**5. Biotechnological Processes**

Despite a few useful features already referred to above, microalgae are in general expensive to produce, so concerted efforts have been on the way toward more costefficient modes of mass cultivation. 

With regard to open systems, the best choice seems to be the open shallow pond— made of leveled raceways, 2–10 m wide and 15–30 cm deep, which run as simple loops or meandering pathways; each unit may cover an area of several hundred to a few thousand m2. However, this configuration poses several problems—which restrict its use to strains that, in view of their weed-like behavior (e.g., *Chlorella*) or their ability to withstand adverse growing conditions (e.g., *Spirulina* or *Dunaliella*), can outgrow other microorganisms. 

