Two most common cultivation systems for microalgae are open ponds and closed photobioreactors. Open ponds come in many different shapes and forms, each hav­ing certain advantages and drawbacks. The types of ponds that are currently used in research and industry include raceway ponds, shallow big ponds, circular ponds, tanks and closed ponds. The location of the pond is a critical factor in determining the type of pond, microalgal strain and intensity of available light for photosynthe­sis. Due to the lack of control associated with open systems, the pond efficiency is a function of the local climate [19] . They are limited by key growth parameters

including light intensity, temperature, pH and dissolved oxygen concentration. Contamination by predators is another issue involved with open ponds. Local cli­mate and contamination can limit the cultivation system to unwanted algal strains which grow under severe conditions [13]. The cultivation system cost is a vital fac­tor when comparing open and closed cultivation systems. It is well known that the cost involved with cultivation ponds are significantly less than that of closed sys­tems. The construction, operating and maintenance costs of cultivation ponds are lower than photobioreactor options, the design of open ponds is technically less challenging, and they are more scalable [15]. Although cultivation ponds result in a relatively lower biomass concentration, the aforementioned features of the pond system make it a competitive cultivation option.

Tubular reactors are considered to be more appropriate for outdoor cultivation. The large illumination surface of the reactor which is created by translucent tubing is the main factor behind its outdoor suitability. The tubing can be arranged in vari­ous configurations and the appropriateness of the configuration depends on the specifications of the system. Common configurations include straight line and coiled tubing [35]. The geometry of the reactor is also important, as tubular reactors can be configured in a vertical, horizontal or inclined plane. Harun et al. [13] states that the major difference between the configurations is that the vertical design allows greater mass transfer and a decrease in energy usage, while the horizontal reactor is more scalable, but requires a large area of land. Tubular reactors make use of either airlift or air pump aeration for culture mixing. The airlift system is more preferred, espe­cially for scale-up purposes. Previous studies have shown that the scale-up of a tubular photobioreactor can play havoc with the mass transfer of the culture [22]. Large build-up of dissolved oxygen may occur within the tubing during scale-up and this can inhibit cell growth. Figure 1 shows a schematic diagram for open ponds, horizontal tubular reactor (HTR) and external loop tubular reactor (ELR).