Bioreactor Types, Design, Parts, Applications, Limitations

A bioreactor is a device or system that is used to cultivate and grow biological cells, tissues, or organisms under controlled conditions. Bioreactors are essential in many industries, including food, pharmaceutical, and biofuels production.

The history of bioreactors can be traced back to the early 20th century, with the development of the first stirred-tank bioreactors. These early bioreactors were used primarily for the cultivation of microorganisms and were relatively simple in design, consisting of a tank with a stirrer to mix the contents.

During the 1950s and 1960s, bioreactor technology advanced significantly with the development of new types of bioreactors, such as airlift and bubble column bioreactors. These bioreactors improved oxygen transfer and mixing, allowing for the cultivation of more oxygen-sensitive organisms.

In the 1970s and 1980s, the use of bioreactors in the production of pharmaceuticals and other high-value products began to increase. Biotechnology companies began to develop new types of bioreactors, such as perfusion bioreactors and hollow fiber bioreactors, which allowed for the cultivation of cells and tissues on a large scale.

In recent years, the field of bioreactor technology has continued to evolve, with the development of new types of bioreactors, such as microfluidic bioreactors and 3D bioprinting bioreactors, as well as the integration of advanced technologies such as automation and process control. This has led to an increase in the efficiency and effectiveness of bioreactor processes, making them more widely used and accessible in various industries.

Overall, the history of bioreactor technology has been marked by a steady evolution and improvement in design, capabilities, and applications.

What is a Bioreactor? (Definition of Bioreactor)

The bioreactor can be described as a vessel-like apparatus which provides a stable environment for microorganisms to flourish and maintains a steady balance in the biochemical processes that these microorganisms carry out to create desired substances.

• The importance of bioreactors lies in their ability to efficiently and effectively produce a wide range of products. For example, bioreactors are used to produce enzymes, proteins, and microbial biomass for use in the food industry, and to produce vaccines and other pharmaceuticals in the healthcare industry. Bioreactors can also be used to produce biofuels, such as ethanol and biodiesel, as well as other products like bioplastics.
• In addition, bioreactors can also be used in the field of tissue engineering to grow replacement tissues and organs for medical use. Bioreactors provide a controlled environment for the cells to grow, which allows for the production of high-quality, consistent products.
• Moreover, bioreactors can also be used in research and development, providing a controlled system to study the growth and behavior of cells and organisms under different conditions.
• In summary, bioreactors play an important role in many industries, allowing for efficient and effective production of a wide range of products, and also in research and development.

Principle of Bioreactor

The bioreactor is the central component of any biochemical process, since it provides the conditions for microorganisms to achieve optimal development and create metabolites for the biotransformation and bioconversion of substrates into desirable products. The reactors can be designed or produced based on the organisms’ growing requirements. Reactors are machines that can be constructed to turn materials derived from living organisms into desirable products. They can be utilised for the manufacture of different enzymes and other biocatalytic processes.

An ideal Bioreactor Should Have Following Qualities

• The vessel is can be operated aseptically for a few days.
• Proper agitation and aeration.
• The power consumption must remain as minimal as is possible.
• Control of temperature and pH must be made available.
• Facilities for sampling should be made available.
• The losses of the fermentation process from evaporation shouldn’t be too high.
• A minimal amount of labor is required during production cleaning, harvesting, and maintenance.
• Internal smooth surfaces.
• Containment is the process of preventing the leakage of cells that are viable from fermenters or equipment downstream.
• Aseptic operations require protection from contamination.

Bioreactor Design

• The design and mode of operation of a bioreactor are determined by the creation of an organism, the optimal conditions necessary for the formation of the intended product, the product’s value, and its production scale.
• A well-designed bioreactor will increase productivity and produce higher-quality products at cheaper costs.
• A bioreactor is a device with a number of components, including an agitator system, an oxygen delivery system, a foam control system, as well as temperature and pH control systems, sampling ports, a cleaning and sterilising system, and lines for charging and emptying the reactor.
• The material used to make a bioreactor must possess the following essential characteristics:
  • It must be noncorrosive.
  • It should not introduce any toxins into the fermentation medium.
  • It should be resistant to steam sterilisation.
  • It must be able to withstand high pressure and pH variations.
• Depending on the application, bioreactor sizes can vary substantially.
• Some bioreactors are designed for small scale fermenters and others for large scale industrial applications, ranging from the microbial cell (a few mm3) to the shake flask (100-1000 ml) to the laboratory-scale fermenter (1 – 50 L) to the pilot level (0.3 – 10 m3) to the plant scale (2 – 500 m3) for large volume.

Important factors need to be consider in designing Bioreactors

Low value and large volume alcohol-based beverages require simple fermenters. They don’t require aseptic conditions. The high-value and low-volume products require a more complex process and aseptic conditions. The design of a bioreactor must also be able to take into account the specific Biochemical Processes’ Characteristics:

1. The levels of the starting substances (substrates) and the products that are in the reaction mixture are usually inadequate; both substrates and products can hinder the process. Cell development, the structure of intracellular enzymes and their production of products are influenced by the nutritional requirements that the cell has (salts and oxygen) and the maintenance of the optimal conditions for biological activity (temperature as well as the concentration of reactants and pH) within a narrow range.
2. Specific substances or inhibitors and effectors and metabolic products, such as precursors, can influence the rate and nature of the reactions and intracellular regulation.
3. Microorganisms can be metabolized by metabolizing unconventional or even contaminants in raw substances (cellulose Molasses, cellulose minerals oil, starch ore waste, air pollution and biogenic waste) this process is often carried out in highly viscous mediums.
4. Contrary to isolated enzymes and chemicals, the mo’s adjust the structure and function of their enzymes in response to process conditions, and thus the their productivity and selectivity can alter. The microorganisms are susceptible to mutations, which can occur under low conditions in the biological world.
5. Microorganisms are usually vulnerable to high shear stress, as well as chemical and thermal influences.
6. Reactions are typically seen in gas-liquid solid systems, with the liquid phase typically being Aqueous.
7. Continuous bioreactors typically exhibit complex dynamic behavior.
8. The mass of microbial cells can grow when biochemical conversion is progressing. There are many effects, such as growth on walls, flocculation, and autolysis of microorganisms could be observed during the process.

Fermenter Design

A good fermenter must have the following features: Heat and oxygen transfer settings Sterilization processes and foam control, a fast and thorough cleaning system A proper monitoring and control system.

• Traditional designs are open-circular or rectangular vessels constructed from stone or wood.
• The majority of fermentations are conducted in close systems to prevent contamination.
• It should be constructed of non-toxic and corrosion-resistant materials.
• Small fermenters with a capacity of just a few liters are made of glass or stainless steel.
• Pilot scales and a variety of production vessels are constructed from stainless steel, with polished internal surfaces.
• Large fermenters are usually constructed of mild steel, and then lined with plastic or glass to cut down on costs.
• If an aseptic process is required the pipelines that transport inoculum, air and ingredients for fermentation have to be sterilized, normally with steam.
• The majority of vessel cleaning processes are now automated with spray jets and are referred to as Cleaning in Place (CIP). It is located inside the vessel.
• The pipework must be designed to limit the chance of microbial contamination. There shouldn’t be joints in the horizontal direction, or any unnecessary pipes and stagnant spaces that are dead where substances can gather; otherwise, the result could be ineffective sterilization.
• Typically, fermenters with a capacity of 1000 liters capacity are equipped with an outer jacket. larger vessels come with internal coils.
• Safety and pressure gauges valves should be used, (required during sterilization and operation).
• To transfer media, pumps are employed. Centrifugal pumps (generate high shear forces and provide a routes for easy contaminations) magnetically coupled jet and the peristaltic pumps.
• Alternative methods for liquid transfer include gravity feeding or vessel pressure
• In ferments operating at high temperatures or that contain volatile compounds A sterilizable condenser could be needed to stop the loss of evaporation.
• Fermenters are usually operated with positive pressure to stop the entry of contaminants.

Parts of the bioreactor and their function

1. Fermenter Vessel/Vessel

The vessel is designed so that it uses the least work and maintains it and work is carried out in a clean manner under carefully controlled conditions. The inside that the vessel has is smooth, and is constructed of low-cost substances that provide the best outcomes. There are two kinds of fermentor vessels such as glass fermenter and stainless steel fermenter, for small-scale glass is the preferred choice, and for industrial use stainless steel is employed.

• Glass is not toxic and is resistant to corrosion. It is easy to study the internal reaction within the vessel. Sterilization is performed using autoclave. They are small fermentors that measure around 60 centimeters.
• The majority of stainless steel is employed for large-scale fermentations. The vessels are able to withstand corrosion and pressure. The sterilization process is performed in situ.

2. Heating and Cooling Apparatus

The vessel that is used to ferment food is generated by the activity of microbes and the an agitation. The temperature in the vessel is controlled by adding or removing heating from the unit. Baths that are thermostatically controlled and internal coils usually employed to supply heat, and silicone jackets are utilized to eliminate heat. It features a double-silicon mats with heating wires sandwiched in between mats. If the dimensions are exceeded and the mat is covered with a surface with the jacket the removal of heat is a pain in the internal coils cold water must be circulated to keep the temperature at a constant level.

3. Sealing Assembly

The sealing assembly is utilized for the sealing of stirrer shafts to ensure proper agitation. it is able to function for longer periods of time aseptically. There are three kinds of seals used within the fermenter. Seals for the packed gland The shaft is sealed by many asbestos packing rings which are pulled by a glands that are pushed to the shaft. To ensure that the heat is not absorbed packing rings are periodically tested and replaced.

Mechanical seals: This kind of seal is composed of two components, one stationary within the bearing, and a rotating shaft. Two components are joined using springs. In the process, stem condensate is utilized to cool and lubricate the seals. Magnet drives comprise two types of magnets which is a driving magnetic and driven. The driving magnet is secured to the exterior of the head plate within a bearing and linked with the shaft of drive. A second, the magnet that is driven is located on the other side of the shaft, and secured in bearings on the face of the head plate’s interior.

4. Baffles

Baffles stop vortex from expanding the capacity of aeration and are composed of metal strips welded in a radial direction to the wall. Baffles are able to reduce the growth of microbial colonies on the sides of the fermenter.

5. Impeller

Impellers are utilized to provide homogeneous suspensions of microbial cells in a homogeneous medium for nutrient delivery by stirring. Impellers mix the bulk liquid with solid particles and gas phases of the culture of suspension. Impellers with variable impellers are employed in fermenters and can be classified according to.

• Disc turbines: They comprise disc with a set of rectangular vanes. They allow an air stream from the sparger to strike on the disc’s underside and then move the air toward the vanes, breaking large air bubbles down into smaller ones.
• Variable pitch open turbine: They also comprise a an agitator shaft that is vanned and joined to propeller blades of the marine on the shaft for the agitator. The air bubbles that make up this turbine don’t touch any surface prior to dispersing.

6. Sparger

• The sparger is used during fermentation to stir and aerate the wort. 
• It lets oxygen in the fermenter, allowing yeast to convert sugars from fruits, vegetables, grains, and juices into alcohol. 
• Spargers made from stainless steel, brass and glass are the best. 
• It must fit into the fermenter’s opening without clogging. 
• Two main functions of the sparger are: It creates air bubbles that help disperse oxygen throughout wort during fermentation. And it pushes out unwanted trub from the fermenter. This makes it easier to clean the equipment and keep the beer clear.
• It pushes air through pipes into fermenters to aerate them. 
• The sparger keeps the contents of fermenters mixed so they don’t get clumped together. 
• This prevents oxygen from getting into the fermenter. This could cause bad smells.
• Three kinds of spargers are utilized. Porous spargers consist of sintered or ceramic and are used in vessels that do not have agitation at the scale of a laboratory.
  • Nozzle Sparger: It’s an open or partially opened single pipe. This kind of sparger is typically employed because they don’t block and offer less pressure.
  • Combined sparger-agitator: They insert air through a hollow shaft, and then release it from the holes in the disc that is drilled to connect directly to the bottom of the primary shaft. If the agitator operates in a range of rpm they will provide an adequate amount of air in an agitator with a baffle.

7. Feed Ports

• Feed ports help you to add ingredients at the right times to the bioreactor.
• Feed ports allow for small amounts of liquid to pass through them, so nutrients can be added to or removed from fermenters without having to open them. 
• This allows you to monitor your fermentation process continuously and makes it easy to add nutrients or remove byproducts.
• Feed ports enable you to add feeds at various stages of fermentation to your fermenters. You can control the characteristics of your beer by controlling when each ingredient is added. You can add malt extract to your wort at the beginning of the brewing process to give it time to turn sugars into alcohol. However, if you wait too long, the beer may taste flat and lack character.
• The feed ports consist of tubes made of silicone.
• In-situ sterilization is carried out prior to either the removal or the addition of ingredients.

8. Foam Control

This is among the most important components of the fermenter, as the volume of foam within the vessel must be reduced to prevent contamination. The level of foam can be controlled with two components: foam sensing and control. In the fermenter the probe is placed through the top and is set to a certain level that is above the surface of the broth. If the level of foam rises and it touches the probe’s tip there will be a current carried across the circuit. The current will activate the pump, and antifoam will be released immediately to fight the issue.

9. Valves

Valves are employed in the fermentor for controlling the flow of liquid inside the vessel. There are around five kinds of valves used including globe valves, butterfly valves ball valve diaphragm and butterfly valve. Globe valves can be used for general use, but they don’t control flow. Butterfly valves are not appropriate for use in aseptic conditions. They are utilized for pipes with large diameters that operate at low pressure. Ball valves can be used in aseptic conditions. They can handle mycelial broths and operate at a high temperatures. Diaphragm valves aid in adjusting flow.

10. Safety Valves

The safety valve is integrated into the pipe and air layout to function under pressure. Through these valves, the pressure remains within the safe boundaries.

11. Aeration System

Anaerobic digestion requires oxygen for microorganisms that metabolize organic matter into biogas. Methane gas is formed when biodegradable materials are broken down. Carbon dioxide, which is produced during the oxidation process, is the main component of the gas. The bacteria won’t be able to grow and digest the material if there is no air. This will lead to lower production of biogas as well as higher levels of carbon dioxide.

To keep cells alive and growing, oxygen is added through spargers (aerating device) during aerobic fermentation. The rate at which yeast consumes sugar during fermentation is affected by the level of aeration. Higher levels of dissolved oxygen allow yeast to grow more quickly, but with a lower cell density. High cell concentrations result in a decreased space per cell, and therefore lower productivity.

• A fermentor’s aeration system can be one of its most important components.
• To ensure adequate oxygen supply throughout the culture, it is crucial to select a reliable aeration system.
• It has two separate aeration devices, an impeller and a sparger, to ensure that fermentors are properly aerated.
• Two things are accomplished by the stirring:
  • It allows you to mix the gas bubbles in the liquid culture medium.
  • It allows the microbial cells to be mixed through the liquid culture medium, which makes sure that they have equal access to the nutrients.

12. Foam-Control

• Two functions are provided by the foam control system in the bioreactor. It prevents foaming by removing air from the solution. 
• It also helps stabilize bubbles that form during fermentation by adding gas. Because less oxygen diffuses through liquid, this results in a better product. It’s also useful for high yields of fermentable sweeteners; adding CO2 to the liquid will increase sugar intake without affecting yield.
• The continuous feedback loop of the foam control system optimizes foam generation and stability according to input flow rates. 
• This device produces foam that has been shown to increase cell growth and proliferation. It creates a favorable environment for the growth of cells from different tissue/organ sources.
• The foam control system adjusts the air supply to maintain the desired levels of dissolved oxygen. 
• This system makes the most of energy and reduces water consumption by half compared to manual operations. This reduces costs and greenhouse gas emissions.
• North Carolina State University’s Dr. Robert Davis designed a foam-control system. A computer algorithm is used to regulate the flow of air into the bioreactor based on the level of dissolved oxygen in the vessel. This keeps cells alive and prevents them growing too large.
• To avoid contamination, the foam level in the vessel should be reduced. This is an important aspect to the fermentor.
• Two units control foam: a foam sensing unit and a control device.
• An inlet is provided to the fermentor that allows for the installation of a foam-controlling device.

13. Controlling devices for environmental factors

• Bioprocess industries have always struggled with controlling devices. Bioreactor design must consider many parameters such as temperature, pH, dissolved oxygen and carbon dioxide concentrations. These should all be controlled at certain levels during the process. This will control growth, reduce contamination, improve production rate and increase product-quality.
• This will allow you to better control the environment in a bioreactor. 
• These devices will enable us to monitor the temperature, carbon dioxide, oxygen concentration, and pH of the reactor at any time. 
• We also want to offer an interface that allows users to program parameters such as the amount of nutrients provided and the rate at which these are added.
• Many devices can be used to regulate environmental elements such as temperature, oxygen concentrations, pH, cell mass and essential nutrients levels.

14. Fermenter using Computer

• Fermentors can be paired with semi-automatic and automated computers to improve process efficiency, data collection, and monitoring.
• Students will have more information because computers are used in fermenters. The output of each fermentation chamber will be visible to students. Students will be able view temperature and progress of each fermenter, keep track of activity and compare results between batches. This will help them understand how microbiology works at every stage of the process.
• Although the fermenter’s computer cannot be used continuously, some users claim that it can do a decent job maintaining temperature stability if it is switched off between batches.
• The fermenter is a computer-controlled device that monitors fermentation activity and automatically adjusts pH levels. It also pumps CO2 into the mixture to maintain a constant level.

Bioreactor Types

The different types of fermentors are the continuous stirred tanks including airlift, the fluidized bed membrane fermenter, photobioreactor along with bubble column fermenters.

1. Continuous Stirred Tank Bioreactors 

• Stirred tank bioreactors are cylindrical vessels with a motor-driven central shaft that supports one or more agitators. The shaft can enter either the top or bottom of the reactor vessel.
• In order to prevent fluid swirling and vortices, most microbiological culture vessels are outfitted with four baffles that protrude from the vessel’s walls.
• One-tenth or one-twelfth of the diameter of the tank is the width of the baffle. The aspect ratio (height to diameter ratio) of the vessel ranges from 3 to 5, with the exception of animal cell culture applications, where aspect ratios do not typically surpass 2. Typically, animal cell culture vessels (especially small-scale reactors) are unbaffled in order to decrease turbulence that may cause cell damage.
• Aspect ratio determines the number of impellers. Approximately one-third of the tank’s diameter above the bottom of the tank is the location of the bottom impeller. Additional impellers are spaced between one and two diameters of the impeller apart.
• For gas dispersion impellers such as Rushton disc turbines and concave bladed impellers, the diameter of the impeller is around one-third of the diameter of the vessel. Larger hydrofoil impellers with diameters ranging from 0.5 to 0.6 times the tank diameter are very excellent bulk mixers and are employed in fermenters for highly viscous mycelial broths.
• Typically, animal cell culture vessels use a single, large-diameter, low-shear impeller, such as a marine propeller. Gas is sparged into the reactor liquid below the bottom impeller using a perforated pipe ring sparger with a slightly smaller ring diameter than that of the impeller.
• A single-hole sparger may also be utilised. In applications involving animal or plant cell culture, the impeller speed in vessels more than 50 litres typically does not exceed 120 revolutions per minute.
• Higher stirring rates are utilised in microbial culture, with the exception of mycelial and filamentous cultures, for which the impeller tip speed (i.e., impeller diameter rotational speed) does not typically surpass 7.6ms1.
• There is evidence that certain mycelial fungi can be harmed at even slower rates. The superficial aeration velocity (volumetric gas flow rate divided by the cross-sectional area of the vessel) in stirred vessels must remain below the value required to flood the impeller.
• Flooding occurs when an impeller gets more gas than it can adequately disperse. A flooded impeller is an ineffective mixer.
• In general, the velocity of superficial aeration does not exceed 0.05 m s1. Stirred tanks are one of the most popular forms of bioreactors, particularly for the manufacture of antibiotics and organic acids.

Features of Stirred Tank Bioreactors

Stirred tank bioreactors, also known as stirred tank fermenters, are bioreactors that are designed to hold and mix a liquid culture of microorganisms or cells. They typically consist of a cylindrical tank with a stirrer or impeller to mix the contents and provide oxygen for respiration. Some key features of stirred tank bioreactors include:

• Agitation: The stirrer or impeller is used to mix the culture and provide oxygen for respiration. The type and speed of agitation can be adjusted to optimize growth conditions.
• Temperature control: The temperature of the culture is usually maintained at a specific value by heating or cooling the tank.
• pH control: The pH of the culture is usually maintained at a specific value by adding acid or base as needed.
• Aeration: Oxygen is supplied to the culture either through the stirrer or by bubbling air or oxygen through the culture.
• Sterilization: The bioreactor and its associated equipment can be sterilized to prevent contamination of the culture.
• Monitoring and control: Various sensors and control systems are used to monitor and control the conditions inside the bioreactor.
• Scalability: Stirred tank bioreactors can be scaled up or down depending on the desired production volume.

Working Mechanism of Stirred Tank Bioreactors

• In bioreactors with stirred tanks, it is possible to add air into the medium under pressure using an instrument called a sparger.
• The sparger could be a ring with a number of holes or a tube having only one orifice.
• The sparger in conjunction together with the impellers (agitators) allows for a better gas distribution throughout the vessel.
• The bubbles produced by the sparger are crushed down to smaller ones through impellers and scattered across the medium.
• This creates an even and uniform environment within the bioreactor. This allows the bioprocess to run efficiently.
• The bioprocess continues to produce the desired end product through the vent.

Advantages of Stirred Tank Bioreactors

• Continuous operation.
• Excellent temperature control.
• It is easy to adapt easily to easily adapt to.
• Control of parameters is good and also the environmental conditions.
• The simplicity of construction 6. Flexible and low operating (labor) costs and investment requirements.
• Clean and easy to maintain.
• can handle the highest concentrations thanks to its high heat transfer.
• Efficacious gas transfer to developing cells, and mixing of contents.

Disadvantages of Stirred Tank Bioreactors

• The requirement for bearings and shaft seals.
• Limitation of size by motor size as well as shaft length and weight.
• The problem of foaming can be a major one.
• Power consumption is increased because of the Mechanical pressure pumps.

Application of Stirred Tank Bioreactors

• The most effective continuous methods to date have relied on the yeast and bacteria where the most desired products are cells.
• Production of the primary metabolites, enzymes and amino acids.
• The process of producing alcohol(product evidently linked with growing or energy-producing mechanisms).
• The most popular is the process of activated sludge employed in the wastewater treatment industries.

2. Bubble column bioreactors

• Typically, the column has an aspect ratio between 4 and 8. Gas is sparged through perforated pipes, perforated plates, sintered glass or metal microporous spargers at the foot of the column.
• Transfer of oxygen (O2), mixing, and other performance variables are primarily determined by the gas flow rate and rheological qualities of the fluid. Internal devices, such as horizontal perforated plates, vertical baffles, and corrugated sheet packings, can be installed in a vessel to increase mass transmission and alter its fundamental architecture.
• As long as the column diameter exceeds 0.1 m, the reactor behaviour is unaffected by column diameter. A notable exception is the performance of axial mixing. For a given gas flow rate, increasing vessel diameter enhances mixing.
• Mass and heat transfer, as well as the predominant shear rate, increase as the gas flow rate rises. In bubble columns, the highest aeration velocity rarely exceeds 0.1 m s−1. 
• As long as the surface liquid velocity remains below 0.1 m s−1, the liquid flow rate has no impact on the gas-to-liquid mass transfer coefficient.
• For the biological treatment of wastewater and other somewhat less viscous aerobic fermentations, bubble columns are ideally suited.

Features of Bubble column bioreactors

• The ratio for height-to-diameter is usually between 4-6.
• Gas is sucked at the bottom by perforated pipes or plates , or metal spargers with porous materials.
• O2 transfer, mixing , and other performance parameters are affected mostly by the gas flow rate as well as the rheological characteristics of the gas.
• Mixing and mass transfer could be improved by putting perforated plates, or baffles with vertical sides within the vessel.
• Doesn’t contain any draft tubes.

Mechanisms of Bubble column bioreactors

1. In the bioreactor bubble column the gas or air is introduced into the bottom of the column by perforated pipes, plates, or through metal micro porous spargers. This creates an unstable stream that allows gas exchange.
2. The flow rate of gas or air affects the performance factors O2 transfer mixing.
3. The bubble column bioreactors can be equipped with perforated plates for improved the efficiency.
4. The reactants are compressed by a finely dispersed catalyst , and so create the product using the process of fermentation.

Advantages of Bubble column bioreactors

• High volumetric efficiency and outstanding heat management.
• Greater utilization of the plate’s area as well as flow distrubution.
• Self-regulating.

Disadvantages of Bubble column bioreactors

• Inefficient compared to other bioreactors.
• Doesn’t have draft tube
• A higher consumption of catalysts that the bed fixed
• Installation costs are higher, and the design is difficult to create

Applications of Bubble column bioreactors

• The reactor is used extensively for the cultivation of herring-sensitive organisms. E.g. Plant cells and mould
• Chemical and pharmaceutical production.
• Also, for fermentation processes.

3. Air-lift bioreactors

• The fluid volume of an airlift bioreactor vessel is split into two interconnected zones by a baffle or draught tube. Only one of the two zones contains air or other gas. The zone receiving gas is known as the downcomer, whereas the zone receiving no gas is known as the riser.
• Since the bulk density of the gas-liquid dispersion in the gas-sparged riser tends to be less than that in the downcomer, the dispersion flows upward in the riser zone and downward in the downcomer zone.
• Occasionally, the riser and the downcomer are two distinct vertical pipes that are joined at the top and bottom to form an exterior circulation loop. The riser-to-downcomer cross-sectional area ratio should be between 1.8 and 4.3 for best gas-to-liquid mass transfer performance. In commercial processes, external-loop airlift reactors are less prevalent than internal-loop designs.
• The internal-loop configuration may consist of either a draft-tube arrangement with concentric tubes or a split cylinder. Airlift bioreactors are significantly more energy-efficient than stirred fermenters, yet their productivities are comparable.
• Due to its suitability for shear-sensitive cultures, airlift devices are frequently used in the mass production of biopharmaceutical proteins derived from fragile animal cells. Airlift devices are also utilised in high-rate biotreatment of wastewater, manufacture of insecticidal nematode worms, and other low-viscosity fermentations.
• The heat and mass transfer capacities of airlift reactors are comparable to those of other systems, and airlift reactors are superior to bubble columns in suspending solids. Ultimately, all performance aspects of airlift bioreactors are dependent on the gas injection rate and the consequent liquid circulation rate.
• In general, the rate of liquid circulation increases proportionally to the square root of the airlift device’s height. Consequently, the aspect ratios of the reactors are optimised. Since liquid circulation is driven by the difference in gas holdup between the riser and the downcomer, circulation is improved if there is little or no gas in the downcomer.
• All of the gas in the downcomer is entrained with the liquid as it flows from the riser near the top of the reactor into the downcomer. At order to limit or eliminate gas carryover to the downcomer, various types of gas-liquid separators are sometimes utilised in the head zone.
• Compared to a reactor without a gas-liquid separator, the installation of a well designed separator will always improve liquid circulation, i.e. the enhanced driving force for circulation will more than compensate for any greater flow resistance caused by the separator.

Features of Air-lift bioreactors

• Two zones are separated The zone that is sparged is referred to as the riser and the zone that is fueled by no gas is called the downcomer.
• The density in the region of riser is less than in the downcomer area which causes the circulation (so the circulation will be enhanced when there is less or no gas in the region down).
• For maximum mass transfer the riser-to-downcomer cross-sectional area ratio should fall between 1.8 to 4.3.
• The rate of circulation of liquid increases by an increase in the square of an airlift system. Thus the reactors are built with large aspect ratios.
• A gas-liquid separator located in the head-zone could reduce gas carry-over to the downstream and, consequently, improve the capacity of the

Mechanisms of Air-lift bioreactors

1. The performance of the bioreactors with airlift depend on pumping (injection) by air as well as the circulation of liquid.
2. It differs than that of the Stirred tank bioreactor, which requires the heating coat or plate around the tank to create a an insulated bioreactor. It is obvious to see that Airlift bioreactor is more efficient in removing heat in comparison to the Stirred tank.

Two-stage airlift bioreactors

• Two-stage airlift bioreactors are utilized for the formation of temperature-dependent batches of substances.
• Cells that are growing from an individual bioreactor (maintained at 30degC) are transferred to another bioreactor (at temperature of 42degC).
• There is a need for the airlift bioreactor with two stages as it is difficult to quickly raise the temperature from 30degC up to 42degC in an identical vessel.
• Each of the bioreactors is equipped with valves and are connected via pumps and transfer tubes.
• The cells are produced within the bioreactor, and the bioprocess itself is carried out in the second one.

Advantages of Air-lift bioreactors

• Highly efficient in terms of energy efficiency and productivity. are similar to stirred tank bioreactors.
• Simple design, no moving parts or an agitator to ensure lower maintenance and less chance of a defect.
• Easier sterilization (no agitator shaft parts)
• Low energy requirement vs. stirred tank clearly doesn’t require energy for the moving components (agitator shaft).
• More efficient heat removal vs. stirred tank In the Airlift bioreactor, there is no need for the heat plate in order to control the temperature since the Draught-Tube that is within the bioreactor is able to function as an the internal exchanger of heat.

Disadvantages of Air-lift bioreactors

• More air flow and higher pressures are needed.
• The agitation in the Airlift bioreactor is controlled by the supply air . This allows it to regulate the supply air, and the required pressure.
• the greater pressure of air required, then more energy consumption required and more costs must be paid.
• Ineffectively break the foam when foaming takes place.
• There aren’t any bubble breakers, there aren’t any blades used to break the bubbles that result from in the supply of air (sparger).

Applications of Air-lift bioreactors

• The reactor is commonly used in the culture of shear sensitive organisms.
• Airlift bioreactors are commonly employed for aerobic bioprocessing technology. They ensure a controlled liquid flow in a recycle system by pumping.
• Due to high efficiency, airlift bioreactors are sometimes preferred e.g., methanol production, waste water treatment, single-cell protein production.

4. Packed Bed Reactors

• A packed bed is a bed of solid particles, typically with limiting walls. The biocatalyst is supported on or within a porous or homogenous nonporous solid matrix. Solids may consist of particles of a compressible polymeric material or a more hard substance.
• A fluid containing nutrients circulates constantly across the bed to supply the immobilised biocatalyst with the necessary nutrients.
• Metabolites and byproducts are released into the fluid, which is then drained away. Flow may be uphill or downward, but under gravity, downward flow is the norm. If the fluid ascends the bed, the maximum flow velocity is restricted because it cannot exceed the minimum fluidisation velocity; otherwise, the bed will fluidise.
• The depth of the bed is constrained by a number of parameters, including the density and compressibility of the solids, the necessity to maintain a specific minimum level of a key nutrient, such as O2, over the entire depth, and the required flow rate for a given pressure drop.
• For a certain void volume (i.e. solids-free volume fraction of the bed), as bed depth increases, the gravity-driven flow rate across the bed decreases.
• As the fluid flows down the bed, the concentration of nutrients declines and the concentrations of metabolites and products increase.
• Consequently, the environment of a packed bed is heterogeneous; nevertheless, concentration fluctuations along the depth can be reduced by increasing the flow rate. If the reaction consumes or generates H+ or OH, pH gradients are possible. Due to poor mixing, controlling the pH by adding acid and alkali is almost impossible.
• The concentration of the biocatalyst in a given volume of bed decreases as voidage (void volume) increases. If the packing, i.e. the solids supporting the biocatalyst, is compressible, its weight may compress the bed if the packing height is not kept low.
• Flow through a compressed bed is problematic due to the decreased voidage. Widespread usage of packed beds as immobilised enzyme reactors. Thus, just a portion of the biocatalyst is exposed to amounts of the product that impede its activity.

Features of Packed Bed Reactors

• A bed of particles are confined in the reactor. The biocatalyst (or cell) is immobilized on the solids which may be rigid or macroporous particles.
• A fluid containing nutrients flows through the bed to provide the needs of the immobilized biocatalyst. Metabolites and products are released into the fluid and removed in the outflow.
• The flow can be upward or downward. If upward fluid is used, the velocity can not exceed the minimum fluidization velocity.

Advantages of Packed Bed Reactors

• Higher conversion per unit mass of catalyst than other catalytic reactors
• Low operating cost.
• Continuous operation.
• No moving parts to wear out.
• Catalyst stays in the reactor 6. Reaction mixture/catalyst separation is easy
• Design is simple
• Effective at high temperatures and pressures

Disadvantages of Packed Bed Reactors

• Undesired heat gradients.
• Poor temperature control.
• Difficult to clean.
• Difficult to replace catalyst.
• Undesirable side reactions.

Application of Packed Bed Reactors

• These are used with immobilized or particulate biocatalysts.
• High conservation per weight of catalyst than other catalytic reactors. Thus mostly preferred fermentor.
• Used is waste water treatment.

5. Fluidized Bed Bioreactor

• Fluidised bed bioreactors are appropriate for reactions involving a fluid-suspended particulate biocatalyst, such as immobilised enzyme and cell particles or microbial flocs. A liquid stream flowing upwards is utilised to suspend or fluidize the particles.
• The reactor resembles a bubble column geometrically, except that the top part is enlarged to reduce the surface velocity of the fluidizing liquid to a level below that required to keep the solids in suspension.
• As a result, the solids sediment in the expanded zone and fall back into the narrower reactor column below; hence, the particles remain in the reactor while the liquid exits. A gas-liquid-solid fluid bed can be created by sparging a liquid fluidised bed with air or another gas.
• If the solid particles are excessively light, they may need to be artificially weighted, for as by inserting stainless steel balls in a solid matrix that is otherwise light.
• Solids with a high density enhance mass transfer between solids and liquids by raising the relative velocity between the phases. Denser solids are also simpler to settle, but their density should not exceed that of the liquid otherwise fluidisation will be challenging.
• The addition of a gas significantly increases turbulence and agitation in liquid fluidised beds, which are typically rather calm. Even with relatively light particles, the surface liquid velocity required to suspend the solids may be so high that the liquid exits the reactor far too soon, i.e. the solid-liquid contact duration is inadequate for the reaction.
• In this instance, it may be necessary to recycle the liquid in order to achieve a sufficiently lengthy cumulative contact time with the biocatalyst.
• The minimum fluidisation velocity, i.e., the superficial liquid velocity required to suspend solids from a settled condition, is determined by a number of variables, such as the density difference between the phases, the particle diameter, and the viscosity of the liquid.

Features of Fluidized Bed Bioreactor

• Suitable for reactions involving a fluid-suspended particulate biocatalyst such as immobilized enzyme and cell particles.
• Similar to the bubble column reactor except that the top section is expanded to reduce the superficial velocity of the fluidizing liquid to a level below that needed to keep the solids in suspension.
• Consequently, the solids sediment in the expanded zone and drop back, hence the solids are retained in the reactor whereas the liquid flows out.
• The properties include:
  • Extremely high surface area contact between fluid and solid per unit bed volume
  • High relative velocities between the fluid and the dispersed solid phase.
  • High levels of intermixing of the particulate phase.
  • Frequent particle-particle and particle-wall collisions.

Mechanism of Fluidized Bed Bioreactor

• For an efficient operation of fluidized beds, gas is spared to create a suitable gas-liquid-solid fluid bed.
• It is also necessary to ensure that the suspended solid particles are not too light or too dense (too light ones may float whereas to dense ones may settle at the bottom), and they are in a good suspended state.
• Recycling of the liquid is important to maintain continuous contact between the reaction contents and biocatalysts. This enable good efficiency of bioprocessing.

Advantages of Fluidized Bed Bioreactor

• Uniform Particle Mixing
• Uniform Temperature Gradients
• Ability to Operate Reactor in Continuous State

Disadvantages of Fluidized Bed Bioreactor

• Increased Reactor Vessel Size
• Pumping Requirements and Pressure Drop
• Particle Entrainment
• Lack of Current Understanding
• Erosion of Internal Components
• Pressure Loss Scenarios

Application of Fluidized Bed Bioreactor

• These reactors can utilize high density of particles and reduce bulk fluid density.
• Fluidized beds are used as a technical process which has the ability to promote high levels of contact between gases and solids.
• In a fluidized bed a characteristic set of basic properties can be utilized, indispensable to modern process and chemical engineering
• The food processing industry: fluidized beds are used to accelerate freezing in some individually quick frozen (IQF) tunnel freezers.
• The fluid used in fluidized beds may also contain a fluid of catalytic type.
• Fluidized beds are also used for efficient bulk drying of materials.
• Fluidized bed technology in dryers increases efficiency by allowing for the entire surface of the drying material to be suspended and therefore exposed to the air.

6. Photobioreactor

• Photobioreactors are utilised for the photosynthetic cultivation of microalgae and cyanobacteria to produce astaxanthin and β-carotene, among other products. Photosynthetic cultures require either natural or artificial light.
• Artificial illumination is impractically costly, and outdoor photobioreactors appear to be the only viable option for large-scale production. Open ponds and raceways are frequently used to cultivate microalgae, particularly in wastewater treatment procedures.
• When monoseptic cultures are required, hermetically sealed photobioreactors must be utilised. Because photosynthesis requires light, it can only occur at relatively shallow depths. Typically, algal ponds are little deeper than 0.15 metres.
• However, excessive light induces photoinhibition; in this case, a little reduction in light intensity will increase the rate of photosynthesis. As cell population increases, the self-shading effect of cells further restricts light penetration.
• In addition to light, photosynthesis-dependent algae cells require a carbon source, typically carbon dioxide. Closed photobioreactors for monoculture are arrays of glass or, more frequently, transparent plastic tubes. As depicted in Figure, the tubes may be positioned horizontally or organised as long rungs on an upright ladder.
• A continuous single-run tubular loop form is also employed, or the tube may be spirally looped around a vertical cylindrical support. In relatively small-scale operations, thin flat or inclined panels may be utilised in addition to tubes.
• A solar receiver is constituted by an array of tubes or a flat panel. Various mechanisms, such as centrifugal pumps, positive displacement mono pumps, Archimedean screws, and airlift devices, are utilised to circulate the culture via the solar receiver.
• Airlift pumps contain no mechanical components, are simple to operate aseptically, and are suitable for shear-sensitive applications.
• The flow in a solar receiver tube or panel should be sufficiently turbulent to facilitate the periodic movement of cells from the darker, less illuminated interior to the regions closer to the walls. The velocity should be sufficient everywhere to prevent the sedimentation of cells.
• The average linear velocity across receiver tubes is between 0.3 — 0.5 m s-1. Due to the need to maintain enough sunlight penetration, it is not possible to scale up a tubular solar receiver by merely expanding the tube’s diameter.
• In general, the diameter should not exceed 6 cm. Light penetration is dependent on biomass density, cellular shape and colour, as well as the absorption properties of the cell-free culture media.

Advantages of Photobioreactor

• Higher productivity
• Large surface-to-volume ratio
• Better control of gas transfer.
• Reduction in evaporation of growth medium.
• More uniform temperature. Powerpoint Templates

Disadvantages of Photobioreactor

• Capital cost is very high.
• The productivity and production cost in some enclosed photobioreactor systems are not much better than those achievable in open-pond cultures.
• The technical difficulty in sterilizing Powerpoint Templates

Application

• The main applications of photobioreactors are in photosynthetic processes, involving vegetable biomass growth or microalgae growth under restricted conditions.

7. Membrane Bioreactor

Membrane bioreactors (MBR) are been used since 90s. It basically combines traditional treatment system with filtration via membranes resulting in removal of organic and suspended solid matters that also removes high level of nutrients.

Membranes in the MBR system are submerged in an aerated biological reactor. The pore size of the membrane ranges from 0.035 microns to 0.4 microns.

However, membrane fouling is a chief obstacle to the extensive application of MBRs. Moreover large-scale use of MBRs in waste water treatment will involve a notable worthy decrease in price of the membranes

Advantages

• The loss of enzyme is reduced.
• Enzyme lost by denaturation can be made up by periodic addition of enzyme.
• Substrate and enzyme can be easily replaced.

Applications

• The use has widely extended and is rapidly growing both in research and commercial applications.
• Several variations of MBR systems have evolved and presently, an MBR system is widely used in treatment of waste water from several sources.

8. Rotary Drum Bioreactor

The rotating-drum bioreactors comprise a horizontally rotating drum, that may or may not have a paddle mixer and rotates slowly for proper mixing of fermentation substrate. For scaling-up purposes, many assumptions need to be made concerning the rotating-drum bioreactors.

• The bioreactor is cylindrical (with a length L and diameter D) and partially filed;
• since the solid materials are degraded during fermentation, it will be considered that only the density of the bed is affected;
• the dry gas remains constant in the headspace;
• the gas flow rates remain the same between the inlet and the outlet of the bioreactor;
• the solid particles and gas phase are in equilibrium (moisture and thermal) and the diffusion from the axe is negligible.

Advantages of Rotary Drum Reactor

• High oxygen transfer.
• Good mixing facilitates better growth and impart less hydrodynamic stress.

Disadvantages of Rotary Drum Reactor

• Difficult to scale up.

9. Mist Bioreactor

Mist bioreactors are hydraulically-driven bioreactors for root cell cultures (see plant cell cultures). Their key feature is a disposable bag (single-use or multi-use) in which the roots are immobilized and aerated on a frame. A two-component jet or an ultrasonic atomizer befogs the culture medium which gets distributed around the supporting frame. The largest disposable mist-bioreactors are currently the 60 liter systems produced by former ROOTec.

Advantages of Mist Bioreactor

• High oxygen transfer.
• Hydrodynamic stress elimination.
• Low production cost.

Disadvantages of Mist Bioreactor

• Mesh trays and cylindrical stainless steel meshes are required.

10. Immobilized cell bioreactor

• The immobilized cell reaction (ICR) operates in accordance with the principle of immobilization. The process of limiting the cell’s mobility within a certain space.
• The interaction between hydrogen and hydrophobic and the formation of salt bridges between the adsorbent as well as the cells are the driving factors for immobilization.
• In general, immobilization can be divided into two kinds which are passive and active.
  • In the passive model cells, they are stuck naturally in the matrix of solids, leading to the creation of biofilm.
  • In active techniques Immobilization can be induced by a physical or chemical method. This can occur in a variety of ways like attachment, entrapment gathering, and confinement.
• The management of cells that are immobilized inside the reactor is difficult since these cells are not dependent on liquid or gas phases.
• The performance of these immobilized cells may be enhanced by a an appropriate design of the reactor.
• To design a good reactor various criteria must be considered.
• The amount of shear forces must be minimal.
• The reactor should be able to hold the maximum amount of particles.
• The heat and mass transfer must be kept in check.
• Immobilization is usually done with sodium alginate (2 percent) where cell beads immobilized are made.
• The substrate or nutrient is introduced into a reaction vessel that includes the cells that are immobilized, they interact and then create a product and a byproduct.
• In the majority of cases, ICRs are created with two stages. These include an enricher stage as well as an a stripper stage. This stage is utilized to remove and treat the byproducts when they are they are present in large amounts
• After the process of fermentation is complete then the inner and outside surface of the beads will be examined to determine whether the cells initially resided in the inner part of the beads but as time passes, the cells move and are located on the outside of the beads.
• ICRs are employed in the industry of fermentation in which the production and growth phases are easily separated.

Advantages

• The harvesting of the product you want is a breeze and requires any effort if the substances are released in the medium

Disadvantages

There are some restrictions in the ICR like;

• A limitation in mass transfer due to the intraparticle diffusion resistance, which restricts the ability of the substrate to get to cells. This type of problem occurs in aerobic reactions, where there is an oxygen shortage to cells, resulting in lower reactor performance.
• Another issue is inhibition of the product in which the concentration of the product is reduced within the inner core and, consequently, the rate of reaction is also decreased.

11. Activated sludge bioreactor

When an active sludge bioreactor is used the proportion of microbes as well as the amount of oxygen and substrate are all the same as the reactor is equipped with a homogeneous tank in which the feed is dispersed throughout the. In the active sludge with plug flow, the reactor has an extended channeled inlet which restricts any growth in microorganisms as well as improves the ability of sludge to settle.

Step feed reactors are an improvement that is a variation of the plug flow system, in where sewage is injected in multiple points in the Aeration tank. Food-to-microbes (F/M) ratio of the step feed reactor is significantly higher than those using the plug flow. Oxidation ditches are a different kind that is an activated sludge-based bioreactor. It uses a modified activated sludge treatment procedure employing long SRTs to eliminate organic matter that is biodegradable. It is fitted with an aeration rotor , or brushes to ensure adequate circulation and Aeration. The reactor is similar to the whole mix reactor.

Mechanism/Process of activated sludge reactor

The activated sludge system consists of:

• water being pumped to the aeration tank an microbial suspension
• solidliquid separation,
• the disposal of treated waste and
• the remaining biomass is returned to the Aeration tank.

In the process of activated Sludge in the activated sludge process, sewage with organic matter is pumped to the tank for aeration that is then metabolized because it is filled with microorganisms. The organic matter that is metabolized is converted to CO2 and water in order to generate energy. A portion of the cells that have formed during the process are eliminated from the process in sludge. The remaining sludge returns to an aeration tank in which this process is carried on.

Application

• The reactor is employed in the treatment of wastewater and sewage.
• This particular reactor is used to produce biofuels such as biogas, bioethanol and so on. such as biofuels that are made by milk-based waste.

Advantage

• The reactor can be operated with high organic loading rates.

Limitation

• This reactor is a major consumer of energy and also capital.
• The operating expenses are high.

11. Immersed membrane bioreactor

Immersed membrane bioreactors (IMBRs) are a form of membrane bioreactor where two fundamental principles, suspended growth bioreactor and the separation are performed in tandem to create an effect synergistically. The IMBR is built on a filtration system which has membranes that are encased within the biomass. The filtration is accomplished through the application of a vacuum on the membrane’s interior. The membranes are placed in the bioreactor, or in an additional tank. The membranes may be hollow, flat, or a mixture of both. An online backwash system is integrated to reduce the possibility of surface fouling. Additionally, aeration is needed to ensure air scour and lessen the possibility of fouling. Membrane reactors with hollow fibers are typically used in large-scale and medium-sized facilities.

Process of immersed membrane bioreactor

The procedure of the IMBR relies on five fundamental components. They are:

1. it’s membrane and its structure and its maintenance of permeability
2. the feedwater, their properties and pretreatment
3. Aeration of the bulk biomass and the membrane;
4. Sludge withdrawal and residence time and
5. Bioactivity and the nature of biomass.

Advantage

• Low energy consumption
• lower capital costs.

Application

• This kind of bioreactor is mainly used to treat textile and tannery wastes along with wastewaters and for the reuse of aquaculture wastes. 

12. Reverse membrane bioreactor

Bioreactors with reverse membranes (rMBRs) are a unique method of combining the traditional membrane bioreactor as well as cell encapsulation methods wherein cells are separated from feed and then encased within the form of a membrane which is then fixated within the reactor. The basic principle behind this type of system is similar the membrane reactor with an immersed design in which that membrane gets submerged within the reactor. However, within the rMBR microorganisms are enclosed inside membrane layers which create an Sachet.

The integrated permeate channels (IPC) along with packed columns are two other types of membrane setting. The choice of the configuration for the membrane is based on the product you want to produce and the byproducts it produces. For example, a multilayer membrane column is utilized to create biogas, and for the production of ethanol it is an IPC membrane-configuration-based reactor is utilized. A majority of synthetic membranes are utilized to allow certain nutrients to flow through, for example, in plants where the cytoplasm is separate from the other components of the cell.

The goal of surrounding cells within this membrane layer is increasing cell’s density and tolerance. For example, the yeast cell concentration can be increased as high as 309 grams/L. Because of this dense concentration the cells could be exposed to greater stress because of the deficiency of nutrition. This results in an immune response to stress through the expression of stress-related genes.

Mechanism

In general, in rMBRs, the diffusion mechanism is carried out in three distinct stages.

1. The dispersion of the substrate from the feed side of the membrane’s surface, and the reverse process to the product
2. the transport of substances (substrate or metabolites) through the membrane and
3. the dispersal of feed and products on the cell’s side via biofilm.

The rate at which compounds diffuse through the membrane is influenced by different parameters, including hydrophilicity, tortuosity and porosity and concentration gradient etc.

Cultivation principles

1. Submerged cultivation

• Submerged cultivation or fermentation is the process of inoculating a microbial culture into a liquid media in order to produce the desired product. The two distinct fermentation processes are aerobic and anaerobic fermentation.
• ​
• By mixing and aerating the entire working volume of the bioreactor, the cells of the producer (microorganism) are supplied with nutritional medium and oxygen (in the event of an aerobic process) during submerged fermentation (or cultivation).
• This makes the process extremely cost-effective. The bioreactor generates optimal conditions for amassing a substantial volume of actively operating producer biomass and, consequently, the desired product.
• As an example relevant to the history of biotechnology, substituting surface fermentation (in flasks and bottles) with submerged fermentation enabled a rapid increase in the production of penicillin, which was most urgently needed during World War II.
• Fermentation submerged may be aerobic or anaerobic. Antibiotics and enzymes, for instance, are created via aerobic fermentation, which involves the incorporation of oxygen into the liquid medium, whereas butanol is produced via anaerobic fermentation, in which the presence of oxygen has an inhibiting impact.
• Certain fermentation processes, such as ethanol production, employ facultative anaerobic organisms, such as Saccharomyces cerevisiae, which may grow and create cell biomass in the presence of oxygen before switching to anaerobic mode during the ethanol fermentation phase. Frequently, enzymes (amylases and proteases, amylases, etc.) are produced through aerobic submerged fermentation.
• Depending on the desired end result, submerged fermentation procedures can be modified. The product of interest may be biomass, ferments, or low-molecular molecules (for example, ethanol, methanol, acetates, oxalic and formic acids).
• Metabolites can be either fundamental or secondary. A primary metabolite is a type of metabolite that directly contributes to normal development, growth, and reproduction. Lactic acid and various amino acids are frequent examples of primary metabolites.
• Secondary metabolites are produced during or near the end of the stationary phase of growth and have no effect on growth, development, or reproduction. Antibiotics such as erythromycin and bacitracin and atropine are examples of secondary metabolites.
• Finally, fermentation processes can be distinguished in terms of technology and product type. The goal result may be biomass, a high-molecular-weight molecule (such as a constitutive or inducible enzyme), or a low-molecular-weight metabolite. In turn, metabolites might be either primary or secondary.
• Consequently, the requirement for inductors and precursors, as well as the timing of their entry into the medium, is dependent on the product of interest. Certain stages of the development of the producer’s culture are characterised by the manufacture of secondary metabolites, which is increased in stressful settings. In this regard, the addition of inducers and precursors is required when the fermentation process’s end product is a secondary metabolite.
• Typically, the curve of biomass accumulation correlates with the curve of primary metabolites but does not overlap with the curve of secondary metabolites.
• Submerged fermentation is ideal for microorganisms, such as yeasts and bacteria, that demand high levels of wetness. This approach also simplifies product purification, which is a benefit. Most frequently, submerged fermentation is utilised to extract secondary metabolites that must be utilised in liquid form.
• In particular applications, stirred tank, bubble column, and airlift type bioreactors, as well as photobioreactors and membrane bioreactors, are utilised to conduct submerged fermentations.

2. Solid state fermentation

• The substrate for solid state fermentation (SSF) is a solid phase. On a solid substrate, bacteria are developing in the absence or near lack of free water.
• The substrate must produce enough moisture to sustain the microorganism’s growth and metabolism. SSF are used to produce fermented foods like as bread, meat cheese, pickles, and yoghurt.
• Using SSF, agro-industrial leftovers can be recycled to obtain enzymes, organic acids, food fragrance compounds, biopesticides, mushrooms, pigments, xanthan gum, and vegetable hormones, among other products.
• SSF requires minimal equipment, and the construction of bioreactors is quite straightforward. However, scale-up is hindered because it is difficult to ensure exact monitoring and control, and environmental conditions of microorganisms cannot be controlled.
• Because the growth rate of microorganisms on solid substrate is sluggish, SSF are lengthy. There are processes that can only be successfully executed by SSF. For instance, the sporulation of certain fungus can only be achieved using SSF, as these fungi cannot sporulate on liquid media.
• For SSF, horizontal drum, tray, packed-bed, and bench-scale bioreactors are utilised.

3. Immobilization

• Immobilization is the binding of an enzyme to an insoluble carrier while preserving its function, i.e., its catalytic activity. In many applications, the final product must be absolutely devoid of enzyme residues in order to avoid immunological reactions, which necessitates immobilisation.
• The immobilisation of enzymes not only considerably improves their stability, but also permits the extended use of a single batch or series of commercial biocatalysts.
• The term “immobilisation of a biological item” refers to the physical separation of a biocatalyst and a solvent in which molecules of the substrate and reaction products can readily pass from a liquid to a solid medium and vice versa.
• In other words, the substrate in the solvent flow is delivered to the bio-object associated with an insoluble carrier, whilst the reaction product in the solvent flow is removed from the bio-object and employed as the target product.
• Enzymes and whole cells can both be immobilised. Immobilization, or the attachment of cells to a carrier, has a number of advantages, for instance in ethanol production. The cells are reusable and have a longer lifespan.
• Because the targeted end product is essentially devoid of biological chemicals and organisms, not all of the standard purification techniques are necessary. The inclusion of cells in gels, which involves combining a cell solution with gel-forming chemicals, is one of the most used immobilisation techniques.
• Small molecules such as glucose can pass through the pores of the gel to reach the cells, whilst their metabolic byproducts (alcohol and carbon dioxide) can leave the beads. Therefore, the yeast cells stay intact.
• Various immobilisation techniques, including the attachment of cells in stable porous gels (e.g., alginate, collagen, chitosan, agarose, cellulose, -carrageenan, or gel-matrix polymers such as polyacrylamide-hydrazide) or hydrogels or immobilisation in solid macroporous carriers, have been established and are used on both laboratory and industrial scales for a variety of applications, such as the food, dairy, In the production of pharmaceutical preparations, the target substance will not contain culture liquid components (mycelium, products of partial lysis of cells, components of a complex nutrient medium, etc.), which greatly simplifies the task of isolating and purifying the target product and ensures the absence of proteins and other harmful impurities.
• Utilizing immobilised biological items under industrial circumstances has evident economic benefits. Utilizing immobilised systems allows for the standardisation of biosynthetic conditions and the consolidation of production as a whole. The resulting biological item has a lengthy lifespan. Additionally, less raw materials are utilised per unit of output.
• The cells may include multiple catalytically active enzymes, which might result in undesirable side reactions, and the cell membrane may act as a diffusion barrier, so limiting productivity. Due to the difficulty in controlling the physiological condition of microorganisms in immobilised cell bioreactors, process variability and adaptability cannot be guaranteed.
• Bioreactors for immobilised cells can be divided into stirred tank, fixed bed, fluidized bed, moving bed, and packed bed reactors.

Operation modes in Bioreactor

1. Batch

Batch culture is a closed system in which the growth rate of biomass goes toward zero due to substrate depletion and inhibitor buildup. These systems are perpetually unstable.

​Microorganisms undergo six growth phases during batch cultivation.

1. The lag phase or induction period begins when the microbe is inoculated into the nutritional medium and is the period of their adaption. During this phase, the micromolecular and macromolecular components of the microbial culture undergo reorganisation, as well as the creation or inhibition of enzymes or structural components of the cell. Depending on external variables, the duration of this phase can range from one hour to three or more hours. During this phase, the cell mass may alter without the cell count changing.
2. The lag period is followed by an exponential growth phase. This is a stage of fast biomass and reaction product buildup.
3. The phase of linear growth is defined by balanced growth in the steady state, that is, the growth rate remains constant during the cultivation process, and the chemical composition of the culture medium changes as nutrients are absorbed and metabolic products are created. As a result, the microorganisms’ surrounding habitat is continually changing, yet the development rate does not depend on the amounts of nutrients.
4. The phase of linear growth is replaced by the phase of growth retardation, during which the growth rate of the culture declines to zero.
5. Furthermore, the culture can enter a somewhat stable stationary phase, with the rate of microbe death being compensated by the rate of biomass gain.
6. With the depletion of the nutritional medium (substrate) and the accumulation of growth-inhibiting products, considerable physiological changes (lysis) occur in the culture, and the so-called phase of culture withering begins.

2. Fed-batch

• Fed-batch is based on the addition of a substrate containing a growth-limiting nutrient to a culture. By altering the feeding method, the cell development and fermentation process can be managed.
• In a bioreactor, fed-batch is typically initiated with a batch fermentation phase until the consumption of one or more substrates and/or inducers. Using various feeding regimes, the new medium can be added.
• During the course of the procedure, only a fixed or variable volume of a new medium or substrate can be added as feeding. During the fed-batch phase, this feeding might be continuous, exponential, or in pulses over a short or long duration.
• When the goal product is positively correlated with microbial growth, fed-batch fermentation is particularly advantageous for bioprocesses aiming for high biomass concentration or high yield. A typical fed-batch method for producing a product consists of three steps.
• First, in batch mode, the biomass grows to a concentration that allows the process to continue with a limitation on the substrate (without the accumulation of the substrate in the medium). The second stage is biomass cultivation, in which a substrate that stimulates rapid development of the culture is added to the medium (glucose, sucrose or glycerin).
• When a biosynthesis inducer is added to the medium, the last stage of product synthesis (recombinant protein or other substances) is initiated. This could be an activator of gene transcription (such as IPTG) or an auto-inducer substrate (for example, methanol for P. pastoris or lactose for E coli).
• The most straightforward method of feeding strategy is the calculation of a time-dependent, modified feeding profile. Typically, this feeding profile is determined using mathematical models. To avoid catabolism repression and prevent the repressive effects of excessive substrate concentrations, the operator must regulate the feeding regime. Controlling fed-batch is hard due to the lack of good on-line biomass and substrate concentration measuring techniques.
• By utilising customised feeding profiles, the operator must always modify these profiles during a fed-batch. Due to the impossibility of delivering entirely repeatable fermentations, this is the case. At some phases of the fermentation process, automatic feeding based on the values of the dissolved oxygen (DO) sensor is feasible.
• However, there are various issues here. When reasonably high biomass densities are reached, it is also impossible to perform this regulation in its best form.
• Model-based fed-batch control is capable of resolving the aforementioned issue. The solutions are not yet available for purchase. Typically, model-based control systems are designed for particular applications. Attempts have been made to design systems with wider application.
• In general, model-based control operates as follows: During the fermentation process, samples are collected so that the software can be updated with the most recent findings of tests on biomass and substrates.
• Following the input of test results, the software performs an automatic comparison between these findings and the outputs of calculations based on the mathematical model in use. If variations exceed the established standards, the software calculates a new feeding profile.
• The amended feeding profile is automatically sent to the process controller PLC, and the substrate feeding is then adjusted to correspond with the new profile (up to a following update). The mathematical model is implemented as a PC software that is connected to the PLC via an OPC server.

3. Continuous

• Continuous cultivation is distinguished by the constant addition of fresh nutrient medium to the bioreactor and the constant selection of a suspension or a spent medium. Continuous culture is an open system that strives to achieve a dynamic balance.
• Thus, it is possible to maintain constant environmental conditions within the growing medium. If the product’s demand potential is sufficient, continuous cultivation may be utilised. The second typical application of continuous cultivation is in the treatment of wastewater using wastewater as the in-flow substrate.
• If a consistent amount of product is required, continuous cultivation is utilised. As a result of the incorporation of additional devices into the bioreactor connection schema, it is structurally more complicated and necessitates additional automatic control.
• In a batch culture, conditions are constantly changing: the culture’s density increases and the substrate concentration decreases.
• However, it is frequently required that cells can remain in the exponential growth phase for an extended period of time at a constant substrate concentration and under other unaltered conditions.
• This can be accomplished by continuously introducing a new nutrient solution into a vessel containing a cell culture while simultaneously removing a suitable amount of cell suspension.
• Chemostat and auxostat cultivation methods are commonly used in microbiological research.
• The chemostat method of cell cultivation is based on the use of a bioreactor into which a nutrient medium is supplied at a constant rate and, simultaneously (for example, drainage according to the level), the cell suspension is removed.
• Likewise, the volume of the grown suspension remains constant. The concentration of the substrates governs the growth of the culture within the chemostat. The system’s stability is dependent on this limitation of the growth rate by the concentration of one of the necessary substrates.
• Auxostats are closed-loop systems governed by feed-back regulation of some state variable, such as biomass, substrate concentration, or pH. Auxostats can be classified as turbidostats, nutristats, or pH-auxostats depending on their operational control principle.
• In a turbidostat, an optical density (turbidity) controller adjusts the feed rate to maintain a constant biomass concentration over time. Under nutrient-rich conditions, the turbidostat facilitates growth rates close to the maximum.
• The application problem of turbidostat is associated with some technical difficulties of sensor readings, such as fouling of the sensor due to microbial growth on its surface and signal transfer disturbances caused by air bubbles or coloured and particulate media.
• Nutristat’s operation is predicated on the measurement and control of substrate concentration through substrate feeding. The application of nutristat is constrained by the absence of suitable analytical tools for the on-line measurement of the majority of relevant substrate concentrations.
• The pH-auxostat is based on the measurement of pH, which is frequently correlated with the rate of biomass production, but is easier to measure and regulate than turbidity and substrate concentrations.
• In a titration mode, the signal from the pH sensor is used to control the medium in-flow so that the addition of fresh medium restores the pH to the setpoint and the same amount of medium is removed from the bioreactor in out-flow. The pH-auxostat method is applicable to microorganisms whose growth results in changes in the medium’s pH.

What are Aerobic and anaerobic bioreactors?

Aerobic reactors

• The product conversion rate or degradation rate in an aerobic reactor is primarily determined by the bubble size and gas-to-liquid mass transfer rate.
• Because the presence of salt might inhibit oxygen’s solubilizing property, oxygen solubility is a crucial parameter that must be optimally maintained. Simple aerobic bioreactors can be made with aerated lagoons or oxidation ponds for waste storage in the open environment and a rotating disc containing the microbe as a biofilm for periodic churning.
• Stirred tank bioreactors, airlift bioreactors, and inverse fluidized bed bioreactors are examples of industrially prevalent aerobic bioreactors. Air is sparged from the bottom of the stirred tank reactor, which is the most frequent type of aerobic reactor.
• In the airlift bioreactor, mixing is facilitated by gas turbulence. In airlift reactors, the oxygen transfer coefficient is greater than in stirred tank reactors.
• As depicted in Fig., air is injected from the bottom of the reactor, resulting in a revolving motion of the reactor’s contents that maximises gas transfer.
• The inverse fluidized bed reactor (FBR) is utilised for wastewater treatment in which inert particles coated with biofilm represent the solid phase, oxygen or air supply is the gaseous phase, and wastewater is the liquid phase.
• The gas travels in the opposite direction of the liquid, which increases the mass transfer rate and makes it easy to re-fluidize the bed.

Anaerobic reactors

• Anaerobic reactors are comparable to aerobic reactors with the exception of a few parameters that must be met to sustain an anaerobic environment. This reactor has a simple design, a high loading capacity, and can handle high levels of hazardous and organic chemicals.
• Methanogens are specialist bacteria used to assist anaerobic processes; they have a propensity to form immobilised granules that settle to the bottom as sludge, which is the fundamental principle utilised in anaerobic bioreactors.
• The UASB reactor employs this operating principle. It is possible to create an anaerobic fluidized bed in which the microbe mixer is in the form of a biofilm grown on carrier particles and fluidized utilising energy from the feed stream.
• The substrate diffuses through the biofilm and is transformed into volatile fatty acids and CH4 before diffusing out of the bulk liquid. As the biofilm expands, the particles within the reactor become larger and leave the reactor, resulting in a drop in particle density and concentration. Additionally, membrane reactors can be designed as anaerobic reactors.
• They are created in several forms. Enzymes are suspended in the reactor, and the mixture is withdrawn along with the enzymes and passed through a membrane, where the enzymes are retained and the products are collected; alternatively, the membrane filter can be designed so that it is immersed inside the reactor, but the permeate will be devoid of enzymes, which are retained within the reactor.
• In comparison to conventional bioreactors, membrane bioreactors produce less sludge. This is the primary advantage of membrane bioreactors (MBRs). These reactors’ shortcomings include membrane fouling and high operational expenses.

Difference between bioreactor and fermenter

Bioreactor and fermenter are similar names, however there is a significant distinction between them. Bioreactor is frequently associated with the cultivation of mammalian, plant, and stem cells.

​Fermenter is employed when the application involves the culture of bacteria, yeast, or fungi. It would not be incorrect to use the term bioreactor in these situations, however only the term bioreactor is used when discussing cell cultivation. The term fermenter refers to reactors in which fermentations, metabolic processes that cause chemical transformations in organic substrates by the action of enzymes, are conducted. The primary distinction between bioreactors used for cell and microbe cultivation is the mixing and aeration requirements, as well as the ratio of height to diameter H/D. The aeration rate for microbe cultivation is between 0.5 and 3 vvm (volume of cultivation media/volume of air flow per minute). Cell cultures require gentle mixing and an aeration rate between 0.01-0.1 vvm. The ideal H/D ratio of the vessel for microbe cultivation is 3:1, but it is 2:1 for cell cultures.

What is the Difference between conventional membrane bioreactor and reverse membrane bioreactor?

Conventional membrane bioreactor

The design of the traditional membrane bioreactor is either internal or external. The pressure gradient functions as an accelerating force. The mechanism for mass transfer occurs via the process of convection as well as diffusion. Microbial or other types in living cells get fed with the feed , and they are able to freely circulate within the medium.

Reverse membrane bioreactor

For RMBR, it’s an type of bioreactor with an embedded structure in which the concentration gradient functions as a force driving the process and the mechanism for mass transfer occurs by diffusion. Living cells are not able to be fed alongside the feed. They have to be isolated and kept inside their membranes.

Application example of bioreactors

The cultivation for acquiring vaccines against the SARS-CoV-2

Recent efforts have been focused on the development of a vaccine against the SARSCoV2 virus. Bioreactors are utilised in the manufacture of each and every vaccination. For instance, the procedure for developing two types of coronavirus vaccines that have already been clinically validated is as follows:

​RNA vaccines are produced utilising a novel technique that was formerly exclusive to veterinary medicine. No RNA vaccination for human use has yet been licenced. This vaccine comprises a viral component that resembles the structure of human messenger RNA (mRNA). After entering human cells, the ribosomes use the mRNA to produce a viral protein. The protein in question promotes an immunological response in the human body, so creating natural resistance to the virus. Vaccine operations including Pfizer-BioNTech and Moderna utilise this technology.

​For the production of these types of vaccines, a host organism that can cultivate huge quantities of viral mRNA is necessary. The Escherichia coli bacterium is the organism most commonly used for such applications. It is possible to develop the bacteria in bioreactors with a working volume of up to 10 m3 (and even bigger) because the procedure can be carried out in bioreactors made of stainless steel. This indicates that the procedure is easily scalable. Nonetheless, this type of cultivation method cannot be conducted in disposable (single-use) bioreactors, as they cannot typically offer the extensive mixing and aeration required for E. coli growth and mRNA generation. The other type of vaccinations utilise a weakened adenovirus that carries a specific viral protein for inducing an immune response.

​These vaccines are examples of non-replicating viral vectors, as they employ an adenovirus shell encoding a SARSCoV2 protein and carrying DNA that encodes the protein. Vaccines against the coronavirus based on viral vectors are non-replicating, meaning they are incapable of creating new viral cells; instead, they produce just the antigen that induces a systemic immune response. This category of vaccines includes the COVID-19 vaccine by Oxford–AstraZeneca, Sputnik V (Russia), Convidicea (China), and Ad26.COV2.S by Johnson & Johnson.

​For the production of these types of vaccinations, mammalian cell cultures are utilised. In such cultivations, single-use bioreactors can be utilised, allowing for comparatively straightforward bioreactor-based production facility growth. The highest working volume of reliable single-use bioreactors now available on the market is around 2,000 litres. However, announcements of bigger volume single-use bioreactors have been reported very lately. ABEC, a global provider of integrated solutions and services for biopharmaceutical manufacturing, recently announced the availability of single-use bioreactors with a maximum operating volume of 6,000 litres.

Applications of bioreactor

Bioreactors are used in a wide variety of applications, including:

• Industrial biotechnology: Bioreactors are used to produce a variety of bioproducts such as enzymes, antibiotics, and biofuels on a large scale.
• Cell culture: Bioreactors are used to culture and grow cells, such as stem cells, for research and therapeutic purposes.
• Tissue engineering: Bioreactors are used to create three-dimensional structures of living tissue for medical applications.
• Environmental biotechnology: Bioreactors are used to treat waste materials and pollutants, such as sewage and industrial waste, by using microorganisms to break down the contaminants.
• Food and beverage production: Bioreactors are used to ferment foods such as yogurt, cheese, beer, and bread.
• Pharmaceuticals: Bioreactors are used to produce vaccines, monoclonal antibodies, and other therapeutics.
• Research and Development: Bioreactors are used in research to study the behavior of cells, microorganisms and their metabolic pathways in different conditions.

Overall bioreactors are used in a wide range of fields, from the production of consumer goods to the development of new medicines, and from the treatment of waste to the production of biofuels.

Type of bioreactor	Applications
Stirred tank fermenter	Stirred tank bioreactors (STBRs) are the reactors most widely employed for culturing of biological agents such as cells, enzymes, or antibodies. They are contactors where the well-mixed among phases is obtained mainly by internal mechanical agitation. Antibiotics, citric acid, Exopolysaccharides, cellulose, Chitinolytic enzymes, Laccase, Xylanase, Pectic, and pectate lyase, Tissue mass culture, Lipase, Polygalacturonases, Succinic acid
Bubble column fermentor	Bubble column reactors are extensively used in carrying out gas-liquid and gas-liquid-solid reactions in a variety of important industrial reactions, including hydrogenation, oxidation, hydroformylation, chlorination, bioreactions and so on.
Airlift fermentor	Antibiotics, Chitinolytic enzymes, Exopolysaccharides, Gibberelic acid, Laccase, Cellulase, Lactic acid, Polygalacturonases, Tissue mass culture
Fluid bed fermentor	Today, fluidized bed reactors are still used to produce gasoline and other fuels, along with many other chemicals. Many industrially produced polymers are made using FBR technology, such as rubber, vinyl chloride, polyethylene, styrenes, and polypropylene.
Packed bed fermentor	These bioreactors are widely applied for valorization of food, beverage, nutraceutical synthesis, as well as waste treatment.
Photobioreactor	A photobioreactor (PBR) refers to any cultivation system designed for growing photoautotrophic organisms using artificial light sources or solar light to facilitate photosynthesis. PBRs are typically used to cultivate microalgae, cyanobacteria, macroalgae, and some mosses.
Membrane bioreactor	Membrane bioreactors have been in use for a variety of applications during the last few decades. This ranges from food and biofuel production to amino acids, antibiotics, proteins, and fine chemicals manufacturing; the removal of pollutants; and wastewater treatment.
Rotary Drum Bioreactor	Rotating drums bioreactors were first utilized for the production of amylase in SSF by. Then, in the 1940s, rotating drum bioreactors were improved and applied in the commercial-scale production of penicillin.
Mist Bioreactor	for in vitro culture of differentiated tissue, in plant micropropagation, and in the culture of transformed (hairy) roots for secondary metabolite production.
Immobilized cell bioreactor	A variety of immobilized cell bioreactors have been developed to optimize the fermentation processes. Immobilized cells are currently being used industrially for vinegar, organic, and amino acid production, as well as in wastewater treatment.
Activated sludge bioreactor	municipal as well as industrial wastewater treatment.
Immersed membrane bioreactor	The EMBR is used to prevent contact between the SRB and wastewater. In EMBR, the wastewater is selectively passed over one surface of the membrane, while the microbial culture is maintained on the other side.

Advantages of bioreactor

Bioreactors offer several advantages over traditional methods of growing microorganisms or cells, including:

• Controlled conditions: Bioreactors allow for precise control of temperature, pH, oxygen levels, and other environmental factors, which can greatly enhance the growth and productivity of the culture.
• Scalability: Bioreactors can be scaled up or down to meet the desired production volume, allowing for easy expansion or reduction of production.
• Automation: Bioreactors can be fully automated, allowing for precise control and monitoring of the culture without the need for constant human intervention.
• Sterility: Bioreactors can be sterilized to prevent contamination, which is important for industrial and medical applications.
• High productivity: Bioreactors can produce large amounts of cells, microorganisms or products per unit volume, that make it cost-effective.
• Repeatability: Bioreactors provide a controlled environment which allows for reproducibility of experiments and production.
• Versatility: Bioreactors can be used to grow a wide variety of microorganisms and cells, making them useful in many different fields.
• Bioreactors can be used to simulate the growth conditions of microorganisms in nature, and to study their behavior under different conditions.

Overall, bioreactors can be used to improve the efficiency, productivity, and repeatability of many industrial, medical and research processes, allowing for the production of high-quality, consistent products.

Disadvantages of bioreactor

1. High cost: Bioreactors can be expensive to purchase and maintain, which can be a barrier for some companies or researchers.
2. Technical complexity: Bioreactors can be difficult to operate and require specialized knowledge and skills to run effectively.
3. Contamination risk: Bioreactors are closed systems, which can increase the risk of contamination if not properly sterilized and maintained.
4. Scale-up challenges: Scaling up a bioreactor process from a small lab scale to a large-scale commercial operation can be challenging and may require significant optimization and experimentation.
5. Limited product diversity: Bioreactors are typically optimized for specific types of products, such as proteins or enzymes, and may not be well-suited for producing other types of products.
6. Environmental concerns: Bioreactors require a steady supply of energy, which can contribute to greenhouse gas emissions and other environmental impacts.

FAQ

References

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