*2.1.5.1 Ultrafiltration (UF)*

Ultrafiltration has been utilized to remediate a wide range of waterways around the world. According to reports, surface waters, including lake waters, rivers, and reservoirs, have been employed in 50% of UF membrane plants. This technology has been used to treat municipal drinking water for over a decade [12]. UF pores are typically between 0.01 and 0.05 mm (roughly 0.01 mm) in diameter or less. Larger organic macromolecules can be retained by UF membranes. They used to be defined by a molecular weight cut-off (MWCO) rather than a definite pore size [13]. Since the osmotic pressure of the feed solution is low, hydrostatic pressures in UF are typically in the range of 2–10 bar. The operation of a pressure-driven UF process can be separated into three distinct pressure ranges based on the relationship of permeate flow on


### **Table 1.** *Pressure-driven membrane process.*

applied pressure (i) linearly increasing flux (sufficiently low), (ii) intermediate, (iii) and limiting flux (sufficiently high).

Even though its concentration polarization layer has not formed appreciably in the linearly increasing flux pressure range, the membrane is the only source of permeate flux resistance. Permeate flux in the limiting flux pressure range, on the other hand, is unaffected by the applied pressure. The process performance is primarily determined by these boundary layer phenomena, just as it is in MF [14]. Water and wastewater can be treated in a variety of ways using the UF process, including the manufacture of ultra-pure water for the electronics industry, COD levels are decreasing in maize starch plants, chemical treatment of groundwater combined with selective removal of dissolved hazardous metals, the dairy industry's whey treatment, wine, or fruit juice clarification.

The UF technology has several benefits such as perfect pore size range thus can be applied for the separation of most of the feed components, low energy usage owing to the unavailability of phase transition during separation, and simple and compact design makes it simple to use. In addition, for temperature-sensitive culinary, biological, and pharmaceutical applications, the most advanced membrane separation technology is UF. However, the application of this technology is faced with some drawbacks including an inability to desalinate saltwater because it cannot isolate dissolved salts or low molecular weight species. UF is ineffective at separating macromolecular mixtures; it can only be efficient if the species have a molecular weight difference of 10 times or more.

### *2.1.5.2 Microfiltration (MF)*

Microfiltration is a pressure-driven membrane technology that can retain particles of molecular weight greater than 100 kDa and a diameter smaller than 1000 nm. The membrane pore size determines the separation or retention capacities. MF membrane pore size spans from 100 nm to 10,000 nm. Because the MF pore size is large, the separation pressure is low, ranging from 10 kPa to 300 kPa. Suspended particles, sediments, algae, protozoa, and bacteria are all separated with MF. Furthermore, the separation method is impractical since particles smaller than the pore size pass readily while larger particles are rejected. Darcy's law describes volume flow through MF membranes, where the applied pressure (Δ*P*) is directly proportional to the flux, *J* through the membrane:

$$J = A.\,\,\Delta P\tag{1}$$

Where permeability is a constant *A* containing structural elements like pore size distribution and porosity. MF can be utilized in a variety of industrial settings, where particles with a diameter > 0.1 mm must be controlled in a suspension. The most fundamental operations still rely on cartridge-based dead-end filtering. However, crossflow filtration will gradually replace dead-end filtering in larger-scale applications. Clarification and sterilization of all types of drugs and beverages are two of the most common industrial applications. Ultrapure water in semiconductors, drinking water treatment, wine, beer, and fruit juice clarification, pre-treatment, and wastewater treatment are some of the other applications.

Microfiltration has shown to be viable due to its low energy consumption, operating pressure, and maintenance which result in low operating cost, fouling is not as bad as it could be because of two factors: larger pore sizes and low pressures. The application of this technology is limited due to its sensitivity to oxidizing agents, bacteria and suspended particles can only be eliminated, particles that are hard and sharp can disrupt the membrane, and cleaning pressures of more than 100 kPa can damage the membrane.

### *2.1.5.3 Nanofiltration (NF)*

Nanofiltration is a filtration technology that separates different fluids or ions using membranes. Due to its broader membrane hole structure than the membranes used in RO, "Loose" RO is a term used to describe NF. More salt can pass through the membrane as a result of this. NF is employed in conditions where strong moderate inorganic removal and organic removal are sought since it can function at low pressures, typically 7–14 bars, and absorbs some inorganic salts. NF may concentrate proteins, sugars, bacteria, divalent ions, particles, colors, and other compounds with a molecular weight of more than 1,000 [15]. NF membranes are constructed of aromatic polyamide and cellulose acetate, displaying salt rejection rates ranging from 95% for divalent salts to 40% for monovalent salts and a molecular weight cut-off (MWCO) for organics of 300 [16]. Organics of low molecular weight, including methanol, are unaffected by NF.

Although NF membranes have strong molecular rejection properties for divalent cations such as magnesium and calcium and may be used instead of traditional chemical softening to effectively remove hardness, they can also be utilized to generate drinking water. Organics with a higher molecular weight that cause odor and taste, or that mix with chlorine to produce trihalomethanes or other particles, can be rejected by NF membranes, boosting the effectiveness of downstream disinfection treatments [17]. Rai and co-workers [18] reported using NF for tertiary treatment of distillery effluent, that the NF membrane had a very high separation efficiency for both inorganic and organic chemicals (around 85–95%, 98–99.5%, 96–99.5% removal of TDS, cooler, and COD, respectively). The advantage of nanofiltration is the lower operating pressure, which results in lower energy costs and potential pump and piping investment savings. The most important drawback of NF membranes is the difficulty in controlling membrane pore size and pore size distribution repeatability. Furthermore, NF membranes are prone to fouling, which could result in significant flow reduction.

### *2.1.5.4 Reverse osmosis (RO)*

Reverse osmosis (RO), in general, is the reverse of the osmosis process. When a semi-permeable barrier is established between two solutions, a solvent flows from lower to higher solute concentrations. Reverse osmosis occurs when an external force causes a solvent to flow from a higher to lower solute concentration. The driving force in the typical osmosis process is a drop in the system's free energy, which diminishes as the system seeks to achieve equilibrium. When the system reaches equilibrium, the osmosis process comes to a stop. An external force larger than the osmotic pressure of the system drives the RO process. RO is like other pressure-driven membrane processes; however, other processes employ size exclusion or straining as the mode of separation and RO employs diffusion.

RO membranes are usually dense membranes having pore sizes less than 1 nm. They are generally a skin layer in the polymer matrix. The membrane material (polymer) forms a layer and a web-like structure. The water follows a tortuous path to *Available Technologies for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.103661*

get permeated through the membrane. RO membranes can reject the smallest entities from the feed. These include monovalent ions, dissolved organic content, and viruses, almost everything that other membrane processes are not capable of. RO membranes can also be used in both cross-flow and dead-end configurations, but on the other hand, crossflow is frequently favored due to its low energy usage and low fouling qualities. Spiral wound modules, in which the membrane is wound around the inner tube, are the most prevalent. RO has several applications, of which desalination is the most important and widely used. RO is also used in wastewater treatment, and dairy and food products.

Using RO technology, desalination of the sea and brackish water is possible when compared to other membrane processes where separation occurs without a phase change. In comparison to other desalting systems, it is compact and hence takes up less space while ensuring low maintenance and easy scalability. High-pressure requirements, energy-intensive process, lower flux, fouling, and the need to pre-treat feed before use are some of the shortcomings of RO.

### *2.1.5.5 Forward osmosis (FO)*

The FO process is a designed osmotic process in which the treated water is on one side of a semi-permeable membrane and a draw solution (DS) is on the other. Even though FO is built on the osmosis principle, the word "forward osmosis" (FO) was most likely coined to differentiate it from "reverse osmosis," which has been the term for membrane desalination technology for decades. Forward osmosis (FO) employs a concentrated draw solution to create high osmotic pressure, which extracts water from the feed solution across a semi-permeable membrane [19]. As a result, the volume of the feed stream drops, the salt concentration rises, and the permeate flux to the draw solution side reduces [20]. The general equation characterizing water movement over the RO membrane, according to Lee et al. [21], is:

$$J\_W = A \left( \sigma \Delta \pi - \Delta P \right) \tag{2}$$

where *J*W is the water flux, *A* is the membrane's water permeability coefficient, σΔ*π* the effective osmotic pressure difference in reverse osmosis, σ is the reflection coefficient, and Δ*P* is the applied pressure; for FO, Δ*P* = 0; for RO, Δ*P* > Δ*π* [21]. Since the parameter A and the reflection coefficient are calculated using the pressure applied to the brine, this equation is not suited for FO operations; also, the driving force employed is the difference between osmotic pressure and the applied hydraulic pressure (Δ*P*) [22, 23]. **Figure 1** displays the principles of osmotic processes.

The primary benefit of FO is how little energy is required to extract pure water from wastewater or recycled feed, with just the energy needed to recirculate the draw solution requiring additional energy [18]. The ultimate flux reduction of concentration polarization is a fundamental limiting element impacting the performance of FO systems [25, 26]. Since forward osmosis is gaining attention as a viable method for lowering the cost of wastewater treatment and generating freshwater, many potential applications for FO membranes have been investigated, including desalination, dilute industrial wastewater concentration, direct potable reuse for enhanced life support systems, food processing, landfill leachate concentration, pharmaceutical industry processes, and concentration of digested sludge liquids [26].

### **Figure 1.**

*Principles of osmotic processes: the initial state of the solutions, forward osmosis (FO), pressure retarded osmosis (PRO) and reverse osmosis (RO), adapted from Rao [24].*

### **2.2 Chemical wastewater treatment technologies**

Chemical methods employed in waste-water treatment are designed to create change through chemical reactions. They are always combined with physical and biological methods. Chemical methods, in comparison to physical ones, have an inherent disadvantage considering that they are additive processes. That is, the dissolved elements of wastewater usually increase. If the wastewater is to be reused, this is an important consideration. A brief description of chemical methods of wastewater treatment is given below.

### *2.2.1 Neutralization*

The pH value of wastewater is adjusted through neutralization. Acids or alkalis are used to neutralize industrial wastewaters after operations such as precipitation and flocculation. Metal-containing acid wastewaters can be treated by adding an alkaline reagent to the acid waste, forming a precipitate, and collecting the precipitate. As a result, the pH of the input solution is adjusted to the optimal range for metal hydroxide precipitation. To meet the overall wastewater treatment objectives, the step is performed before the major phase of wastewater treatment [27].

### *2.2.2 Precipitation*

By lowering their solubilities, dissolved contaminants become solid precipitates, which can be easily skimmed from the water's surface during precipitation [27]. While it effectively removes metal ions and organics, the accumulation of oil and grease may produce precipitation issues. Adding chemicals or reducing the temperature of the water reduces the solubility of dissolved pollutants. Adding organic solvents to the water could theoretically decrease the contaminant's solubility, however, this procedure is costly on a large scale. Precipitates form when these compounds react with soluble contaminants. The most used substances for this function include ferric chloride, lime, ferrous sulphate, sodium bicarbonates, and alum. The most critical moderating parameters for the precipitation process are temperature and pH. Precipitation can eliminate approximately 60% of pollutants [28]. This method can be used to recycle water and remediate wastewater from the chromium

and nickel-plating industries. Among the applications are water softening and heavy metal removal and phosphate from water. The handling of the vast amount of sludge produced is the main issue related to precipitation [29, 30].

### *2.2.3 Ion exchange*

An ion exchanger, a solid substance, exchanges hazardous ions in wastewater for non-toxic ions [31–35]. There are two types of ion exchangers: anion and cation exchangers, which can exchange anions and cations, respectively. Ion exchangers are resins with active sites on their surfaces, which might be natural or synthetic. The most used ion exchangers include metha-acrylic resins, zeolites, acrylic, polystyrene sulfonic acid, and sodium silicates. It is a reversible process that utilizes very little energy. Low amounts of inorganics and organics are removed using ion exchange (up to 250 mg l–1). Concentrations of inorganic and organic compounds can be reduced by up to 95%. Potable water production, industrial water, pharmacy, fossil fuels, softening and other sectors are among the applications. It's also being utilized to cut down on pollution. If there is oil, grease, or large quantities of organics and inorganics in the water, it may be necessary to pre-treat it.

### *2.2.4 Oxidation/reduction*

Redox reactions are commonly used in chemical wastewater treatment and potable water treatment. Chlorinated hydrocarbons and pesticides are effectively removed from drinking water using ozone and hydrogen peroxide oxidation methods. Oxidation techniques are utilized in wastewater treatment to remove problematic biodegradable chemicals. Photochemical purification, which uses UV light to create hydroxyl radicals from hydrogen peroxide or ozone, is very effective. These Advanced Oxidation Processes (AOP) destroy antibiotics, cytostatic medications, hormones, and other anthropogenic trace chemicals. Advanced Oxidation Processes (AOPs) are efficient methods to remove organic contamination not degradable through biological processes in water and wastewater. Ozone also helps with the oxidation of iron and manganese in well water. To convert heavy metal ions, for example, into easily dissolvable sulfides, reduction procedures are necessary [36].

### *2.2.5 Electrodialysis*

Ion-selective semi-permeable membranes allow water-soluble ions to pass through them when an electric current passes through them [37, 38]. Ion-selective membranes are ion exchange materials that are selective. They can be anion or cation exchangers, allowing anion and cations to flow out of the system. The technique uses two electrodes to which a voltage is supplied in either a continuous or batch mode. The membranes are arranged in a series or parallel pattern, to obtain the required degree of demineralization [39, 40]. Factors such as pH, temperature, the type of contaminants, membrane selectivities, scaling and fouling of wastewater, the wastewater flow rate, and the volume and design of phases all affect dissolved solids removal. The creation of drinkable water from brackish water is one of the applications. Furthermore, this technology has been utilized to reduce water sources. Total dissolved solids (TDS) concentrations of up to 200 mg l−1 can be decreased by electrodialysis by up to 90% [41]. Membrane fouling happened in the same way that reverse osmosis does. Carbon nanotubes have been used in composite membranes to alleviate this problem and increase flow.

### *2.2.6 Disinfection*

Disinfection in wastewater treatment aims to limit the number of microorganisms in the water that will be released back into the environment for later use as irrigation water, bathing water, drinking water, and so on. The quality of the treated water (pH, cloudiness, and other parameters), the type of disinfection used, the disinfectant dosage (time and concentration), and other external conditions all influence disinfection efficiency. Due to the obvious nature of wastewater, which contains several human enteric organisms linked to a variety of waterborne diseases, this technique is critical in waste-water treatment [42]. Physical agents such as heat and light, mechanical means such as screening, sedimentation, and filtration, radiation, primarily gamma rays, chemical agents such as chlorine and its compounds, bromine, iodine, ozone, phenol and phenolic compounds, alcohols, heavy metals, dyes, soaps, and synthetic detergents, quaternary ammonium compounds, hydrogen peroxide, and various alkali and acids are among the most used disinfection methods. Oxidizing chemicals are the most frequent chemical disinfectants, and chlorine is the most widely utilized of these.

### **2.3 Biological wastewater treatment technologies**

Biological water treatment technologies are critical components of a wastewater treatment strategy since they are utilized to produce safe drinking water. Aerobic, anaerobic and bioremediation processes are the techniques employed for this. These operations are outlined below.

### *2.3.1 Aerobic processes*

Aerobic and facultative bacteria cause biodegradable organic matter to break down aerobically when oxygen or air is freely accessible in wastewater in the dissolved form [43, 44]. Temperature, retention time, oxygen availability, and the biological activity of the bacteria all limit the extent of the process. Furthermore, the addition of specific compounds essential for bacterial development may increase the rate at which organic pollutants are biologically oxidized. This approach can remove phosphates, nitrates, volatile organics, dissolved and suspended organics, chemical oxygen demand (COD), biological oxygen demand (BOD), and other pollutants. It is possible to reduce the number of biodegradable organics in the environment by up to 90%. The method's downside is that it produces a huge number of bio-solids, which necessitates additional costly treatment and management. Oxidation ponds, aeration lagoons, and activated sludge processes are used to carry out the aerobic process [44]. The following Eq. (3) gives a simple depiction of aerobic decomposition.

Organic matter O Bacteria H O Bacteria Byprodu + +<sup>2</sup> →*CO*2 2 ++ + cts (3)

## *2.3.1.1 Oxidation pond*

Oxidation ponds are aerobic systems in which the heterotrophic microbes consume oxygen that is supplied by both the atmosphere and photosynthetic algae. In this process, algae utilize the inorganic substances (N, P, CO2) generated by aerobic bacteria to fuel their growth, which is powered by sunlight. They discharge oxygen into the fluid, which the bacteria then use to complete the symbiotic cycle [44].

### *2.3.1.2 Aeration lagoon*

Aeration lagoons are deeper than oxidation ponds, because aerators supply oxygen rather than algal photosynthetic activity, as in oxidation ponds. The aerators maintain the microbial biomass afloat and supply enough dissolved oxygen for the aerobic process to be maximized. Although there is no deposition or sludge return, this process relies on properly mixed liquor formation in the tank/lagoon. As a result, aeration lagoons are appropriate for effluent that is both strong and biodegradable, such as wastewater from the food industry [44].

## *2.3.1.3 Activated sludge*

The activated sludge method works by suspending a substantial bacterial colony in wastewater under aerobic conditions. Greater levels of bacterial proliferation and respiration can be achieved with limitless nutrients and oxygen, resulting in the conversion of accessible organic compounds to oxidized end-products or the formation of new microbes. The activated sludge system is comprised of five interconnected components: bioreactor, activated sludge, aeration and mixing system, sedimentation tank, and returned sludge [44]. The biological mechanism employing activated sludge is a widely utilized technology for wastewater remediation that has low operating costs.
