Photobioreactors for the production of microalgae

Microalgae have a large biotechnological potential for producing valuable substances for feed, food, nutraceutical, and pharmaceutical industries . Furthermore, other applications can be attributed to the photosynthetic process performed by these microorganisms such as CO2 mitigation, wastewater treatment, and biofuels production. Whatever the process, it must be designed considering the specific characteristics of these microorganisms. Thus microalgae (according to applied phycology) are photosynthetic microorganism able to perform oxygenic photosynthesis. Both cyanobacteria with a prokaryotic cell structure and microalgae with a eukaryotic cell structure are usually included in this category. These microorganisms are photoautotrophs, although they may also grow under mixotrophic or heterotrophic conditions. For the production of microalgae under phototrophic conditions, it is necessary to use photobioreactors that must be adequately designed, built, and operated to satisfy the requirements of the selected microalgae. Multiple designs and configurations of photobioreactors have been proposed, but no optimal design still exists. For whatever application, the photobioreactor to be used must be adequately selected according to the requirements of process. Thus the establishment of the requirements of the biological system to be used is required to adequately design the optimal photobioreactor, which constitutes the starting point when designing a microalga-based process. Two major categories of photobioreactors are considered: open and closed. As open cultivation systems (having direct contact with the environment), artificial ponds, tanks, raceways (shallow racetracks mixed by paddle wheels), and thin-layer (i.e., inclined-surface systems) platforms are often used. As closed cultivation systems (having no direct contact between the culture and the atmosphere), bubble columns, tubular loops, and flat-panels are typically used. At present, open systems are feasible for the production of thousands of tons of biomass significantly cheaper than that from closed systems.