**4.3. Nanofiltration (NF)**

NF is capable of removing ions that contribute significantly to the osmotic pressure hence allows operation pressures that are lower than those RO. For NF to be effective pre-treatment is needed for some heavily polluted waters; Membranes are sensitive to free chlorine. Soluble elements cannot be separated from water [41]. In the study reported by Yang and co-workers, PMIA/GO composite nanofiltration membranes were used for water treatment. The prepared composite membrane had greater hydrophilic surface which gave rise to high pure water flux compared to that of the pure polymer (PMIA). The results obtained showed high dye rejection and enhanced fouling resistance to bovine serum albumin (BSA) [42]. Xu and others reported NF membrane for textile wastewater treatment, the prepared membrane displayed good removal of heavy metal ions, common salts and dyes, showing high removal efficiency toward metal ions and cationic dyes [43]. Lin and others reported nanofiltration membranes for dye (Congo red and direct red) and salt rejection, the results showed high dye rejection and low salt rejection which shows the possibility of the salt reuse in FO.

of ZnO coated MWCNTs increased pure water flux due to the hydrophilic properties added. The results showed increase in antifouling properties as well as decrease in surface roughness brought by the embedded nanoparticle. ZnO/MWCNTs composite membrane showed greater dye removal compared to pure PES membrane [50]. Polymeric membranes in water treatment can reject up to 98% Cd ions through asymmetric polysulfone membrane [51]. Hybrid membranes are also used in removal of water contaminants as they introduce adsorptive capability, photocatalytic and antibacterial capabilities. This will lead to improved water flux and rejection value [52]. Aromatic polyamide is among other polymers that have been used in membrane industries. High pressure membrane includes tight UF, NF and RO, these are operated at high transmembrane pressure (>200 kPa) and low pressure membrane includes lose UF and MF. Usually fouling turn to occur when transmembrane increases, as to maintain

Wastewater Treatment Using Membrane Technology http://dx.doi.org/10.5772/intechopen.76624 33

Qiu and others have reported the use of hybrid microfiltration-osmosis membrane bioreactor to remove nitrogen and organic matter in municipal wastewater. Results showed decrease in fouling and reduced bacteria deposition [54]. In the study reported by Ochando-Pulido and others in olive mill wastewater and the rejection efficiency was 99.1% [55]. Microfiltration membrane has been applied in domestic wastewater and the amount percentage recovery of phosphorus was found to be 98.7% [56]. Combination of UF/NF/RO have been used in rendering plant wastewater (RPW) and the rand filtration was used as an effective pre-treatment for UF hence lowering membrane fouling [57]. Another form of membrane called membrane with a molecular weight cut-off (MWCO) was used to treat municipal and industrial wastewater, the obtained results showed complete resistance to irreversible fouling and high dye rejection [58]. UF and NF membranes have been used for waste stream purification also known as backwash water, which is obtained by washing filtration beds from swimming pool

**Matrix/pollutants Membrane type Performance References** Oily water MF 90.2% removal of organic additives [60]

24–32 L h−1 m−2 permeate flux

membranes showed complete resistance to irreversible fouling and high

phosphorus

FO and MF 86–99% removal efficiency for nitrogen

100% for phosphorus

rejections of dyes

[55]

[56]

[61]

[58]

Olive mill wastewater RO COD rejection 97.5–99.1% and

membranes with a molecular weight cut-off

(MWCO)

Domestic wastewater MF >97% removal of total nitrogen and total

Chlorophenol RO Improved unit performance [62]

flux or when there is decrease in flux [53].

water system [59] (**Table 1**).

Nitrogen and phosphorus in

Municipal and industrial wastewater streams

**Table 1.** Membrane applications.

microalgae

### **4.4. Forward osmosis (FO)**

FO is a natural occurrence where the solvent moves from a region of lower concentration to the region of higher concentration across a permeable membrane [44]. This method is found to be highly efficient with low rate production of brine and is well studied as it promise to solve water problems worldwide, however regeneration of the draw solution is highly expensive for desalination processes hence the use of nanofiltration or reverse osmosis for regeneration of draw solution [45].

#### **4.5. Reverse osmosis (RO)**

RO is pressure driven technique used to remove dissolved solids and smaller particles; RO is only permeable to water molecules. The applied pressure on RO must be enough so that water can be able to overcome the osmotic pressure. The pore structure of RO membranes is much tighter than UF, they convert hard water to soft water, and they are practically capable of removing all particles, bacteria and organics, it requires less maintenance [46]. Some disadvantages include the use of high pressure, RO membranes are expensive compared to other membrane processes and are also prone to fouling. In some cases, high level of pretreatment is required [47]. RO has extremely small pores and able to remove particles smaller than 0.1 nm [48]. Huang and others, reported RO membranes coated with azide functionalized graphene oxide hence created smooth, antibacterial and hydrophilic membrane, which removed *Escherichia coli* and reduced BSA fouling [49].
