A photobioreactor (PBR) refers to any cultivation system designed for growing photoautotrophic organisms using artificial light sources or solar light to facilitate photosynthesis. PBRs are typically used to cultivate microalgae, cyanobacteria, and some mosses. PBRs can be open systems, such as raceway ponds, which rely upon natural sources of light and carbon dioxide. Closed PBRs are flexible systems that can be controlled to the physiological requirements of the cultured organism, resulting in optimal growth rates and purity levels. PBRs are typically used for the cultivation of bioactive compounds for biofuels, pharmaceuticals, and other industrial uses.
Open systems
The first approach for the controlled production of phototrophic organisms was 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. Raceway ponds are still commonly used in industry due to their low operational cost in comparison to closed PBRs. However, they offer an insufficient control of reaction conditions due to their reliance on environmental light supply and carbon dioxide, as well as possible contamination from other microorganisms. Using open technologies also result in losses of water due to evaporation into the atmosphere.
Closed systems
The construction of closed PBRs avoids system-related water losses and minimises contamination. Though closed systems have better productivity compared to open systems due to the advantages mentioned, they still need to be improved to make them suitable for production of low price commodities as cell density remains low due to several limiting factors. 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. However, light attenuation and increased carbon dioxide requirements with growth are the two most inevitable changes in phototrophic cultures that severely limits productivity of photobioreactors. The accumulation of photosynthetic oxygen with growth of microalgae in photobioreactors is also believed to be a significant limiting factor; however, it has been recently shown with the help of kinetic models that dissolved oxygen levels as high as 400% air saturation are not inhibitory when cell density is high enough to attenuate light at later stages of microalgal cultures. 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.
Tubular photobioreactors
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 carbon dioxide. Tubular photobioreactors are established worldwide from laboratory up to production scale, e.g. for the production of the carotenoid Astaxanthine from 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 at laboratory research scale. Being controlled by a complex process control system the regulation of the environmental conditions reaches a high level.