The harvesting of Chlorella vulgaris was investigated by magnetic flocculation, where the natural magnetite was used as magnetic seeds and the polyaluminium chloride (PACl) and polyacrylamide (PAM) were used as the coating polymer on the Fe3O4 surface. The composite modes of PACl, PAM, and Fe3O4 and their effects on harvesting were studied. The results showed that adding the composite PACl/Fe3O4 first (at (0.625 mmol Al/L)/(10 g/L)) followed by the addition PAM (at 3 mg/L) was the optimum dosing strategy. Following this strategy, 99% of cells could be harvested in less than 0.5 min, and it could overcome negative impacts from pH and algal organic matter. Compared to PACl, zeta-potentials of PACl/Fe3O4 were found to be increased substantially from -4.9-8.5 mV to 1.5-19.5 mV at pH range 2.1-12.3. The charge neutralization of PACl/Fe3O4 and sweeping of PAM play an important role in magnetic harvesting of microalgal cells. (C) 2015 Elsevier Ltd. All rights reserved.