Process variables affecting harvesting efficiency of Nannochloris oculata by AlCl3 flocculation such as, cell density, ionic strength, coagulant dosage, media pH, and cell surface charge were investigated. Initial cell density and coagulant dosage had a significant effect on the removal efficiency; however, levels of ionic strength tested were not significant. Best flocculation conditions of investigated variables were: 0.0016 ng of AlCl3/cell, 3.0 x 10(7) cell/mL, and pH 5.3. Removal efficiency at optimum conditions and salt concentrations of: 0, 15, and 30 g/L NaCl was 96, 98, and 97 %, respectively. Low cell density cultures similar to 10(6) cell/mL, required five times greater AlCl3 dosage to achieve the same removal efficiency. Destabilization of algal cultures using 0.0032 ng of AlCl3/cell was observed by reducing the zeta potential to -22 mV. Acidification with HCl for conducting flocculation at pH 5.3 could be a significant cost burden unless is mitigated by selecting a low-buffering-capacity media. (c) 2012 Elsevier Ltd. All rights reserved.