Bioreactor Design and Scale-Up

Bioreactor design is quite a discipline and forms the main topic of interest for anyone involved in producing biomass and products such as antibiotics to wine and cider. It is best to start with an overview of the requirements for a bioreactor before drilling down further into key elements.

Reviews abound on this subject and plenty of these are recommended. Please read Cooney (1983) which offers an overview of the subject and is summarised below. Also look at Pollard and Shearer (1977) and Maiorella et al., (1984).

Bioreactor Design

The starting point in an any bioreactor design is knowing what the product requirement(s) are, i.e. the product specification. Is it about generating biomass or a product? Biomass might be producing yeast or bacteria for consumption or it could be a product such as xanthan gum or an antibiotic (Asenjo & Merchuck, 1994).

Having identified the output of the reactor, it is a case of then identifying and selecting a biological system to generate that output. That means understanding what medium is needed and the stoichiometry around the inputs generating these outputs. Once there is an idea on what the outputs are to be, there are three elements that come to be considered:-

• the upstream constraints
• kinetics
• heat and mass transfer constraints

When enough knowledge has been gathered on these three elements, it is then feasible to decide on the type of bioreactor to be used and then to consider the fundamental issue of bioreactor system design.

Bioreactor design relies on deep knowledge about heat and mass transfer rates, on mixing requirements, the type of instrumentation and control needed, what media is needed and how the whole bioreactor is sterilized and contained.

Once those features are established it then comes down to downstream processing which is about separating the product of interest from what is not wanted. In some cases it could just be the whole fermentation that is asked for with biomass and media as in a live-yeast cider or untreated live yeast beer. It might be more specific however such as a purified antibiotic. 

Throughout this process, there are refinements and feedback to earlier points in the design to improve production and make it easier and more straightforward to produce the product to specification.

Scale-Up Considerations   

Scaling up a bioreactor is an important process and economic step in the biotechnology and bioprocessing industries, especially when transitioning from laboratory-scale production to larger industrial-scale operations. 

The stages of bioreactor can be thought of as scales i.e.

• bench scale (2 -20 litres)
• pilot scale (100-500 litres)
• industrial and plant scale (500-20,000 litres).

The process of scale-up can best be thought of as increasing the size and capacity of the bioreactor whilst maintaining the performance and integrity of the bioprocess.

Another way of presenting the subject is to discuss scale-up as a ‘careful calculation of the geometric relations of the large-scale bioreactor to avoid heterogeneities’ (Palomares & Ramirez, 2009). It means that when increasing the size of the bioreactor which means increasing the volume of the bioreactor, it brings about alterations in the physical processes that contribute to bioreactor performance. The heterogeneities are the unforeseen consequences that occur to the performance of the bioreactor. 

In most instances scale-up will primarily be about using sizing-up to improve the economy of the process because producing a product or biomass becomes cheaper the more that is made. It certainly does not mean making the production of the product whatever it happens to be more expensive! It is also the case that some manufacturer’s avoid scale-up issues in production by scaling-out which means increasing the number of units operating. The size of each unit remains the same but the number involved in production rises.

The Basic Function Of The Bioreactor

The function of a bioreactor is based on a number of chemical engineering requirements. It is the growth of a microorganism using substrates mostly in a liquid medium but in some cases on semi-solid and solid structures.

• Containment – to keep the fermentation or bioreactor within a containment facility that it is separated from the external environment. It ensures sterility and is designed to prevent contamination.
• Introduction of gaseous reactants – to ensure an adequate supply of oxygen for the microorganism to use as an energy source, in aerobic fermentations only.
• Introduction of liquid reactants -These are intended for the microorganism to thrive on. Most carbohydrates are energy molecules and liquid sugar is the most prevalent.
• Removal of gaseous products – removal of carbon dioxide as gas is the biggest effluent.
• Control of the physical environment – control of temperature and pH is normally required. Shear rates have to be limited to avoid cell damage.
• Suspension – cells and particulate have to be suspended in solution to make sure there is an appropriate amount of substrates they can feed upon whilst effluent out and product is removed.
• Dispersion – there should be mixing of the system to ensure homogeneity.

Metabolic Processes

Changes in scale will impact microbial kinetics and thus physiology. If mixing is poor, then concentration gradients are created. Poor mixing also produced shear stress. In any large-scale aerated reactor there will be gradients with respect to oxygen. In fed-batch and continuous operation, it is likely to be the provision of substrate which then becomes limiting.

Process Development and Optimization

Before scaling up, it’s essential to thoroughly understand and optimize the bioprocess at the laboratory scale. This includes determining the optimal growth conditions, media composition, and other process parameters. From lab-scale we move to pilot plant-scale which is a demonstration of whether increasing the size of the bioreactor presents physical issues or alters the metabolic processes of the microorganism and then onto factory-scale production where lessons learnt at the pilot-scale level will have offered up solutions.

Scale-Up Criteria

We have to define the criteria for scaling up, such as the desired production volume, productivity, and product quality. Consider factors like mixing, oxygen transfer, and heat transfer rates as the top three priorities.

Bioreactor Design

Design Criteria

There are a number of general design issues which need to be considered. This section is a precis of what Carl-Fredrik Mandenius considers important in bioreactor design (Mandenius, 2016). We present them here:

(1) Gas transfer in submerged culture. The purpose  of working on this aspect is to achieve a high growth rate as well as avoiding oxygen starvation. The design methods and parameters encompass reactor geometry (aspect ratios), sparger design (K_la), position and size of baffles (OTR), impellar geometry (CER) and use of overpressure (OUR).

(2) Mixing efficiency. Understanding this parameter means avoiding gradients of heat, nutrients and additives. It also reduces stress on the microorganism and on reducing power. The design methods and parameters encompass impellar geometry (aspect ratios), baffles (mixing time t), mixing analysis (power number) and CFD.

(3) Nutrient supply and addition. Understanding this parameter concerns the efficient transfer of nutrients to bioreactor volume. The means of design concerns feeding regimes and the use of multiple ports. The parameters assessed are the linear and exponential profile.

(4) Liquid-solid transfer. Its purpose is to enhance reaction rate and reduce gradients in transfer. The means of design cover flow distributors and porous supports. The parameter of interest is the Thiele modulus.

(5) Heat transfer. The purpose of knowing this is the efficient removal of metabolic heat. Design means using internal coils, the recycling of media, employing heating and cooling jackets and using cooling media. Heat transfer relies on dimensionless numbers.

(6) Sterility. In this situation we must ensure the whole unit is free of foreign microorganisms to avoid infection. In this situation the design means covers sterilization procedures, using overpressures, exploiting barriers for containment and the use of microfilters. The parameters measured here are sterilization time and temperature.

(7) Strain selection. Finding strains with properties adapted to media and reactor constraints. It relies on microbial analysis and various ‘omics’. It covers parameters on specific rates (u, qp, qs) and on inhibition constants.

(8) Scale-up Procedure. This concerns we have the same conditions at large-scale. It covers design geometry (aspect ratios, scale-up rule parameters) of vessels and impellars and the range of mixing (dimensionless numbers).

(9) Rheology. This covers the type of fluids in a bioreactor. One way of influencing this is assessing viscosity and exploiting additives that affect viscosity. It also brings in fluid flows and the way computational fluid dynamics (CFD) is now used. The parameters used here cover Reynold’s numbers, Navier-Stokes equations and CFD data.

(10) Homogeneity of culture. This means avoiding gradients for ideal reactor conditions. It exploits CFD and the parameter covers zonal analysis data.

(11) Media composition. The purpose here is to achieve a balanced culture media. It exploits factorial analysis and uses omics methods. Parameters are covered off using models.

Select or design a bioreactor suitable for the intended scale-up. Different types of bioreactors, such as stirred-tank reactors, airlift reactors, and fermenters, have different scaling considerations.

The stirred tank bioreactor (CSTR) which is usually devoted to batch fermentation has been the mainstay of the industry and has been designed with this type of operation in mind. There are some examples of large-scale plug-flow bioreactors but these are not as common.

A bioreactor is cylindrical with a ratio of reactor diameter to height (aspect ratio) of between 1:2 and 1:5. Type 316L stainless steel is the preferred construction material. It often has a rounded bottom to help with cleaning and sterilization in place and it avoids stagnant zones during operation.

All fermenters whatever their shape need some stirring system so that fresh medium can reach the microorganism. In a CSTR, it is commonly a shaft with blades on it. This stirrer shaft can be top-mounted or bottom-mounted, the latter being now more common. The placement of the stirrer into the vessel is designed in a way to prevent contamination. A double mechanical seal is most popular.

Foam breakers are often needed and are mounted on the stirrer shaft. These are commonly thin blades mounted above the liquid surface level and with a much larger diameter.

The bioreactor needs to be temperature controlled to avoid the fermentation getting out of control. Cooling can be through the reactor wall or by using internal coils. Sometimes the fermentation medium is pumped out of the reactor to be cooled or heated as desired. With small reactors, only wall cooling is needed but with larger reactors it may be necessary for internal cooling coils. Reactors can have baffles – usually 4. The baffle breaks any vortex that occurs in the reactor which would reduce mixing efficiency. The baffle is designed to be about 1/12 to 1/10 of the tank diameter.

A good example of how bioreactor design influences fermentation is the work over many years on beer production. Studies at a bench scale show that brewing yeasts are not affected by mechanical agitation up to 4.5 W/kg. Fermentations can be mixed by CO₂ gas evolution. When the brewing fermentation is scaled up in cylindroconical fermenters but without mechanical agitation, it is relying then on gas evolution. This is not enough to keep the yeast suspended and much of the yeast sediments into the cone for 50% of the bioreactor’s time and that also produces poor temperature control. In that study, a new jet mixer was employed which overcame any difficulties reported at pilot-scale (Nienow et al., 2011)

Solid-State Fermentation Scale-Up

One particular type of scale-up which is unique is to scale-up a solid-state fermentation. Various approaches have been tried including fixed- or packed-beds, rotating drums, gas–solid fluidized beds and various stirred bioreactors. 

Most of the strategies used for scale-up depend on semi-empirical methods. Heat removal is one major issue in the design of these types of bioreactor. The other, especially in submerged liquid fermentations is the supply of oxygen gas. 

Fixed Bed and Fluidized Bed Bioreactors

Whilst stirred tank fermenters are the most typical type of bioreactor, the fixed (packed bed bioreactors) and fluidized bed bioreactors have started to become more popular. These types are especially popular in the cultivation of mammalian cells because they allow for growth of immobilized cells within macroporous carriers at very high cell densities.

The fixed bed bioreactor is a vessel filled or packed with carrier material used as support for immobilization of cells. In fixed bed bioreactors the stationary bed of carriers is not agitated and in fluidized bed bioreactors the carrier is allowed to float.

The packed bed (fixed bed reactor) is often used in solid-state fermentation (SSF) If the air flows uniformly through the substrate bed, they provide good oxygen supply to the particles whilst avoiding or minimizing the need for mixing. These are used primarily for growing filamentous fungi. Too vigorous a mix and the fungi lose growth and suffer reduction in product formation because the fungal hyphae are damaged. The fixed-bed is ideal for this type of cell morphology. Some packed beds which are rarely mixed can be up to 12 metres high and are used for industrial koji manufacture. 

Large carriers are used for fluidized and fixed bed reactors because they have a high sedimentation rate. For fluidized bed reactors, carriers of a size from 0.6 to 1 mm are used. For fixed beds, much larger carriers between 3 and 5mm are preferred because they prevent blockage of free channels between the carriers. Larger carriers suffer from limitations in mass-transfer effects especially where oxygen supply is concerned.

Generally, in fluidized bed the specific gravity must be greater than 1.5 of the carrier is typically chosen to achieve homogenous fluidization in a fluidized bed bioreactor. Fluidized beds have been scaled up from laboratory scale (about 1.5L) to 40 Litres. 

In fixed bed reactors the specific gravity is not relevant because the carriers are retained within a fixed bed column using a porous mesh. In some cases, large fixed bed reactors are portioned using sinters to minimise the pressure drop through the column. 

One of the issues in scaled-up fixed bed reactors is the control of temperature because there is no mixing of any particular sort to disperse heat or warm up a reactor. An example where heat transfer limitations were explored with scale-up was the production of the enzyme pectinase. The scale-up was from 12 grams of dry substrate to 30 kg. One issue witnessed was bed compaction. With compaction the bed temperature rose from ambient to 47C. The level of pectinase activity varies from 17 to 20 × 10³ U/kg. 

 Airlift Reactor

Scale-up is based on the principle of geometric similarity. An airlift reactor is easier to scale-up compared to CSTRs. Greater height produces a pressure increase which means the oxygen content needs to rise and cause a rise in kLa.

The higher pressure produces more stress on microbial cells. The multiple feed points are needed. The time in a downcomer means cells will survive. Placing of baffles impacts coalescence and hold-up.

Scale-Up Ratios

Calculate the scaling ratios for various parameters, including volume, impeller speed, aeration rate, and nutrient feed rates. These ratios should maintain similar fluid dynamics and mass transfer characteristics as the laboratory-scale bioreactor.

One of the most important design considerations is how to use the data from laboratory-scale measurements such as a shake flask that can be modelled to estimate performance of a fermentation on a larger scale.

A major strategy was developed by Seletsky et al., (2007) who made empirical correlations from the volumetric mass transfer coefficient (k_L a) and the pH. They applied their knowledge to understanding the behaviour of Corynebacterium glutamicum on lactic acid in shake flasks and fermenters in either batch or continuous mode. They established the following values: maximum growth rate (µ_max = 0.32 h⁻¹) and the oxygen substrate coefficient ( = 0.0174 mol/l). These were the same for shake flask, fermenter, batch, and continuous cultures.

Of great interest was the biomass substrate yield that was independent of the scale, but was lower in batch cultures (Y_X/S = 0.36 g/g) than in continuous cultures (Y_X/S = 0.45 g/g).

In the fermentation of beet molasses by Aspergillus niger to produce citric acid it is key to maintaining a particular redox potential profile. The effective regulation of redox potential is by regulation of aeration and agitation (Berovic, 2000). The scale-up level is from 10 Litres to 1000 litres. and applies even when different types of stirred-tank reactors are used.

Physical Behaviour in Bioreactors

To make the point clearer, many of these approaches come from studies such as those of Oldshue (1966) who tabulated a series of scale-up criteria. The paper lists these as energy input, the energy input relative to volume, the impeller rotation number, impeller diameter, the pump rate of the impeller, the pump rate of the impeller relative to volume, the maximum impeller speed and finally the Reynolds Number. They set each parameter to 1 for a small fermenter of 80 litres. They then looked at how these parameters changed when they started using a production fermenter of 10,000 litres. They arbitrarily fixed particular parameters at 1 and noticed how these other parameters would change. It was clear that keeping one parameter constant always produced significant changes in all the other parameters. There was no consistency in terms of scale-up.

It is often assumed the equipment used in fermentation, especially the bioreactor will remain geometrically similar. The effects of different reactor sizes are compared using the characteristic length D which is the tank diameter. When the tank diameter increases 10-fold, all the other reactor parts such as tank height, impeller, etc. increase by the same proportion of 10-fold.

When scaling up these processes, the most important variable(s) need to be kept constant or within an acceptable range.  The common design objectives for scale-up are:

(1) keeping kLa (volumetric mass transfer coefficient) constant so that mass transfer is maintained.

(2) keeping the impeller tip speed constant so that the critical value of high shear velocity to avoid mechanical damage to cells, or to break up agglomerating pellets of mycelial cells in fungal fermentation.

(3) keeping the power input per volume constant usually with less power-intensive processes 

(4) keeping the mixing time constant

Mixing is a critical parameter because it means:-

• an even distribution of nutrients
• minimizing waste concentration
• control of pH, dissolved oxygen and temperature
• minimizing shear stress which is related to the impellar and fluid dynamics

The critical dimensionless number is the Reynolds Number which is used to predict flow patters in turbulent conditions. For turbulent flow in a fermenter, the Renolds Number Re must be greater than 10,000. Poor mixing will occur when the flow is laminar which is less than 10. It enters a transition period when Re is between10 and 10,000. 

The Reynolds Number is given as:-

Re = ρ*N*D2 / μ

where ρ is the fluid density, N is the impellar agitation rate, D is the impellar diameter and μ is the fluid dynamic viscosity.

The Re is based on a cylinder tank design with a single centered rotating impellar. It does not take into account impellar design itself.

Mixing Time

When it comes to highly viscous non-Newtonian broths the usual design equations do not apply. In such cases, robust and high quality agitation is the main goal. Such robust mixing is needed when adding antifoam agents, fed-batch ingredients and other solutions to the fermentation broth.

Providing robustness with a larger impeller or higher number of impellers is easier and yet much less energy efficient. You can however opt for a taller fermenter where the impeller diameter does not need to increase for robustness however a deep stack of broth can definitely be more troublesome. We can find the oxygen solubility at the the bottom of a deep fermenter near the sparger is significantly higher than at the surface. It inevitably generates an undesirable oxygen gradient but also significantly increases the gas power input (P/Vad³) and thus power requirements become prohibitive in large-scale fermenters. There is thus a trade-off. In such a case, engineers may choose to scale-up based on equal mixing or blending time.

Mixing time is defined as:-

t_m = V/Nd³

where V is the working volume (m3), N is the impeller rotational speed (rpm) and d is the impeller diameter (m).

If one of your strategies on scale-up is to keep a particular parameter constant then you notice detrimental changes in other parameters. For example the energy input per unit volume drops off if you keep the impellar tip speed constant which means that the oxygen transfer level also drops off.

 Keeping Energy Inputs Constant

Keeping energy input per unit volume Po/V constant will have a negative effect on which parameter. You see heat transfer drops and carbon dioxide – not for high energy inputs as it causes stress on the cells (Oldshue, 1966).

Impellers

One of the most important features of scaling-up a reactor is how the impeller used for mixing is set up. Each impeller decides the flow pattern, with how material is suspended and and then how it is dispersed. The two main types of impeller are the axial flow impeller which is a propeller type and radial flow impellers such as the flat blade turbine impeller. The most famous design is the Rushton turbine which has 6 equally spaced blades mounted on a disk. The impeller diameter is around 1/3rd of the reactor diameter for Rushton turbines, whereas fluid foil impellers can have a diameter exceeding half of the tank diameter.

The Rushton disk turbine is the most popular type used in fermenters. The rotational speed varies from 500 rpm in small laboratory fermenters to about 100 rpm in the larger fermenters. 

Aeration means introducing air or sometimes oxygen into the bioreactor via a sparger located below the lowest part of the impellers. The sparger can be a single open tube or a ring of fine holes. The sparger typically has a diameter slightly smaller than the impeller.

Combined with a large power input, the oxygen transfer is significantly raised using a Rushton turbine. Agitation breaks up the air bubbles. It increases the interfacial area for transfer an entraps air bubbles in the liquid. 

The key measure is the ratio between the volumetric air flow rate v_g and the cross-sectional area of the reactor which is called the superficial gas velocity, u_s. This latter value cannot be too large because it needs to be a measure of efficient dispersion and use of gas. At too high a superficial gas velocity, the impeller becomes fully surrounded by gas, and the dispersion capacity falls dramatically. This is about preventing flooding.

Power Consumption

All mixing requires energy. Power input, P (units: Watts) has two parts: power for stirring and the compression power for aeration.

Power is needed for operating the agitator in a stirred tank reactor. The compression power is required for pumping in a gas so that mixing occurs in an airlift bioreactors and bubble column. Mixing power is commonly the biggest cost in any fermentation, especially aerobic processes.

In chemical engineering systems, the term “power number” is a dimensionless parameter used to characterize the flow behaviour of fluids in mixing and agitation processes. The power number is denoted by Np​ and is defined as the ratio of the impeller power to the fluid power in a mixing system.

The power number is a dimensionless quantity that provides insights into the efficiency and effectiveness of an impeller in transferring energy to the fluid. It is commonly used in the analysis and design of agitated vessels, where various types of impellers are employed to mix and blend fluids.

The formula for the power number Np is given by:

N_p = P_m/(ρN³D⁵)​​

where:

• P_m​ is the impeller power (measured in watts or other power units),
• ρ is the fluid density (measured in kg/m³),
• N is the impeller speed (measured in revolutions per second),
• D is the impeller diameter (measured in meters).

The power number is crucial in assessing the efficiency of the mixing process and is used in the context of dimensionless correlations that relate mixing parameters. Different impeller designs and operating conditions result in different power numbers, and scale-up engineers use these values to optimize mixing processes for specific applications.

The viscosity is also an important factor – the more viscous the medium, the more power is needed to mix the medium.

The power number is expressed as a function of the Reynolds number, Re which is defined as:-

Re = fluid density * N * impeller diameter²

In the majority of cases, Np and fluid density remain constant so P is proportional to N³D⁵. 

The degree of power decreases in a gassed system when compared to an ungassed system. This is given as:

P_g/P = function (N_a)

N_a is the aeration number which is the apparent velocity of gas (or air) divided by the tip velocity of the impeller.

i.e. (F/Di²)/nDi = F/nDi³

Given Na remains constant, we see that F is proportional to nDi³

Agitation Speed: Scale-Up Effects on Tip Speed

The tip speed (m/s) is related to the shear rate produced by an impellar as it moves through the cell culture media. High shear rates will cause cell membranes to break up because they are torn apart by shear forces which destroys the cells. If scale-up is based on maintaining a constant tip speed then the P/V and mixing time will decrease.

So N₂ = N₁ * (D₁/D₂) 

where:-

N1 is the agitation speed in the scaled-down reactor

N2 is the agitation speed in the scaled-up bioreactor

D1 is the impellar diameter of the scale-down bioreactor

D2 is the impellar diameter of the scale-up bioreactor.

The criterion for shear force is typically 2 m/s maximum.

The highest shear zones in a stirred-tank bioreactor are often described as existing within the impellar zone. The outer edges of the impellar blades create shear as they rotate through the liquid, the impellar tip speed is often considered during bioreactor comparisons:

Tip Speed = π*D*N

where D = impellar diameter and N is the impellar agitation rate.

The tip speed calculation does not take into account impellar design.

Gas-Liquid Mass Transfer

One of the features of the scaled-up process are the changes occurring in gas–liquid mass transfer. Typically, a bioprocess is strongly affected and influenced by the hydrodynamic conditions in the bioreactor. These conditions are known to be a function of energy dissipation that depends on the operational conditions, the physicochemical properties of the culture, the geometrical parameters of the bioreactor and if it’s an aerobic process, on the presence of oxygen consuming cells. Oxygen transfer is the rate-limiting step in aerobic bioprocesses because oxygen solubility in most fermentation media is low.

Scaling-up of an aerobic fermentation is performed by trying to keep a constant oxygen transfer coefficient, k _L a, to ensure the same oxygen supply rate to support normal growth and metabolism of the desired high cell populations (Ju & Chase, 1992). Measurement and prediction of the k _L a is one of the most critical measurement steps in the scale-up of bioreactors.

An example of maintaining focus on k _L a as a scale-up factor comes from a study on improving ethanol productivity using Escherichia coli strain MS04 in mineral medium supplemented with xylose, glucose, and sodium acetate (Fernández‐Sandoval et al., 2017). The volume of the bioreactor was increased from 750mL to 9.16L and then 110L which is roughly a 10-fold increase each time.

Having established a k_La value of 7.2 h⁻¹ this was the main design criterion of scale-up through the next two volumetric increases. In this example, the controlled supply of low oxygen levels helped increase the concentration of biomass which favoured production of ethanol. By ensuring k_La was maintained throughout, similar ethanol yields and productivity was maintained.

Other Challenges of Scale-Up

One of the main issues with increasing vessel size is the way in which a fermentation medium and resulting biomass are mixed. We’ve already discussed some design features but there are other aspects that come into play.

Studies with dye tracers illustrate the point that mixing in a small vessel is usually uniform with the dye rapidly dispersed throughout the volume. As the vessel size increases 1000-fold, the tracer is not so well dispersed with the time taken for full mixing taking longer and even having the tracer not being fully dispersed at all save by diffusion. This could be the situation with substrates including gases such as oxygen. In some cases there is flooding which means that gas such as oxygen is retained around the impeller so the gas does not get to all the cells and they die off. It also implies that reactants and products of metabolism are not dispersed properly.

Changes to the surface to volume ratio produce notable alterations in performance. The height to diameter ratio in a CSTR can usually be 3:1 to 2:1. If you scale-up with a constant H/D ratio it changes oxygen supply and carbon dioxide removal.

The surface aeration is a critical factor especially for shear sensitive organisms. Growth of a microorganism can also occur at the walls of vessels.

In practice, the rules for scale-up tend to be empirical.

The common rules are:

• constant power to volume
• constant gas flow rate to unit volume
• geometric similarity
• constant kLa
• constant tip speed
• maintenance of constant substrate or product level (oxygen)

Instrumentation and Control

Upgrade or adapt control systems and instrumentation for the larger bioreactor. This may involve more advanced sensors and control algorithms to maintain optimal process conditions.

Sterilization

Ensure that the larger bioreactor can be effectively sterilized to prevent contamination. Autoclaves, steam-in-place systems, or other sterilization methods may be required.

Upstream and Downstream Processes

Consider the entire production process, including upstream (inoculum preparation, media preparation) and downstream (harvesting, purification) processes. Ensure they are compatible with the scaled-up bioreactor. If greater volumes of material need processing, or there is more biomass and metabolite to manage it means a greater expenditure on other process equipment. Likewise, effluent and waste management costs rise too.

Validation and Quality Control

Develop protocols for validation and quality control of the scaled-up process. This includes ensuring that product quality, yield, and consistency meet regulatory and industry standards.

Computational fluid dynamic (CFD) models  can be used to predict bioreactor performance and check scale-up parameters. Other mathematical models for microbial behaviour can also be examined but each type of fermentation tends to be unique and trialling production which reveal the pinch-points and issues that require resolution.

Risk Assessment

Identify potential risks associated with scaling up, such as equipment failure, contamination, or deviations from expected performance. Develop mitigation plans for these risks.

Testing and Monitoring

Conduct thorough testing and monitoring during the scale-up process. This includes assessing key performance indicators and making adjustments as needed.

Data Collection and Analysis

Collect data on process parameters, yield, and product quality throughout the scale-up process. Analyze this data to identify trends and make informed decisions.

Regulatory Compliance

Ensure that the scaled-up process complies with regulatory requirements and guidelines. This may involve coordination with regulatory agencies for approvals and inspections.

Personnel Training

Train operators and staff on the operation of the scaled-up bioreactor and associated processes.

Environmental and Safety Considerations

Consider environmental and safety aspects of the scaled-up operation, including waste management, containment measures, and emergency response procedures. With scale comes an increasing level of feedstock and waste material which requires extra holding capacity.

Cost Analysis

Assess the cost implications of scaling up the bioreactor, including capital expenses, operating costs, and potential return on investment.

Scaling up a bioreactor is a complex and iterative process that requires careful planning, execution, and ongoing monitoring. It is essential to maintain the integrity of the bioprocess and ensure that the larger-scale production meets the required standards for product quality and yield. Collaboration among multidisciplinary teams, including bioprocess engineers, biologists, chemists, and regulatory experts, is often necessary to successfully scale up a bioreactor.

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