Application of nanofiltration membrane to water softening

Many fields need soft water but not deioned water.Soft water for different purposes can be preparated by nanofiltration according to the retention characteristics of nanofiltration membranes that high retention for bivalent ions such as Ca~(2+),Mg~(2+),SO_4~(2-) and low retention for monovalent ions such as K~+,Na~+,Cl~-.The principles of water softening by nanofiltration,the separation performances of some commercial nanofiltration membranes and the selection principles for nanofiltration membranes according to the quality of raw water and softened water are discussed in this review.The factors including the quality of raw water and softened water,operation pressure and the recover rate are also discussed.The applications in the production of softened drinking water and industrial softened water and pre-softening for seawater desalination are instructed.Finally,the economy of water softening by nanofiltration is discussed by comparisons with lime softening and ion exchange softening.As a new softening process,nanofiltration is an advanced and economic water treatment technology,which can overcome the drawbacks such as high energy consumption in reverse osmosis and environmental pollution caused by wastewater or watse residue in lime softening and ion exchange softening.It is concluded that as a softening process,nanofiltration could be in competition with lime softening and ion exchange softening and reverse osmosis,and could be widely used for the preparation of softened water for different purposes.