The two main methods of infrastructure considered suitable for cultivation of algae are open (raceway) ponds or photo-bioreactors (PBRs) [55], and are compared in Table 3. Raceway ponds are similar to oxidation ditches used in wastewater treatment systems being large, open basins of shallow depth and a length at least several times greater than that of the width. Raceway ponds are typically constructed using a concrete shell lined with polyvinyl chloride (PVC) with dimensions ranging from 10 to 100 m in length and 1 to 10 m in width with a depth of 10 to 50 cm [55]. Oswald considered the open pond to be the most viable method of combining algal cultivation and wastewater treatment in the 1950s [56].

Photobioreactors are more commonly used for growing algae for high value commodities or for experimental work at a small scale. Recently, however, they have been considered for producing algal biomass on a large scale as they are capable of providing optimal conditions for the growth of the algae [55]. A closed reactor allows species to be protected from bacterial contamination, shallow tubing allows efficient light utili­sation, bubbling CO2 provides high efficiency carbon uptake and water loss is minimised. PBRs provide very high productivity rates compared to raceway ponds. In their life-cycle assessment (LCA) study, Jorquera et al. [55] estimated volumetric productivity to be at least eight times higher in flat-plate and tubular PBRs. The reason why PBRs however have not become widespread is due to the energy and cost intensity of production and operation. PBRs require a far higher surface area for the volume of algal broth compared to alternative infrastructure. Much higher volumes of material are therefore required which in turn requires a higher capital energy input and increases environmental impacts [57]. During operation algal biomass must be kept in motion to provide adequate mixing and light utilisation. These increase productivity but also require additional energy for pumping. So far in comparison to raceway ponds the benefits of PBRs do not outweigh the necessary energy requirements identified in the LCA study published by Jorquera et al. [55]. A net energy ratio (i. e., energy produced/energy consumed) of 8.34 has been reported for raceway ponds as compared to a net energy ratio of 4.51 and 0.20 for flat-plate and tubular photobioreactors, respectively [55]. It is likely that ponds will continue to provide the most effective infrastructure for algal cultivation due to their low impact design and low energy input requirement. PBRs will continue to be important however, for laboratory work, developing cultures and producing biomass with high economic value. As research continues it may also be possible to develop infrastructure that will provide the ben­efits of both PBRs and open ponds together.