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Nanofiltration (NF) and low energy reverse osmosis (LERO) are pressure driven, selective membrane separation techniques that progressively acquire new fields of application in treatment of surface, ground and wastewater (WW) (Deraniyagala, 2006) in view of their continuous development. Trends of their development are lower energy requirements, higher efficiency and specificity solute rejection, longer working lifetime at steady performance, and consequently higher process cost effectiveness, in addition to promotion of environmental safety. In fact, according to the results presented in the present chapter, NF is expected to replace, in the near future, several conventional techniques of WW treatment (Khedr, 2004, 2008, 2009). NF membranes are generally characterized by relatively high surface charge (Childress et al, 2000), usually negative, and pores in the range of nanometer diameter (Jiang et al, 2004). In contradiction with the usually stated oversimplified description of NF membranes as intermediate type between loose RO membranes and tight ultrafiltration ones, the permeation performance of these membranes is mainly determined by their surface charge density, pores diameter and distribution, surface roughness and fouling susceptibility depending on the simultaneously working mechanisms of solute species size exclusion and electrostatic interaction (Thanuttamavong et al, 2002). In general, NF membranes have high permeate flux and rather high rejection of polyvalent ions at low or moderate rejection of TDS. Therefore, they have lower opposite osmotic pressure and, consequently, lower operation pressure and lower energy consumption than RO under the same operation conditions (Khedr, 2008). The moderate salt rejection in NF also maintains the stability of the water and prevents it from being aggressive to network piping. Efficient rejection of organics by NF and LERO was reported in a wide range of applications in wastewater treatment, food, chemical and pharmaceutical industries, for purification of industrial effluents and minimizing waste discharge. NF was also applied for rejection of natural organic matter which imparts color (Jensen & Thorsen, 1995). Efficiency of NF in removal of organics, suspended solids, colloids, microorganisms, and hardness components enabled its application for pre-treatment of sea water prior to SWRO or MSF in hybrid desalination processes. Such pre-treatment promoted the percent recovery of the subsequent RO step to 65% instead of 40% and lowered the fouling potential of RO membranes so as to lower the maintenance rate (Hassan et al, 1998, 2000; khedr, 2004).

Nanofiltration and Low Energy Reverse Osmosis for Advanced Wastewaters Treatment