Although they are more expensive to build and run than open systems, the promise of improved yields, and the possibility of growing a wider range of species, has led to significant interest in closed reactors. It is much easier to control contamination and environmental parameters in closed systems, allowing the cultivation of more sensitive strains and expanding the potential product range. Biomass concentra­tions obtained are higher than in open systems, thus reducing the cost of harvesting. However, the capital and operating costs of closed reactors are higher than those of open ponds (Carvalho et al., 2006).

A large variety of PBR designs have been proposed, only a few of which have been commercialized (Greenwell et al., 2010). Most designs are based on the premise of optimizing light provision by maximizing the area-to-volume ratio, while ensuring a reasonable working volume, cost of reactor material, and mixing pattern (Carvalho et al., 2006). One of the major problems with closed reactors is temperature control, and the larger the area-to-volume ratio, the more susceptible the temperature of the medium is to changes in environmental temperature. The optimum light path length is 2 to 4 cm (Borowitzka, 1999), but most closed reactors have a larger diameter for ease of mixing, cleaning, temperature regulation, and to increase the working volume while reducing the cost of construction materials. Sedimentation is prevented by maintaining turbulent flow through mixing mechanically or by airlift.

An important and often overlooked feature of closed reactors is the ease with which they can be cleaned and sanitized (Chisti, 2007). Closed microalgal reactors are often presented as having the advantage of a decreased risk of contamination. Contamination can be avoided in closed reactors, but only if they are operated under sterile conditions, which adds to the cost (Scott et al., 2010). Due to their large size and surface area, closed reactors cannot be effectively sterilized by heat, and therfore require chemical steriliz­ers. These are not always 100% effective and sometimes require large volumes of sterile water to flush out the chemical agent. Most closed PBRs do not satisfy the good manu­facturing practice requirements for production of pharmaceutical products (Lee, 2001).

The most common designs are tubular (Miyamoto et al., 1988; Richmond et al., 1993; Borowitzka, 1996; Vonshak, 1997) or flat-plate (Hu et al., 1996; Vonshak, 1997) reactors. Both types usually operate with culture circulated between a light-harvesting unit, consisting of narrow tubes or plates, to provide a high surface area, and a reser­voir or gas exchange unit in which CO2 is supplied, O2 removed, and harvesting car­ried out. The circulating pump must be carefully designed so as to avoid shear forces disrupting algal cells. A variety of microalgae, including Chlorella and Spirulina, have been successfully maintained in both tubular and flat-plate PBRs (Molina Grima, 1999; Lee, 2001; Pulz, 2001; Carvalho et al., 2006).

Scale-up of any PBR design is challenging due to the difficulty in maintaining optimum light, temperature, mixing, and mass transfer in large volumes (Ugwu et al., 2008). Large-scale closed systems will likely be based on the integration of multiple units rather than increasing the size of a single reactor (Brennan and Owende, 2010).