Composite nanofiltration (NF) hollow fiber membranes desirable for water softening under low operating pressure ( <2 bar) were successfully developed in the current study. The thin-film selective layer of the composite hollow fiber was formed through interfacial polymerization on the inner surface of a polyethersulfone (PES) ultrafiltration (UF) membrane substrate with branched polyethyleneimine (PEI) and trymesoyl chloride (TMC) employed as the monomers in aqueous and organic phases, respectively. It is found that a proper molecular weight of PEI and the presence of sodium dodecyl sulfate (SDS) in the aqueous phase are important for a successful interfacial polymerization reaction. The resulting membrane prepared with optimized preparation parameters possesses a positively charged thin-film selective layer with pure water permeability (PWP) of about 17 l/m(2) h bar and a molecular weight cut-off (MWCO) of around 500 Da, which translates into the effective pore diameter of about 1.29 nm. With combined separation mechanisms of Donnan exclusion and steric hindrance, it was possible to achieve MgCl2 and MgSO4 rejection of 96.7% and 80.6%, respectively, when tested for 1000 ppm feed solutions at 2 bar operating pressure. In addition, for a 3000 ppm TDS feed stream containing salt mixtures, the membrane rejections for Mg2+ and Ca2+ ions were found to be around 90% while the water flux was about 20 l/m(2) h at 2 bar pressure, suggesting the potential of the newly developed composite hollow fibers for effective water softening application. (C) 2012 Elsevier B.V. All rights reserved.