Bioreactor

A bioreactor, often referred to as the fermenter is a container, cultivated under the best possible conditions in which certain microorganisms, cells, or small plants (also: fermented) are. The operation of a bioreactor is therefore an application of biotechnology, biological processes ( biotransformation, biocatalysis) use in equipment or makes available.

Important factors, which are controllable or controllable in most bioreactors, the composition of the nutrient medium (including broth or substrate), the oxygen supply, temperature, pH, sterility, and others. The purpose of the cultivation in a bioreactor can be the recovery of the cells or components of cells, or the production of metabolic products. These can be used, for example as the active ingredient in the pharmaceutical or as basic chemicals in the chemical industry. Also, the degradation of chemical compounds can take place in bioreactors, such as in wastewater treatment in sewage treatment plants. The production of beer, wine and other generated for thousands of products also takes place in bioreactors. Unlike modern applications are usually meant for these classic examples of bioreactors but not used, the traditional expressions (eg brewing kettle in beer production ).

In bioreactors variety of organisms for various purposes can be cultivated. Therefore, several reactor types are available in various designs. Typical stirred tank reactors are made ​​of metal, which can have up to thousands of liters a volume of a few and be filled with nutrient solution. But even widely differing variants, such as fixed-bed reactors, photobioreactors, etc. are used.

• 3.1 reactor types 3.1.1 distinction by Rührtechnik
• 3.1.2 distinction on the structure
• 3.1.3 reusable and disposable reactors

• 3.2.1 sewage treatment plants
• 3.2.2 biogas plants
• 3.2.3 breweries and wineries
• 3.2.4 pharmaceutical and cosmetic industries

History

Since brewing kettle technically count in breweries to the bioreactors, one can equate the appearance of the first bioreactors with the appearance of the first breweries about 5500 years ago. The common devices for thousands of years for producing different milk products with the aid of bacteria, or enzymes may be referred to as bioreactors.

With the advancement of biotechnology, especially by significant advances in microbiology in the 19th century and the genetics, molecular biology and genetic engineering from the mid-20th century, more and more applications could be developed as in the chemical industry and in the field of pharmaceutics. In many biotechnological processes thereby bioreactors are used.

Operating parameters

A bioreactor has the main purpose to provide the highest possible yields. This is achieved in particular through the creation of optimal conditions for the organism being used. This is adapted to various parameters that prevail in its natural habitat. The importance of the nature and concentration of nutrients, temperature, oxygen content of the pH - value, etc., are usually also an agitator or other means is necessary to provide a homogeneous setting of these parameters over the whole reactor area. In addition to the requirements of the organisms other technical, organizational and economic factors must be considered that influence the choice of the operating parameters. Examples include the prevention of foam formation and the choice of either a continuous or a batch operation.

With the help of probes or sensors, many of these parameters are measured directly in the culture medium or in the exhaust air. Using these parameters, the process development also is usually assessed. The cell density can be determined by measuring the absorbance ( optical density) determined, which can in turn deduce the amount of product. An alternative is often to measure the concentration of a typical chemical compound, such as the increase in the concentration of a metabolic product or the decrease of the substrate concentration.

At the beginning of fermentation, the nutrient medium is mixed with a small amount of recovered from a microorganism pre-cultivation. This amount is called the inoculum, the process is often referred to as seeding. The suspension obtained from the fermentation process ( broth ) is processed in the so-called downstream processing in several steps.

Nutrient

With the growth medium the organisms must be provided with all the nutrients needed for growth. This information can include the required in larger quantities main nutrients ( Makronährelemente ), such as carbon, nitrogen and phosphorus. Also, various trace elements ( Mikronährelemente ) are required. Depending on the organism, other compounds necessary which can not be synthesized even (vitamins, essential amino acids, etc.). And an energy-supplying compound, such as is often the sugar glucose, are required (except for phototrophic organisms).

Temperature

Organisms have an optimum temperature at which they reproduce best. Exceeding this temperature can cause irreversible damage by denaturing the proteins, a shortfall leads to lower metabolic rates and thus to longer process times. Temperature control is achieved by heating and cooling cycles. When the reactor, the entire reactor contents are heated or warmed up to operating temperature. Partial produce the cultured organisms by their metabolism so much heat that from a certain cell concentration, only the cooling circuit is active. In this cycle, a heat exchanger can be integrated or the energy-carrying medium is fed directly. The heat exchange surfaces to the reaction chamber are usually here only twice the tank wall, in rare cases, built-in cooling registers available.

Oxygen content

Fermentation batches, depending on the organism and product are carried out aerobically ( in an atmosphere containing oxygen ) or anaerobic ( oxygen-free). Oxygen is poorly soluble in water, so that a sufficient supply of aerobic approaches is difficult. The solubility of oxygen in a fermentation medium having a temperature of 37 ° C, for example, at 3-5 mg / L. The oxygen partial pressure may be controlled by various methods:

• Changing the gas flow rate,
• Changing the stirrer speed,
• Change in the Rührwerkzeuggeometrie,
• Evolution of the gas mixture composition,
• Change in the pressure head ( in this case, however, is also the solubility of other gases, for example carbon dioxide, is increased).

However, the disturbing foam formation is increased by vigorous injection of gas or high stirrer.

In obligately anaerobic organisms, however, a supply of oxygen must be avoided, since it can be toxic. In anaerobic approaches with facultative anaerobic organisms oxygen supply would allow unwanted aerobic reactions which could reduce the process yield.

The cultured organisms usually have a limited pH tolerance range with a pH-optimum. The pH can be controlled automatically by a pH sensor coupled to the pump, which, as required for the acidification, for example, phosphoric acid (H3PO4 ), hydrochloric acid (HCl) or to increase the pH, for example, caustic soda (NaOH ) in the bioreactor pumps. In certain cases, the pH can also be achieved on the rate of feeding with the substrate.

Homogenization

Most bioreactors have an agitator, such as a stirrer or a gas injection, through which the medium is circulated. This ensures a homogeneous set various parameters in the entire reactor and thus for a smoother process flow.

Foaming

The problem is often the development of foam by agitating, what the air filter clogged and the cultured cells can strain mechanically. Chemical defoamer ( antifoaming agent ), act by reducing the surface tension. Negative is the influence of gas transport and the poor separability of the reaction solution in downstream processing ( product processing ).

Mechanical defoaming agents such as foam breakers crush the foam, but do not remove the foam-forming factors, such as dead cells. When the foam is derived Schaumabscheidern and liquefied again, and can be subsequently pumped out.

Continuous or batch operation

In the operation of a fermenter can be distinguished:

• Batch mode: charging the reactor, no addition or subtraction during the fermentation process, easy control, contamination unlikely
• Fed - batch operation: similar to the batch mode; However, for example, substrate addition during the process as an initially high substrate concentration may be inhibitory
• Continuous operation in the chemostat bioreactor: continuous operation by substrate addition and product removal, complex control system, contamination is problematic, but expensive and complex downstream processes can also be carried out continuously, and thus optimal utilization

In research rather batch fermentations are performed while for larger production facilities the establishment of a continuous operation can be economically viable.

Reactors

Reactor types

The mandatory for the test organism or for technical, organizational and other reasons, the operating parameters are very different. Therefore an appropriate bioreactor is to design for the specific use or it can be used a type of reactor, in which the various parameters can be controlled in a wide window so that it can be used for various purposes. A common type is the begasbare stirred tank reactor in different variants ( material, size, etc.).

Distinction by Rührtechnik

In each bioreactor, the three phases solid ( biomass), liquid ( broth ) and gaseous ( for example, air, oxygen, carbon dioxide, nitrogen) find. In the bioreactor, whose distribution is kept homogeneous with different measures:

• Moving mechanical components ( agitators): eg in stirred tank reactor
• External pump circuit: the liquid is circulated by a pump, eg jet reactor
• Blowing gas: the gas phase is blown into the liquid part such as airlift reactor or bubble column reactor

If these forms reactor provided with guide tubes, then there are the following types of reactors:

• Propeller loop reactor ( a reactor, in which energy is introduced by an axially -enhancing downward stirring element and is provided with a guide tube )
• Jet loop reactor ( a jet reactor with a draft tube )
• Lift loop reactor ( an airlift reactor or a bubble column reactor with a draft tube )

Distinction on the structure

A further distinction can be made according to the type of reactor construction:

• Stirred tank reactor: common type; the liquid phase is circulated by an agitator; as needed done fumigation
• Fixed-bed reactor: The reactor is filled with a solid, porous matrix on which the organisms can be immobilized ( or enzymes ); the organisms remain so in the reactor, instead of being flushed out with the medium ( or the enzyme is reduced demand ) so that the growth of the organisms is a less limiting factor
• Rieselstromreaktor ( trickling filter ): a fixed bed is sprayed with liquid (eg for clarifying waste water); the degrading organisms sit on the surface; countercurrently for the degradation required air (oxygen) is introduced
• Photobioreactor (algae reactor, hydrogen bioreactor ) for the cultivation of photosynthetic organisms ( algae, plants (cells) ); The reactor is made of glass so that the light reaches to the necessary organisms; the light utilization is optimized through a plate or tubular reactors (see below: tubular reactor )
• Tubular reactor: in tubular reactors can result in a plug flow, which is in certain fermenters exploited eg in biogas plants; already degraded and fresh material (substrate ) is not completely mixed, which may have several advantages
• MBR: a reactor in which ( depending upon application), the reaction product, or the biomass, the purified water may be removed continuously through membranes. Applications include wastewater treatment (MBR ), recovery of lactic acid and pharmaceutical products.

Several stirred tank reactors in series form a cascade reactor ( ' cascade of stirred vessels '). Especially in the research and development process of parallel bioreactor systems of four, eight or sixteen reactors are used increasingly.

In research and laboratory fermenters or stirred tank reactors often small Erlenmeyer flasks are used, which are attached to the stirring of the medium to a so-called shaker.

Previously dominated solids bioreactors due to the simpler process control in some areas. The liquid culture, also called submerged fermentation was difficult to control, dominate today but because of various advantages, such as better ways to supply oxygen by stirring and gassing.

Reusable and disposable reactors

Most bioreactors made ​​of metal ( stainless steel) or glass are made. This allows an easy cleaning and sterilization, and thus the multiple use.

In the animal cell culture technology, however, single-use bioreactors are used in the form of pre-sterilized disposable bags increased. These consist of composite film. By the way the technology -consuming cleaning and sterilization processes are avoided, resulting in particular in the production of biological preparations to significant shorter makeready times and thus cost savings. Most single-use bioreactors are not stirred tank reactors. The circulation takes place instead through a teeter-totter.

Applications

Sewage treatment plants

Very large bioreactors can be found in wastewater treatment plants with biological process stages. In the activated sludge process initially takes place an aerobic step will be bound in the dissolved compounds by microorganisms in the form of biomass formed. This biomass can ultimately ( a digester gas ) are fermented in the digester for methane-rich sewage gas. Another method is the aerobic trickling filter.

Biogas plants

The bioreactors for biogas plants are usually referred to as a fermenter. The biomass used is degraded in an anaerobic process with several steps ( hydrolysis, acidogenesis, acetogenesis and methanogenesis ) into biogas and digestate. The tanks are hermetically sealed and have an agitator and different measuring, control and regulation technology devices ( MSR ) for process control.

Breweries and wineries

Also in the brewery or winery bioreactors are used, but which are here referred to, for example, as fermentation vats. The microorganisms used are here yeasts that convert the sugar in the must or the grape juice to alcohol and carbon dioxide ( CO2).

Pharmaceutical and cosmetic industries

The most valuable products are manufactured in bioreactors medical- pharmacological products such as the become known as a doping agent erythropoietin ( EPO) or modern insulins. As to drugs, a significantly higher standard of purity is provided as to foods, apply here with the Good Manufacturing Practice standards particularly strict regulations. All operating parameters of the bioreactor must be kept within narrow limits, and even the smallest deviation have the consequence that the entire batch must not be put into circulation. To exclude as many uncertainties, only very rarely full biological nutrient media used in these processes are used, but it is an optimized, synthetic mixture of the required nutrients used. This avoids that by variations in the substrate mitschwankt quality product quality. Depending on the desired product will come in the pharmaceutical industry different, mostly genetically modified microorganisms used.

Modeling

The processes in bioreactors can be described by the reaction kinetics, where you have to consider the specifics of biological processes in the modeling (eg Michaelis- Menten kinetics, Monod kinetics, enzyme kinetics, enzyme inhibition, etc.).