In a semi-continuous system the fresh medium is delivered to the culture all at once, by simply opening a valve in the medium delivery line. Fresh medium flows into the culture vessel, and spent culture flows out into a collecting vessel. Once the required medium has entered the culture, the valve is closed, and the culture is allowed to grow for 24 h, when the procedure is repeated. The semicontinuous technique prolongs the use of large tank cultures by partial periodic harvesting followed immediately by topping up to the original volume and supplementing with nutrients to achieve the original level of enrichment. The culture is grown up again, partially harvested, etc. Semi-continuous cultures may be indoors or outdoors, but usually their duration is unpredictable. Competitors, predators, or contaminants and metabolites eventually build up, rendering the culture unsuitable for further use. As the culture is not harvested completely, the semicontinuous method yields more algae than the batch method for a given tank size.
Existing commercial microalgae culture systems range in volume from about 102 l to more than 109 l. However, aside from the specialized small-scale culture systems (<1000 l) other types of culture systems predominate: large open ponds, circular ponds with a rotating arm to mix the cultures, raceway ponds, or large bags.
There are several considerations as to which culture system to use. Factors to be considered include: the biology of the alga, the cost of land, labor, energy, water, nutrients, climate (if the culture is outdoors), and the type of final product. The various large-scale culture systems also need to be compared on their basic properties such as their light utilization efficiency, ability to
control temperature, the hydrodynamic stress placed on the algae, the ability to maintain the culture unialgal or axenic and how easy they are to scale up from laboratory scale to large-scale. The final choice of system is almost always a compromise between all of these considerations to achieve an economically acceptable outcome. Successful further development of the industry requires significant improvements in the design and construction of the photobioreactors as well as a better understanding of the physiology and physical properties of the microalgae to be grown.
A common feature of most of the algal species currently produced commercially (i.e. Chlorella, Spirulina, and Dunaliella) is that they grow in highly selective environments, which means that they can be grown in open air cultures and still remain relatively free of contamination by other algae and protozoa. Thus, Chlorella grows well in nutrient-rich media, Spirulina requires a high pH and bicarbonate concentration, and Dunaliella salina grows at very high salinity. Those species of algae which do not have this selective advantage must be grown in closed systems. This includes most of the marine algae grown as aquaculture feeds (e.g., Skeletonema, Chaetoceros, Thalassiosira, Tetraselmis, and Isochrysis) and the dinoflagellate Crypthecodinium cohnii grown as a source of long-chain polyunsaturated fatty acids, as well as almost all other species being considered for commercial mass culture. In the particular case of C. cohnii, a large scale fermentation plant is operated by Martek Biosciences in Winchester (USA).