**3. Promising solutions**

quantity are the main problems that need to be solved [3]. Removal of contaminants/water pollutants is required as to avoid negative effects on the environment as well as human health [4]. Several techniques have been developed for treatment of wastewater; such methods include reverse osmosis [5] ion exchange [6] gravity [7] and adsorption [8] among others. Adsorption has been widely used to remove water contaminants due to its low cost, available of different adsorbents and easy operation. Different adsorbents that have been used include use of magnetic nanoparticles [9] activated carbon [10], nanotubes [11] and polymer nanocomposites [12]; these can remove different contaminants including heavy metals that are very harmful even at low concentrations. Even though adsorption can remove most of water pollutants, it has some limitations such as lack of appropriate adsorbents with high adsorption capacity and low use of these adsorbents commercially [13]. Hence there has been a need for more efficient techniques such as membrane technology. Membrane separation or treatment process mainly depends on three basic principles, namely adsorption, sieving and electrostatic phenomenon [14]. The adsorption mechanism in the membrane separation process is based on the hydrophobic interactions of the membrane and the solute (analyte). These interactions normally lead to more rejection because it causes a decrease in the pore size of the membrane [15]. The separation of materials through the membrane depends on pore and molecule size [16]. For this reason, various membrane processes with different separation mechanisms have been developed. These include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), forward osmosis (FO) and reverse

Therefore the aim of this chapter is review different membrane technology processes used for treatment of wastewater in the last 5 years (2014–2018). The advantages, challenges/limitations associated with the use of each membrane technology and possible solutions are also

Membrane processes such as MF, NF, UF and RO are currently used for water reuse, brackish water and seawater [17]. Polymer based membranes are mostly used membrane material but because polymers such as polysulfone and polyethersulfone, are hydrophobic [18], polymeric membranes are prone to fouling [19]. This leads to blockage of membrane pores and decrease membrane performance [20], also increases operation cost by demanding extra cleaning process. There are factors causing membrane fouling, such as deposition of inorganic components onto the surface membrane/solute absorption pore blocking, microorganism and feed chemistry [21]. This results to either reversible or irreversible membrane fouling [22]. Reversible fouling formed by attachment of particles on the membrane surface, irreversible which occurs when particles strongly attach the membrane surface and cannot be removed by physical cleaning. When there is a formation of strong matrix of the fouled layer with the solute during continuous filtration process will turn reversible fouling to irrevers-

osmosis (RO).

30 Wastewater and Water Quality

discussed briefly.

**2. Challenges**

ible fouling layer [23].

For polymeric membranes, surface modification of the polymer is essential; such surface modification includes grafting, blending and incorporation of nanomaterials such as TiO<sup>2</sup> [24]), ZnO [25], Al2 O3 [26], carbon nanotubes [27] and graphene oxide [28]. Among these, graphene oxide membranes (GMs) are very promising in water treatment application such as desalination and wastewater treatment, due to their hydrophilic properties, flexibility and high mechanical strength; GMs have been reported to give wide range of pure water flux [28–32].
