Photobioreactors

Industrial production of microalgae is prevalently carried out in raceway ponds. There is, however, the diffuse perception that open systems have reached their technological limit and that closed photobioreactors (PBR) will prevail in the future even at very large scale. PBR are flexible systems that can be optimized according to the characteristics of the cultivated species and permit to cultivate microalgae that can not be grown in open ponds because of contamination. In the last decades PBR have much evolved and plenty of new designs have been proposed. This review focuses on those systems used outdoors at pilot level and at commercial scale. The main design criteria for PBR (construction materials, surface-to-volume ratio, orientation and inclination, mixing and mass transfer, temperature control) are outlined, and the principal PBR categories (tubular and flat; horizontal, inclined, vertical and spiral; manifold and serpentine) are illustrated, with examples of commercial plants closed down and currently in operation. The performance of ponds and PBR is also compared and the issue of productivity evaluation is discussed.