Water Pollution Sources and Purification: Challenges and Scope

Review on Water Purifications Techniques and Challenges

R. M. Belekar¹, *, S.J. Dhoble²

¹ Department of Physics, Institute of Science, Rabindranath Tagore Road, Nagpur-440 001, M.S., India

² Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India

Abstract

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.

Keywords: Adsorption, Biological treatment, Chemical methods, Electrodialysis, Fenton process, Membrane treatment, Purification methods, UV treatment, Water pollution.

* Corresponding author R. M. Belekar: Department of Physics, Institute of Science, Rabindranath Tagore Road, Nagpur-440 001, M.S., India; Tel: +91-9822292336; E-mail: [email protected]

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.

Table 1 Properties of some commercially available coagulants [21].

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]:

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.

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-

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].

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].

These holes and electrons further reacts with OH- and H2O adsorbed on the surface of TiO2 produces hydroxyl radicals-

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-

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-

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