We compared potential pre-concentration techniques for Nannochloropsis gaditana (Nng) by testing natural sedimentation; flocculation with aluminium sulphate, polyaluminium chloride and chitosan; and induced pH. Promising flocculation efficiencies and concentration factors were obtained in a short time with alkalinity-induced flocculation at an adjusted pH of 9.7 and with chitosan at an adjusted pH of 9.9 using a concentration of 30 mg L-1. The sedimentation rates of alkalinity-induced flocculation were also evaluated. Additionally, viscosity, particle size distribution and Ca/Mg ions were analysed for pre-concentrated samples of N. gaditana (Nng) and the previously studied Phaeodactylum tricornutum (Pht) which were obtained by various different harvesting methods under optimal conditions. The theological properties of the concentrated algae suspensions of two microalgal species showed Newtonian behaviour. The mean diameters of the flocs were between 39 and 48 mu m. The Ca/Mg analysis showed that Mg+2 is the triggering ion for alkalinity-induced flocculation in the conditions studied. (C) 2013 Elsevier Ltd. All rights reserved.