The choice of cultivation systems is an important aspect that significantly affects the efficiency and cost — effectiveness of a microalgal biofuel production process (Lee, 2001; Pulz, 2001; Carvalho et al., 2006). A wide vari­ation exists among the microalgal cultivation systems for the production of biomass. Raceways, PBRs, and fer­menters, which are the three most widely used microal­gae culture systems, will be discussed below.

OPEN-POND CULTURE

Large-scale cultivation of microalgae in outdoor open-pond systems is well documented (Benemann and Oswald, 1996; Borowitzka, 2005). Open ponds most closely resemble the natural milieu of microalgae. Indeed, ponds can be natural bodies of water, excavated ditches that are unlined or lined with impermeable ma­terials, or they can be constructed above ground with walls (Figure 10.2). Despite a certain variability in shape, the most common technical design for open-pond sys­tems is raceway cultivators driven by paddle wheels and usually operating at water depths of 15—20 cm (Figure 10.1). At these water depths, biomass concentra­tions of up to 1000 mg/l and productivities of 60—100 mg/(l/day), i. e. 10—25 g/(m2/day) are possible. Similar in design are the circular ponds commonly seen in Asia and the Ukraine (Becker, 2007). Such circular ponds usually have the provision of a centrally located rotating arm (similar to those used in wastewater treat­ment) for mixing and may have productivities ranging between 8.5 and 21 g/m2 day (Benemann and Oswald, 1996). On the other hand, thin-layer, inclined ponds consist of slightly inclined shallow trays and may achieve productivities up to 31 g/m2 day (Doucha and Livansky, 2006). Because these ponds are open to the
environment, they are most suitable for algal species that can tolerate extreme environmental conditions (salinity, pH, nutrient loads, etc.) to the exclusion of invasive species. Such algal species include fast growers such as Chlorella, Spirulina, and Dunaliella, which thrive in highly alkaline or saline environments (Chisti, 2007).

Limitations to successful scale-up of microalgae in open-pond systems include contamination, evaporation, limited species suitability, low-volumetric productiv­ities, and the need for large land area.

PHOTOBIOREACTORS

The problems associated with open systems have encouraged the development of closed system PBRs. PBRs can be located indoors under supplemental illumi­nation or outdoors utilizing natural sunlight. Various types of PBRs have been designed depending on growers’ needs; these include tubular PBRs, vertical bubble columns and airlift reactors, combined bubble column and inclined tubular reactors, helical PBRs, and flat-plate PBRs (Tredici and Zittelli, 1998; Sanchez et al., 1999; Berzin, 2005; Ugwu et al., 2005) (Figure 10.3). Closed PBRs allow for tighter regulation and control of nearly all the biotechnologically important parameters

and confer the following fundamental benefits: a reduced contamination risk, reduced CO2 losses, repro­ducible cultivation conditions, controllable hydrody­namics, and temperature (Pulz, 1992). However, widespread implementation has been hampered by the high capital costs associated with PBRs.