25 Types of Bioreactor

Dr Kashyap Kumar Dubey

Learning objectives:

• To study controlled growth conditions of microbes.
• To understand the application of bioreactor in environment.
• To study various type of reactors with their advantages and limitations. To study the strategies for selection of bioreactor.

TABLE OF CONTENTS

1.Introduction

2.Bioreactor types

2.1 Bioreactor used for microbial cell culture

2.1.1 Aerobic reactors

2.1.1.1 Stirred Tank Bioreactor

2.1.2 Anaerobic reactors

2.1.3 Immobilized cell bioreactors

2.1.3.1 Moving Bed/Fludized bed reactor (FBR)

2.2 Bioreactors foe animal cell culture

2.2.1 Hollow Fibre Membrane bioreactor

2.3 Photobioreactor for Algae

3.Application in Environmental problems

4.Membrane based bioreactors

5.Disposable bioreactors

6.Landfill Bioreactors

7.Strategies for selection of Bioreactors

8.Summary

1.Introduction

Bioreactors are vessels to perform biochemical reactions by using microorganism, plant cells and animal cell for the production of different biological products. These reactors basically provides controlled environment for optimal growth of biological systems for the production of desired products.

Fermentor is used in fermentation industries, in which low value substrate is consumed by microbial cells or enzymes to produce high value product. Fermentors are widely used in food processing, fermentation, waste treatment etc. purpose. The term Fermentor is basically used as synonym to bioreactor. The first pilot scale fermentor was established in India at Hindustan Antibiotic Ltd., Pune in the year 1950.

Bioreactors are different from chemical reactors, where the more support and control required for the growth of biological systems. Bioreactor provides more sophistication due to sensitivity from foreign microbes, sensitivity to physical properties of organisms, biochemical reactions because they are more sensitive than chemical processes. Though, the shear forces and shear stresses arises during microbial growth and product formation frequently the medium changes to Newtonian to non-Newtonian as the reaction happens. Henceforth, regular agitation and aeration is expected within a bioreactor so as to ensure efficient heat and mass transfer.

Bioreactors are highly selective to produce the desired product (above other unwanted) is of main significance. Selectivity is very much important in the production of antibiotics, steroids, vitamins, proteins, certain sugars and organic acids. Often, in biological processes, desired activity and selectivity come together only within a considerably narrow range of reaction conditions than that of the conventional chemical reactions.

In bioreactor, rate of reaction is critical as in the biological reactions, a proper incubation time is required to assist the growth of microbes to achieve the desired product formation. This incubation period may depend on the reaction type, product type and type of organism. Besides that, consider the reaction parameters like temperature, pH, substrate concentration, moisture content, salt concentration, vitamins, oxygen (for aerobic processes), gases generated, and product & by-product removal prior to design of a bioreactor. Additionally, a reactor type varies with the mode of operation and the product generated.

1.1 Bioreactor types: Bioreactors can be classified:

Type of cultures

• Bioreactors used in microbial cell culture
• Bioreactors used in animal cell culture
• Bioreactors used in algae cell culture
• Bioreactors used in microbial cell culture

Microorganisms are used in industry for production of commercially important products can be divided on the basis of oxygen demand of the microbial culture and the need for stirring. Based on these factors the following bioreactor classification has been framed:

Aerobic reactors

• Mechanically driven Example- Stirred Tank Reactor (STR)
• Air-driven Example- Air-Lift Reactor (ALR) /Bubble Column Reactor (BCR)

Anaerobic reactor (UASB), Packed Bed Reactor (PBR)

1. Aerobic reactors
2. Stirred Tank bioreactor

This is a conventional glass (borosilicate) or stainless steel reactor with provision for mixing. The stirrer is either incorporated on upward or downward of the reactor vessel. As shown in Figure 1, the sterile medium and the inoculum are fed into the sterilized tank, and air is supplied from the downward. To ensure proper mixing, additionally impellor and baffles are also provided within the vessel. These aid to overcome a whirl effect that could hinderance proper mixing. The number of baffles generally varies within four to eight, depending on the diameter of the vessel.

During this process, the bubbles generated to air supply are broken down by the agitator system as they move upward. Most widely used impellers are the four-bladed disk turbine (Rushton Type). And the exhaust gas generated during the course of reaction is removed from the top of the tank through gas exhaust pipe, while the product came down.

Advantages

1. Less capital and operation cost
2. Due to high stirring mass and heat transfer limitations are overcome.

Limitations

Foaming is the major drawback associated with STR. Antifoaming agents must be incorporated in the reaction mixture to tackle the problem. Yet, this has to be practiced cautiously as certain antifoaming agents inhibit microbial growth. Sometime, foaming will also lead to contamination.

(B) Air Lift Reactor (ALR) and Bubble Column Reactor (BCR)

This reactor is also called as a tower reactor or bubble column with a draft tube. Air is generally flown into the bottom of a central draught tube through a ring type sparger that regulates the air and medium circulation (Figure 2). ALR are used when process need more oxygen transfer rate and less power consumption. Air flows as tiny bubbles carrying the medium upward towards the headspace of the reactor along the draft tube, and excess gas and/or exhaust gases generated are sapped out of the reaction medium at the top of the column. The degassed broth along with the product gradually passes down through down-comer and is collected from the bottom of the tank. The draft tube can be used as internal heat exchanger, axial mixing throughout the whole reactor, reduction of bubble coalescence and equalizes shear force. ALR has three configurations: i. Divide vessel with draft tube, ii. Draft tube internal loop reactor, iii. External loop reactor.

ALR are having four components like, riser, gas-liquid separator, down-comer and bottom.

Advantages of bubble column reactor are absolute sterile operation, less internal parts, easy cleaning, and no sealing problem.

Application: In Wastewater treatment, production of Baker’s yeast, beer etc. ALR used in tissue culture.

Advantages

• Simple reactor devoid of additional mixing parts or agitator system and thus less maintenance cost and reduce shear stress.
• This system is suitable for both plant and animal cells due to low shear rate
• It enables homogenous gas distribution obligatory for aerobic processes
• Low energy input due to lack of agitation
• High degree of heat and mass-transfer due to oxygen circulation
• Enhanced area/volume inside the tank due to absence of mixing shaft

Limitations

1. Additional investment for air supply especially significant in case of large scale processes
2. High air throughput and pressure is needed for uniform circulation
3. Difficult to maintain constant concentration of substrate, nutrients, oxygen and organisms within the reactor at a given time
4. In this reactor system, foaming impedes proper gas/liquid separation

Nevertheless, these limitations can be overruled by appropriate designing of airlift systems. For instance, instead of single feeding point, multiple feeding provisions must be made to ensure the organisms experience continuous cycles of high growth throughout the vessel, resulting in desirable products, high yields and low mortality rates. The same applies for entry point of oxygen; therefore it must be delivered at different places within the reactor, with the main stream entering from the bottom of the vessel.

Anaerobic bioreactors

Anaerobic bioreactors operate in the complete absence of aeration. These reactors are conventionally employed in the production of ethanol, wine, beer and wastewater treatment. The various by-product gases evolved from the reaction mixture upon fermentation ensure proper mixing thereby ruling out necessity for mixing apparatus.

(i) Upflow Anaerobic Sludge Blanket (UASB) bioreactor

Upflow Anaerobic Sludge Blanket (UASB) reactor is a type of anaerobic reactor which contains a blanket of granular sludge suspended within the reactor. Biomass growth occurs on the fine sludge particles, which then form sludge granules having high specific gravity. A stream of feed is pumped upwards through a mechanical pump so that the contact between the feed and the granular sludge is ideal. The movement of feed in the upward direction and the pull of gravity upon the sludge bed in downward direction aid in proper suspension of the blanket. This reactor is widely employed for waste water treatment.

UASB can be distinguished into three parts, sludge bed, sludge blanket and GLS (Gas-Liquid-Solid) separator present at the top of the reactor. The sludge bed is comprised of high concentration of anaerobic bacteria (40 – 100 g/L) which occupy 40 to 60% of reactor volume. Greater than 95% organic matter degradation occurs in this zone. During this treatment process, the feed (wastewater) comes in contact with the granular sludge leading to useful gas formation that in turn causes internal mixing in the bioreactor. A fraction of this gas evolved gets attached to the biological granules making them lighter. These particles along with free gas rise towards the top of the reactor.

In between the sludge bed and GLS separator, a thin layer of sludge is maintained, called as sludge blanket. This is necessary for auxiliary dilution and treatment of the wastewater stream that escaped the sludge bed zone due to the rising biogas. This accounts to 15 to 25% of reactor volume while the GLS separator located at the top of the reactor occupies 20 to 30% of the vessel volume. The particles that reach the liquid surface hit the base of the degassing baffles, thus resulting in the disengagement of attached gas bubbles. These gas free granules fall back on the sludge bed surface where as the free gas and the gas disengaged from these granules is collected in the gas collection domes situated at the top of the reactor.

The granular sludge containing biomass of the current reactor can be used as inoculum to initiate a new UASB reactor. In case of unavailability of such material, other non-granular materials such as anaerobic digested sludge, waste activated sludge and cow dung manure can be considered as other alternatives which subsequently assume granular form. Prior pretreatment techniques namely sedimentation, neutralization of feed material is generally required before treating waste in UASB. With hydraulic retention time of 4-24 h and organic loading rate of 1-20 kg COD /m3.d, waste water treatment efficiency can range between 75 to 85 %.

Packed Bed Reactor (PBR)

It consists of a hollow tube in which packing material is filled in order to improve the contact between the different phases involved in the reaction mixture. There are generally two types of packings: random packing with objects like raschig rings and other is organized packing with structured packing material. Diagrammatic representation of a packed bed reactor is shown in figure 3. Packed bed reactors have successfully been used in solid state fermentation processes consisting of the substrate pre-inoculated with the desired inoculum.

Microbial culture generally functions in two different forms: submerged and immobilized form. Immobilized forms of microbial cultures are advantageous over the submerged forms in terms of reduced toxicity caused by inhibitory products, reuse and minimal/ no washout of cells.

Enzyme reactors/Immobilized cell bioreactors

This reactor system is particularly ideal for microbial cells which are sensitive to friction/ shear. It is based on immobilized cells as these are advantageous over the other reactor types. This practice enables continuous functioning of the vessel at any set liquid throughput without any cell wash out. In this reactor type, cells are protected from toxic substances and thus result in higher concentration of cells/ related enzymes in the reactor. Also, this is used for the production of intracellular and unstable enzymes, low molecular weight products and several amino acids, organic acids etc. Generally, fluidized bed reactors and hollow fiber membrane bioreactors come under this type.

(A) Moving Bed /Fluidized Bed Reactor (FBR)

Fluidized bed reactor is a type of reactor in which a variety of multiphase biochemical reactions can be performed. Generally, a fluid (gas or liquid) is pumped upwards through a granular sludge bed at high velocity so as to suspend the sludge properly within the reactor. This process of suspending the sludge granules by maintaining a high pressure of fluid upwards is known as fluidization. In this kind of reactors, heat and mass transfer coefficients are very high between particle-gas and bed-surface. These reactors are mostly used for solid-state fermentation because of the following reasons.

1. Removal of the heat produced during the reaction by the gas phase involved in fluidization
2. Healthy growth of microbes due to efficient aeration by the fluidizing air
3. Instantaneous removal of metabolic products which might inhibit the fermentation process
4. No temperature gradients due to proper mixing of the substrate
5. High productivity in comparison to traditional solid state fermentation.

A schematic diagram of the fluidized bed reactor is shown in figure 4.

Fluidized bed reactors have also been successfully used for the continuous production of ethanol

(B) Hollow fibre membrane bioreactors

These reactors possess hollow fibres arising from various polymer materials such as cellulose acetate, acrylic polymers, polysulphone etc. Hollow fibres have successfully been used as immobilized bioreactor systems. For instance, target enzymes immobilized into the wall of hollow fibres allows easy reaction between immobilized enzyme and the diffused substrate to convert into desired product. The product thus formed subsequently diffuses out and is further processed to meet the desired standards.

Advantages

1. Scale up is relatively easy as many parallel fibres can be incorporated
2. Simultaneous extraction of extracellular products from the reaction mixture is possible
3. High productivity can be expected

Limitations

1. Clogging of pores possess problems
2. High cell growth may deform and rupture the hollow fibres
3. Accumulation of any toxic by-products within the fibre may inhibit the microbial growth.

2.Bioreactors used in animal cells culture

Animal cells are predominantly used for the production of vaccines and therapeutic proteins. Animal cells grow in either anchorage dependent mode or anchorage independent mode. Anchorage dependent cells necessitate their adhesion to a particular surface while the anchorage independent cells do not relay on any surface. These factors make the design of bioreactor for animal cell culture difficult for culturing and separation.

Most of the time FBR is used for anchorage dependent (microcarriers) animal cell cultures and successfully explored in CSTR. However, upon increasing the cell density, the problems of improper mixing arises and hence alternative means of providing efficient mass transfer of oxygen and nutrients is required which can be solved by fluidized bed reactors, since the contents in this scheme are always in a fluidized state. Advantages of fluidized bed reactors for animal cell culture are as follows:

• Both anchorage dependent and independent cells can be cultured.
• High cell densities can be obtained
• Efficient mass and heat transfer can be achieved
• Less energy requirements

3. Bioreactors used for algae culture (Photobiorectors)

Photobiorectors are basically designed for wastewater treatment, water quality management, and algae culture. Photoautotrophs, photohetrotrophs, BGA (Blue Green Algae) are used in photobiorectors. Photobiorectors is a system which regulates its environment as per the need of the specific algae grown within a photobioreactor. Various factors which affect the reactors are carbon dioxide, light, pH and temperature.

Photobioreactors are mostly categorized on the basis to increase surface area of vessel for enhanced aeration which could be used for the growth of microorganisms like

Tubular photobioreactor: These are made up of borosilicate glass or plastic tubes which are mounted horizontally or vertically (See Figure).

Plate photobioreactor: This is an economic modification of the earlier schemes in which the reactor is in the shape of a plate made up of glass or plastic.

Foil photobioreactor: This consists of PVC or PE foils which are mounted on a support to form a bag which serves the purpose of the reactor.

4. Application in Environmental problems

Bioconversion of wastes to wealth or value added products has potential role in environmental pollution control and simultaneously resource utilisation. Bioreactors required for waste management do not require stringent aseptic conditions. The mixed microbial and aquatic system stabilise the waste.

Gaseous effluents will be managed by biofilters and bioscrubbers; in biofilters pollutants are removed through sorption and oxidised by the microorganisms immobilised in the bed. When biofileters failed due to heavier pollutant loading, activated sludge mixed and further contacted with gas effluents in packed bed reactor.

5.Membrane based bioreactor for wastewater treatment

Membrane based Bioreactor is an improved version of Activated Sludge Process (ASP), where secondary clarifier replaced with membrane unit for separation of treated water.

6.Disposable bioreactor

Disposable bioreactors are based on non-invasive agitation and can be used at least 500L. They are biocompatibale, easy to use, and suitable for GMP operations. Disposable bioreactor is mainly useful in suspension and adherent animal culture.

7.Landfill Bioreactor

Bioreactor landfill is innovative technique where landfills are treatment vessels/ reaction site used to treat solid wastes into value added products. They are the sustainable solution for conversion of “waste to best”. Landfill bioreactors can work for quick biodegradation of organic waste; recirculation of leachate; operated near the field capacity of the waste; could be anaerobic, aerobic or both

Why Landfill Bioreactor needed?

• Less leachate treatment required which reduces costs
• Energy recovery from waste
• Potential for 2R (Recovery and Recycling)
• Reduction in Green House Gaseous

8. Strategies for selection of bioreactor

Type of microorganisms

• Oxygen requirement
• Sterilization method
• Requirement of Light
• Material of construction

Summary

Bioreactors are basically used for growth of microorganisms under controlled conditions. They are useful for the production of microbial metabolites on scale-up level to fulfill the demand of market. Recently, bioreactors are proven track record for bioremediation purpose of solid and liquid waste. Presently, Landfill bioreactors are used in the treatment of solid waste generated from house hold.