DESIGN AND OPERATION OF THE FERMENTER

A fermenter is simply defined as an apparatus that maintains optimal conditions for the optimum growth of microorganisms used in large-scale fermentation and in the commercial production of economically useful products. Fermenter can also mean fermentation vessel (i.e. an apparatus in which microbes including bacteria, yeasts and moulds causes fermentation to take place). It is a vessel in which a particular microorganism is cultivated under controlled environmental conditions to produce a desired byproduct or biomass (Figure 1). Fermenters provide a stabilized environmental condition for the unperturbed and optimal growth of the microbe used for fermentation as well as ensure a stable and better production of the desired metabolite by the organism. Fermenters are bioreactors used for containing and controlling fermenter microorganisms in industrial productions. The phrase fermenter and bioreactor is used synonymously to mean the same thing.

A fermentation apparatus or vessel must be environmentally suited to support the growth of the fermenting microbes; and it must also provide important physical, biological and/or chemical parameters such as pressure, oxygen (for aerobic organisms), temperature and nutrients required for optimal microbial growth. Fermenters should be designed in such a way that it controls the conditions necessary to raise any non-phototrophic microorganisms as well as provide the necessary environmental conditions that will also stimulate the growth of anaerobic microbes – which are used in most fermentation processes. Fermenters should be able to maintain anaerobic conditions, while removing product and adding substrate intermittently. The system components of a fermenter include temperature control systems, system for aeration, impellers, spargers, systems for inflow nutrients and outflow of wastes or products, a reservoir pump, inlets and outlets for air, cooling jacket, sensors for pH control, and possibly a computer systems for on-process or in situ controls (Figure 2). A fermenter must provide all necessary environmental conditions such as optimal pH, optimal temperature, and oxygen for the optimal growth of the fermenting organisms. Fermenters assume different sizes and shapes; and the sizes of a fermenter vary – depending on the type of fermentation process it is intended for. Laboratory-scale fermenters are small as I liter or up to about 20 liters. Laboratory fermenters may be carried out in conical flasks that can be shaken at intervals – in order to provide the necessary aeration required for the optimal growth of the organism.

The conical flasks are usually covered with cotton wool to prevent microbial contamination of the process. However, when a larger production is anticipated, a large-scale fermenter is constructed. Production or factory fermenters range from 100,000 liters to 200,000 liters (Figure 3). It is noteworthy that only about 75 % of the total size or volume of a fermenter is actually used for the fermentation process. The remaining 25 % of the volume of the fermenter is meant to contain other conditions such as foam formation and the buildup or production of exhaust gases. There are various types of fermenters including the aerated stirred tank batch fermenter (for cultivation of aerobic organisms), anaerobic batch fermenters (for cultivation of anaerobic organisms), fed-batch fermenters and fluidized bed fermenters. Aerated stirred tank fermenters are commonly used in most fermentation process because it provides all the features that can create both aerobic and anaerobic conditions for the cultivation of aerobic and anaerobic organisms respectively. A fermenter should be constructed with non-corrosive cylindrical steel that is able to withstand high temperatures (e.g. 150-200oC). The material must be able to withstand and resist corrosion. An ideal fermenter must have provisions for the cooling of the vessel or fermentation broth especially when the incubation temperature rises beyond the required temperature for growth. It must also have provisions that sense and regulate the pH of the system as well as the concentration of dissolved oxygen. An ideal fermenter must be provided with several inlet and outlet pipes or channels that control the addition or removal of gases, nutrients, acids or bases as well as outlets for the harvesting or evacuation of byproducts or wastes. An ideal fermenter must also be fitted with agitators – that ensures the even or uniform mixture of the fermentation media.

Figure 1: Schematic illustration of an aerated-stirred tank batch fermenter.

Figure 2: Illustration of a fermenter showing systems for process control (arrows).

Figure 3: Illustration of fermenters for large scale productions.

Agitation in fermentation vessels helps in the distribution of incoming air (especially in aerobic-stirred tank fermenters) as well as in the uniform distribution of temperature in the vessel. The agitators also help to maintain the uniform suspension of the microbial cells in the fermentation medium as well as the gases and temperature conditions of the vessel. Process control in a fermenter is critical to the entire success of the fermentation process. It is usually carried out by monitoring various environmental factors such as pH, temperature, effluent gases, nutrients and air or oxygen, biomass formation and metabolite production. It is critical to monitor these factors in order to ensure optimal progression of the fermentation process so that the desired end-product or microbial metabolite will be produced in the right form and quantity.

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