Harvesting of algae depends on the type of algae being cultivated. Some of the harvesting methods commonly used include – sedimentation, membrane filtration, flocculation, floatation, centrifugation and more. The mode of harvesting depends on the algae characteristics.
Harvesting of Dunaliella salina should be carried out carefully as the cells lack rigid cell wall. Improper handling could lead to damaging of the cell wall and loss of algae cells and beta carotene content. Commercially suggested two-stage harvesting techniques for Dunaliella include membrane filtration and Evodos spiral plate centrifugation. These techniques have higher efficiency in harvesting the algae without any damages to the algae cells.
This is the initial stage of algae harvesting. This stage, also called pre-concentration stage, aims to increase the concentration of the algal slurry to about 0.5-1% of the suspended material. Usually 500 mesh nylon filter bag is used for this harvesting technique. These membranes retain the microalgae while allowing the solutes and other contaminants to pass through.
While there are some industrially used centrifugation techniques such as disc-stack centrifuges for Dunaliella salina harvesting, Evodos spiral plate centrifugation technique is widely recommended owing to rupturing of the cells caused by the former.
After the centrifugation step, the algae is in the form of a paste with the final concentration of 15-30%.
An important factor to consider in algae cultivation is that dewatering should be done as soon as the cell density and the beta carotene content in the red stage reaches the required level, else algae would start undergoing fermentation process which would lead to decrease in beta carotene content. Further, at each stage, only 50% of the algae is transferred and finally harvested. The remaining algae content is stored as inoculum to avoid contamination of the entire process.