Study of laboratory fermenter design and identification of its main parts using a bench-top stirred tank fermenter
Aim: To study the design of a laboratory fermenter and identify its important parts used for microbial fermentation in a stirred tank fermenter system.
Principle: A fermenter is basically a controlled vessel where microorganisms grow and produce useful compounds. Sounds simple. But the inside conditions must be maintained very carefully. Temperature, pH, aeration and mixing all matter. In microbiology and biotechnology labs we mostly use the stirred tank fermenter. Inside the vessel, an impeller rotates and keeps the culture mixed. Air is supplied through a sparger so the microorganisms get oxygen. Sensors monitor temperature and pH continuously. If mixing is poor, cells settle. If aeration is poor, growth slows down. Small mistakes give bad fermentation. So fermenter design is not decoration. Every part has a clear job, and when students see the system directly, the whole fermentation process becomes much easier to understand.
Types of Fermenter
1. Stirred Tank Fermenter (STR)
The stirred tank fermenter is the most common fermenter used in microbiology laboratories and fermentation industries. Inside the vessel a rotating impeller continuously mixes the culture medium. This mixing helps oxygen dissolve properly in the broth, which aerobic microorganisms need for growth. Air enters through a sparger placed at the bottom of the tank. Baffles are fixed on the inner wall so the liquid does not just spin around uselessly. Because of good mixing and oxygen transfer, bacteria and yeast grow very well in this system. Many antibiotics, enzymes and organic acids are produced using this fermenter only in industry.
2. Airlift Fermenter
An airlift fermenter works without mechanical agitation. Instead of a motor and impeller, compressed air is used to circulate the liquid medium. The fermenter usually has two zones called the riser and the downcomer. Air bubbles rise through the riser section and push the liquid upward. When the bubbles escape, the liquid moves down through the downcomer and the cycle continues. This circulation keeps the microorganisms suspended in the medium. Because there is no mechanical stirring, cell damage is less. I feel like this design is very clever in a simple way. It is often used for growing delicate cells like algae and some animal cell cultures.
3. Packed Bed Fermenter
In a packed bed fermenter, microbial cells are immobilized on solid support materials such as glass beads, ceramic particles or synthetic polymer beads. These particles fill the column forming a packed bed structure. Nutrient medium flows slowly through this packed material while the microorganisms remain attached on the surface. As the medium passes, the cells carry out metabolic reactions and produce the desired product. The advantage is that cells remain inside the reactor for long time, so the system can run continuously. I am knowing this since long from fermentation lectures only. Packed bed fermenters are often used in enzyme production and some wastewater treatment processes.
4. Fluidized Bed Fermenter
A fluidized bed fermenter looks similar to a packed bed reactor at first, but the working is slightly different. Here the support particles containing immobilized microbial cells are lifted upward by the flowing liquid or air. Because of this upward flow the particles move freely inside the vessel, almost like boiling motion in liquid. This movement improves contact between nutrients and microbial cells. Mixing is better compared to packed bed systems. Sometimes students get confuse between these two fermenters in exam, I also did once during my practical test. Fluidized bed fermenters are used in biotransformation reactions and certain enzyme based industrial processes.
5. Continuous Stirred Tank Fermenter (CSTR)
A continuous stirred tank fermenter operates in a steady process where fresh sterile medium continuously enters the fermenter while an equal amount of culture broth leaves the vessel. Because of this constant inflow and outflow, the volume inside the fermenter stays almost the same. The impeller keeps the culture uniformly mixed throughout the process. Microbial cells grow at a stable rate when conditions like nutrient concentration and dilution rate are controlled properly. This system is useful for large scale industrial fermentation where production needs to run for long periods. But maintaining contamination free condition in this fermenter are little challenging.
Parts of Fermenter:
- Bench-top laboratory fermenter (2–5 L capacity stirred tank fermenter)
- Stainless steel fermenter vessel with glass jacket or viewing window
- Control panel with temperature and pH controller
- Agitator motor and shaft assembly
- Rushton turbine impeller or marine impeller
- Sparger for air supply
- Air compressor or laboratory air pump
- Sterile air filter unit
- Foam sensor and antifoam inlet port
- Sampling port with sterile valve
- Inoculation port
- Exhaust gas outlet with condenser
- pH probe
- Temperature probe
- Dissolved oxygen probe (if available)
- Silicone tubing and connectors
- Power supply
Fermentation medium for demonstration (1000 mL preparation):
- Glucose : 10 g/L
- Yeast extract : 5 g/L
- Peptone : 5 g/L
- Sodium chloride : 5 g/L
- Distilled water : 1000 mL
(This medium is only for demonstration of fermenter operation. No heavy fermentation is required.)
Procedure:
- Gather all the students near the fermenter setup placed on the laboratory bench. Observe the instrument carefully before touching anything.
- Identify the fermenter vessel. It is usually made of stainless steel and holds the fermentation broth.
- Locate the agitator motor placed at the top of the fermenter. This motor rotates the shaft inside the vessel.
- Observe the agitator shaft running vertically inside the vessel. At the end of the shaft you will see the impeller.
- Identify the impeller type. In most laboratory fermenters, a Rushton turbine impeller is used.
- Locate the sparger at the bottom of the vessel. This part introduces sterile air into the culture medium.
- Follow the air line from the air pump to the fermenter. Notice the sterile air filter placed in the line. This filter prevents contamination.
- Observe the pH probe inserted through the top port of the fermenter. This probe measures acidity of the fermentation broth.
- Locate the temperature probe. It is connected to the temperature control system.
- Check the sampling port provided on the side of the fermenter. Samples of culture broth are collected from here during fermentation.
- Identify the inoculation port. Microbial culture is added through this port under sterile condition.
- Observe the exhaust outlet at the top of the fermenter. This allows gases like carbon dioxide to escape.
- Locate the control panel. It controls agitation speed, temperature and sometimes aeration rate.
- Fill the fermenter vessel with around 1000 mL prepared fermentation medium.
- Switch on the fermenter control system and set the agitation speed around 200 rpm.
- Start the air supply slowly and observe air bubbles coming from the sparger inside the vessel.
- Allow the system to run for 10 to 15 minutes so students can observe mixing and aeration clearly.
- Carefully note each part and write its function in the practical record.
Observation: Air bubbles can be seen rising from the sparger while the impeller rotates and mixes the liquid medium. The broth moves continuously inside the vessel, showing proper agitation and aeration.
Result: The structure and working parts of a laboratory stirred tank fermenter were observed and identified successfully.
Conclusion: Seeing a fermenter directly makes many theoretical things clear. In textbooks it looks complicated, but once you stand near the instrument and trace each pipe and probe, the system starts making sense. I remember first time seeing one during my MSc practical. Whole class crowd around it like it was some space machine. Every part inside the fermenter has the small but very important role. If air filter leaks, contamination will happen. If impeller not rotating properly, cells will not grow good. Fermentation is sensitive work. Even a small mistake shows its effect very fast.