Low cell concentrations of Dunaliella salina in production scale cultivations require high energy input for biomass harvesting. Flocculation is a potential preconcentration method to lower the dewatering costs for the beta-carotene production. In the present study, optimal flocculant dosages were determined for several metal salts, NaOH, Ca(OH)(2) and Al-electrolysis. Beside harvesting efficiency eta(H) and concentration factor CF, also the recyclability of the separated medium as well as the influence of the cell physiology on the harvesting performance were analyzed for selected flocculants. To assess the possible recycle of non-sedimented cells for the inoculation of new cultivations, cell vitality and the photosynthetic activity of D. salina were analyzed after the flocculation. As a result, the flocculation with NaOH led to a clear inhibition of both, the algal growth on recycled medium and the algal photosynthetic activity. The addition of FeCl3 seems most promising to flocculate D. salina. (C) 2015 Elsevier Ltd. All rights reserved.