of the type of fermentation process. It seems in the past the engineers building or designing the fermentors are not able to think outside the box. The rigidity of their thinking is as rigid as the fermentor design

The fermentation process is considered complex with physical chemical and microbiological continually changing. With this in mind the design of fermentors must always be responsive. The fermentor design must not only be looked upon as fixed unit processes undergo by the fermentor such as standard stirrer specifications, aeration and even the sterilizing specifications. It is doubtful one standard design fermentor can bring out the best for a variety of fermentation carried out in the lifetime of the fermentor.

So, it is back to the drawing board in the design of the fermentors and a compulsory re education in fermentation technology for those involved in the design of fermentors. Reengineering and novel engineering is a must for the future of fermentors and fermentation.

The trouble with the design of fermentors in the past is that there is too much emphasis on engineering and math with poor understanding of the fermentation process. Sheets and sheets of the blueprint is the testimony to this

It is recommended that:

1 Understand the fermentation process first
2 Determine the rate limiting steps of the fermentation process
3 Understand the characteristics of the fermentation broth
4 Carry out feasibility lab studies before the design is accepted
5 Determine the flow and mixing pattern of the fermentor before determining the final optimal geometry
6 And other relevant studies

The key performance index in all the designs of the fermentors is cost. One of the major liabilities is energy. A fermentor is not a dead or innate structure metaphorically speaking. It consumes and even generates energy depending on the situation. Energy costs may not be a problem if you are dealing with small fermentors or working in government fermentation laboratories. But in industries, the energy costs could be very prohibitive and will add cost to the final fermentation products

If we analyse the energy consumption in a typical fermentor, most of the energy requirements are for:

1 Aeration
2 Stirring
3 Heating and cooling activities

Aeration is necessary in any aerobic fermentation process. The amount of aeration should reflect the oxygen demand of the fermentation process at that temporal and spatial status. What is important is that the aeration process should always be able to provide enough oxygen in the narrow window during the fermentation process. Providing more not only add up to the energy costs but might even complicate the fermentation process. Insufficient aeration might result in certain zones of the fermentor being deprived of oxygen and affect the fermentation process

In deciding the aeration requirements we have to consider many factors such as:
1 Type of aerators or spargers used
2 Location of spargers within the fermentors
3 Demand of oxygen as function of time of fermentation progress
4 Presence of mixers or stirrers in fermentors
5 Feed back loops to stirrer and aerator volumetric rate

Easier said than done. There is really no such thing of using the optimal aeration conditions of one fermentation process to be used on another. No two fermentations are the same. The optimal aeration conditions for fermentation have to be individually determined. This itself is the problem of having customized designed fermentor as it might only be suitable for one type of fermentation and not another

It is a known fact that experience in fermentation beats all the formal knowledge or information from books.

The choice of aerator is important and will decide whether the aeration costs will be higher or lower. It has been shown that aeration by air compression is only about 40% efficient in terms of electrical energy while agitation by turbine is about 90% efficient. Surprisingly in my experience as consultant on a trouble shooting job I have seen many clients are convinced by very qualified engineers who proposed air compression. Poor clients!

Use of sensors n feed back lops important. We need a reactive fermentor to detect changes and operate within narrow window of process optimization

Whole objective of stirring to homogenize or keep broth in suspension. How powerful we stir are often determined by the nature of the rheology of the fermentation broth. Mixing and aeration should be interplayed where a compromise is made between keeping the components of the broth in suspension yet still keeping the minimal amount of dissolved oxygen concentration enough to support the needs of the microorganisms. Excessive stirring may not only be demanding on the energy but also damaging to the microorganisms


Type rest of the post here.
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GAS HOLDUPS IN FERMENTOR

The behaviour of gases in fermentors is not simple. One could not simply visualize the behaviour of gases in the broth merely as the formation of gas bubbles that once formed will simply rises to the surface of the broth where it just collapse in the head space. The behaviour of gas bubbles in the fermentor is complex and one of the main problems faced in any fermentation studies is the problem of gas hold ups.

Gas hold up could be defined as the total volume of gas bubbles in the liquid at any instant during the gas-liquid contact processes. The gas bubbles constituting the gas holdups could be large or even microscopic bubbles. In prolonged cases of gas hold ups in fermentor there is the tendency for all the gas bubbles to come together or coalesce to form a very big bubble giving appearance of a static void within the fermentation broth


Inadvertently gas hold up will lead to the increase in total liquid volume of the broth in the fermentor compared to ungassed broth.

Gas hold up occurs either in situations such as:
1 Failure for the gas to rise up and explode into the head space of the fermentor
2 The bubbles are to small and recirculated in the fermentation broth for sometime
3 The gas bubbles are trapped in foams
4 Overgassing resulting in broth enriched with gas bubbles
5 The fermentation broth is too viscous that exert the pressure on the gas bubbles from rising

In theory having lots of gas bubbles is good because gas represents reservoirs of air or air carriers. A lot of gas means a lot of air supply ready to diffuse out into the broth. But the real situation is not really as imagined.

There are many factors which affect mass transfers of oxygen from the gas bubble to the broth.
A big bubble held up in the broth is not effective in the transfer of oxygen to the environment. A thick coating of foam or surfactant will reduce the efficiency of mass transfer of oxygen to the environment.

A stagnant bubble in the broth would even be anoxic as oxygen would have been depleted by the microorganisms and it will become toxic pockets instead

Gas hold up might even increase the chance of contamination especially as foams in the headspace.

Gas hold ups in the form of foam formation at the surface of the broth will lead to:
1 Separation of microbial cells from the broth
2 Death and lyses of the cells in the foam
3 Separation of other solids and nutrients from the broth
4 Poor control of mass transfers
5 Growth on walls
6 Poor ph control










REDUCING OR PREVENTING GAS HOLD UPS
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There are a few possible ways of preventing or reducing gas hold ups. Some of these measures may not be economical or may even interfere with the fermentation process over a long run.

1 Breaking the gas bubbles physically by physical foam breakers
2 Adding anti foam to break the foam
3 Adjusting fermentation conditions











to be continued..... Read more!

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
---------------------------------

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
--------------------------------
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
--------------------------------------------

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 Read more!

AERATION OF AEROBIC BIOLOGICAL WASTEWATER TREATMENT PLANT

INTRODUCTION

(PICTURE TAKEN FROM WWW.WASTEWATER.COM)

Aerobic biological wastewater treatment is one of the most popular options in wastewater treatment. Biological treatment usually constitutes the secondary treatment stage in a conventional wastewater treatment plant. However, despite its widespread use and popularity of choice, the aerobic biological wastewater treatment plant is often befuddled by many problems leading to inefficient treatment process.

One of the main causes of failure in aerobic biological wastewater treatment plant is due to the poor understanding of the basic processes involved. This often results in the poor design and operation of the aerobic biological wastewater treatment plant.

In this article, we will discuss the various problems associated with the aeration processes in aerobic biological wastewater treatment plant.

Aeration process of the wastewater treatment plant should not be looked simply as just pumping of air into the wastewater treatment plant. Poor understanding of the aeration requirements have often resulted in plant failures and economically inefficient operation.

It is a common knowledge that most plant operators seemed satisfied as long as they see large bubbles of air generated in their treatment plants. Treatment plant operators still could not understand why their treatment plants failed despite being continuously aerated. Most plant operators do not even know that their treatment plants are in fact not fully operating under aerobic conditions.

Poor aeration management of treatment plant is often cited as one of the main causes of treatment plant failure. Problems generated by aeration failure in biological wastewater treatment plants range from emission of foul odours to failure to comply to effluent discharge standards.

There are no serious attempts to optimize or monitor the aeration processes, unless the aeration system is out of order. Aeration of wastewater treatment plant has always been considered as one of the costliest components in treatment plant operations and yet very little attempts are made to optimize the process and make it cost efficient.

This scenario occurs because of the lack of understanding and the impact of the aeration processes on biological wastewater treatment.

AEROBIC BIOLOGICAL WASTEWATER REACTORS

For the waste treatment to be effective, the reactions will have to be carried out by high concentration of microorganisms. This is only possible if the microorganisms are cultivated in a wastewater bioreactor which is designed to support the growth of high concentration of microorganisms.

The wastewater bioreactor is able to support high concentration of microorganisms and treat high concentration of wastewaters within small space confinement because:

1 Provision of good mixing and circulation to ensure that nutrients

and oxygen will reach every microbe in the bioreactor

2 Provision for intense aeration to supply all the oxygen needs of all

the microorganisms

There are various types of wastewater biological reactors that exploit the activity of the aerobic microorganisms in treating wastewaters. The wastewater bioreactors are either based on attached or suspended microbial growth forms such as found in biofiltration and activated sludge systems respectively.

In our discussions we will be concentrating on the Continuous Stirred Tank Reactor as exemplified by the activated sludge system

PRINCIPLE OF AEROBIC BIOLOGICAL TREATMENT

The crucial element in all these aerobic biological wastewater treatment systems depends on the ability of the system to provide sufficient oxygen to support the growth of the aerobic microorganisms. Although oxygen is supplied in the gaseous form to the wastewater, the microorganisms are only able to metabolize the oxygen in the form of dissolved oxygen.

Generally it is agreed that a minimum steady state dissolved oxygen value of about 2 mg/litre should be maintained throughout the mixed liquour to support the growth of aerobic microorganisms. If the value of the dissolved oxygen falls below 2 mg/litre, the wastewater treatment system might be anaerobic and will instead support the growth of anaerobic microorganisms.

Even if the dissolved oxygen value is maintained above 2 mg/litre, throughout the mixed liquor, there are anaerobic zones developing within the microbial aggregates due to poor mass transfer of dissolved oxygen from the surrounding environment into the microbial aggregates.

PROBLEMS OF OXYGEN SUPPLY

In reality, it is difficult for wastewater bioreactors to achieve uniform dissolved oxygen concentration throughout the bioreactor due to the large size of the bioreactor, poor mixings and liquid circulations within the bioreactor. This will often result in oxygen gradients and stratifications being formed within the bioreactor.

Another main contributory factor to the low dissolved oxygen in the mixed liquour is attributed to the poor solubility of oxygen in water. Oxygen is a gas under normal temperature and pressure. Even with very clean water, the saturation value of dissolved oxygen is only about 8 mg/litre. In dirty wastewaters the amount of dissolved oxygen at saturation is far less due to the presence of high concentration of dissolved solids.

The presence of oily films on the surfaces of wastewaters also helps in reducing the mass transfer of oxygen into the mixed liquor.

Dissolved oxygen readings above 2 mg/litre are often exhibited at the surfaces of the mixed liquour and within proximity of aeration or mixing zones. These liquid-air interphase zones exhibit higher oxygen diffusion due to efficient mass transfers. At deeper zones the dissolved oxygen readings could decline rapidly making the bulk of the bioreactor anaerobic.

Low dissolved oxygen values could also be attributed by higher oxygen utilization rate due to higher number of microorganisms or high substrate loadings. The rate depletion of dissolved oxygen is not matched with the rate of oxygen replenishments to the system

There is often not enough dissolved oxygen to sustain the growth of all aerobic microorganisms throughout the reactor despite the visual observations that the aerators are fully working.

PROBLEM OF SUPPLYING OXYGEN IN AEROBIC BIOLOGICAL WASTEWATER TREATMENT PLANTS

There is a huge reservoir of oxygen gas in the atmosphere. Oxygen constitutes about 20% of the composition of air. However, these huge supply of oxygen could not be efficiently exploited by the microorganisms due to:

1 Microorganisms can only use oxygen in the form of dissolved

oxygen in their metabolism

2 Oxygen under normal temperature and atmospheric pressure is a

gas with very limited solubility in water.

The concentration of dissolved oxygen in water is very low. Only very clean water will be able to maintain dissolved oxygen saturation value of about 8mg/litre. In dirty and polluted water the saturation dissolved oxygen value could be very low.

Oxygen from atmosphere can enter the wastewaters either by passive diffusion or by forced aeration. Forced aeration may be brought about by mechanical mixers or by compressed air into the wastewater.

In both these methods, the mass transfer of oxygen to the wastewaters is still by diffusion and governed by the laws of diffusion.

The oxygen diffusion process is based on the molecular motion of molecules with the direction of diffusion dictated by the direction of the concentration gradient.

The diffusion process is affected by various physical parameters such as:

1Thickness of diffusion barrier

2 Surface area of diffusion

3 Difference in concentration of diffusion molecules

4 Temperature

5 Time

OXYGEN PATH

We can follow the fate of oxygen from its source to the targeted microorganisms by following the diffusion path of the gas. Analyzing the diffusion path will help us determine the rate limiting step where improvements can be made.

The oxygen path for forced aeration as in the activated sludge system is as shown:

Air (Bubble)®Mixed liquour®Mixed liquour/floc interphase®Floc/microorganism interphase

For oxygen to be transferred from the air bubble to an individual microbe, several independent partial resistances must be overcome.

Resistances for oxygen transfer from air bubble to the microbial cell:

• Resistance within the gas film to the phase boundary

• Penetration of the phase boundary between gas bubble and liquid

• Transfer from the phase boundary to the liquid

• Movement within the wastewater medium

• Transfer to the surface of the cell

At each step of resistance encountered will result in the sharp decrease of oxygen diffusion rate, which will lead to the decline in oxygen transfer. This would mean lesser oxygen servicing the microorganisms.

In aerobic biological wastewater treatment, the most inefficient stage of the diffusion path is usually movements through the mixed liquour.

The distance covered and the viscosity of the wastewater could affect the efficiency of the oxygenation process resulting in little or no oxygen reaching the microorganisms.

MASS TRANSFER OF OXYGEN FROM AIR BUBBLES

The most common method of supplying oxygen to activated sludge systems is by the forced aeration. Air bubbles are generated as compressed air is released at the nozzles or spargers.

Initially, small air bubbles are generated. These small bubbles rapidly increase in size as it rises to the surface. On reaching the surfaces, these bubbles collapse, releasing the gas into the atmosphere.

Gas exchanges occur between the gas bubbles and the surrounding medium as the bubbles rise to the surface.

Oxygen being higher in concentration inside the gas bubbles will diffuse out of the bubbles into the surrounding environment where the concentration of oxygen is lower. The lower concentration of oxygen in the medium is attributed to the rapid consumption of oxygen by the microorganisms in the mixed liquour.

Carbon dioxide is generally higher in the mixed liquour compared to the concentration of carbon dioxide in the gas bubbles. The higher concentration of carbon dioxide is due to the product of microbial metabolism which generates carbon dioxide. This difference in concentration will result in the diffusion of carbon dioxide into the gas bubbles from the surroundings.

The efficiency of mass transfers through the gas bubbles is affected by various parameters. Smaller size bubbles have more efficient diffusion rates compared to large bubbles due to their high surface area to volume ratio.

At the point of bubble generation where the bubbles are smaller, the mass transfer of oxygen is highest as small bubbles exhibit higher surface area to volume ratio.

As the bubbles of air rises it expand in size resulting in a lower surface area to volume ratio resulting in less efficient mass transfer of oxygen

Fresh air bubbles surfaces are usually free from deposition of organic matter or foam. However, with time the bubbles surfaces will be rapidly coated with layers of foam. This will result in a thicker bubble wall which will impede further the gas exchanges occurring between the bubble and the environment.

Refreshment of bubbles could occur in the mixing zone where old bubbles collapse and new bubbles regenerated.

The efficiency of the oxygen diffusion process is affected by the amount of time the bubbles resides in the mixed liquour. Longer bubbles residence time would mean longer time for gas diffusion to occur

The bubbles residence time is affected by the length of the oxygen path or distance traveled by the bubble. Longer oxygen path would result in longer bubble residence time.

Smaller bubbles due to the higher surface area to volume ratio will be exposed to higher drag forces acting on the bubbles as it rises. This explains why small bubbles rise slowly. Bigger bubbles have lower surface area to volume ratio and less drag forces resulting in rapid rise of air bubbles and shorter bubble path and time for diffusion of oxygen to the surroundings.

The bubble path can be extended by intense mixings of the mixed liquour. Under intense mixing, the bubbles generated will be subjected to a long and tortuous path, thus improving the diffusion process.

IMPROVEMENT OF MASS TRANSFER BY MIXING

Mixing of the mixed liquour will also confer other advantages to the aeration efficiency of the wastewater treatment system

In a turbulent system, the mixed liquour is agitated by a complex combination of aeration, stirring and baffles systems. Transfer of oxygen into the broth is brought about by:

· Bubbles of air

· Entrainments of air

· Shear reactions

GASES ENTRAINMENT

During active agitation, the mixed liquour will show a chaotic dispersion of fluid. The surface of the mixed liquour will be destabilized and fluid vortex formed by stirring will be disrupted. In such situations gases such as oxygen will be easily entrained or trapped within the fluid motion resulting oxygen diffusing effectively into the mixed liquour.

SHEAR ACTIONS

High turbulence resulting in high shear forces will be generated during active agitation. Shear forces occur due to development of eddy currents, laminar shearing, bubbles dispersions and stirrer generated shearing. All these shearing will result in the fluid being stretched thin, frictional forces between the phases resulting in higher amount of gas diffusion.

LEVELS OF MASS TRANSFERS IN BIOREACTOR

During the mass transfer of oxygen through the mixed liquour to the microorganisms, there are two main stages of mass transfer. The first stage is governed by the homogenous macro mixing of the mixed liquour to ensure that oxygen and other nutrients reach every point in the bioreactor. The second stage of mass transfer is observed at a microscopic scale in the intimate proximity of the microorganisms. At this stage, mass transfer process of oxygen is affected by the boundary layer effect which influences the diffusion process of oxygen molecules to the microorganisms. Only shearing forces brought about by mixing will reduce the thickness of the boundary layer to improve the diffusion of oxygen into the microbial cells

WHY TREATMENT PLANT NEED TO BE CONTINUALLY AERATED

The very low solubility of oxygen in water is considered the greatest challenge in supplying oxygen to the microorganisms in aerobic biological wastewater treatment plant. This is often the costliest rate and rate limiting step in the operation of the treatment plant.

Due to these problems the treatment plant has no choice but to continue aerating despite very little of the oxygen pumped will be used by the microorganisms. A lot of oxygen is just wasted unused and returned to the atmosphere.

Oxygen utilization rate studies have shown that the microorganisms rapidly utilize oxygen in the wastewaters despite the low steady value of dissolved oxygen in the mixed liquour. It is therefore the problem of solubility of oxygen gas to form dissolved oxygen as the rate limiting step

The amount of oxygen which can diffuse into the wastewaters at steady state therefore depends on the amount removed or utilized by the microorganisms. Those unused are simply returned wasted to the atmosphere.

The alternative solution taken is to fine tune the supply and demand of oxygen to make the treatment process efficient and economical. This requires close monitoring and understanding of the process in order to control the amount of oxygen supplied as required.

FINE TUNING THE OXYGEN SUPPLY

Fine tuning of the oxygen supply is where when the amount of oxygen provided should be equivalent to amount needed by all the microbes. Nothing more and nothing less. In such situations excess oxygen is not wasted and there will be no lack of oxygen to jeopardize the treatment process.

This objective can be achieved if we know just how much oxygen can be utilized by the microorganisms in a unit volume of wastewaters and how much oxygen can be transferred to a unit volume of wastewaters at any one time during the treatment process.

The measure of how much oxygen needed is measured by the oxygen utilization rate or OUR, and the measure of the oxygen transfer into the wastewater by oxygen transfer rate or OTR. Ideally OTR should equal OUR.

The values of OUR and OTR could be affected by various parameters such as composition of wastewaters, microbial content and phase of growth among others. Thus field and laboratory studies have to be carried out for each specific wastewaters and operating conditions. Close on site monitoring of the process is essential to keep the treatment plant working within the optimal range of parameters.

IMPROVING THE AERATION PROCESS

There are a few strategies that can be taken to improve the aeration efficiencies of biological wastewater treatment plant

Improve sparger design that will result in the generation of smaller size bubbles. These small size bubbles will have higher surface area to volume ratio leading to better mass transfer of oxygen.

Improve stirrer’s design that will encourage good mixings and fluid circulations

Improve design of bioreactor that will promote good flow circulation

and avoiding dead ends.

Improve circulation pattern that will allow longer bubble path for longer time for mass transfer to occur.

Give oxygen where it’s needed most and less oxygen where the demand for oxygen is less.

Increase the efficiency of solid removals upstream in wastewater treatment to reduce the organic load and oxygen demand downstream.

Read more!

APPRECIATING GAS BUBBLES IN FERMENTATION

(PICTURE TAKEN FROM WWW.MASTERMEDIA.ORG)
One of the most striking features in any fermentation process is the observation of gas bubbles in the fermentation process. Aerobic fermenters exhibit gas bubbles produced from the sparger at the bottom of the fermenter. Anaerobic fermenters such as anaerobic digesters produce bubbles which are predominantly methane gas.

It is very mesmerizing to see the behavior and the dance of the bubbles in any fermentor (even fish aquariums produce such beautiful train of bubbles!). But bubbles are more than just a beautiful graphic or artistic representations in fermentors. Bubbles do have very important roles to play in any fermentation process.

Bubbles are basically 'carriers' of gases. Gases richer in concentration such as oxygen in the bubbles will diffuse out into the broth medium powered by the differences in the concentration gradients. Gases or volatiles which occur in higher concentrations in the broth medium will diffuse inwards from the environment into the bubbles and released into the headspace

To a certain extent especially for large gas bubbles, their movement or generation do contribute to the mixing and even circulations within the fermentation broth

There are various parameters which affect the size, shape, buoyancy velocity or how fast it rise and stability of the bubbles.

For our discussion on bubbles in fermentors we will be concentrating on air bubbles generated in aerobic fermentation system.

The bubbles in the aerobic fermentation system are result of forced air which escaped through the nozzles at the bottom of the fermentor. Initially the bubbles generated are small at the moment it leaves the nozzles. It immediately increase in size as it rises upwards through the fermentation broth. As the bubbles rise it increase in size rapidly and rate of ascension.

We can divide the bubbles formed during the fermentation process into two types:
1) Small tiny and microscopic bubbles
2) Large bubbles

Both types of bubbles show different behaviour in the fermentors

TINY BUBBLES
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Tiny bubbles are very small and rises very slowly upwards. The slow rise in tiny air bubbles to the surface is attributed to the increase drag forces experienced by the bubble with the surrounding fluid

Due to their very small size tiny bubbles have very high surface area to volume ratio, thus optimizing mass transfer reactions to occur between the gas bubbles and the surrounding environment.These tiny bubbles are easily recirculated again and again the fermentation broth especially carried by fluid circulations caused by the impeller movements. Tiny bubbles do have the tendency to coalesce among themselves and reformed into large bubbles.

LARGE BUBBLES
-------------------
Big bubbles are generally generated in nozzles with large orifice. These large bubbles have lower surface area to volume ratio thus making them not so efficient in mass transfers between the gas bubbles and the environment.

Large gas bubbles have a rapid lift and rises to the surface very fast. There is lower drag forces acting upon large bubbles. In terms of the bubble pathway, the large bubbles do not show circulation by the impeller mixing

ARE BIGGER BUBBLES BETTER?
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One most common illusion confronting a person doing fermentation is that the sight of large air bubbles in the fermentation broth means that the fermentor is supplied with enough oxygen. In reality this is not so. Whether there are enough oxygen or air is not indicated by the sight of large air bubbles bubbling aggressively in the fermentor. To indicate the real status of the dissolved oxygen values in the fermentor, one has to measure it using a dissolved oxygen meter.

Large air bubbles need not mean there will be sufficient dissolved oxygen in the fermentor. Due to the large size of the air bubbles and the rapid rise of the air bubbles to the surface and short distance the bubble takes to rise will mean there will not be enough oxygen mass transfer occurring between the bubbles and the surrounding medium








BUBBLES EXPLOSION AT HEAD SPACE
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All bubbles on reaching the surface or the liquid/air interphase in the headspace of the fermentors will collapse or explode due to sudden release in the pressure of the bubbles Read more!