High-pH-induced flocculation–flotation of the hypersaline microalga Dunaliella salina

Natural autoflocculation was not observed in a Dunaliella salina hypersaline culture and the microalgae did not float without destabilization of the algal suspension. High-pH-induced flocculation by sodium hydroxide addition was chosen to induce flotation. Recovery efficiencies greater than 90% and concentration factors of around 20 were reached. An autoflocculation mechanism, with precipitation of magnesium hydroxide, is proposed to explain a sweeping flotation of D. salina cells. The influence of the flow rate of sodium hydroxide addition was also studied to anticipate the constraints related to the industrialization of this process. The flow rate of sodium hydroxide addition had no effect on the recovery efficiency and reduced the concentration factor only for abrupt injections. Natural increase of culture pH by photosynthetic activity could reduce the amount of base consumed. Non-harvested cells remained viable during pH increase and could be used as inoculum for a new culture.