**3. Membrane technology**

Membranes, as a thin layer barrier for size differential separation, are usually integrated with chemical and biological treatment or standalone systems in secondary treatment of wastewater settings [28–30]. In a typical membrane mechanism, there is usually a driving force, such as a semi-permeable barrier which controls the rate of movement of components by fractional permeation and rejection through pores of different sizes as depicted in **Figure 4** [32]. The permeation and selective rejection is a function of the membrane pore size and chemical affinity, which helps

**Figure 4.**  *Membrane selective permeation for various solutes adapted from [31].* 

*Treatment of Water and Wastewater for Reuse and Energy Generation-Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.84474* 

to have a product stream devoid of target components [33]. Due to the relatively low energy requirement and wastewater treatability efficiency, membrane technology has tremendously improved by the development of new materials and configurations for industrial applications. Some of these applications include microbial fuel cells, removal of organic and inorganic components, disinfection, pathogen removal and desalination [30, 33, 34].

### **3.1 Types of membrane technologies**

 Generally, the major driving force for selective filtration is a potential gradient of variables such as hydrostatic pressure, electrical voltage, temperature, concentration or a combination of these driving forces [29, 32]. These variables including nature (natural and synthetic) and structure (porous or non-porous and heterogeneous or homogenous) have been used in the classification of membranes [28, 31, 32, 34]. However, most commercially available and industrially used membranes are pressure-driven and energy driven (electrodialysis and electrodialysis reversal) membranes [35]. Pressure driven types are namely microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO) and nanofiltration (NF). These are also classified by their pore sizes or molecular weight cut-off (MWCO). MWCO is expressed in Daltons (1 Da = 1 g mol<sup>−</sup><sup>1</sup> ) and is the minimum or smallest component that can be retained with at least 90% efficiency [28, 30, 32, 35]. It should be noted that as the pore size of these membranes decrease, the driving force for the operation increases. For instance, the MF and UF are referred as low pressure driven processes while RO and NF are high pressure driven processes [28, 29, 34].

#### **3.2 Applications of membrane technology**

#### *3.2.1 Microfiltration (MF) application*

 Microfiltration utilises a sieving mechanism to retain macromolecules or particles more than 0.1 μm, specifically in the range of 0.1–10 μm [30]. Unlike UF, RO and NF, the transmembrane pressure (TMP) for both sides of the membrane as a result of the small particle retention is puny, hence requiring a relatively small TMP lower than 2 bar but varies from 0.1 to 2 bar [28, 30]. As indicated, the larger pore sizes of MF membranes limit removal to suspended solids, bacteria, some viruses (up to 2-log), protozoan cyst, turbidity and on a lesser extent, organic colloids within the region [28, 29, 32].

#### *3.2.2 Ultrafiltration (UF) application*

 The role of UF is increasing as a pretreatment for desalination and membrane bioreactors. Ultrafiltration (UF) like MF utilises physical sieving as a separation mechanism. The pore size, MWCO and pressure for a UF membrane ranges from 0.05 μm to 1 nm, 1–500 kDa and operating pressure of 1–7 bar [30, 33]. As such, UF with a definite MWCO are impermeable to compounds with molecular weights exceeding the MWCO and have shown a 3–6 log removal of chlorine resistant protozoan cysts, active *Giardia lamblia*, colloids, viruses and coliform bacteria. The use of MF and UF as pretreatment to RO has gradually emerged as an industry standard. Both are often used as pretreatment for NF and RO processes to reduce membrane fouling and is also applied as a post treatment to chemical precipitation for organic chemical removal and pH adjustment, phosphorus, hardness and metals [29–31, 33, 34]. Fouling is highly eminent in UF due to the high molecular weight of fractions retained in relations with the small osmotic pressure differentials and

liquid phase diffusivity. However, this does not negatively influence the demand for UF's as any design, configuration and application will be fouled [28, 30, 36]. The configuration for application is influenced by the mechanical stability, hydrodynamic requirement and economic limitations.

#### *3.2.3 Nanofiltration (NF) application*

Nano filtration is a pressure related process where the mechanism of separation is based on molecular size for the removal of dissolved micro pollutants and multivalent ions. The NF is a complex process characterised by solvent diffusion, transport and electrostatic repulsion effects at the membrane surface and within the nanopores [29, 30]. The difference between the pore diameter and particle size forms the basis of the sieve effect. Based on the membrane pore size, NF is often referred to as 'loose RO' with separation taking place at the lower end of UF and upper end of RO as it covers a MWCO of 100–500 Da, a pore size of 0.5–5 nm and operates at relatively low pressure of 5–35 bar [28, 30, 34]. The NF is usually deployed in the removal of polyvalent cation, reduction in colour, tannins, turbidity and disinfection by-product precursors such as organic matter as their potency lies in the high rejection of divalent ions (98%), permeation of monovalent ions (20–80% rejection) and high flux. Nano filtration is often used as a post treatment or polishing step in conventional treatment processes. Although it is not advisable to be used in desalination processes, it is used to reduce the salt content of slightly saline water. Recent applications have used NF as a pretreatment to RO reducing the operating pressure in RO providing savings in operational and maintenance costs [31, 32, 35]. Second stage fouling is usually reduced in NF systems through ozone pretreatment and non-thermal crystallisation while cleaning is done using suitable alternatives that also exist for MF and UF [30, 33].

#### *3.2.4. Reverse and forward osmosis*

 Reverse osmosis, often referred to as tight membrane has been widely used in brackish water and wastewater treatment with its effectiveness in desalination against conventional thermal Multi stage flash. High external pressure of 15–150 bar which is a function of the hypertonic feed and greater than the osmotic pressure is applied to retain dissolved solute and solvent permeation at a MWCO around 100 Da through diffusion mechanism [37–39]. Using concentration gradient as the driving force, separation and concentration in forward osmosis (FO) occurs as the concentrated solution (e.g., salts such as NaCl) draws water from a less concentrated feed solution. Characteristic advantages include low energy consumption, simple configuration and operation, low membrane fouling tendency and high rejection of a wide range of contaminants. The use of FO operates at ambient conditions, hence irreversible fouling is low [37, 39]. However, FO technology is faced with the lack of recyclable and economical drawing solutions, internal and external concentration polarisation and a difficulty of developing effective large scale plants [37]. To achieve desired process flow and optimum configuration, ROs are arranged in stages and passes. The sequence of the stages has the concentrate stream of the first stage as the feed inlet of the second stage. Permeate streams from both streams are summed into one discharge channel. However, passes involve either a one path recovery of permeate or the rechannelling of permeates from the first RO into the second RO to improve quality [40, 41] as summarised in **Table 1**.


*Treatment of Water and Wastewater for Reuse and Energy Generation-Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.84474* 

*PBI, polybenzimidaole; CA, cellulose acetate; PSO, polysulfone; PP, polypropylene; PVDF, polyvinylidene fluoride; PES, polyethersulfone; TFC, thin film composite.* 

#### **Table 1.**

*Summary of pressure-driven membranes [39–41].* 

#### *3.2.5 Membrane bioreactors*

Low-pressure driven MF and UF used for critical solid-liquid separation has been integrated with biological treatment into a hybrid activated sludge process termed as membrane bioreactors (MBR) for wastewater treatment. Unlike conventional wastewater treatment processes with treatment limitations, MBRs have shown wide range treatment efficiency in the removal of organic and inorganic emerging micro pollutants including pesticides, antibiotics, analgesics, antiepileptic, biodegradable organic compounds, microplastics, industrial chemicals and nutrients [30, 42, 43]. The MBRs can be configured into gravity or submerged vacuum driven systems by using hollow fibre or flat sheet modules, while pressure driven membranes on the other hand are mostly configured with external pipe cartridge systems. The MBR is usually combined with conventional systems including thermophilic or mesophilic bioprocesses, AOPs, powdered activation carbon to enhance improve the water quality and treatability efficiency [39, 40, 42, 44]. Some of the advantages of MBRs include small foot-print requirement, simple transport configuration and the ability to handle high biomass concentrations [45].

#### *3.2.6 Ion exchange membrane*

Ion exchange membranes are classified as anion exchange membrane (AEM) if the polymer matrix is embedded with fixed positive charge groups and vice

**Figure 5.** 

*Configurations of (a) electrodialysis, (b) reverse electrodialysis and (c) diffusion dialysis [35].* 

 versa for cation exchange membranes (CEM) [35]. This involves the permeation of anions/cations and rejection of cations/anions in the effluent. Electrodialysis (ED), reverse electrodialysis (RED), diffusion dialysis (DD) and Donnan membrane process (DMP) are examples of these, which usually involves the exchange of ions between the solutions across the membrane as shown in **Figure 5**. The application of these processes is usually based on the type of effluent and is reported to be nonfouled and energy resourceful as mechanism of separation is by potential gradient. The DD is applied to reclaim free mineral acids and alkalis while DMP is used in recovery of toxic and valuable metals from various feeds. In both DD and DMP, a Donnan equilibrium is established hence their difference lies the in application areas. Unlike ED and RED, DD and DMP are being applied on bench scale, have osmotic limitations and therefore kinetic studies are been considered for various effluents. However, ED and RED constructions requires compatibility of feed stream and stack materials, electrical safety consideration and larger footprint to produce equivalent water quantity and quality [30, 32, 33, 36].

## **4. Advanced oxidation process**

 Basically, there are two stages which are usually employed in wastewater treatment settings via a pre-treatment step involving mechanical and physicochemical systems to reduce the heterogeneous components of the effluents followed by an advanced treatment process. The physiochemical process enhances the efficiency of the advanced treatment by agglomerating the containments into a larger size for easy filtration or removal [46, 47]. However, degradation of emerging recalcitrant components with membrane and bioremediation in advanced treatment processes attests to be complex. So, in addressing this problem, advanced oxidation process (AOP) has gained much attention due to its potential to degrade a wide range of organic micro-pollutants [47, 48]. This process involves the generating of potent reactive hydroxyl radicals (E0 = 2.8 eV) with photon energy and without further additional chemical treatment. Examples are chemical oxidation (O3, Fenton reagents), photochemical oxidation (Ultraviolet-UV/O3, UV/H2O2), heterogeneous photocatalysis (UV/TiO2), electrolysis and sonolysis [46, 49–52] as shown **Figure 6**. These technologies use UV-A with long wavelengths of 315–400 nm, and UV-C with short wavelength radiation of 100–280 nm for degradation of most environmental contaminants. Generally, UV/O3 and UV/H2O2 processes consume large amounts of oxidant, which makes them uneconomical to operate [43]. On the other hand,

*Treatment of Water and Wastewater for Reuse and Energy Generation-Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.84474* 

**Figure 6.** 

*Types of advance oxidation process (AOPs) adapted from [46].* 

the hazards associated with ozone being an unstable gas limits its application and is usually coupled with an ozone-water contacting device to convert the ozone into its liquid phase thus increasing the cost of production. However, the considerable operational conditions of ambient temperature and pressure and the use of a lowcost and chemical stable catalyst (TiO2) are predominantly attractive for complete mineralisation of contaminants and by-products. This makes heterogeneous photocatalysis techniques to be advantageous over other AOP's. Other advantages include no sludge production, quick reaction rate, low cost and operating well at ambient temperature and pressure conditions [46, 49].

#### **4.1 Photocatalysis**

There are several semiconductors favourable for water treatment such as TiO2, ZnO, Fe2O3 CdS and ZnS, which are active within a band gap energy of 2.3–3.2 eV and wave length of 413–539 nm [53, 54]. TiO2 has been the most widely used photocatalyst and exists in three major crystalline polymorphs such as anatase, rutile and brookite (**Figure 7**). There have been intensive studies to determine the crystalline structure, surface area, density of surface hydroxyl groups and adsorption/desorption characteristics of TiO2 [46, 49]. As mentioned, the surface hydroxyl group concentration has been documented to play a vital role in the photocatalytic degradation process, such that an increase in concentration of the hydroxyl groups on the catalyst surface might have a positive effect on the reaction rate [46, 49, 51]. Thus, the hydroxyl radicals produced (OH<sup>−</sup>) are a very combative species and are therefore able to oxidise a wide range of organic pollutants in a swift and effectual means.

 **Figure 7.**  *Crystal structures of TiO2 adapted from [51, 52, 55].* 

#### *4.1.1 Principles of heterogeneous photocatalysis*

The heterogeneous photocatalysis process involves series of reactions such as oxidation, dehydrogenation, hydrogen transfer and metal deposition, among others [55]. Besides, the normal thermal and chemical catalytic actions that ease any distinctive chemical reaction require simultaneous activation of the molecules. As such, in photocatalysis, a light source with energy equal to or higher than the bandgap energy of the catalyst is employed to stimulate the catalyst to be active during the reaction bringing about the reduction or oxidation of the adsorbed species on the surface, for instance, oxidation of organic compounds into their subordinates until carbon dioxide and water are formed [56, 57]. This results in charge separation as the electron (e<sup>−</sup>) moves from the valence band to the conduction band of the semiconductor catalyst, resulting in a hole (h+ ) in the valence band as shown in **Figure 8**.

#### *4.1.1.1 Selection of catalysts*

Some of the parameters to consider in selecting a photocatalyst to enhance the degradation capacity include the nature and intensity of light source, the amount of photons to activate the catalyst, the reaction medium and the water chemistry to generate the hydroxyl radicals, and the nature and concentration of the pollutants [46, 51, 55]. However, the relationship between the energy levels of conduction and valence bands with respect to the energy for reduction and oxidation is one of the cardinal points to consider in selecting a photocatalyst [46, 47]. The chemical capacity of the photogenerated electrons and holes has high influence on the conduction band energy level of the photocatalyst. Thus the photo-generated electron of a catalyst should be able to

#### **Figure 8.**

*Schematic illustration of the photocatalytic mechanism in the presence of a water contaminant adapted from [46, 49, 52].* 

*Treatment of Water and Wastewater for Reuse and Energy Generation-Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.84474* 

have essential energy to ease the mineralisation of the micro pollutants. The conduction band energy level of the photocatalyst in terms of mineralisation has to be more negative, while the valence band has to be more positive with respect to the energy for oxidation of water. This makes semiconductors with large band gaps to be considered as suitable for photocatalytic activities [46, 47, 50]. Thus, semiconductor materials are able to provide adequate negative and positive redox potentials in conduction bands and valence bands, respectively. In addition, the wider the band-gap of semiconductor material the higher the energy input to enhance its efficiency. Furthermore, the poor photo-corrosion stability of many semiconductors limits significantly the number of potential photo-catalytic materials for mineralisation in wastewater settings [55, 57].

#### *4.1.1.2 Modification of catalyst*

 Photocatalytic activity of a distinctive semiconductor, such as TiO2, can attract light energy equivalent to or higher than the band gap energy, resulting in generating electrons and holes. This electron-hole pair then migrates to the TiO2 surface and amalgamates with the adsorbed reactants to hasten the reduction and oxidation process. The lack of such an energy transfer leads to recombination of the pairs, which then competes with the desirable redox process with a high loss of energy. Therefore, to increase photocatalysis efficiency, suitable modification of the semiconductor band gap is essential to attract the recombination charge carriers which might hinder the photocatalytic efficiency. Some of these processes include (i) doping with metal cations and anions, (ii) coupling with other semiconducting oxides, (iii) sensitization with light harvesting compounds/dye molecules and (iv) plasmon resonance induced by specific metals [49, 51].

#### *4.1.2 Parameters affecting photocatalysis*

As with most processing techniques, several parameters affect the performance of the photocatalytic degradation process. Photocatalytic activities are being influenced by experimental conditions such as the amount of catalyst, light intensity, lighting area, reactor volume, pH type of reactor, temperature and pressure. Likewise, the inherent structural and properties of semiconductor photocatalyst influences its performance. This includes the particle size, phase composition, surface area, surface hydroxyls, lattice defects and the type of dopants (metals and non-metals) [46–48, 52]. The amount of light radiation absorbed is the driving force for semiconductor with threshold wavelength to provide the appropriate photon energy to overcome the band gap between the valence and conduction bands. This threshold wavelength is very important to promote electrons to be in an excited state corresponding to the band gap energy as depicted in **Table 2**.

### *4.1.3 Applications of photocatalysis*

 With the current greenhouse emission problems and energy deprivation situation, photocatalysis emerges as one of the alternative ways of providing feasible solutions to the global front in relation to energy and the environment. For instance (**Figure 9**), in the phenomenon of photosynthesis, green plants trap the solar energy from the sunlight and with a series of enzyme catalysed redox processes, converts the CO2 into water and carbohydrates by releasing oxygen into the atmosphere where most living organisms depend on to survive. In this scenario, photocatalysis which works with the same principle has gained incredible status and hence can be explored for divert applications in seeking sustainable energy, social economic growth and environmental impact.


**Table 2.** 

*Semiconductors and their band gaps at specific wavelengths.* 

### *4.1.3.1 Photocatalytic degradation in wastewater*

 The importance of photocatalytic degradation technology in pre- and posttreatment of water and wastewater using sunlight on a large scale is at the verge of development, with few of them like self-cleaning, anti-fogging and anti-bacterial applications currently being practiced. For instance, two slurry reactors coupled with TiO2 were built in New Mexico (USA) and Almeria (Spain); however, the environmental conditions, low photonic yield and efficiency under the visible light make it challenging to be commercialised [27, 60–63]. There are numerous studies ongoing seeking to address some of these limitations in order to improve the photocatalyst, reactor design and light efficiency [55, 59]. Furthermore, the kinetics study to understand the mechanistic pathways of mineralisation of recalcitrant organic and micropollutants has also received attention.

#### *4.1.3.2 Photocatalytic energy production*

In the early 1900s, the photocatalytic techniques commonly referred to as "artificial photo-synthesis" was employed in reducing CO2 into useful hydrocarbons (**Figure 9**). This has now attained incredible status on the global level. Currently researchers, scientists and engineers are exploring the mechanism to control the current atmospheric carbon dioxide levels (green-house gas effect) by altering it into to fuels and useful chemicals, and the role of CO2 as a source of energy. However, the

**Figure 9.**  *Schematic comparison of photo-catalysis and photo-synthesis [58, 59].* 

#### *Treatment of Water and Wastewater for Reuse and Energy Generation-Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.84474*

use of solar energy via chemical storage can be attained by photocatalytic or photoelectrochemical initiation of light-sensitive catalytic surfaces. Due to the simplicity of the photocatalytic process, it can be employed in converting solar energy into other useful forms of energy like hydrogen via splitting of water and hydrocarbons (methane, methanol, etc.), commonly known as solar fuels [52, 55, 59].
