**1. Introduction**

The global water scarcity is one of the critical issues faced by human beings. Sustainability of the available water resources is very important for society's development which renders the transformation of wastewater into clean water is mandatory. One of the most challenges in the treatment of industrial and municipal wastewater is the quality and the corresponding cost of the treated water. Recent improvements in membrane technology have emerged as the most important and reliable treatment method for wastewater separation and recycling by the unique features including no need for chemical additives, thermal inputs, and spent media regeneration. The fact that the membrane market in the water and wastewater segment around the world is projected to reach USD 39.2 billion with a compound annual growth rate (CAGR) of 10.8% from 2013 to 2019 is a true indicator of this appeal [1]. Among different kinds of membrane materials, polymer-based membranes have the most common use owing to their relatively cheap manufacturing costs and simple fabrication processes [2, 3]. Polymeric UF, NF, and RO membranes have been successfully used for the production of clean water and recent improvements have been summarized by Deng and Yin [4].

The hydrophobic nature of the polymeric membranes with their inherent permeability/selectivity trade-off is the most prominent problem that causes membrane fouling and lower throughput [5]. Applying one of the surface modification strategies (coating, grafting, blending, etc.) to convert surface non-polar groups into strong polar groups by the introduction of -OH, -COOH, -NH2 has been accepted as a facile and robust way for the manufacturing of the membranes with desired hydrophilicity, leading to improved performance in terms of permeability, selectivity, and antifouling properties [6, 7]. It must be pointed out that the number of modification steps during the membrane fabrication process makes it difficult for large-scale production and the bulk structure of the membrane can be worse affected by the complex technological process, which will result in impairing the separation performance and mechanical strength of the membrane.

The LbL self-assembled surface modification via polyelectrolytes provides a defect-free ultra-thin surface accomplished on any negatively or positively charged surface by a single-step process. In addition, it is an environmentally benign process involving aqueous solution as the media at moderate temperatures. Another approach adopted to improve membrane performance is the impregnation/decoration of inorganic nanomaterials in/on the membrane. According to the literature, TiO2, ZnO and Ag NPs [8–10] provide antibacterial, SiO2 NPs [11] electrical conductance, carbon nanotubes (CNTs) such as single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) [12] and graphene oxide (GO) [13] new water pathways, Fe catalytic property, and FeO NPs [14] magnetic property to the membrane. The hydrophilic nature of the nanomaterials with their high surface area to volume ratio, photocatalytic, antibacterial, and adsorptive capabilities have been widely utilized to modify the conventional polymeric membranes, aiming to overcome their limitations. For example, GO and TiO2 nanoparticles have been attracted considerable attention, in which the former has abundant of oxygen-containing functional groups (e.g., carboxyl, carbonyl, epoxy groups, and hydroxyl), making them hydrophilic, hence improves membrane permeability [15], while the latter can contribute to continuous oxidation reactions, result in destruction or lethal effect on bacteria, virus, fungi, and algae [16, 17]. Zeolite nanoparticles with high ion exchange capability, on the other hand, add new functionality to the above-mentioned nanoparticles. Zeolites have well-defined porous structures and offer mobility of alkali and alkaline earth metals, in order to compensate net negative charge between Si4+ and Al3+ in the framework makes zeolites excellent adsorber for the removal of many target solutes [18–20]. The nanomaterials can be incorporated into polymer dope by physical blending [21] or deposited as a thin layer on the active layer of the membrane via layer-by-layer selfassembly [22], interfacial polymerization [23], surface grafting [24], or filtration [25, 26] methods.

In the following sections, recent developments in the fabrications and applications of membranes that meet the required throughput, selectivity, mechanical integrity, resistance to fouling, and low manufacturing cost will be discussed. Throughout the chapter, membrane modification techniques via layer-by-layer self-assembly and decoration/incorporation of inorganic nanoparticles (hybrid membranes) will be focused. The effect of variable parameters including size and charge of polyelectrolyte, ionic strength of the media, number of bilayers, and different types of nanomaterials on the bulk and surface property, water permeability, selectivity, antifouling, antibacterial, and adsorptive properties of the resultant composite membranes will be highlighted. Benefits and drawbacks of blending and coating methods will be discussed.
