Mixed polyamide-based composite nanofiltraiton (NF) hollow fiber membranes with suitable characteristics for water softening under ultrafiltration (UF)-range low operating pressure were successfully developed. The thin-film selective layer of the composite membrane was formed via interfacial polymerization on the inner surface of a microporous polyethersulfone (PES) hollow fiber substrate with trymesoyl chloride (TMC) being the organic phase monomer, and a mixture of branched polyethyleneimine (PEI) and piperazine (PIP) being the monomers in the aqueous phase. In was found that there was a synergetic effect of PEI and PIP on the formation of the selective layer. The water permeability and salt rejection of the resultant membrane were both enhanced with a small amount of PIP added into the PEI aqueous phase, but dropped quickly with a higher PIP to PEI ratio. The optimized NF membrane possessed a molecular weight cut-off (MWCO) of 380 Da, an effective pore diameter of 1.27 nm, and pure water permeability (PWP) of 18.21/m(2) h bar. Under an operating pressure of 2 bar, the membrane exhibited rejection of 96.3% and 93.8% to 1000 ppm MgCl2 and MgSO4 feed solutions, respectively. The capability of the newly developed membrane for low-pressure water softening was evaluated by employing simulated hard water feed solutions with different ionic compositions and total hardness. By the combining effect of electrostatic repulsion and size exclusion, the mixed PEI/PIP-based composite hollow fiber offered superior water softening performance compared with the membranes made with PEI or PIP alone as the aqueous phase IP monomer. (C) 2014 Elsevier B.V. All rights reserved.