Bioreactors- The flagbearer of fermentation process

FERMENTATION AND FERMENTERS

Bio+reactor is a self-explanatory word that generalizes the whole process of fermentation. Bio means a vast variety of micro-organisms and a reactor is a vessel that contains micro-organisms with their food for their multiplication purpose. The most simple example of fermentation is the production of beer/wine or making of dough through bacteria and yeast.

Not only this, but other useful products nowadays rely on this process. Fermentation reactions need a favorable environment that has appropriate temperature, agitation, pH, dissolve oxygen, pressure other conditions. To meet these conditions, customized fermenters are manufactured for specific kinds of products. You people might have read these information but, this blog will tell you what are the major differences in the types of bioreactors, their classification, and their working procedure. The following figure can show that on the basis of what important things these bioreactors are classified:

• Bioreactors based on the culture in use are usually Microbial bioreactors (for cultures like bacteria, yeast, and fungi) and cell culture bioreactors (for cultures like mammalian cell culture and insect cell culture).

MICROBIAL BIOREACTORS

Microbial cultures have a history in many industrial applications, one of them is fermentation. From the above passage we get that for fermentation processes microbes need a particular environment for their growth, a microbial bioreactor is a piece of equipment that provides a favorable surrounding environment for cultures. In these kinds of bioreactors, there is a provision of rapid agitation and aeration for good oxygen transfer through impellers and spargers. Impellers by breaking up the gas bubbles increase surface area and high rate of gas exchange into the medium. One of the disadvantages in bioreactors is foaming or froth which comes on the surface of medium blocking growth and process inflow. For such cases ports are provided in the reactors for anti-foam, acid/ base for pH change, additional feed. Temperature sensors are also there to regulate the change in the environment of suspension.

CELL CULTURE BIOREACTORS

For cultures like insect cells or mammalian cells, the process is highly sensitive where cells need extra care and gentle mixing. The chances of contamination are also very vulnerable in tissue cultures. Due to these restrictions, broad impellers are used for mixing purposes, and gas is used for oxygen transfer. Slow-speed mixing protects cells from shear stress and mechanical problems. The products from these reactors are widely used in medical applications and are avoided for industrial use.

• We all know that there are two types of reactions aerobic and anaerobic reactions. one required oxygen and the other does not. On the basis of these reactions fermenters are classified respectively. Aerobic and anaerobic fermentation is related to aerobic and anaerobic respiration. In Cellular respiration, one type includes the presence of oxygen and the other is devoid of oxygen. Ethanol fermentation and lactic acid fermentation come under anaerobic respiration. Aerobic respiration mainly occurs in plants and animals, anaerobic occurs in microorganisms. Keeping this in mind bioreactors are also differentiated. For the aerobic fermentation process, a fermenter needs an oxygen provision. So bioreactors like airlift fermenters, bubble column reactors come under aerobic fermentation, anaerobic process does not need an air supply only agitators are required for mixing.

BUBBLE COLUMN BIOREACTORS

It is a top column bioreactor where the air is introduced for proper oxygen transfer and mixing by perforated pipes or metal porous spargers. Gas flow rates and rheological properties of broth are the main factors for oxygen transfer and mixing. The vessel used is usually cylindrical with an aspect height to diameter ratio of 4-6. It is a self-regulated reactor that has excellent heat management capacity.

AIRLIFT BIOREACTORS

As compared to bubble column reactor, airlift contains draft tube which helps in improvised circulation, oxygen transfers and equalize shear forces in the reactor. These reactors are more efficient particularly for denser suspensions of microorganisms. The reactor divides into two halves, one where the air is passed through the sparger (riser) and the other where no gas is received (downcomer). For optimal mass transfer, the aspect ratio is set increasing the rate of circulation of liquid.

• Based on low scale and high scale production, bioreactors are differentiated.

LAB-SCALE, PILOT SCALE, PRODUCTION SCALE BIOREACTORS

Depending upon the production volume bioreactors are classified into lab scale, pilot scale, and production level. For laboratory practices or limited production of valuable products small scale bioreactors are used, easy handling procedures, and easy to use. Parts of the reactor can be autoclaved even with utility parts and there are fewer chances of contamination, good control on temperature, and other affecting factors. Generally during media optimization procedures or research work small scale reactors are used and when optimizing is done capacity is shifted to pilot scale and further to production level. Working volumes of vessels are mostly describing 1-10lts as small scale, 50-200lts pilot scale, and above 200lts large scale. Vessels used are typically made of glass or stainless steel and have an aspect ratio concerning the requirement. For sterilization at a small scale, steam sterilization is there (in situ) whereas for large fermenters is CIP (cleaned automatically in place).

• For a special type of fermentation, customized fermenters are used in applications.

SOLID-STATE FERMENTERS

Solid-state fermentation is an opposite process to submerged fermentation where solid substrates containing low moisture levels are used instead of liquid broth as food for organisms. Most used solids are grains like wheat, rice, barley, corn, and a wide range of plants like sugarcane moss and apple pomace. When compared to submerged, factors affecting SSF bioreactors are temperature regulation, the humidity of substrate, aeration, and cooling rate. For this reason, bioreactors are generally equipped with humidifiers, slow agitator units, and perforated bases. For the production of antibiotics enzymes, this type of reactor is used but its major role is in the biofuel and bio-pesticides industries.

PHOTOBIOREACTORS

There are a few phototrophic microorganisms that can be used as food supplements and one of them is algae. Microalgae give a lot of health benefits and can also be used in cosmetics and aquaculture industries. Many techniques are implied for their production at smaller as well as larger scales. The most important part is to expose algae to the environment because they are good exchangers of gases like oxygen and carbon dioxide and can grow only in the appropriate light. Open systems like ponds, lakes, artificial ponds are useful in algal production but due to evaporation loss and the chance of contamination, not prominently used. Closed systems consist of tubular photobioreactors, horizontal photobioreactors of different shapes, sizes and made of transparent materials like plastic/foil. Tubular PBR on an industrial scale is the largest success in the production of dry biomass providing proper exposure of light and aeration.

• On the basis of feed/ substrate utilization fermenters are as follows:

BATCH, CONTINUOUS AND FEDBATCH REACTORS

In a fermentation process, culture is inoculated to a medium and Initially, growth is slow in the lag phase but after some time when organisms adapt to the environment, they multiply rapidly (log phase). When nutrients are consumed by a microorganism, by-products accumulate but only up to a certain limit. After continuous consumption of feed, a point of time comes when nutrient depletion is observed and the growth of organisms slows down (stationary phase). At this point harvesting should be done for a batch process but, cell density obtained is low. Whereas in the case of fed-batch, batch process is used for some time and later on due to substrate limitation feed is added respectively. Similar to the batch here also the product is harvested at the end of the process. In continuous fermentation, fresh medium is continuously added to the fermenter, while used medium and cells are harvested at the same time.

• Industrial bioreactors are totally different from the ones used in pilot scale especially in effluent treatment plants:

WASTEWATER TREATMENT REACTORS

The major principle of bioprocess lies in the implementation of actions to provide a stable environment free of pollutants and we know chemical sedimentation, sludge waste is deteriorating our surroundings. Microorganisms have helped many industries and for remedies bioreactor plants are used for waste treatments. Commonly used bioreactors are:

Sequencing batch reactors (SBR)- Carbon, phosphate, and ammonia along with impurity water are poured into the reactor where the aerobic reaction occurs. Agitators are there for mixing and spargers for aeration purposes meanwhile biological processes take place in reactors. Biological processes include BOD removal, nitrification, and denitrification causing impurities to precipitate and settle. After the whole process sediment sludge is removed and treated water is collected.

Moving bed biofilm reactors (MBBR)- Microbes are first grown over carriers which can free flow in the reactor containing liquid. These carriers then react with impurities present in the water and via biological metabolism, unwanted components are removed. The advantage of this reactor is no sludge recirculation. This process can be aerobic or anaerobic, agitators plus the effect of air are operated in case of aerobic and in case of the anaerobic process, shaft is used to carry carriers for the movement.

Packed bed reactors (PBR)- A bed is formed which has immobilized catalyst-based packing material upon which nutrient is given either from top or bottom of the reactor. The catalyst here acts as an active ingredient that is responsible for the exchange of impurities with itself. This capacity of exchange depends on the net solid weight of the catalyst. The most commonly used packing materials are ceramic pieces. Other materials like volcanic rocks and clay balls can also be used as a pack.

Membrane bioreactors (MBR)- Biodegradation is done with the help of microbes whereas physical separation is done by membrane bioreactors. Based upon the porosity of membranes and particle size of waste the process is set up. Constituents containing different sizes need to be passed through respective membranes. Ultrafiltration and diafiltration are majorly used for wastewater treatments. Water with impurity is passed through these membranes with pressure separating solutes and solvents.

Note: Images were taken online, from valid sites.