Kinds of Closed Systems
Tubes made from glass or polymer are being arranged vertically or horizontally. A pump makes the algae culture pass the tubular system, after which the culture is collected by a tank and recycled again into the tubular system. Closed tubular photobioreactors exist in laboratory and production size as they are the only systems, which are able to produce high-quality biomass for a cost-efficient production of high-end algae products like DHA, Astaxanthin or Spirulina. Algae for dietary supplements are best grown in closed systems as they provide a high degree of purity and productivity, which results in high quality of the end product.
This system uses plates made from glass or polymer. The plates are arranged vertically or horizontally with a thin layer of algae culture in-between. The systems provide good lighting conditions. But the systems suffer from heating-problems and the tendency to build up biofilm formations, in particular when polymer plates are used. Such biofilms are difficult to clean as they are mechanically hard to access.
Foils made from PVC or PE are fixed on special holder systems to intercept the culture supernatants. The invest costs are low, but strong biofilm formation followed by a frequent exchange of bags makes the technology labor intensive and causes a lot of waste.
Here, tubes are arranged as truncated cones. Visually the systems look like fir trees, which is why the system’s nickname is “Christmas tree”. The motivation for the design is the desire to provide as much light as possible to the algae. However, the polymer tubes involved tend to biofouling and darkening under UV-light.
A fermenter is typically a specific tank, which is used for the cultivation of microorganisms or animally or herbal cells in a special nutrient medium. As a consequence, these glass-containers are also suited for the cultivation of moss, as an example. In the case of algae production the container is supplied with light from outside, with the advantage of being able to determine the wavelength of the light. The process monitoring and gas exchange is made via the cover. This kind of photobioreactor can often be found in laboratories, but they are usually not put to use at industrial algae production, as the capacity of the fermenter is strongly limited.
Fermentors are closed bioreactors for the heterotrophic production of algae where energy for growth originates from organic sources such as sugar. As closed systems the growth conditions can be well adjusted to optimum conditions. Growth concentrations and growth rates of microorganisms can be much higher when compared to phototrophic systems, but not all algae (with their valualbe ingredients) can be grown heterotrophically.
Hybrid systems combine two or more of the above mentioned methods, for example a small photo bioreactor which includes large open ponds.