Closed photobioreactors are flexible systems that can be optimized according to the biological and physiological characteristics of the microalgal species involved. An overview of several closed or semiclosed culture systems used for mass cultures of microalgae outdoors is presented in Figure 3. Schematic diagram of a closed photobio reactor is illustrated in Figure 4. Compared to open systems, photobioreactors have a number of advantages, namely reproducible cultivation conditions with regard to environmental influence; reduced risk of contamination; possibility oftemperature regulation; low CO2 losses; and smaller area requirements. On the downside, closed sys tems are more difficult to clean, the tube material might partially decrease sunlight penetration, and the system must be effectively cooled and degassed since excessive oxygen produced by the growing cultures can reduce growth. Furthermore, the cost of construction is about 1 order of magnitude higher than that of open ponds (about 100 US$m 2).
Plexiglas, acrylic, and glass tubes and flexible plastic coils can act as solar collectors - the microalgal suspen sion being circulated continuously through rows of connected transparent tubes or flexible coils. Here, a much greater biomass density can be maintained than in open ponds. Outdoor photobioreactors for commercial production are designed as modules.
Generally, the tubes are positioned horizontally or vertically, arranged as a serpentine loop or manifold rows. The culture suspension is circulated by a pump -or more preferably by air lifting (injecting a stream of compressed air in upward pointing tube). Peristaltic and membrane pumps are physically more 'friendly' to cells than centrifugal pumps, which cause high sheer stress. The cooling is maintained by submerging the tubes in a pool of water, by heat exchangers, or by spraying water on the tube surface. A further improvement is represented by the two plane horizontal tubular photobioreactor built in Florence, Italy, which led to a high Spirulina productivity of 30 g dry weight m 2 day 1 (Figure 3a). The biggest production plant to date has been built in a greenhouse in Klotze, Germany, with a volume of 700 m (20 modules of 35m ). Horizontal glass tubes are arranged in a vertical fence like system in order to utilize diffuse light (Figure 3b).
Helical tubular system, commonly called 'biocoils', seem to be another alternative for microalgae cultiva tion (see diagram in Figure 4). It consists of coiled, flexible, transparent tubes (3-6 cm in diameter) around an open cylindrical frame realized by the company Addavita Ltd. in UK. In Australia, 1001 laboratory photobioreactors have been scaled up to 1000 l outdoor pilot plants.
Vertical column photobioreactors are relatively sim ple systems in which mixing is achieved by air + CO2 bubbling from the bottom. A variation of this column system are annular photobioreactors, which consist of two glass or Plexiglas cylinders of different diameters placed one inside the other to form a culture chamber, some 3-5 cm thick and 50-1501 in volume (Figure 3d). Illumination is provided by either natural or artificial light. In column photobioreactors, sensitive strains with fragile cells or filaments can be grown as the culture mixing is very gentle (e.g., Nostoc, Microcystis, Nannochloropsis).
Flat panel photobioreactors with a short light path, with horizontal, inclined, or vertical orientation, have a high surface to volume ratio and give high volumetric productivities. These photobioreactors are made from plexiglass or polycarbonate alveolar sheets, 2-4 cm thick. In the system of vertical panels, where the flow is horizontal in alveolar channels using pumps, the panels are packed 20 cm apart in series (Figure 3c). Compared with tubular systems, flat panel photobior eactors have several disadvantages: in temperature control, oxygen degassing, and fouling due to the lack of turbulent flow in the narrow, rectangular channels.
A similar principle of cultivation has been used in panel type photobioreactors developed as flat glass chambers connected in a series of plate type photobio reactors, vertical arranged or inclined to the sun, and mixed by air bubbling from the bottom.
Despite the higher yields attainable with photobior eactors, its high construction and maintenance costs still make them uncompetitive for industrial production of microalgal biomass. Their use can be foreseen for the production of high value bioactive substances, which require the adoption of sterile conditions.
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