While bioreactor is a general term for any reactor that uses a biological process catalyzed by microbes to produce a desired product, the term typically refers to biological applications of continuous stirred tank reactors or chemostats, plug flow reactors, or fixed film reactors. For more information on these types of reactors please refer to the CSTR, PFR, or FFR pages of the encyclopedia. A fermenter, often used to convert sugars to acids or alcohols, is the most common type of bioreactor due to its simplicity.
Fermenters
The fundamental function of a fermenter is to provide a suitable environment in which microorganisms can efficiently produce target products such as ethanol through the metabolization of sugars by yeast.
General Information
Fermenters can be run in either batch or continuous modes, depending on the application, and can be agitated either mechanically, with an impeller, or pneumatically, using injected gas or fluid. Some of the most common types of fermenters are listed below.
- Airlift fermenters forego standard mechanical mixing systems for a stream of gas injected into or outside of a riser tube at the bottom of the tank. The rising action from the gas causes the contents of the tank to circulate around and through the riser tube, thus mixing the system.
- Stirred tanks are the most basic fermenters, consisting of a vessel with a 3:1 aspect ratio and a mixing system that incorporates an impeller driven through the base or top. The top also has ports for the addition of reactants, as well as instrumentation.
- Tower fermenters are characterized by large height to diameter ratios, upwards of 15:1, and aeration occurs by the introduction of gas streams at the bottom of the tower
- Bubble columns are continuous tower fermenters. Gas is continuously sparged in from the bottom and acts to agitate and react with the downward flowing liquid stream
Equipment Design
A typical fermenter, as shown below, consists of a stainless steel housing containing an impeller or agitator for uniform mixing of microorganisms and nutrients. Inlets are located at the top of the body, a sparger is present at the bottom to introduce gas streams into the liquid if needed, and probes measure and control various process parameters. Often times a jacket will surround the body to allow process engineers to facilitate heating or cooling of the slurry contained within.
Usage Examples
Fermenters are used in a variety of industries that require biological manufacturing, such as the pharmaceutical and food industries. In the pharmaceutical industry, they are used to grow cells and bacteria for drugs such as antibiotics or penicillin. They can also be used for alcoholic fermentation in breweries or even to grow yeast for bread-making.
Advantages
- Simple, inexpensive batch design
- Process parameters can be easily varied
- Can be used for a variety of biological reactions
- Agitation from aeration as cost-saving means
- Can operate at lower temperatures compared to reactors using non-biological reagents and processes
Disadvantages
- Long retention times
- Constant monitoring
- Difficult sampling
- As process parameters change with time, microbes can begin to produce unwanted products
- Contamination of microbes can ruin batches and result in large costs
- Higher mixing levels can damage microbes
Other Bioreactors
In addition to fermenters, some types of bioreactors use more complex processes to achieve an efficient mixing to increase reactor conversion. These reactors rely on fluidization, membrane filtration, and the recycling of moving bed particles, among other methods.
General Information
Chemical processes in bioreactors can be aerobic, anaerobic, or a hybrid of the two. Bioreactors typically involve the flowing of a liquid phase through a medium containing microbes that catalyze the process. These microbes are typically affixed to the surface of particles such as granular activated carbon (GAC) or larger industrially-produced plastic pieces. Later on, the media is filtered out and the liquid product is recovered. A significant portion of operating costs goes toward controlling fouling, the unwanted buildup of filtrate. Three of the most commonly used bioreactors are :
Fluidized Bed Reactors
In fluidized bed reactors, gas or fluid is bubbled through a bed of biological media. This action causes the bed to rise slightly and behave as a fluid. This allows for a longer retention time of reactant in the bed, as well as better contact between reactants, and therefore a higher conversion.
Membrane Bioreactor
In membrane bioreactors (MBRs) a biological reaction occurs through the use of microorganisms and then a membrane is used to filter out the product based on molecular size and weight. The microorganisms that carry out the biological reactions in MBRs degrade the pollutants in the medium being filtered, which can generate significant levels of organic matter that must be removed. . MBRs allow for the microbial flocs to be easily separated from the aqueous phase and often removed in sedimentation columns. The most common application of this process is in wastewater treatment, where the bioreactor supports desired bacteria in flocculated forms known as activated sludge flocs. Such flocs allow for microorganisms to convert biodegradable organic substances into carbon dioxide and new biomass. This generated biomass is easily removed with membranes before the distribution of the treated water.
Membrane pore sizes can range from 0.01-0.4 µm. Over time the pores can be fouled, which can raise costs due to constant cleaning. It has been shown that the proper amount of aeration in the bioreactor can both lead to greater microorganism activity as well as reduced propensity to membrane fouling.
When fouling occurs, physical cleaning is necessary. One such method is called relaxation, a process in which permeate flow is stopped for short periods while aeration is continued. The material that causes fouling is returned to the aqueous phase and removed from the membrane surface. Relaxation is often used in conjunction with backwashing to “clean” the membrane. While this method can reduce the amount of irreversible fouling, the efficiency of relaxation and backwashing decreases over time. When a certain amount of irreversible fouling occurs, the need for chemical cleaning becomes significant for the membrane to remain functional.
Moving Bed Biofilm Reactors
Moving Bed Biofilm Reactors, such as the one pictured below, allow carrier pieces, to which biofilms are affixed, to flow as a bed along with the reactant fluid through the system. As the carriers and liquid flow, the reaction occurs. The carriers are then filtered out using a sieve at the exit and recycled back to the reactor entrance, seen in the picture below.
These reactors have several advantages as populations in industrialized areas grow and the amount of wastewater increases. Biofilm reactors function better with high biomass concentrations and achieve better removal of organic compounds. Additionally, these biofilm reactors can be designed for more compact spaces and the moving biofilm does not require traditional sludge recirculation for cleaning. Instead, the biomass growth occurs on carriers that move freely inside the reactor tank and discourage blocking on the biofilm. Moving bed biofilm reactors have significantly reduced head loss and thus show higher volumetric treatment capacity.
Equipment Design
The design of bioreactors varies upon the type of bioreactor used but there are still commonalities between the different units. Commonly, gases are sparged in from the bottom plate of a unit, and reactants flow in through the top. Often bioreactors will incorporate some carrier, such as those pictured below, that has microbes fixed to its surface and acts as the site of the reaction. These carriers are often made from high-density polyethylene, as its density is very similar to water.
Usage Examples
Bioreactors can be used to create biodiesel from algae. In the tubular algae bioreactor shown below, algae within the tubes grow with the assistance of a light source and carbon dioxide inputs. Once enough algae has grown, it is scraped from the tube and used for its natural oils
Advantages
- Can be designed for many different processes
- Can operate at lower temperatures compared to reactors using non-biological reagents and processes
- Continuous operation more feasible than in fermenters
Disadvantages
- Require constant monitoring
- Difficult to pull samples
- More complex than fermenters thus more difficult to control
- Microbes can produce unwanted products as process parameters change
Acknowledgments
- GEA Process Engineering Inc., Columbia, MD
- New Brunswick Scientific Co., Inc., Edison, NJ; now part of Eppendorf
- W2 Energy, Inc., Carson City, NV
- Process Engineered Water Equipment LLC, Camas, WA
- BioprocessH2O, Portsmouth RI
- Mott Corp. Farmington, CT
References
- Banz, Gregory. “Piloting Bioreactors for Agitation Scale-Up.” Chemical Engineering Progress. 104.2 (Feb. 2008): 32-34. Print.
- Daigger, Glenn T. Membrane Bioreactors. Alexandria, Virginia: Water Environment Federation, 2011.
- Dezotti, Marcia, et al. “Advanced Biological Processes for Wastewater Treatment.” University of Michigan Library, Springer International Publishing, 2011.
- McNeil, B. and Harvey, L. Practical Fermentation Technology. Chichester, England: Wiley, 2008.
- Stanbury, Peter F. Principles of Fermentation Technology. Tarrytown, New York, USA: Elsevier Science inc. 1984.
- Winkler, M. A. Chemical Engineering Problems in Biotechnology. Essex, England: Elsevier Science inc. 1990.
Developers
- Sam Catalano
- Kelsey Kaplan
- Thomas Plegue
- Amani Ramli
- Joel Holland