Poly(styrene sulfonic acid) sodium salt (PSSS)/polypiperazine-amide nanofiltration (NF) membrane and PSSS/SiO2/polypiperazine-amide NF membrane were prepared by interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) on polyethersulfone (PES) supporting membrane. Effect of PSSS and silica concentrations were discussed. The active surface of the membrane was characterized by using SEM and AFM. The results showed that PSSS/polypiperazine-amide NF membrane prepared under the optimum conditions exhibited Na2SO4 rejection of 96.1% and water flux with 55.7 L m(-2) h(-1) at operation pressure of 0.6 MPa. The rejection of Na2SO4, MgSO4, MgCl2 and NaCI follows a decreasing in order of 96.1%, 85.2% 45.4% and 21.1% respectively. After addition of 0.2% (w/v) silica in the aqueous phase, the water flux increased 42.7% compared with only polypiperazine-amide NF membrane. The PSSS/polypiperazine-amide NF membrane shows a promising capability for softening highly concentrated seawater from 8-20 wt.%. (C) 2013 Elsevier B.V. All rights reserved.