Under phototrophic growth conditions, microalgae absorb solar energy and assimilate car­bon dioxide from the air and nutrients from aquatic habitats. However, commercial produc­tion must replicate and optimize the ideal conditions of natural growth. The choice of the reactor is one of the main factors that influence the productivity of microalgal biomass.

Open tanks come in different forms, such as raceway, shallow big, or circular (Masojidek and Torzillo, 2008). Circular ponds with a centrally pivoted rotating agitator are the oldest large-scale algal culture systems and are based on similar ponds used in wastewater treat­ment. The design of these systems limits pond size to about 10,000 m2 because relatively even mixing by the rotating arm is no longer possible in larger ponds. Raceway tanks are the most widely used artificial systems of microalgal cultivation. They are typically constructed of a closed loop and have oval-shaped recirculation channels. They are usually between 0.2 and 0.5 m deep, and they are stirred with a paddlewheel to ensure the homogenization of culture in order to stabilize the algal growth and productivity. Raceways may be constructed of concrete, glass fiber, or membrane (Brennan and Owende, 2010).

Compared to closed tanks, the raceway is the cheapest method of large-scale microalgal production (Chisti, 2008). These tanks require only low power and are easy to maintain and clean (Ugwu et al., 2008).

The construction of open tanks is low cost and they are easy to operate; however, it is dif­ficult to control contamination, and only highly selective species are not contaminated by other microalgae and microorganisms. Environmental variations have a direct influence, and the maintenance of cell density is low due to shadowing of the cells (Amaro et al.,

2011) . Light intensity, temperature, pH, and dissolved oxygen concentration may limit the growth parameters of open tanks (Harun et al., 2010).

Open photobioreactors have lower yields than closed systems due to loss by evaporation, temperature fluctuations, nutrient limitation, light limitation, and inefficient homogenization (Brennan and Owende, 2010). The amount of evaporated water can be periodically or contin­uously added to the raceway. The amount of evaporated water in raceways depends on the temperature, wind velocity, solar radiation, and pressure of water vapor. Water can also be lost during harvesting; however, recycling of the medium reduces this problem, and nutrients from the culture medium can be reutilized (Handler et al., 2012).

Open ponds are the microalgal cultivation systems that have been studied for the longest time. These reactors are used on an industrial scale by companies such as Sosa Texcoco, Cyanotech, Earthrise Farm, Parry Nutraceuticals, Japan Spirulina, Far East Microalga, Taiwan Chlorella, Microbio Resource, Betatene, and Western Biotechnology (Spoalore et al., 2006). Earthrise Farm began cultivation on a large scale in 1976 with Spirulina. Currently the company produces Spirulina and Spirulina-based products. The cultures are grown in 30 open ponds that are 5,000 square meters in size, each one mixed by a 50-foot paddlewheel (Earthrise, 2012).

Since 1981 Parry nutraceuticals has produced Spirulina in powder form, capsules, pills and tablets, and extracts astaxanthin from Haematococcus pluvialis. The company is located in South India (Oonaiyur), and the crops are grown in open ponds, covering an area of 130 acres (Parry Nutraceuticals, 2012). Cyanotech, located in Kailua Kona, Hawaii, on the Pacific Ocean, develops and markets astaxanthin from Haematococcus in gel capsules and Spirulina in tablet form in an area of 90 acres. Since 1984, Spirulina has been cultivated in open ponds, with the medium supplemented with water from the Pacific Ocean and agitation by paddlewheels (Cyanotech, 2012).

In Brazil, since 1998 the Laboratory of Biochemical Engineering (LEB) at the Federal University of Rio Grande (FURG) has been developing a project that studies the cultivation of Spirulina on a pilot scale in an open pond (Figure 1.1) on the edge of Mangueira Lagoon (Morais et al., 2009), for addition to meals for children. Products that are easy to prepare, con­serve, and distribute have been developed. These products include instant noodles, flan, powdered mixture for cakes, cookies, chocolate powder, instant soup, isotonic sports drinks, gelatin powder, and cereal bars (Costa and Morais, 2011).

The LEB, along with the President Medici Power Plant (UTPM), operated by the Society of Thermal Electricity Generation (CGTEE) since January 2005, has carried out the cultivation of microalgae for the biofixation of CO2 that is emitted in the combustion of coal at UTPM in an open pond (Figure 1.2) (Morais and Costa, 2007).

In southern Brazil, the company Olson Microalgae began commercial production of Spirulina capsules as a nutritional supplement in 2012, with an annual target of 6,000 kg in open ponds (Figure 1.3).

The Company Ouro Fino produces biomass and protein from microalgae for human and animal feed using the culture medium vinasse, cane husks, and carbon dioxide generated from the alcohol industry (Figure 1.4).

Several studies of open systems have taken place. According to Lee (2001), only species with high resistance are grown in open systems, such as Dunaliella (resistant to high salin­ity), Spirulina (grown in high alkalinity), and Chlorella (grown with high nutrient concentrations).