Water Pollution Sources and Purification: Challenges and Scope E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
The book helps readers to understand the fundamentals of water purification processes. Chapters in the book cover industrial purification techniques, while also exploring the future scope and current challenges in this field.
Key Features
- seven chapters arranged and structured in a clear, coherent manner for understanding the broad topics.
- Covers basic water purification techniques for safe drinking water
- Covers defluoridation techniques
- Explains the parameters affecting photocatalytic degradation of substituted benzoic acids.
- Includes a case study for seasonal variations in pond water
- Covers the role of nanotechnology in wastewater treatment
- Covers the impact of water mismanagement on the environment with suggestions for preventive measures for sustainable water utilization
This reference informs advanced readers (sustainable development professionals, post-graduate and research scholars) interested in water treatment processes. It also serves as a resource for courses in environmental chemistry, waste management and sustainability.
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Seitenzahl: 474
Veröffentlichungsjahr: 2003
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FOREWORD
Dr. K. G. RewatkarThe authors of this edited book are renowned researchers in the field of water research, photocatalytic degradation, and solar cells. With their research experience, they have edited this book by referring to and analyzing a wide range of literature. This edited book not only covers a critical review of some water research problems but also contains some original research work regarding catalytic degradation. In short, this book reveals the causes of water pollution and its impact on animal life. Moreover, it also discusses various water purification methods developed so far. This edited book further focuses on fluoride contamination in drinking water, its effect on human life, and fluoride removal by using activated alumina modified with different materials. The authors of this book have included original research like that on the degradation of substituted benzoic acid by nanoparticles and analysis of seasonal and spatial variations of water quality of Dulhara and Vedponds in Ratnapur, Chhattisgarh, India. Few chapters of this book cover wastewater treatment using modern methods, the impact of water mismanagement on the environment, and suggestions on the preventive measures for proper water utilization. The authors have made all possible efforts to enhance the usefulness of the book for the research community.
PREFACE
Water pollution and its impact on human health are frequently discussed in this age. Several research studies and projects are undertaken and accomplished year after year. However, sustainable water resource management is still a serious concern in most developing countries. Moreover, the problem of water contamination and its purification are greater challenges and a lot of the research tends to be futile. There is, therefore, a need to design and develop appropriate methodologies in order to improve the quality of water. By keeping this view in mind, the present book is written with clear objectives: to develop a basic idea about various challenges in water pollution and their removal.
Regarding the organization, the book consists of seven chapters, well-arranged in a coherent manner.
Chapter one deals with the different water purification techniques used for safe drinking water production, potential threats, and challenges.Chapter two focuses exclusively on fluoride removal by adsorption method using activated alumina modified with different materials and isothermal studies.Chapter three is the result-oriented chapter that discusses different parameters affecting photocatalytic degradation of substituted benzoic acids.Chapter four covers the analysis of seasonal and spatial variations of water quality of Dulhara and Ved ponds in Ratnapur, Chhattisgarh, India.Chapter five examines the degradation of benzoic acid by iron nanoparticles as a photo-catalyst using an advanced oxidation process (AOP). This chapter also discusses the synthesis of Fe nanoparticles via hydrothermal process at ordinary and elevated temperatures.Chapter six deals with wastewater treatment using modern methods supported by nanoscale materials.Chapter seven discusses the impact of water mismanagement on the environment and suggests preventive measures for proper water utilization.This book is meant for postgraduate and research scholars in the field of physical sciences, chemistry, and material sciences interested in water treatment, photocatalytic degradation, advanced oxidation process, and solar cell. The book will explore and help readers understand fundamental as well as advanced studies on these processes. Chapters in the book also provide future scope and challenges in both the phenomena, which allow readers to understand basic and current status in the fields. It is hoped that the book shall provide guidelines to all interested in research studies of one sort or the other.
We are highly indebted to our students and learned colleagues for providing the necessary stimulus for writing the book. We are grateful to all those persons whose writings and works have helped us in the preparation of this book. We are equally thankful to the reviewers of
this edited book who made extremely valuable suggestions and have thus contributed to improving its quality.
We will feel highly rewarded if the book proves helpful in the development of genuine research studies. We look forward to suggestions from all readers, researchers, and scholars for further improving the content of the book.
List of Contributors
Review on Water Purifications Techniques and Challenges
R. M. Belekar1,*,S.J. Dhoble2Abstract
Nowadays, the whole world is facing water containment issues caused by anthropogenic sources, including household waste, agricultural waste, and industrial waste. There is a huge impact of wastewater on the environment; hence, the public concern over it has been increased. This led researchers to be motivated and find radical and cheap solutions to overcome this problem. Several conventional techniques, including boiling, filtration, sedimentation, and chlorination, are used for wastewater treatment; however, they have limited scope. Some other methods like coagulation, flocculation, biological treatment, Fenton processes, advanced oxidation, membrane-based processes, ion exchange, electrochemical, adsorption, and UV-based processes have been applied to remove pollutants, but there are still some limitations. This review chapter sheds some light on these traditional and modern methods applied for water treatment, along with their advantages and disadvantages. These methods have the potential to remove pollutants from wastewater, such as natural organic matter, heavy metals, inorganic metallic matter, disinfection byproducts, and microbial chemicals. The potential threats and challenges of using water treatment methods for safe water production have also been discussed in this chapter.
INTRODUCTION
Water is an essential element in natural resources required for the survival of all living organisms, cultivation, and food production. Today, many cities around the world face severe water shortages. About 40 percent of the global food supply requires irrigation, and the industrial process depends on the extensive use of water [1]. Environment and economic development are severely affected by the
seasonal availability of water and its quality. Water quality is affected by human activity and is being reduced due to urbanization, population growth, industrial production, and climate change. As a result, water pollution has a severe impact on the earth and its inhabitants [2]. Water treatment produces drinkable water that is chemically, biologically, aesthetically, pure and healthy. The treatment cost for clean raw water is less as it requires fewer purification steps. In rural areas, water usually comes from commonly shared wells, ponds, or hand pumps, whereas in urban areas, it is supplied by municipal corporation water supply [3]. The purification of water involves many steps that are different in different regions and depend upon the quality of water and contaminants. More than 70% of the earth’s surface is covered with water, but only around 1% of water is drinkable as per standards. There are many contaminants that make the water unhealthy for drinking purposes, like aluminum, ammonia, arsenic, fluoride, barium, cadmium, copper, etc [4-6]. There are common treatment methods that include coagulation, sedimentation, biological oxidation, photo-Fenton treatment, advanced oxidation processes (AOPs), oxidation with chemical oxidants, photocatalytic oxidation, membrane processes, electrochemical oxidation/degradation, adsorption, and combined methods [7].
TRADITIONAL WATER PURIFICATION METHODS
The rural communities have adopted simple and traditional methods for removing visible impurities present in the water collected from various sources. Though these methods are not sufficient to provide quality water in urban areas as per international standards, they are more useful in rural areas where the degree of harmful contamination is almost negligible [8]. These methods can easily remove certain bacteria, pathogens, undissolved matter, dust, etc.
Filtration
This is the most simple and convenient technique for removing wind-borne impurities like plant debris, insects, dust particles, or coarse mud particles. The raw water collected from various sources passes through a cotton cloth or winnowing sieves, and the impurities get filtered. However, this method cannot be used effectively when water is highly turbid or muddy as cotton cloth or sieve cannot filter fine suspended particles. This method of filtration is popular in many villages of India and other parts of the world, where water is collected from wells or clean ponds [9]. To filter highly turbid water, clay vessels with suitable pore sizes can be used. The turbid water is collected in a clay vessel and allowed to settle. The water in the clay vessel trickles through it, and clear water is collected in another jar. This method of filtration is common in Egypt. In the southern part of India, water purification is carried out using plant parts. The turbid water is allowed to settle and coalesce out using nuts and roots of some locally available plants. It was found that nuts excrete coagulation chemicals upon soaking, which settles most fine suspended particles. Besides that, the wiry roots of some plants are placed in a clay jar that has tiny holes at the bottom. In some artificial ponds in Indonesia, Jempeng stone filters are used for the filtration of water. This Jempeng stone is porous in nature and capable of filtering even highly turbid water [10].
Boiling
Boiling with fuel is the oldest and most commonly practiced water treatment method that kills many bacteria, parasites, cysts, worms, and viruses. It is the simplest and easiest method to remove waterborne pathogens from water. This method of water purification can be implemented anywhere and at anytime as it does not require many accessories. According to WHO, water must be heated until the first big bubble appears in it, which ensures that water is pathogen-free [11]. In an emergency situation such as a flood, pandemic, or war, it is advised to drink boiled water. Besides these advantages, there are certain disadvantages as well. The boiling of water can only kill pathogens and does not remove chemical pollutants like fluoride, arsenic, etc. It also cannot remove the turbidity of the water; therefore, pretreatment is required for highly turbid water. Moreover, it consumes traditional fuels (wood, gas, kerosene), which are costly, contributing to deforestation and indoor air pollution. The boiling of water also alters the taste of natural water as it drives out dissolved gases.
Chlorination
Chlorination involves adding a measured amount of chlorine into the water to kill bacteria, viruses, and cysts. Besides, chlorination can also be used for taste and odor control and to remove some gases such as ammonia and hydrogen sulfide. Chlorine is an effective disinfectant widely used in rural common wells to kill most of the bacteria which are responsible for many diseases. Chorine is added to the water as a final stage of water treatment. Chlorine is widely used in many developing countries to prevent waterborne diseases like typhoid and dysentery. The chlorine is added to the water resources in the form of sodium hypochlorite, bleaching powder, or chlorinated lime in a measured amount. The chlorine is also available commercially in tablet form as halazone, Chlor-dechlor, and hydrochlonazone. Depending upon the water quality, the appropriate amount of freshly prepared chlorine is added to water by trained personnel. Chlorine can produce some harmful effects in some cases [12]. The halogen chlorine can easily react with organic compounds present in the water producing trihalomethanes and haloacetic acids. These materials are hazardous to human health and shows symptoms like sleepiness, and slower brain activity. Chronic exposure to trihalomethanes can be responsible for kidney cancer, heart disease, and unconsciousness [13]. However, WHO states that the risk to health from these by-products is negligible than the risk associated with drinking water without disinfectant. The enhanced filtration method to remove organic matter should be employed to prevent producing hazardous compounds in the treated water.
Sedimentation
In rural areas, most of the regions are underprivileged and there is no availability of filters, disinfectant chemicals, and trained workers. Sedimentation is the only method to treat turbid water. In the sedimentation process, the suspended particles in water are allowed to settle down under the effect of gravity [14]. The sedimentation is mostly implemented before coagulation as it reduces the concentration of the particles in suspension and fewer coagulation chemicals are required. In the sedimentation technique, the turbid water is filled in the tank and left for a longer time to settle the particles, and decant off the clear water. There are many types of sedimentation techniques like horizontal flow tanks, radial flow tanks, inclined settling, ballasted sedimentation, floc blanket sedimentation, etc. The efficiency of sedimentation is depended upon the nature of the suspended particles, size, and characteristics of suspended matter. There are few chemicals that assist sedimentation, but in rural areas use of such chemicals is not feasible [15].
COAGULATION AND FLOCCULATION
The ground water, soil water, and surface water contain suspended or dissolved particles. These suspended particles vary in shape, size, source, charge, and density. The suspended particles in water possess a negative charge; therefore when coming closer, they repel each other. The result is these small particles cant clump together to form larger structures (flocs) and settle down hence proper coagulation and flocculation are required. In the coagulation process, the repulsive potential of electrical double layers of colloids is reduced and microparticles are produced. The coagulation process removes turbidity, color, and pathogens. In the coagulation process, coagulant chemicals with charges opposite to that of suspended particles are added, which neutralizes negatively charged particles [16]. Such chemicals are usually used for non-settlable solids like clay and organic substances. After neutralizing the charge, the suspended particles stick together and micro flocs are formed which are not visible to the naked eye. The water gets clear after the formation of complete flocs. The rapid mixing of coagulants is required to promote particle collisions and achieve good coagulation. The flocculation is the next step after coagulation which increases the size of submicroscopic micro flocs particles to visible suspended particles. Tiny and neutral micofloc particles collide and bond together to form larger visible floc particles called pin flocs. The coagulant chemical interacts with these flocs and their size continues to grow with collision. The coagulant chemical is usually high molecular weight polymers that help to bind, add weight and settle the flocs. The general process of the coagulation-oriented filtration mechanism is shown in Fig. (1).
Fig. (1)) General Coagulation and filtration mechanism.Besides polymers, there are many inorganics coagulants such as aluminum and iron salts [17]. In water, these salts dissociate into trivalent ions Al3+ and Al3+. These ions get hydrolyzed and form positively charged soluble complexes on the surface of negatively charged suspended particles [18]. When the pH of the water is higher than the minimum solubility of the coagulants, the hydrolysis products are HMM polymers whereas when the pH of the water is lower than the minimum solubility of coagulants the hydrolysis products are monomers or medium polymers [19]. The most commonly used coagulant is an alum (aluminum sulfate) and some ferric salts. The leaching of aluminum in drinking water may pose a risk of Alzheimer, and hence the use of ferric salts has become more popular now a day [20]. Table 1 describes the features and properties of some coagulants being available in the market.
BIOLOGICAL TREATMENT
The biological treatment employs natural processes to decompose organic contaminants present in wastewater. Biological treatments use bacteria, nematodes, and other small organisms to break down organic waste using a normal cellular process [22]. Organic waste usually consists of vegetables, waste foods, garbage, and pathogenic organisms. There are two types of biological treatment aerobic and anaerobic. The aerobic treatment involves the oxidation of organic material (termed biochemical oxygen demand, BOD) and the oxidation of ammonium (NH4+) in the presence of oxygen. The organic materials present in the water mineralized to H2O, CO2, and NH4+. The biological treatment fosters the accumulation of large biomass to affect rapid and complete oxidation in a relatively short liquid detention time. Many water scientists are trying to control and refine biological processes to achieve optimum removal of an organic substance from water. If we use an activated sludge process, the microorganisms usually accumulate into larger particles called flocs as discussed in the previous section. These flocs can settle out in quiescent settlers as they are larger than normal bacteria cells. The settled cell mass can proceed to the aeration tank to build up activated sludge. In another method called the trickling filter system, the cell mass retained in the filter is attached to a fixed and solid surface. In this type organic and NH4+ ions could be removed and new cell mass (called a biofilm) growth occurs. The wastewater moves from filter to settler for improving the quality of effluents. In aerobic biological plants, bacteria responsible for the oxidation of organic contaminants alone and with NH4+ are physiologically different. The oxidizers are heterotrophs and nitrifiers are autotrophs. The heterotrophs employ organic molecules as a source of carbon to acquire electrons and energy to synthesize new cell mass. Whereas, the autotroph reduces carbon from CO2 and can oxidize NH4+ or NO2- to acquire energy and electrons. The CO2 reduction demands huge energy and electrons from the autotroph hence the yield of new cell material per unit of oxidized electron donor substrate is lower for autotrophs than heterotrophs. Hence, specific growth rate for autotrophs is much lower than heterotrophs under the same favorable conditions for both microorganisms [23].
Activated sludge process widely used in secondary treatment of domestic and industrial wastewater employs aerobic biological treatment. This method is suitable for treating wastewater streams generated from municipal sewage, pulp and paper mills, meat processing, and other industrial waste streams, which contains carbon molecule. Another process is called membrane aerated biofilm reactor (MABR), which uses 90% less energy than another biological reactor [24]. In the MABR reactor, oxygen diffuses through the gas-permeable membrane. This oxygen is supplied into the biofilm side of the membrane where oxidation of pollutants takes place. This method is suitable for high rate organic carbonaceous pollutant oxidation, organic compound biodegradation, nitrification, and denitrification. High oxygen concentration on the biofilms membrane supports nitrification and an anoxic layer close to liquid-biofilm interface allows denitrification. An aerobic heterotrophic layer supports carbonaceous pollutant removal. It is important to study the location of individual layers of microbial activity in membrane aerated biofilms.
In the anaerobic treatment of wastewater, degradation of organic material into gaseous products and biomass occurs as shown in Fig. (2). These gaseous products are usually methane and carbon dioxide. This treatment is remarkably useful for the treatment of highly polluted wastewater [25]. The anaerobic biological water treatment has low energy input hence no energy is required for oxygenation. Besides that, it has lower sludge production and lower nutrient requirement due to lower biological synthesis. The degradation of waste organic materials also produces biogas which is also a valuable source of energy. The anaerobic digestion is used for the stabilization of sludge from sedimentation tanks in the closed digester or open lagoons. The anaerobic open lagoons are generally employed for the treatment of industrial wastewater.
Fig. (2)) Anaerobic biological treatment plants.FENTON OXIDATION PROCESS
Fenton oxidation process employed for direct mineralization of organic matter present in wastewater or improvement of biodegradability of organic pollutants through oxidation. There are many reactions that represent the Fenton process but the following is the general core reaction [26]:
(1)The H2O2 and homogeneous solution of iron ions are called Fenton reagents. They are chemically unstable and concentrated H2O2 is harmful to humans. Therefore, these reagents increase transformation and storage costs as well as create human health issues [27,88]. The degradation of the organic matter in wastewater is strongly affected by pH, the concentration of Fenton reagent, and the initial concentration of pollutants. The single Fenton optimization process is of three types: heterogeneous Fenton process, photo-Fenton process, and electro-Fenton process. The conventional Fenton process is limited to a narrow pH range and produces a heavy amount of iron sludge. In the case of the heterogeneous Fenton process, Fe2+ catalyst is replaced by a solid catalytic active component. This prevents the leaching of iron, facilitates a wide pH range, and reduces iron sludge formation. However, the heterogeneous Fenton process is suitable for laboratory scale use due to harsh synthesis conditions, complicated synthesis routes, and high synthesis costs [29]. Therefore, the heterogeneous Fenton process cannot be directly implemented in large-scale industrial applications.
In the case of the photo-Fenton process, ultraviolet or visible light is used in combination with the conventional Fenton process which enhances the catalytic capacity of the catalyst. The use of light also increases the degradation efficiency of the organic pollutants and reduces iron sludge production. The energy provided through light photons reduces Al3+ ions to Fe2+ [30, 31]. The photons present in light trigger metal charge transfer excitation from Fe(OH)2+ and regenerate Fe2+ which promotes decomposition of H2O2. The decomposition of H2O2 produces OH- ions, which finally degrade organic pollutants present in the wastewater.
(2)(3)The use of ultraviolet sources shows remarkable increase in degradation but it consumes more energy and has a short life span. Therefore, it is advisable to use a natural light source i.e. sunlight, which is renewable and free. Thus, solar photo Fenton processes have gained more attraction for the removal of TOC. However, it has certain disadvantages like utilization of light energy, high operation cost, and design of photo-reactor on large-scale operations.
The electro Fenton process employs in-situ generations of H2O2 by electrochemical reduction of O2 on the cathode. The Al3+ ions generated in the Fenton process can be reduced to Fe2+ on the cathode which reduces iron sludge formation [32]. The electro-Fenton process has four types: cathode electro-Fenton process, sacrificial anode electro-Fenton process, Fe2+ cycling electro-Fenton process, cathode and Fe2+ cycling electro-Fenton process. In case of the cathode electro-Fenton process, H2O2 is generated by the electrochemical process on the cathode, and Fe2+ is added externally. The sacrificial anode electro-Fenton process involves the addition of H2O2 externally while Fe2+ is generated electrochemically using a sacrificial anode. In the Fe2+ cycling electro-Fenton process, both H2O2 and Fe2+ are added externally, but Al3+ generated by the Fenton reaction is reduced to Fe2+ on the cathode. It reduces the iron sludge production and the requirement of initial Fe2+ concentration input. The cathode and Fe2+ cycling electro-Fenton process H2O2 is generated within the reaction by reduction of O2 and Fe2+ is regenerated through the reduction of Al3+ on the cathode, which not only avoids the addition of H2O2 but also reduces the iron sludge production and the initial Fe2+ concentration input. Thus, the major challenge in the electro-Fenton process is the development of the electrode material. The electrode must possess good efficiency, high catalytic activity, corrosion resistance, long working life span, and low preparation costs. Finally, it can also be added about the Fenton process that the maximum organic pollutants removal capacity is strongly influenced by optimum pH range, nature of the catalyst and H2O2 concentration.
ADVANCED OXIDATION PROCESS (AOP)
The advanced oxidation process (AOP) involves the generation of hydroxyl radicals (OH-) in sufficient quantity for water purification. The sulfate radicals (SO4-) also play a vital role in oxidative processes in AOP [33]. The function of AOP involves the destruction of organic or inorganic pollutants present in wastewater. The radicals like OH- and SO4- have a short half-life, hence, they are feebly effective in the inactivation of pathogens. However, these radicals are
powerful oxidizing agents which destruct water pollutants and convert them into less toxic products.
The sulfate radical (SO4-) has a standard oxidation potential (E˚) of 2.6 V, which is sufficient to initiate a sulfate-based advanced oxidation process. The SO4- radicals can be produced from persulfate S2O82- (with E˚=2.01V) by heat, UV irradiation, or with transition metals as follows-
(4)(5)There are many ways to activate persulfate: by increasing pH, varying temperature in the range 35˚C to 135˚C, ultraviolet irradiation process, or transition metal activation. The transition metals used for activation are usually Fe(II), Fe(III), Cu(I), or Ag(I) however, the metal activation process can generate 50% radicals (eq.2.5) therefore it is not an efficient method. The sulfate radicals remove electrons from organic waste material and transformed them into organic radical cations [34]. The hydroxyl radicals can also be generated from sulfate radicals in alkaline conditions. The hydroxyl radical is the most reactive radical with standard oxidizing potential (E˚) 1.95 V to 2.8 V [35]. Hydroxyl radicals can attack organic pollutants through hydrogen abstraction, radical addition, electron transfer, and radical combination. The hydroxyl radicals usually add to the C=C bond or remove H from the C-H bond when reacted with organic compounds. When reacting with organic compounds, hydroxyl radicals produce R• or R•–OH radicals. These radicals transformed into organic peroxide radicals (ROO•) in the presence of O2. As the lifetime of hydroxyl radicals is very short therefore these radicals should be produced in-situ during application in the presence of oxidizing agents. Ozone is a strong oxidizing agent with an oxidation potential of 2.07 V that can react with an ionized and dissociated form of the organic compound directly. The OH˙ can also be produced in an indirect mechanism under certain conditions [36].
(6)There are many oxidants that can significantly improve hydroxyl radical yield. These radicals can also be generated with ultraviolet photons in the presence of catalysts like TiO2 or RO-type semiconductors. When TiO2 is used as a catalyst, they produce positive holes in the valance band and negative electrons in the conduction band. The holes possess oxidizing property whereas electrons possess reducing property [37].
(7)These holes and electrons further reacts with OH- and H2O adsorbed on the surface of TiO2 produces hydroxyl radicals-
(8)(9)The hydroxyl ions can also be produced in Fenton-based AOP by activating H2O2 using iron metal as discussed in the previous section.
The ultrasound irradiation uses sound waves on the cavities made up of vapor and gas-filled micro-bubbles. This generates high temperature (4200-5000K) and high pressure (200-500 atm) which fragments water molecules in the micro-bubbles and hydroxyl radicals can be generated. The electron beam irradiation also generated hydroxyl radicals or reducing radicals by splitting water-
(10)Many studies have demonstrated that AOPs are viable options for water treatment like leachate treatment, effluent organic matters in biologically treated secondary effluent, water reuse. In the future, an effort should be made on producing cost-effective AOP technology for the treatment of wastewater. The detailed information on this method is discussed in the upcoming chapter.
MEMBRANE PROCESS
The membrane process is a very popular water purification method that includes reverse osmosis, nanofiltration, ultrafiltration, microfiltration, and electrodialysis. The membrane serves as a selective barrier for unwanted pollutants and allows only certain particles whose size is smaller than membrane pores. There is a driving force between the two sides of the membrane which is capable of moving the constituents across the membrane. Depending on the types of driving force i.e. pressure, electrical potential, concentration, or temperature, the membrane processes are classified [38]. The membrane is made up of a number of materials based on mechanical, thermal, chemical stability, and fouling tendency [39]. Polymer-based membrane materials are widely used because they are hydrophobic and are prone to fouling [40]. The membrane fouling is generally caused by deposition of inorganic components, pore blocking, microorganisms, and feed chemistry. The fouling is either reversible (loosely attached of particles) or irreversible (strongly attached particles). In order to overcome these issues, surface modifications of the polymeric membrane are employed. These modifications include blending, grafting, and incorporation of nanomaterials such as ZnO, carbon nanotubes, grapheme, Al2O3, and TiO2 [41-44]. The graphene oxide membrane has gained more attention due to its hydrophilic properties, flexibility, and high mechanical strength. The graphene oxide membrane is suitable for desalination and wastewater treatment which gives a wide range of pure water flux. The membrane should have high permeability and high selectivity as well as it should possess both hydrophilic and hydrophobic characteristics.
Reverse Osmosis (RO)
Reverse osmosis is a pressure-driven water purification technique that removes small particles and solids [45]. The RO membrane is permeable to only water molecules. It is well-studied and established technology for various types of water purification. The pressure applied on the membrane must be high enough to overcome the osmotic pressure. The RO technique is capable of removing all particles, bacteria, and organics. It is usually applied in the desalination of brackish water and seawater with less maintenance. It uses a pressure gradient between the water to be treated and permeate side to remove molecules and ions from solutions when it is on one side of a selective membrane. The effective water flow through the membrane is given by the equation-
(11)Where, A-membrane permeability coefficient, ΔP-pressure across the membrane, and Δπ- osmatic pressure difference across the membrane. The RO membrane allows pure water on one side of the membrane (called permeate stream) and rejects ions and salts on another side of the membrane (reject stream). The membrane is composed of a thin polymeric layer along with porous support that provides mechanical strength to the membrane. Besides so many advantages, there are some disadvantages like the use of high pressure, expensive membrane and are also prone to fouling. The RO membrane also removes useful minerals from water therefore additional mineral cartridge is to be installed which adds cost.
Ultrafiltration and Microfiltration
The pore size of the microfiltration membrane falls within the range of 0.05-10 μm whereas the pore size of ultrafiltration falls in between nanofiltration (NF) and microfiltration (MF) i.e. 0.001-0.05 μm. The UF membranes have an asymmetric structure with a smaller pore size and lower surface porosity than the MF membrane, which produces higher hydrodynamic resistance. The UF operating pressure is low (2-5 bars) due to the larger pore size of the membrane than the NF and RO membrane. The water flux J is given by the equation-
(12)Where, η- fluid viscosity, Rm, and Rc are membrane resistance and cake resistance respectively. The UF membrane consumes low energy and is capable of removing pathogenic microorganisms, macromolecules, and suspended matters [46, 47]. The UF/MF membrane separation is decided by membrane pore size, solute membrane interactions, shape, and size of solutes. The UF/MF process is suitable for potable water treatment, RP pretreatment, tertiary water treatment, and water reclamation. Besides these applications, UF has some disadvantages like its inability to remove any dissolved inorganic substances from water and regular cleaning to maintain high-pressure water flow. The MF cannot remove viruses and dissolved solids with a size less than 1 mm.
Nanofiltration
Its operation is similar to that of RO, but it operates at a lower pressure than RO. The NF process is better than the RO and UF processes for the treatment of wastewater because it selectively rejects low molecular weight organic compounds and divalent compounds. It is also useful for removing heavy metals and separating dyes and color compounds in the textile industry. Unlike RO, in the case of NF, the rejection of solute depends upon the molecular size and the Donnan exclusion effect [48]. The equilibrium between the solution and the charged membrane is associated with an electric potential called the Donnan potential so that ions smaller than the pore size are rejected because of Donnan exclusion. Pretreatment is required to remove some heavily polluted water using NF as the membranes are sensitive to free chlorine. It was reported by a few researchers that the PMIA/GO composite nanofiltration membrane was found effective for water purification with a greater hydrophilic surface as compared to pure polymer (PMIA). They have also observed high dye rejection and enhanced fouling resistance to bovine serum albumin [49]. The NF method is also useful for textile wastewater treatment, showing excellent removal of heavy metal ions, organic color compounds, and trihalomethane (THM) precursors such as humic acids.
Electrodialysis
In the dialysis method, the separation of solutes takes place by transport of the solutes, through a membrane instead of using a membrane to retain the solutes whereas water passes through it in reverse diffusion and nano-filtration. Electrodialysis is the most widely used water treatment process that involves the membrane separation process [50]. This process is used to remove salt, acid, and bases from aqueous solutions to separate ionic compounds from neutral; and separate monovalent ions from multivalent ions. In this method, the electric potential is used as a driving force, whereas an ion exchange membrane is used between anode and the cathode. In the presence of electric potential, the negative and positive ions move towards the anode, and cathode, respectively, through the membrane compartment. These membrane compartments are concentrated and diluted alternatively. The electrodialysis process depends upon a number of parameters like pH, cell structure, flow rate, feed water ionic concentrations, and properties of the ion exchange membrane [51]. This process is cost-effective for TDS feed concentrations of less than 3000 parts per million (ppm). When the concentration is above 3000 ppm, RO is more cost-effective than electrodialysis when higher recovery of feed is not required. Membrane fouling is a major problem in electrodialysis, which consumes more energy and declines membrane flux. The fouling problem can be overcome by applying DC polarity to the electrodialysis membrane, which reverses polarity every 15-20 minutes. This prevents the deposition of salts on the membrane surface and eliminates acid and anti-scalant pretreatment. The periodic rinsing of the electrodes also prevents the formation of any gases on the electrodes. The electrodialysis process cannot remove nonionized compounds like silica or other colloids. This method of water purification is useful for the desalination of black water and the treatment of municipal water and wastewater.
ION EXCHANGE PROCESS
The ion exchange method is the most utilised technique for water treatment as well as in separation processes such as chemical synthesis, medical research, food processing, mining, and agriculture. The ion exchange process involves the removal of dissolved ions from water and replacing them with other similar charged ions [52]. The water causes hardness due to calcium and magnesium ions, which can be softened by the ion exchange process. The replacement of ions involves the replacement of hardness-producing ions with no hardness ions. The water softener usually employs sodium ions supplied from a sodium source called brine. The solid phase in the ion exchange process is a synthetic resin that can selectively adsorb the containments. In the ion-exchange process, the contaminated water is passed through an ion exchange resin until all sites of the resin beads are filled with contaminant ions. The exhausted bed is regenerated by rinsing by suitable regenerant by rinsing the ion exchange column. In order to remove fluoride from water supplies, a strongly basic anion-exchange resin can be used (e.g., chloride-fluoride resin). The fluoride ions replace the chloride ions of the resin until all the sites on the resin are occupied. The supersaturated resin is then backwashed with water. The chloride ions replace halide ions present on the surface, showing a higher replacement tendency towards halide ions. The resins can be cation exchange resins or anion exchange resins based on the type of contaminants present in the water. These resins can be regenerated several times and used for the ion removal process. There are several ion exchange materials, like natural organic/inorganic materials, modified natural materials, synthetic materials, etc. These materials may includes: vermiculites, zeolites, clays, polysaccharides, protein, carbonaceous materials, titanates, silicotitanates, transition metal hexacyanoferrate, phenolic and acrylic materials, and many more.
Suppose, the ion exchanger is represented by M+X- with M+ being a soluble ion and placed in the salt solution NY [53]. The salt ionises in the solution and gives N+ and Y- ions and an exchange reaction would take place as-
(13)In a similar manner, we can write the equation for the anion exchange reaction. Hence, ion exchange materials are of two types: cation exchangers and anion exchangers, based on the type of ionic groups attached to the materials.
The mechanism of ion exchange considers the transfer of ions from the interface boundary through a chemical reaction, which is diffusion inside the material and diffusion in the surrounding solution. There are counterions that are exchangeable ions carried by ion exchangers that cause interphase diffusion. These counterions move freely within the framework, and their movement is compensated for by the counter-movement of other ions of the same charge to maintain electroneutrality. The mass action selectivity coefficient determines the thermodynamic equilibrium constants (K) for cation exchange treatment. The activities of adsorbed ions are depends upon mole fractions of the ions adsorbed on the solid, which often varies the ionic composition of the exchanger and the total ionic strength of the solution [54]. According to another model, the variation measured selectivity coefficients with ionic compositions is caused by variation in the activities of adsorbed ions [55]. They have included changes in the chemical potential of water occurring during the ion-exchange reactions. The model proposed by Eriksson is based on a diffuse double-layer theory that estimates selectivity coefficients for heterovalent exchange reactions [56]. This model is based on electrostatic interactions that treat all the cations equally, regardless of their selectivities. In order to represent a mathematical approach to kinetics, diffusion equations have to be considered. The ion exchange interactions are diffusion induced electric forces suggested by Fick’s law [57].
(14)Where, Ji-flux of the ion in moles/time, D-the diffusion coefficient, and Ci- concentration in moles per unit volume. When the species is not subject to any forces besides the concentration inhomogeneity, then the diffusion coefficient is constant and the flux is directly proportional to the concentration gradient Ci. The flux does not depend upon concentration itself because diffusion is a purely statistical phenomenon that does not involve any physical force at the molecular level. The diffusion between two counterions without any co-ion transfermeans that the two fluxes are rigorously coupled [58]. Thus, the diffusion of majority ions and thus rates are not much affected by the presence of the electric field. The ions which are going to exchange are present in the phase with different concentrations and the diffusion between them is affected by electro-coupling interactions. The rate of ion exchange depends upon two factors: diffusion of ions inside the material or diffusion of ions through liquid a film. The diffusion inside the material is enhanced by selecting the material with a low density of the gel. The reduction in bead size also increases the rate as it reduces the time of equilibrium achievement for the particles. The increase in temperature also increases the rate, independent of the rate-controlling step.
ELECTROCHEMICAL METHODS
Electrochemistry is a promising field in cleaner and eco-friendly water treatment. Environmental electrochemistry is developed on the basis of electrochemical techniques used to remove impurities from liquids to minimise environmental pollution. These techniques have certain advantages, like versatility, high energy efficiency, cost-effectiveness, and amenability to automation. The electrochemical process is either employed in pretreatment to increase the biodegradability of pollutants or in advanced steps to reduce COD/color to obtain water standards [59].
In the electrochemical oxidation method, the anodic oxidation of most resistant organic compounds can be broken by both direct and indirect oxidation. Direct oxidation involves the direct transfer of electrons to an anode, and pollutants are broken down after adsorption on the anode surface with no involvement of other substances. This requires more negative potential than is required for water splitting and evolution. The main disadvantage of direct oxidation is the electrode fouling problem, which is due to the accumulation of polymeric layers on the anode surface. This results in poor decontamination performance after a period of time [60]. In an indirect oxidation process, there is no need to add an oxidation catalyst to the solution and it does not produce any byproducts. In this process, oxygen is adsorbed physically or chemically, which leads to decontamination of electro-generated species at the anode. Physio-sorbed oxygen brings complete combustion of organic compounds, whereas chemisorbed oxygen participates in the formation of selective oxidation products. There are active anode electrodes like IrO2, RuO2, or platinum and nonactive anodes like SnO2, PbO2, or boron-doped diamond. The nature of the oxidation electrode influences the efficiency of the process and electrode selectivity. Table 2 represents various electrodes and their properties in an indirect electrochemical oxidation process.
In the direct oxidation method, there is the problem of deactivation of the anode during oxidation, which can be resolved by using the indirect oxidation method. In the indirect oxidation method, pollutants are destroyed by the electrochemical regeneration of chemical reactants like ozone, chlorine, H2O2, persulfate, etc. Active chlorine in the form of Cl2, HOCl, and OCl- can be electrochemically generated and used in the electrochemical oxidation of organic pollutants both in model solutions and in actual wastewater. The active chlorine is produced in the following way.
(15)(16)(17)It is a challenge to develop effective electrodes that do not produce organo-chlorinated intermediates and reduce toxic chemical species in water. Electro-deposition is a metal removal technique based on cathodic deposition, employed in the metallurgical and electroplating industries, printed circuit boards, and the battery manufacturing industry. These heavy metals can be recovered using a chelating agent like EDTA, nitrilotriacetic acid, or citrate. The metal recovery is enhanced by integrating electro-deposition with ultrasonic. The electro-reduction method is an effective method for the dechlorination of chlorinated organic compounds (COC and VOC), polychlorophenols, and polychlorinated hydro-carbons. Cathodic electrochemical denitrification is used for the reduction of nitrate and nitrite ions for the treatment of nitrate-containing groundwater. The rate of reaction on electrodes is determined by the reduction of nitrate to nitrite, the value of the Tafel slope, the effect of co-adsorbing ions, and the kinetic order. Table 3 summarises various advantages and disadvantages of electrochemical reduction methods.
ULTRAVIOLET IRRADIATION TECHNOLOGY
The ultraviolet wavelength (250-270 nm) has a germicidal effect and is used for the disinfection of wastewater. In this wavelength region, UV light is lethal to microorganisms like bacteria, protozoa, viruses, molds, yeasts, fungi, nematode eggs, and algae [62]. UV light has the ability to kill faecal coliform and Escherichia coli bacteria at a wavelength of 260 nm. UV light prevents the division of deoxyribonucleic acid (DNA) and the production of enzymes. The nucleotide bases present in the DNA, i.e., adenine, guanine, thymine, and cytosine, absorb UV light. Thymine and cytosine are ten times more sensitive to UV light than adenine and guanine. Two thymine molecules react with UV light and produce a thymine dimer. The dimerization of adjacent pyrimidine molecules causes more photochemical damage. The microorganisms are deactivated by photochemical damage to cellular RNA and DNA. DNA replication is prevented by the formation of a number of thymine dimers in the DNA of bacteria and viruses, which kill the cells. Such damage caused by UV light can be repaired by visible light or sunlight, which is called photo-reactivation [63].
UV light in the germicidal lamps is emitted as a result of electron transmission through ionised vapour between the electrodes. The glass of the germicidal UV lamp is made up of quartz, which transmits 86% of UV light of its total intensity. The slimline instant start lamp is an excellent choice for wastewater treatment because it produces 26.7 watts of UV-C from 100 watts of power at 0.18 watts of UV-C per centimetre of arc length using a conventional core-coil ballast and operates at 40˚C. The rapid start lamp is another choice for UV light in which a low voltage (4V) is continuously applied through ballast to the lamp cathodes to operate the arc. The low-pressure flat lamp employs a quartz envelope and a much higher current, which decreases the distance so that ions and electrons quickly reach the wall and increases UV output. Such flat UV lamps are capable of producing 3 times the UV-C output per arc length centimeter. In every UV source, a ballast is used to limit current to the lamp and provide sufficient voltage to start the lamp. In the case of a rapid start circuit, the ballast supplies voltage to heat the lamp cathode continuously [64].
The quality of effluent affects the efficiency of UV disinfection as most of the UV photons are absorbed by pollutants that reduce UV intensity. UV adsorption depends upon suspended solids which typically consist of bacteria-laden particles of varying shapes and sizes. These suspended solids present in wastewater absorbs UV light before it can penetrate the solid to kill microorganisms, which reduces disinfection efficiency. The size of the suspended particles decide the UV light demands. When the size of these particles is less than 10 μm, UV light can easily penetrate and requires less UV demand. As the size of the particles increases (beyond 40 μm), UV light cannot penetrate completely; hence UV demands are higher . The presence of iron in the water also affects UV disinfection. Higher iron concentrations absorb UV light before it can kill any microorganisms, or iron will precipitate out on surface and absorb UV light before it enters into the wastewater. The iron can also be absorbed by suspended solids, clumps of bacteria, and other organic compounds that prevent UV light from penetrating the suspended solids. The hardness of the water caused by calcium and magnesium salts also affects UV disinfection [65
