Reverse osmosis (RO) and nanofiltration (NF) are widely used in many water treatment processes, such as seawater and brackish water desalination. Membranes with improved separation performance and anti-fouling properties can be prepared by incorporating nanoparticles into membrane matrix. A number of nanomaterials can serve as potential water transport channels and modify the structure and surface properties of the membrane thin film layers. Mixed-matrix membranes (MMMs) can benefit from the high performance of both the organic matrix and inorganic fillers, which are believed to be the next generation of novel membrane materials. Recent advances of the nanoparticle-filled RO, NF and forward osmosis (FO) MMMs are reviewed in this paper. The effects of nanomaterials with various properties on membrane structure and separation performance are discussed, including zeolites, nanotubes, mesoporous materials, pure metal, metallic oxides, graphene oxide and aquaporin. The dispersibility of nanofillers in MMMs and their compatibility with polymer matrix can be improved by surface modification. Different methods of nanoparticle addition and membrane preparation are also illustrated in detail. Based on the analysis of the recent works, the membrane formation and separation mechanisms are explored and the main problems of membrane synthesis and application are summarized. Finally, the future development of RO, NF and FO MMMs for water treatment process is suggested.

Contents
1 Introduction
2 Mixed-matrix reverse osmosis membranes
2.1 Different types of nanomaterials
2.2 Modification of nanomaterials
2.3 Adding methods of nanomaterials
3 Mixed-matrix nanofiltration membranes
3.1 Interfacial polymerization
3.2 Phase inversion
4 Mixed-matrix forward osmosis membranes
5 Conclusion and outlook