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 CO2. 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[edit source | editbeta]
Open raceway pond
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 CO2, 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 1950ies 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.[SUP][1][/SUP] Photobioreactor types[edit source | editbeta]
All modern photobioreactors have tried to balance between a thin layer of culture suspension, optimized light application, low pumping energy consumption, CAPEX and microbial purity. Many different systems have been tested, but only a few approaches were able to perform at an industrial scale.[SUP][2][/SUP] Redesigned laboratory fermenters[edit source | editbeta]
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.[SUP][3][/SUP] This type is quite common in laboratory scale, but it has never been established in bigger scale, due to its limited vessel size. Tubular photobioreactors[edit source | editbeta]
Tubular glass photobioreactor
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 carotenoidAstaxanthine 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.[SUP][4][/SUP] 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.[SUP][5][/SUP] Christmas tree photobioreactor[edit source | editbeta]
Christmas tree reactor
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.[SUP][6][/SUP] 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 shall prevent biofouling and ensure high final biomass concentrations. The combination of turbulences and the closed concept are ought to reach a clean operation and a high operational availability.[SUP][7][/SUP] Plate photobioreactor[edit source | editbeta]
Plastic plate photobioreactor
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 more simple construction when compared to tubular reactors allows the application of cheap 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 bordered by the limited scalability of plate systems, additionally.[SUP][8][/SUP] In April 2013, the IBA in Hamburg, Germany, a building with an integrated glass plate photobioreactor facade has been commissioned.[SUP][9][/SUP] Foil photobioreactor[edit source | editbeta]
The pressure of marked 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[edit source | editbeta]
The discussion around Microalgae and their potentials in CO2 sequestration and biofuel production has caused high pressure on developers and manufacturers of photobioreactors.[SUP][10][/SUP] Today, none of the mentioned systems is able to produce phototrophic microalgae biomass on a price level, which is able to compete on the crude oil market. 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. The expected influence of increasing crude oil price on a successful microalgae breakthrough is still to come.
An algae bioreactor or photobioreactor is used for cultivating algae on purpose to fix CO[SUB]2[/SUB] or produce biomass. Specifically, algae bioreactors can be used to produce fuels such as biodiesel and bioethanol, to generate animal feed, or to reduce pollutants such as NOx and CO2 in flue gases of power plants. Fundamentally, this kind of bioreactor is based on the photosynthetic reaction which is performed by the chlorophyll-containing algae itself using dissolved carbon dioxide and sunlight energy. The carbon dioxide is dispersed into the reactor fluid to make it accessible for the algae. The bioreactor has to be made out of transparent material.
The algae are photoautotroph organisms which perform oxygenetic photosynthesis.
The equation for photosynthesis:
Some of the first experiments with the aim of cultivating algae were conducted in 1957 by the "Carnegie Institution" in Washington. In these experiments, the monocellular Chlorella were cultivated by adding CO[SUB]2[/SUB] and some minerals. In the early days, bioreactors were used which were made of glass and later changed to a kind of plastic bag. The goal of all this research has been the cultivation of algae to produce a cheap animal feed.[SUP][1][/SUP] Frequently used photo reactor types[edit source | editbeta]
Nowadays 3 basic types of algae photobioreactors have to be differentiated, but the determining factor is the unifying parameter – the available intensity of sunlight energy. Plate photobioreactor[edit source | editbeta]
A plate reactor simply consists of vertically arranged or inclined rectangular boxes which are often divided in two parts to effect an agitation of the reactor fluid. Generally these boxes are arranged to a system by linking them. Those connections are also used for making the process of filling/emptying, introduction of gas and transport of nutritive substances, easier. The introduction of theflue gas mostly occurs at the bottom of the box to ensure that the carbon dioxide has enough time to interact with algae in the reactor fluid. Tubular photobioreactor[edit source | editbeta]
A tubular reactor consists of vertical or horizontal arranged tubes, connected together to a pipe system. The algae-suspended fluid is able to circulate in this tubing. The tubes are generally made out of transparent plastics or borosilicate glass and the constant circulation is kept up by a pump at the end of the system. The introduction of gas takes place at the end/beginning of the tube system. This way of introducing gas causes the problem of deficiency of carbon dioxide, high concentration of oxygen at the end of the unit during the circulation, and bad efficiency. Bubble column photobioreactor[edit source | editbeta]
A bubble column photo reactor consists of vertical arranged cylindrical column, made out of transparent material. The introduction of gas takes place at the bottom of the column and causes a turbulent stream to enable an optimum gas exchange. At present these types of reactors are built with a maximum diameter of 20 cm to 30 cm in order to ensure the required supply of sunlight energy. This type allows for a reduction of the harmful shear forces.[SUP][clarification needed (What forces?)][/SUP]
The biggest problem with the sunlight determined construction is the limited size of the diameter. Feuermann et al.[SUP][who?][/SUP] invented a method to collect sunlight with a cone shaped collector and transfer it with some fiberglass cables which are adapted to the reactor in order to enable constructions of a column reactor with wider diameters. - on this scale the energy consumption due to pumps etc. and the CO2 cost of manufacture may outweigh the CO2 captured by the reactor.[SUP][citation needed][/SUP] Industrial usage[edit source | editbeta]
The cultivation of algae in a photobioreactor creates a narrow range of industrial application possibilities. Some power companies[SUP][citation needed][/SUP] already established research facilities with algae photobioreactors to find out how efficient they could be in reducing CO[SUB]2[/SUB] emissions, which are contained in flue gas, and how much biomass will be produced. Algae biomass has many uses and can be sold to generate additional income. The saved emission volume can bring an income too, by selling emission credits to other power companies.[SUP][2][/SUP]
The utilisation of algae as food is very common in East Asian regions[SUP][citation needed][/SUP]. Most of the species contain only a fraction of usable proteins and carbohydrates, and a lot of minerals and trace elements. Generally, the consumption of algae should be minimal because of the high iodine content. For example, if someone has hyperthyroidism it could be very dangerous for their health. Likewise, many species of diatomaceous algae produce compounds unsafe for humans.
The algae, especially some species which contain over 50 percent oil and a lot of carbohydrates, can be used for producing biodiesel and bioethanol by extracting and refining the fractions. This point is very interesting, because the algae biomass is generated 30 times faster than some agricultural biomass[SUP][citation needed][/SUP], which is commonly used for producing biodiesel. See also[edit source | editbeta]
[*=left]Acien Fernandez, F. G., Fernandez Sevilla, J. M., Sanchez Perez, J. A., Molina Grima, E. und Christi, Y. (2001) Airlift-driven external-loop tubular photobioreactors for outdoor production of microalgae: assessment of design and performance. Chemical Engineering Science 56, 2721–2732
[*=left]Borowitzka, M. A. (1999) Commercial production of microalgae: ponds, tanks, tubes and fermenters. Journal of Biotechnology 70, 313-321
[*=left]Carlsson, A. S., Van Beilen, J. B., Möller, R. und Clayton, D. (2007). Micro- and Macro-Algae: Utility for industrial applications. D. Bowles, University of New York.
[*=left]Chisti, Y. (2007) Biodiesel from microalgae. Biotechnology Advances 25, 294-306
