Nanofiltration hollow fiber development for water softening

• Main Author: Lin, Hongxiang.
• Other Authors: Wang Rong
• Format: Final Year Project
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Environmental engineering::Water treatment
• Online Access: http://hdl.handle.net/10356/53907
• Institution: Nanyang Technological University

Hard water can cause a series of problems in distribution networks and homes such as scaling and failure of household appliances. Water softening is therefore required to remove divalent ions like Ca2+ and Mg2+ in order to reduce the hardness of raw water. Nanofiltration (NF) is a relatively new membrane technology that has the ability to remove divalent ions in the water. Formation of thin film composite NF membranes via interfacial polymerization with piperazine (PIP) and trimesoyl chloride (TMC) as the monomers has been reported in recent studies, but more research is needed for water softening membranes which can operate at low pressures. In addition, current NF membranes are normally constructed in spiral-wound configurations. On the contrary, hollow fibers offer greater packing densities, self-supporting abilities, as well as higher surface area to volume ratios. Research studies have reported outer skin interfacial polymerization of NF hollow fibers to obtain greater surface areas, but inner skin interfacial polymerization allow lower fouling tendencies and concentration polarization, as well as better hydraulic flows. In this study, a thin film composite NF hollow fiber membrane for water softening was developed successfully. The thin film on the inner skin of the membrane was developed through interfacial polymerization with PIP and polyetherimide (PEI) as monomers in the aqueous phase and TMC as the monomer in the organic phase. Polyethersulfone (PES) ultrafiltration (UF) hollow fibers were used as the substrate for the polymerization reaction. It was found that a specific ratio of PIP and PEI concentrations in the presence of sodium dodecyl sulfate (SLS) in the aqueous phase was required for a successful interfacial polymerization reaction. The developed composite hollow fiber underwent a 120s interfacial polymerization reaction with 5% PIP and 95% PEI mixed in 0.25% aqueous monomer solution and 0.1% TMC mixed in cyclohexane. The developed thin-film selective layer has a pure water flux of 17.7 l/m2 h MgCl2 and MgSO4 rejection rates of 96.3% and 87.1% respectively in single-salt feed solutions of 1000 ppm under operating pressure of 2 bar. It has a positive surface charge with an effective pore radius of 0.62 nm and a MWCO of 600 Da. When tested in feed streams containing salt mixtures, the composite hollow fiber achieved Mg2+ and Ca2+ rejections of above 80%, signifying its potential for water softening applications.