A
bioreactor may refer to any manufactured or engineered device or system
that supports a biologically active environment. In one case, a
bioreactor is a vessel in which a chemical
process
is carried out which involves organisms or biochemically active substances derived from such organisms. This process can either be aerobic or anaerobic.
These bioreactors are commonly cylindrical, ranging in size from litres to
cubic metres, and are often made of stainless
steel.
A
bioreactor may also refer to a device or system meant to grow cells or tissues in the context of cell
culture.
These devices are being developed for use in tissue engineering or biochemical engineering.
On
the basis of mode of operation, a bioreactor may be classified as batch, fed batch or continuous (e.g. a continuous stirred-tank
reactor model).
An example of a continuous bioreactor is the chemostat.
Organisms
growing in bioreactors may be submerged in liquid medium or may be attached to
the surface of a solid medium. Submerged cultures may be suspended or
immobilized. Suspension bioreactors can use a wider variety of organisms, since
special attachment surfaces are not needed, and can operate at much larger
scale than immobilized cultures. However, in a continuously operated process
the organisms will be removed from the reactor with the effluent.
Immobilization is a general term describing a wide variety of cell or particle
attachment or entrapment. It can be applied to
basically all types of biocatalysis including enzymes, cellular organelles,
animal and plant cells. Immobilization is
useful for continuously operated processes, since the organisms will not be
removed with the reactor effluent, but is limited in scale because the microbes
are only present on the surfaces of the vessel.
Large
scale immobilized cell bioreactors are:
- moving media, also known as Moving Bed Biofilm Reactor (MBBR)
- packed bed
- fibrous bed
- membrane
· Bioreactor design
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A closed bioreactor used in cellulosic
ethanol research
· Bioreactor design is
a relatively complex engineering task, which is studied in the discipline of biochemical engineering. Under optimum conditions, the microorganisms
or cells are able to perform their desired function with limited production of
impurities. The environmental conditions inside the bioreactor, such as
temperature, nutrient concentrations, pH, and dissolved gases (especially
oxygen for aerobic fermentations) affect the growth and productivity of the
organisms. The temperature of the fermentation medium is maintained by a
cooling jacket, coils, or both. Particularly exothermic fermentations may
require the use of external heat exchangers. Nutrients may be continuously
added to the fermenter, as in a fed-batch system, or may be charged into the
reactor at the beginning of fermentation. The pH of the medium is measured and
adjusted with small amounts of acid or base, depending upon the fermentation. For
aerobic (and some anaerobic) fermentations, reactant gases (especially oxygen)
must be added to the fermentation. Since oxygen is relatively insoluble in
water (the basis of nearly all fermentation media), air (or purified oxygen)
must be added continuously. The action of the rising bubbles helps mix the
fermentation medium and also "strips" out waste
gases, such as carbon dioxide. In practice, bioreactors are often pressurized;
this increases the solubility of oxygen in water.In an aerobic process,
optimal oxygen transfer is sometimes the rate limiting step. Oxygen is poorly soluble in
water—even less in warm fermentation broths—and is relatively scarce in air (20.95%). Oxygen
transfer is usually helped by agitation, which is also needed to mix nutrients
and to keep the fermentation homogeneous. Gas dispersing agitators are used to break up
air bubbles and circulate them throughout the vessel.
· Fouling can harm the overall
efficiency of the bioreactor, especially the heat
exchangers.
To avoid it, the bioreactor must be easily cleaned. Interior surfaces are
typically made of stainless steel for easy cleaning and sanitation. Typically
bioreactors are cleaned between batches, or are designed to reduce fouling as
much as possible when operated continuously. Heat transfer is an important part
of bioreactor design; small vessels can be cooled with a cooling jacket, but
larger vessels may require coils or an external heat exchanger.
· Types of Bioreactors
· Photobioreactor
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· A photobioreactor (PBR) is a
bioreactor which incorporates some type of light source. Virtually any translucent container could be
called a PBR, however the term is more commonly used to define a closed system,
as opposed to an open tank
or pond. Photobioreactors
are used to grow small phototrophic organisms such as cyanobacteria, algae, or moss plants. These organisms use
light through photosynthesis as their energy source and do not
require sugars or lipids as energy source.
Consequently, risk of contamination with other organisms
like bacteria or fungi is lower in
photobioreactors when compared to bioreactors for heterotroph organisms.
· Sewage treatment
· Bioreactors are also
designed to treat sewage
and wastewater. In the most
efficient of these systems, there is a supply of a free-flowing, chemically
inert medium which acts as a receptacle for the bacteria that break down the
raw sewage. Examples of these bioreactors often have separate, sequential tanks
and a mechanical separator or cyclone to speed the separation of water and
biosolids. Aerators
supply oxygen to the sewage and medium, further accelerating breakdown. Submersible
mixers
provide agitation in anoxic bioreactors to keep the solids in suspension and
thereby ensure that the bacteria and the organic materials "meet". In
the process, the liquid's Biochemical Oxygen Demand (BOD) is reduced
sufficiently to render the contaminated water fit for reuse. The biosolids can
be collected for further processing, or dried and used as fertilizer. An
extremely simple version of a sewage bioreactor is a septic tank whereby the
sewage is left in situ, with or without additional media to house bacteria. In
this instance, the biosludge itself is the primary host (activated sludge) for
the bacteria. Septic systems are best suited where there is sufficient
landmass, and the system is not subject to flooding or overly saturated ground,
and where time and efficiency are not prioritized.[citation needed]
· Because they are the
engine that drives biological wastewater treatment, it is critical to closely
monitor the quantity and quality of microorganisms in bioreactors. One method
for this is via 2nd Generation ATP tests.
· NASA tissue cloning
bioreactor
· In bioreactors in
which the goal is to grow cells or tissues for experimental or therapeutic
purposes, the design is significantly different from industrial bioreactors.
Many cells and tissues, especially mammalian ones, must have a surface or other
structural support in order to grow, and agitated environments are often
destructive to these cell types and tissues. Higher organisms, being auxotrophic, also require highly
specialized growth media.
· NASA has developed a new
type of bioreactor that artificially grows tissue in cell cultures. NASA's
tissue bioreactor can grow heart tissue, skeletal tissue, ligaments, cancer
tissue for study, and other types of tissue.
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