A bioreactor may refer to any manufactured or engineered device or system that
supports a biologically active environment.[1] 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 bioreactor may refer to any manufactured or engineered
device or system that supports of cell or particle attachment or entrapment. [2] It can be
applied to basically all types of biocatalysis including enzymes, cellular organelles,
animal and plant cells.[3] 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:
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moving media, also known as moving bed biofilm reactor (MBBR)
packed bed
fibrous bed
membrane
Bioreactor design
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
Moss photobioreactor with Physcomitrella patens
A photobioreactor (PBR) is a bioreactor which incorporates some type of light source
(that may be natural sunlight or artificial illumination). 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.[4] 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.
Personal Bioreactors
A simplified continuous bioreactor that is designed for non-professionals, enables the
growth of E. coli bacteria cells under aerobic or anaerobic conditions. These
bioreactors do not rely on autoclavability, but instead rely on chemical inactivation for
reuse. A good example of an equipment is the Bioproduction lab created by Amino
Labs. A laptop-sized Personal Bioreactor and Transformation station for
bioengineering. It includes: continuous culturing system with remote-monitoring, real
time date streaming, on-screen instructions, plate incubator, heat + ice-cold stations.
Suitable for bacterial growth and culturing.
Personal bioreactor for non-professionals
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.
Up and down agitation bioreactor
Unique up and down agitation in the bioreactor.
Up and down agitators are useful to avoid shear stress to the cells. These are done by
instead of a traditional propeller agitator, which requires an expensive motor and
magnetic coupling. Vertical up and down motion is achieved by a motor together with
an inexpensive membrane perfectly assure sterility and produce an efficient mixing
without formation of a vortex (no baffles needed). At the same time this type of mixing
is gentler on cells and produces less foam. Novel biomimicking “fish-tail” stirring discs
offer maximum mixing efficiency without cutting edges.[5]
NASA tissue cloning bioreactor
A bioreactor used to ferment ethanol from corncob waste being loaded with yeast.
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.[6]
For more information on artificial tissue culture, see tissue engineering.
Modelling of bioreactor
Mathematical models act as an important tool in various bio-reactor applications
including wastewater treatment. These models are useful for planning efficient process
control strategies and predicting the future plant performance. Moreover, these models
are beneficial in education and research areas.
Bioreactors are generally used in those industries which are concerned with food,
beverages and pharmaceuticals. The emergence of Biochemical engineering is of
recent origin. Processing of biological materials using biological agents such as cells,
enzymes or antibodies are the major pillars of biochemical engineering. Applications of
biochemical engineering cover major fields of civilization such as agriculture, food and
healthcare, resource recovery and fine chemicals.
Till now, the industries associated with biotechnology have been lagged behind other
industries in implementing control over the process and optimization strategies. A main
drawback in biotechnological process control is the problem to measure key physical
and biochemical parameters.[7]
Operational stages in a bio-process
A bioprocess is composed mainly of three stages — upstream processing, bioreaction,
and downstream processing — to convert raw material to finished product.
The raw material can be of biological or non-biological origin. It is first converted to
more suitable form for processing. This is done in upstream processing step which
involves chemical hydrolysis, preparation of liquid medium, separation of particulate, air
purification and many other preparatory operations.
After upstream processing step, the resulting feed is transferred to one or more
Bioreaction stages. The Biochemical reactors or bioreactors form the base of the
Bioreaction step. This step is mainly consists of three operations namely, production of
biomass, metabolize biosynthesis and biotransformation.
Finally, the material produced in the bioreactor must be further processed in the
downstream section to convert it into more useful form. The downstream process is
mainly consists of physical separation operations which includes, solid liquid
separation, adsorption, liquid-liquid extraction, distillation, drying etc.[8]
Specifications of a bioreactor
A typical bioreactor consists of following parts:
Agitator – used for the mixing of the contents of the reactor which keeps the “cells” in
the perfect homogenous condition for better transport of nutrients and oxygen to the
desired product(s).
Baffle – used to break the vortex formation in the vessel, which is usually highly
undesirable as it changes the center of gravity of the system and consumes additional
power.
Sparger – In aerobic cultivation process, the purpose of the sparger is to supply
adequate oxygen to the growing cells.
Jacket – The jacket provides the annular area for circulation of constant temperature of
water which keeps the temperature of the bioreactor at a constant value.[9]
Development of modelling equations for bioreactors
Assumptions –
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The reactor contents are perfectly mixed together.
The reactor is operating at a constant temperature (i.e., it is isothermal).
The feed should be clean and pure (i.e., no biomass in the feed stream).
The feed stream and reactor contents have equal and constant density (ρ).
The feed and product streams have the same flow rate (F).
Total continuity equation
Making overall mass balance, we get the following equation:
d(ρV)/dt = Fρ – Fρ = 0 (1)
Equation(1) states that the reactor volume (V) is constant since dV/dt = 0.
Biomass continuity equation
We know,
Flow rate of biomass into the reactor = Fxi
Flow rate of biomass out of the reactor = Fx
Rate of generation of biomass by reaction = Vr1
Rate of accumulation of biomass within the reactor = d(Vx)/dt
Now, apply general mass balance equation i.e,
Rate of Mass In – Rate of Mass Out + Rate of Generation = Accumulation
d(Vx)/dt = Fxi – Fx + Vr1 (2)
Where r1 is the rate of cell generation. Dividing both sides of the above equation by V,
we obtain
dx/dt = (F/V)xi – (F/V)x + r1 (3)
In the chemical reaction engineering, F/V is called space velocity(s−1) and V/F is
called the residence time (s). But in biochemical engineering, F/V is known as
Dilution rate (Dr). Accordingly, equation(3) yields:
dx/dt = Drxi – Drx + r1 (4)
dx/dt = Dr(xi – x) + r1 (5)
Substrate continuity equation
For substrate balance,
Flow rate of substrate into the bioreactor = FSi
Flow of the substrate out of the bioreactor = FS
Rate of generation of substrate by reaction = –Vr2
Rate of accumulation of substrate within the reactor = d(VS)/dt
Now, apply general mass balance equation i.e,
Rate of Mass In – Rate of Mass Out + Rate of Generation = Accumulation
d(VS)/dt = FSi – FS – Vr2 (6)
rearranging above equation, we get
dS/dt = Dr(Si –S ) – r2 (7)
where r2 is the rate of substrate consumption.
Biochemical reaction kinetics
For the chemical reaction,
A ----> P
We can write
( –rA) = k (CA)n (8)
(rA) = – k (CA)n (9)
Where,
( –rA) = rate of disappearance of A
(rA) = rate of formation of A
k = reaction rate constant
CA = Concentration of reactant A
n = order of reaction with respect to component A
For first order reaction, n = 1 and accordingly,
–rA = k CA
The reaction kinetics involved in biochemical operations is comparatively difficult to
obtain than the chemical reaction kinetics. In biochemical operations, the cell kinetics is
used for the unstructured models where balanced growth condition is assumed.
The following equation is used to represent the net rate of cell mass growth:
r1 = μx (10)
where μ is the specific growth rate or specific growth rate coefficient(s−1). Here, μ is
analogous to first order rate constant k but however, μ is not a constant.
In biochemical engineering, yield is defined as the ratio of mass or moles of product
formed to the mass or moles of the reactants consumed. The yield (Y) of product (P)
with respect to reactant A is defined as:
Y = (mass of P formed )/(mass of A consumed) (11)
In case of bioreactor,
Y = (mass of cells formed)/(mass of substrate consumed) (12)
Thus,
Y = r1/ r2
Or,
r2 = r1/Y
Or,
r2 = μx/Y ( from 10) (13)
Final form of equation of modelling
By substituting equations (10) & (13) in equations (5) & (7) respectively, we get,
dx/dt = Dr(xi – x) + μx (14)
dS/dt = S(Si – S) – (μx/Y) (15)
Since we have assumed that the feed stream does not contain any biomass i.e., xi = 0,
then, bioreactor modelling equation finally get the following form:
dx/dt = (μ – Dr)x (16)
dS/dt = S(Si – S) – (μx/Y) (from 15)
Thus,Equations (15) and (16) are the basic equations which are used for the modelling
of any bioreactor.