In order to remove CuO nanoparticle-humic acid composite contaminant (CuONP-HA) and reduce the membrane fouling in coagulation/ultrafiltraion process, sodium alginate (SA) was used as a coagulant aid of polyaluminum chloride (PAC) to pre-treat CuONP-HA in synthetic water. The effect of SA on floc properties and membrane fouling was investigated by a laser diffraction instrument and ultrafiltration fouling model. The results showed that SA, as coagulant aid, could improve the removal efficiency of CuONP-HA by coagulation/ultrafiltration process. CuONP was completely removed, and the removal efficiency of HA was over 80% at PAC dosage = 3.0 mg/L and SA dosage >= 0.2 mg/L Besides, PAC with SA (PAC + SA) could form larger flocs with better recoverability than PAC alone. The average size of CuONP-HA flocs formed by PAC + SA exceeded 1000 um which was about 66% larger than flocs formed by PAC, and the small CuONP-HA aggregation also increased to about 280 um. Moreover, membrane fouling was effectively reduced by pre-treating of SA, and membrane fouling index (MFI) was decreased with the dosage of SA increasing. (C) 2015 Elsevier B.V. All rights reserved.