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- 1 Photobioreactor in general
- 2 Photobioreactor types
- 3 Outlook of photobioreactor development
- 4 See also
- 5 External links
- 6 References
Photobioreactor in general
A bioreactor is an installation for the production of microorganisms outside their natural but inside an artificial environment. The prefix “photo” particularly describes the bio-reactor's property to cultivate phototrophic microorganisms, or organisms which grow on by utilizing light energy. These organisms use the process of photosynthesis to build their own biomass from light and carbon dioxide. Members of this group are Plants, Mosses, Macroalgae, Microalgae, Cyanobacteria and Purple Bacteria. Key objective of a photobioreactor, or PBR, is the controlled supply of specific environmental conditions for respective species. Thus, a photobioreactor allows much higher growth rates and purity levels than anywhere in natural or habitats similar to nature. Basically, photobioreactors can grow phototropic biomass even from nutrient polluted waste water and from flue gas carbon dioxide.
Open versus closed systems
First approach for the controlled production of phototropic organisms was and still is a natural open pond or artificial raceway pond. Therein, the culture suspension, which contains all necessary nutrients and carbon dioxide, is pumped around in a cycle, being directly illuminated from sunlight via the liquid’s surface. This construction principle is the simplest way of production for phototrophic organisms. But due to their depth up to 0,3m and the related reduced average light supply, open systems only reach limited areal productivity rates. In addition, the consumption of pumping energy is relatively high, as high amounts of water containing low product concentration have to be processed.
Grounds in Areas on earth with a dense population are expensive, while water is rare in others. Using open technologies causes high losses of water due to evaporation into the atmosphere. Hence, since the 1950s several approaches have been conducted to develop closed systems, which theoretically provide higher cell densities of phototrophic organisms and therefore a lower demand of water to be pumped. In addition, closed construction avoids system-related water losses and the risk of contaminations trough landing water birds or dust is minimized.
All modern photobioreactors have tried to balance between a thin layer of culture suspension, optimized light application, low pumping energy consumption, capital expenditure and microbial purity. Many different systems have been tested, but only a few approaches were able to perform at an industrial scale.
Redesigned laboratory fermenters
The simplest approach is the redesign of the well-known glass fermenters, which are state of the art in many biotechnological research and production facilities worldwide. The moss reactor for example shows a standard glass vessel, which is externally supplied with light. The existing head nozzles are used for sensor installation and for gas exchange. This type is quite common in laboratory scale, but it has never been established in bigger scale, due to its limited vessel size.
Made from glass or plastic tubes, this photobioreactor type has succeeded within production scale. The tubes are oriented horizontally or vertically and are supplied from a central utilities installation with pump, sensors, nutrients and CO2. Tubular photobioreactors are established worldwide from laboratory up to production scale, e.g. for the production of the carotenoid Astaxanthine form the green algae Haematococcus pluvialis or for the production of food supplement from the green algae Chlorella vulgaris. These photobioreactors take advantage from the high purity levels and their efficient outputs. The biomass production can be done at a high quality level and the high biomass concentration at the end of the production allows energy efficient downstream processing. Due to the recent prices of the photobioreactors, economically feasible concepts today can only be found within high-value markets, e.g. food supplement or cosmetics.
The advantages of tubular photobioreactors at production scale are also transferred to laboratory scale. A combination of the mentioned glass vessel with a thin tube coil allows relevant biomass production rates a laboratory research scale. Being controlled by a complex process control system the regulation of the environmental conditions reaches a high level.
Christmas tree photobioreactor
An alternative approach is shown by a photobioreactor, which is built in a tapered geometry and which carries a helically attached, translucent double hose circuit system. The result is a layout similar to a Christmas tree. The tubular system is constructed in modules and can theoretically be scaled outdoors up to agricultural scale. A dedicated location is not crucial, similar to other closed systems, and therefore non-arable land is suitable as well. The material choice should prevent biofouling and ensure high final biomass concentrations. The combination of turbulences and the closed concept should allow a clean operation and a high operational availability.
Another development approach can be seen with the construction based on plastic or glass plates. Plates with different technical design are mounted to form a small layer of culture suspension, which provides an optimized light supply. In addition, the simpler construction compared to tubular reactors allows the use of less expensive plastic materials. From the pool of different concepts e.g. meandering flow designs or bottom gassed systems have been realized and shown good output results. Some unsolved issues are material life time stability or the biofilm forming. Applications at industrial scale are limited by the scalability of plate systems.
In April 2013, the IBA in Hamburg, Germany, a building with an integrated glass plate photobioreactor facade has been commissioned.
This photobioreactor type consists of a plate-shaped basic geometry with peaks and valleys arranged in regular distance. This geometry causes the distribution of incident light over a larger surface which corresponds to a dilution effect. This also helps solving a basic problem in phototrophic cultivation, because most microalgae species react sensitively to high light intensities. Most microalgae experience light saturation already at light intensities, ranging substantially below the maximum daylight intensity of approximately 2000 Watt/m². Simultaneously, a larger light quantity can be exploited in order to improve photoconversion efficiency. The mixing is accomplished by a rotary pump, which causes a cylindrical rotation of the culture broth. In contrast to vertical designs, horizontal reactors contain only thin layers of media with a correspondingly low hydrodynamic pressure. This has a positive impact on the necessary energy input and reduces material costs at the same time
The pressure of market prices has led the development of foil-based photobioreactor types. The cheap PVC or PE foils are mounted to form bags or vessels which cover the algae suspension and expose it to the light. The pricing ranges of photobioreactor types have been enlarged with the foil systems. It has to be kept in mind, that these systems have a limited sustainability as the foils have to be replaced from time to time. For full balances, the investment for required support systems has to be calculated as well.
Outlook of photobioreactor development
The discussion around microalgae and their potentials in carbon dioxide sequestration and biofuel production has caused high pressure on developers and manufacturers of photobioreactors. Today, none of the mentioned systems is able to produce phototrophic microalgae biomass at a price which is able to compete with crude oil. New approaches test e.g. dripping methods to produce ultra-thin layers for maximal growth with application of flue gas and waste water. Further on, much research is done worldwide on genetically modified and optimized microalgae.
- Lane. G. (2013). "Up To Speed On: Algae Biofuels" 1. Smashwords. pp. 1–9. ISBN 9781301351961.
- Submariner Project: Photobioreactor design principles
- Eva Decker, Ralf Reski (2008): Current achievements in the production of complex biopharmaceuticals with moss bioreactors. Bioprocess and Biosystems Engineering 31(1), 3-9 term=Current+achievements+in+the+production+of+complex+biopharmaceuticals+with+moss+bioreactors
- Pulz. O. (2001). "Photobioreactors: production systems for phototrophic microorganisms" 57. pp. 287–293.
- Algae Observer: IGV Biotech Presents Novel Algae Screening System
- F. Cotta, M. Matschke, J. Großmann, C. Griehl und S. Matthes; “Verfahrenstechnische Aspekte eines flexiblen, tubulären Systems zur Algenproduktion” (Process-related aspects of a flexible, tubular system for algae production); DECHEMA 2011
- Großmann Ingenieur Consult GmbH: Aufbau eines Biosolarzentrums in Köthen, 6. März 2011.
- Handbook of microalgal culture 2nd edition 1. Blackwell Science Ltd. 2013. ISBN 978-0-470-67389-8.
- art-magazin.de: IBA Hamburg - Opening, Algaehouse, Worldquartier
- Spolaore. P. et al. (2006). "Commercial Applications of Microalgae". Journal of Bioscience and Bioengineering 102: 87–96.