4.1.2 Cultivation Vessels

Many different configurations of photobioreactors are possible: from simple unmixed open ponds to highly complex enclosed ones. The configuration of the bioreactor has great influence on carbon dioxide consumption during algal growth. Most of the recent research in microalgal culturing has been carried out in photobioreactors with external light supplies, large surface areas, short internal light paths, and small dark zones. Examples include open ponds (the cheapest ones), tubular reactors, flat panel reactors, and column reactors (stirred — tank reactors, bubble columns, airlift).

The applications of such systems range from the small-scale production of high-value prod­ucts to the large-scale production of biomass for feed. The choice between the different designs of photobioreactors must be specific to the intended application and local circumstances.

Open ponds can be an important and cost-effective component of large-scale cultivation technology, and optimal design parameters have been known for many years. The elongated "raceway type" of open pond, using paddlewheels for recirculation and mixing, was devel­oped in the 1950s by the Kohlenbiologische Forschungsstation in Dortmund, Germany. However, sustained open pond production proved to be feasible for only three microalgae: Spirulina platensis, Dunaliella salina, and fast-growing Chlorella, in all cases because con­tamination by other species can be avoided.

Beyond the economical difference between the types of photobioreactors feasible for algae cultivation, light incidence and CO2 availability are the two main factors influencing algae growth. Large surface areas are essential to ensure enough light diffusion to the media, but they are normally associated with very little time to mass transfer the gas to the liquid phase (short liquid column). The optimal condition of light diffusion and CO2 availability is easily achieved in a closed reactor for logical reasons: In open photobioreactors, the undissolved CO2 is lost to the atmosphere, whereas in closed ones it is possible to increase (and maintain) partial pressure.