Oscillatory flow bioreactors contain equally spaced orifice plate baffles in a tubular style reactor. This reactor has improved heat and mass transfer due to the oscillatory motion that is imposed on the net flow of the fluid, which means that the degree of mixing is independent of the net flow. This results in long residence times in rela­tively low length-to-diameter ratios. This reactor design, therefore, has the potential to decrease the energy required for mixing algae cultures, due to decreased pumping requirements, and also leads to decreased capital costs (Harvey et al., 2003).

5.3.3.2 Hybrid Production Systems

As open and closed systems offer different advantages and disadvantages, it seems practical that a combination of the two could provide the best of both worlds. This idea has been investigated in various configurations, either by circulating culture between open and closed reactors through a single growth stage, or by having a two-stage culture regime where cells are moved from one to the other at a certain point. A simple single-stage intermediate design is produced by enclosing or semi­enclosing open ponds in tunnels or greenhouses to improve temperature control and reduce evaporation and contamination. This is very effective in improving produc­tivity, particularly in colder seasons, but comes at increased capital cost (Vonshak, 1997). Pushparaj et al. (1997) described a system where an alveolar panel oriented toward the sun was coupled with an open raceway for gas transfer. Adding the panels improved the productivity of the pond from 0.18 to 0.31 g L-1d-1.

In two-stage configurations, culture is usually grown initially in closed PBRs to optimize the growth rate and minimize contamination of the inoculum, which is then moved to an open pond for the second growth stage. Integration of open and closed PBRs in this way could provide sufficiently large, clean inoculants to limited — duration culture in outdoor raceways in order to significantly limit adverse events (Greenwell et al., 2010).

The second cultivation stage often involves nutrient stress for accumulation of a metabolite such as lipids or pigments. The nutrient stress stage is suited to open ponds because the growth rate is naturally low and therefore not affected by the low light availability (Brennan and Owende, 2010). Initial culture in closed reactors also implies that the culture entering the pond is relatively dense and therefore less likely to be contaminated, particularly in a nutrient-deprived environment (Singh et al., 2011). This sort of system has been used for the production of astaxanthin from Haematococcus (Huntley and Redalje, 2006) and described for the production of biodiesel from Nannochloropsis (Rodolfi et al., 2009).