Today there are three main types of system for the production of microalgae. These systems are open cultivators (flow reactors, raceway ponds), tubular photobioreactors or fermenters (photobioreactors), and vertical reactors (vertical growth reactors). Open cultiva­tors consist of parallel circular tunnels situated on the earth. Microalgae inside them are moved by a wheel mixer (Salis, 2010). It is difficult to control the conditions under which microalgae are developed in these reactors because they can be contaminated by other microorganisms.

11.1.1 Harvesting of Microalgae

Conventional processes used to harvest microalgae include concentration through centri­fugation, foam fractionation (Csordas and Wang, 2004), flocculation (Knuckey et al., 2006; Poelman et al., 1997), membrane filtration (Rossignol et al., 2000), and ultrasonic separation. Harvesting costs may contribute 20-30% to the total cost of algal biomass (Molina Grima et al.,

2003) . The microalgae are typically small, with a diameter of 3-30 pm, and the culture broths may be quite dilute at less than 0.5 gL-1. Thus, large volumes must be handled. The harvesting method depends on the species and cell density and, often, the culture conditions.