4.1. Physical and Chemical Processes

4.1.1. Aeration

Aeration is an optional part of the water treatment process using scrubbing action and oxidation to remove or modify constituents of the water. Aerators work by increasing the amount of surface area of air coming in contact with water. This may be achieved by passing air through water, as in an air diffusion aerator. In contrast, many aerators pass water through air, as in spray nozzle, cone tray, cascade, and coke tray aerators. Finally, forced draft aerators both pass air through water and water through air (URL 1). Aeration is a unit process in which air and water are brought into intimate contact. Turbulence increases the aeration of flowing streams The contact time and the ratio of air to water must be sufficient for effective removal of the unwanted gas. Aeration is also an effective method of bacteria control. Aeration as a water treatment practice is used for the following operations:

• To reduce concentration of taste- and odor-causing substances and, to a limited extent, for oxidation of organic matter,
• To remove substances that may in some way interfere with or add to the cost of subsequent water treatment. A prime example is removal of carbon dioxide from water for corrosion control and before lime softening,
• To remove VOCs and volatile SOCs considered hazardous to public health,
• To add ozone or chlorine gas to the water for oxidation and/or disinfection purposes,
• To add oxygen to water, primarily for oxidation of iron and manganese found in many well waters so that they maybe removed by further treatment,
• To remove ammonia from the water,
• To remove gases such as radon, hydrogen sulfide, and methane,
• To remove THMs, pesticides and herbicides (Dyksen, 2005; URL 2).

4.1.1.1. How does aeration work?

Aeration removes or modifies the constituents of water using two methods -scrubbing action and oxidation (Figure 4.1). Scrubbing action is caused by turbulence that results when the water and air mix together. The scrubbing action physically removes gases from solution in the water, allowing them to escape into the surrounding air. In the picture above, carbon dioxide and hydrogen sulfide are shown being removed by scrubbing action. Scrubbing action will remove tastes and odors from water if the problem is caused by relatively volatile gases and organic compounds (URL 3).

Figure 4.1. Aeration steps (URL 3)

Oxidation is the other process through which aeration purifies water. Oxidation is the addition of oxygen, the removal of hydrogen, or the removal of electrons from an element or compound. When air is mixed with water, some impurities in the water, such as iron and manganese, become oxidized. Once oxidized, these chemicals fall out of solution and become suspended in the water. The suspended material can then be removed later in the treatment process through filtration (URL 3).

4.1.1.2. Methods of aeration

There are several different methods used to aerate water, but all either involve passing water through air or air through water. Water can be exposed to air by spraying or by distributing it in such a way that small particles or thin sheets of water come in contact with the air. Water can also aerated by pumping large volumes of air through the water (URL3). The method of aeration to be used depends on which materials on the water are to be removed. The chemical characteristics of the water to be treated also influence which treatment method is used. Finally, each method has a different efficiency. In general, pumping water through air is much more energy efficient than pumping air through water. Different types of aeration and other methods of treatment should all be compared to determine the most efficient and practical method of treatment in each case (URL 3). Methods of aeration can be classified into four general categories (Figure 4.2, 4.3) (Dyksen, 2005; URL 4). (1) Waterfall aeration uses the desorption principle and accomplishes gas transfer by causing water to break into drops of thin films, increasing contact surface area between the air and water. There are five common types of waterfall aerators: (a) Spray Aerators have been used in water treatment for many years for iron oxidation and stripping VOCs and dissolved gases. Spray aerator installation generally consists of fixed nozzles or a pipe grid located over an open-top tank. The nozzles spray fine water droplets into the surrounding air, creating the air-water interface necessary for contaminant transfer. The configuration of pipe grid on top of open-top tank is preferable for stripping applications. Spray aerators require a large installation area and pose operating problems during freezing weather; furthermore, for effective application they require supplemental air exchange. (b) Cascade aerators consist of a series of steps that allows water to fall in thin layers from one level to another, where aeration is accomplished in the splash zones. The exposure time of air to water is increased by increasing number of steps, and the area-volume ratio is improved by adding baffles to produce turbulence. The major operating problems include corrosion and slime and algae buildup. (c) A multi-tray or slat tray aerator consists of series of trays equipped with slats or perforated or wire-meshed bottoms. Water is distributed over the trays and is allowed to fall from each tray onto the collection basin at the base. Air is forced or induced to flow perpendicular to water path. Often coke, stone, or ceramic balls of size 2 to 6 in. are used to improve efficiency of gas exchange. There are typically 3-9 trays spaced 12-30 inches apart in multi-tray aerators. Multi-tray aerators are analogous to cooling towers and require adequate ventilation. They provide excellent oxygen absorption and carbon dioxide, ammonia, and hydrogen sulfide removal. (d) Cone aerators are primarily used to oxidize manganese and iron as a pretreatment measure. The design of the aerator is similar to cascade aerators, with stacked pans arranged such that water fills top pan and cascades down to each succeeding pan. (e) Packed tower or air strippers are capable of removing from water fuel, solvents and volatile SOCs, VOCs, ammonia, hydrogen sulfide, and carbon dioxide. A packed column consists of a cylindrical tower, a packing material (usually plastic, steel or ceramic) and a centrifugal blower. The contaminated feed water is pumped into the top of the tower and blowers are used to introduce air countercurrent through the bottom of the tower. The large surface area provided by the packing material allows more liquid-gas transfer compared with other air stripping methods. The number of stages is determined by Henry’s Law constant and the tower loading rate. A most common problem with packed tower aerators is fouling of packing material with solids, resulting in loss of plant capacity, efficiency, and increased pressure drop. (2) Bubble aerators consist of a rectangular concrete tank in which perforated pipes, porous diffuser tubes or plates are installed. Compressed air is forced through these pipes to produce air bubbles that rise through water, producing turbulence resulting in effective water-air mixing. Bubble aerators are effective in removing carbon dioxide, VOCs, gasoline components, hydrogen sulfide, methane, and radon from contaminated groundwater. They are mostly used by small water systems with low flow rates. (3) Mechanical aerators employ motor driven impellers alone or in combination with air injection devices. They are also installed at water reservoirs to control taste and odor. The mechanical aerators can be installed as surface aerators (on the float) or submerged (below the water level) aerators. They are normally used for increasing dissolved oxygen levels, but they can also remove certain contaminants. (4) Pressure aerators are typically used for oxidizing iron and manganese. There are two basic types of pressure aerators in which either compressed air is injected directly in pressurized pipeline or water is sprayed into the top of a closed tank while the tank is continuously supplied with compressed air.

Figure 4.2. Nozzles for sprays and units for aeration or stripping: (a-d) nozzle types;(e) inclined apron that may be studded with riffle plates; (f) perforated plates; (g) spray tower; and (h) cascade (Sincero and Sincero, 2003a)

Figure 4.3. Aeration units: (a) turbine aerator with an air sparger; (b) porous ceramic diffuser; and (c) surface aerator (Sincero and Sincero, 2003a)

The effectiveness of aeration depends upon the aeration method selected, Henry's law constant of contaminant, design factors such as air to water ratio, flow and loading rate, available area of mass transfer, temperature, pH and algae production. As Henry's law constant increases, design factors such as air to water ratio becomes less critical for effective removal (Dyksen, 2005; URL 4).

4.1.1.3. Problems with aeration

Aeration typically raises the dissolved oxygen content of the raw water. In most cases, this is beneficial since a greater concentration of dissolved oxygen in the water can remove a flat taste. However, too much oxygen in the water can cause a variety of problems resulting from the water becoming supersaturated. Supersaturated water can cause corrosion (the gradual decomposition of metal surfaces) and sedimentation problems. In addition, air binding occurs when excess oxygen comes out of solution in the filter, resulting in air bubbles, which harm both the filtration and backwash process (URL 3). Aeration can also cause other problems unrelated to the supersaturated water. Aeration can be a very energy-intensive treatment method, which can result in overuse of energy. In addition, aeration of water can promote algal growth in the water and can clog filters (URL 3).

4.1.2. Adsorption

4.1.2.1. Basic concepts and definitions of adsorption

Adsorption is a phase transfer process that is widely used in practice to remove substances from fluid phases (gases or liquids). It can also be observed as natural process in different environmental compartments. The most general definition describes adsorption as an enrichment of chemical species from a fluid phase on the surface of a liquid or a solid. In water treatment, adsorption has been proved as an efficient removal process for a multiplicity of solutes. Here, molecules or ions are removed from the aqueous solution by adsorption onto solid surfaces. In adsorption theory, the basic terms shown in Figure 4.4 are used. The solid material that provides the surface for adsorption is referred to as adsorbent; the species that will be adsorbed are named adsorbate. By changing the properties of the liquid phase (e.g. concentration, temperature, pH) adsorbed species can be released from the surface and transferred back into the liquid phase. This reverse process is referred to as desorption (Worch, 2012).

Figure 4.4. Basic terms of adsorption (Worch, 2012).

4.1.2.2. Engineered adsorption processes in water treatment

Adsorption processes are widely used in water treatment. Table 4.1 gives an overview of typical application fields and treatment objectives. Depending on the adsorbent type applied, organic substances as well as inorganic ions can be removed from the aqueous phase (Worch, 2012). Activated carbon is the most important engineered adsorbent applied in water treatment. It is widely used to remove organic substances from different types of water such as drinking water, wastewater, groundwater, landfill leachate, swimming-pool water, and aquarium water. Other adsorbents are less often applied. Their application is restricted to special adsorbates or types of water (Worch, 2012).

Table 4.1. Adsorption processes in water treatment (Worch, 2012).

4.1.2.3. Activated carbon

Activated carbon is an adsorbent that is widely used in water treatment, advanced water treatment, and the treatment of certain organic industrial wastewaters because it adsorbs a wide variety of organic compounds and also economically feasible. It is generally used in granular form either in batch, column (both fixed bed and countercurrent bed), or fluidized-bed operations, with fixed bed columns being the most common. Occasionally, activated carbon is used powdered form and is not recovered for regeneration: however, such application is usually limited to water treatment where amounts of carbon used are not appreciable. Adsorbents other than activated carbon are used to lesser extent in environmental engineering (Reynolds, 1982).

4.1.2.4. Why we use adsorption process in drinking water treatment?

For nearly 100 years, adsorption processes with activated carbon as adsorbent have been used in drinking water treatment to remove organic solutes. At the beginning, taste and odor compounds were the main target solutes, whereas later the application of activated carbon was proved to be efficient for removal of a wide range of further organic micropollutants, such as phenols, chlorinated hydrocarbons, pesticides, pharmaceuticals, personal care products, corrosion inhibitors, and so on. Since natural organic matter (NOM, measured as dissolved organic carbon, DOC) is present in all raw waters and often not totally removed by upstream processes, it is always adsorbed together with the organic micropollutants. Since activated carbon is not very selective in view of the adsorption of organic substances, the competitive NOM adsorption and the resulting capacity loss for micropollutants cannot be avoided. The competition effect is often relatively strong not least due to the different concentration levels of DOC and micropollutants. The typical DOC concentrations in raw waters are in the lower mg/L range, whereas the concentrations of organic micropollutants are in the ng/L or ?g/L range. On the other hand, the NOM removal also has a positive aspect. NOM is known as a precursor for the formation of disinfection by-products (DBPs) during the final disinfection with chlorine or chlorine dioxide. Therefore, removal of NOM during the adsorption process helps to reduce the formation of DBPs. Activated carbon is applied as powdered activated carbon (PAC) in slurry reactors or as granular activated carbon (GAC) in fixed-bed adsorbers. The particle sizes of powdered activated carbons are in the medium ?m range, whereas the GAC particles have diameters in the lower mm range. In recent years, the problem of arsenic in drinking water has increasingly attracted public and scientific interest. As a consequence, a number of water works, in particular in areas with high geogenic arsenic concentrations in groundwater and surface water have to upgrade their technologies by introducing an additional arsenic removal process. Adsorption processes with oxidic adsorbents such as ferric hydroxide or aluminum oxide have been proved to remove arsenate very efficiently. The same adsorbents are also expected to remove anionic uranium and selenium species (Worch, 2012).

4.1.3. Membrane Processes

A wide variety of membrane processes can be categorized according to driving force, membrane type and configuration and removal capabilities and mechanisms. Membrane processes in the drinking water are used for desalination, softening and color, particle, microbial and natural organic materials removal that foul water’s taste and taint its clarity and other purposes (Bergman, 2005; URL 5). Water treatment membranes are thin sheets of material that are able to separate contaminants based on properties such as size or charge. Water passes through a membrane; but depending on their size, larger particles, microorganisms, and other contaminants are separated out (URL 5). Some of these systems are pressure driven, depending on water pressure to separate the particles based on size. Microfiltration (MF) employs the largest pore size, and can remove sand, silt, clay, algae, bacteria, Giardia, and Cryptosporidium. Ultrafiltration (UF) can also remove viruses. Nanofiltration (NF) systems provide nearly complete protection against viruses, remove most organic contaminants, and can reduce hardness in water. Electrodialysis (ED) and Reverse osmosis (RO) systems are dense membranes that remove almost all inorganic contaminants and all but the smallest organic molecules (URL 5). The lower limits of ED-RO, NF, UF, and MF particle rejection in potable water are 0.0001 mm, 0.001 mm, 0.01 mm, and 0.1 µm, respectively. However, the size range for each process is broad (Figure 4.5) (Taylor and Hong, 2000).

Figure 4.5. Removal capability of membrane systems (Flynn, 2009)

Electrodialysis combines membrane technology with the application of electrical current, to separate contaminants based on charge. Unlike other membrane processes, the source water never passes through the membranes during electrodialysis. It is not used as much in large water treatment facilities as some of the other technologies described here. Rather, it is mostly used for medical and laboratory applications that need ultrapure water (URL 5). Membranes, which especially reverse osmosis and nanofiltration, can be a good option for small-scale water treatment systems faced with a wide variety of contaminants. However, they often produce larger quantities of wastewater (or “concentrate”) than most other treatment systems -as much as 15 percent of the total treated water volume- and can become clogged with clay or organic materials if particle-rich source water is not filtered first (URL 5). Pressure-driven membrane processes can be designed for Cross-Flow or Dead-End operating mode (Figure 4.6).

Figure 4.6. Membrane flow configurations (Walsh and Gagnon, 2006)

4.1.3.1. Membrane technology for drinking water treatment

The advancement of membrane technology over the past ten years has resulted in an economically viable drinking water treatment solution for both large- and small-scale applications. From a fundamental perspective, membrane technology is based on the principle that these systems act as a physical, size- exclusion barrier to contaminants present in raw water feedstreams. Low-pressure membranes, microfiltration (MF) and ultrafiltration (UF), effectively remove suspended or colloidal particles via a sieving mechanism based on the size of the membrane pores relative to that of the particulate matter (USEPA, 2003). Dissolved substances that are smaller in dimension than the pores in a MF or UF membrane surface will pass through the surface of these membranes. As presented in Figure 4.7, UF membrane filtration with a rated pore size of 0.01 to 0.1 ?m can effectively remove particulate matter and microorganisms via a size exclusion mechanism. However, based on the principle of pore size exclusion, dissolved material present in the feed water or wastewater streams (i.e., viruses, DOC and soluble inorganics) may not be effectively removed (Walsh and Gagnon, 2006).

Figure 4.7. Conceptual drawing of UF membrane pore sieving mechanism (Walsh, 2005)

4.1.4. Ion Exchange

Ion exchange processes are widely used in water and wastewater treatment to remove objectionable ionic contaminants. The first ion exchange processes used inorganic zeolites mined from natural deposits as a cation exchanger for water softening. Modern synthetic polymer-based exchange media are today used as cation and anion exchangers. Ion exchange is also used in many specific applications, for example, to reduce or remove potentially harmful ionic contaminants from potable water supplies in chemical processes and for product purification and recovery for specialty separations, such as chromatographic separations by size, valance charge, and as catalysts (Gottlieb, 2005). Ion exchange removes unwanted ions from a raw water by transferring them to a solid material, called an ion exchanger, which accepts them while giving back an equivalent number of a desirable species stored on the ion exchanger skeleton (Kemmer, 1988). Certain ions in the solution are preferentially sorbed by the ion exchanger solid, and because electroneutrality must be maintained, the exchanger solid releases replacement ions back into the solution. The reactions are stoichiometric and reversible and obey the law of mass action (Reynolds, 1982). Ion exchange is the exchange of ions from one phase to another. In water treatment, the exchange of ions occurs between the solid phase of the ion exchanger and influent water. In a water softener, a cation exchange resin operating in the sodium form exchanges sodium ions for an equivalent amount of calcium and magnesium ions are removed from the raw water by the resin, and an equivalent number of sodium ions are added to the water from the resin (Gottlieb, 2005). The ion exchangers used in water conditioning are skeleton like structures having many ion exchange sites, as shown in Figure 4.8. The insoluble plastic skeleton is an enormously large ion that is electrically charged to hold ions of opposite charge. The ion exchanger has a limited capacity for storage of ions on its skeleton, called its exchange capacity; because of this, the ion exchanger eventually becomes depleted of its desirable ions and saturated with unwanted ions (Kemmer, 1988). The softener is similar in design to a pressure filter, with resins in place of the filter media (Figure 4.8).

Figure 4.8. Left image: Model of a cation exchanger (Kemmer, 1998),Right image: Ion exchange softener (URL 6).

Two types of ion exchange materials are used: the cation exchange material and the anion exchange material. The cation exchange material exchanges cations, while the anion exchange material exchanges anions. The insoluble part of the exchange material is called the host (Sincero and Sincero, 2003b).

4.1.4.1. Resin capacity

Units for expressing resin capacity are equivalents per liter (eq/L), milliequivalents per millilitre (meq/mL), kilograms per cubic foot (kg/ft³), and grams per liter (g/L). Generally, chemists use the first two units; the last two are practical units used by designers and system owners (Flynn, 2009).

4.1.4.2. The ion exchange process

It is important to keep in mind that the ion exchange process only works with ions. Substances that do not ionize in water are not removed by ion exchange. Each type of ion exchange resin exhibits an order of preference for various ions. This can be stated quantitatively through selectivity coefficients. Each ion pair has a unique selectivity value for each ion exchange resin. The higher the selectivity coefficient, the higher the relative affinity of the ion for the resin. The higher the affinity, the easier it is to load the ion, and conversely the more difficult it is to remove during regeneration (Gottlieb, 2005). An ion exchanger tends to prefer: 1) ions of higher valence, 2) ions with a small solvated volume, 3) ions with greater ability to polarize, 4) ions that react strongly with the ion exchange sites of the exchanger solid, 5) ions that participate least with other ions to form complexes. For the usual cation exchangers, the preference series for the most common cations is as follows (Reynolds, 1982):

Ba⁺²>Pb⁺²>Sr⁺²>Ni⁺²>Cd⁺²>Cu⁺²>Co⁺²>Zn⁺²>Mg⁺²>Ag⁺¹>Cs⁺¹>K⁺¹>NH₄⁺¹>Na⁺¹>H⁺¹

For the usual anion exchangers, the preference series for the most common anions is as follows (Reynolds, 1982):

SO₄^-2>I^-1>NO₃^-1>CrO₄^-2>Br^-1>Cl^-1>OH^-1

The operating performance, capacity, and leakage data for ion exchange resins for the common ions found in water are usually provided by resin supplier (Gottlieb, 2005).

higher the affinity, the easier it is to load the ion, and conversely the more difficult it is to remove during regeneration (Gottlieb, 2005).

4.1.4.3. Resin capacity and regeneration

Ion exchange resins have a finite capacity. When this capacity is used up, the resins are exhausted and leakage of the unwanted ions increases. The exhausted resin can be regenerated with a salt, acid, or base solution containing the ion whose “form” the resin will be operated in. This is passed through the resin bed in sufficient quantity and at a sufficiently high concentration to reverse the exchange, desorb, and replace the previously exchanged ions from the resin with the ions from the regenerant solution. The most common regenerant used in potable water applications of ion exchange is sodium chloride. It is used softening, dealkalization, barium, radium, uranium, selenium, arsenic, and nitrate removal (Gottlieb, 2005).

4.1.4.4. Unit operations of ion exchange

The first commercially used ion exchange materials were naturally occurring porous sands that were commonly called zeolites. Zeolites were the first ion exchangers used to soften waters; however, they have been almost completely replaced in recent years by synthetic organic exchange resins which have a much higher ion exchange capacity. Synthetic cation exchange resins are polymeric materials that have reactive groups, such as sulfonic, phenolic, and carboxylic, that are ionisable, and may be charged with exchangeable cations. Also, synthetic anion exchange resins are available that have ionisable groups, such as the quarternary ammonium or amine groups, which may be charged with exchangeable anions (Reynolds, 1982). Figure 4.9 shows the schematics of the unit operations of ion exchange. Figure 4.9.a shows a cation exchanger and Figure 4.9.b shows an anion exchanger. In both units, the influent is introduced at the top of the vessel. The bed of ion exchanger materials would be inside the vessels, where, as the water to be treated passes through, exchange of ions takes place. This exchange of ions is the chemical reaction of the unit process of ion exchange; the mere physical passing through of the water with the attendant head loss and pumping consideration is the unit operation of ion exchange (Sincero and Sincero, 2003b).

Figure 4.9. Unit operations of ion exchange (Sincero and Sincero, 2003b)

4.1.5. Coagulation and Flocculation

Coagulation and flocculation may be broadly described as chemical and physical processes that mix coagulating chemicals and flocculation aids with water. The overall purpose is to form particles large enough to be removed by the subsequent settling or filtration processes. Particles in source water that can be removed by coagulation, flocculation, sedimentation, and filtration include colloids, suspended material, bacteria, and other organisms. The size of these particles may vary by several orders of magnitude. Some dissolved material can also be removed through the formation of particles in the coagulation and flocculation processes (Delphos and Wesner, 2005). The processes of coagulation and flocculation are employed to separate suspended solids from water whenever their natural subsidence rates are too slow to provide effective clarification (Figure 4.10). Water clarification, lime softening, sludge thickening, and dewatering depend on correct application of the theories ofcoagulation and flocculation for their success (Flynn, 2009).

Figure 4.10. Physical-chemical process involved in coagulation-flocculation (URL 7)

Coagulation is the destabilization of colloids by neutralizing the forces that keep them apart. This is generally accomplished by adding chemical coagulants and applying mixing energy. Aluminum salts, iron salts, or polyelectrolytes are the chemicals usually used. Initially small flocs join, creating larger, settleable agglomerates. The destabilization step is coagulation (charge neutralization); the floc-building stage is flocculation (Flynn, 2009). The water treatment literature sometimes makes a distinction between the terms “coagulant” and “flocculant.” When this distinction is made, a coagulant is a chemical used to initially destabilize the suspension and is typically added in the rapid-mix process. In most cases, a flocculant is used after the addition of a coagulant; its purpose is to enhance floc formation and to increase the strength of the floc structure. It is sometimes called a “coagulant aid.” Flocculants are often used to increase filter performance (they may be called “filter aids” in this context) and to increase the efficiency of a sludge dewatering process.(Letterman et al., 1999).

4.1.5.1. Why coagulation and flocculation are used

All waters, especially surface waters, contain both dissolved and suspended particles. Coagulation and flocculation processes are used to separate the suspended solids portion from the water. The suspended particles vary considerably in source, composition charge, particle size, shape, and density. Correct application of coagulation and flocculation processes and selection of the coagulants depend upon understanding the interaction between these factors. The small particles are stabilized (kept in suspension) by the action of physical forces on the particles themselves. One of the forces playing a dominant role in stabilization results from the surface charge present on the particles. Most solids suspended in water possess a negative charge and, since they have the same type of surface charge, repel each other when they come close together. Therefore, they will remain in suspension rather than clump together and settle out of the water (URL 8).

4.1.5.2. How the coagulation and flocculation processes work

Coagulation and flocculation occur in successive steps intended to overcome the forces stabilizing the suspended particles, allowing particle collision and growth of floc. If step one is incomplete, the following step will be unsuccessful (URL 8) (Figure 4.11)

Figure 4.11. Mechanism of coagulation (a) and flocculation (b) (URL7)

4.1.5.3. Coagulation

Coagulation is a complex process, involving many reactions and mass transfer steps. As practiced in water treatment the process is essentially three separate and sequential steps: coagulant formation, particle destabilization, and interparticle collisions. Coagulant formation, particle destabilization, and coagulant-NOM interaction typically occur during and immediately after chemical dispersal in rapid mixing; interparticle collisions that cause aggregate (floc) formation begin during rapid mixing but usually occur predominantly in the flocculation process. For example, using the aluminum sulfate salt known as alum [Al₂(SO4)₃.14H₂O] in coagulation involves formation of an assortment of chemical species, called aluminum hydrolysis products, that cause coagulation. These species are formed during and after the time the alum is mixed with the water to be treated. Coagulants are sometimes formed (or partially formed) prior to their addition to the rapid-mixing units (Letterman et al., 1999).

4.1.5.4. Flocculation

Following the first step of coagulation, a second process called flocculation occurs. Flocculation, a gentle mixing stage, increases the particle size from submicroscopic microfloc to visible suspended particles. The microflocs are brought into contact with each other through the process of slow mixing. Collisions of the microfloc particles cause them to bond to produce larger, visible flocs called pinflocs. The floc size continues to build through additional collisions and interaction with inorganic polymers formed by the coagulant or with organic polymers added. Macroflocs are formed. High molecular weight polymers, called coagulant aids, may be added during this step to help bridge, bind, and strengthen the floc, add weight, and increase settling rate. Once the floc has reached it optimum size and strength, the water is ready for the sedimentation process. Design contact times for flocculation range from 15 or 20 minutes to an hour or more (URL 8).

4.1.5.5. Coagulants and coagulant aids for the coagulation process

Coagulation reactions ocur rapidly, probably taking less than one second (Delphos and Wesner, 2005). Historically, metal coagulants have been most widely used in water clarification (Flynn, 2009). The most commonly used coagulants are:

• Alum (aluminum sulfate), Al₂(SO₄)₃.14H₂O. It is often used in conjunction with cationic polymers.
• Polyaluminum chloride, Al(OH)x(Cl)y. This is efficient in some waters, requiring less pH adjustment and producing less sludge.
• Ferric chloride, FeCl₃. This may more effective than alum in some applications.
• Ferric sulfate, Fe₂(SO4)₃. It is effective in some waters and more economical some locations.
• Cationic polymers can be used alone as the primary coagulant or in conjunction with aluminum or iron coagulants (Delphos and Wesner, 2005.

Difficulties with settling often occur because of flocs that are slow-settling and are easily fragmented by the hydraulic shear in the settling basin. For these reasons, coagulant aids are normally used (Sincero and Sincero, 2003c).

Typical additivies used for coagulant aids are (Sincero and Sincero, 2003c; Delphos and Wesner, 2005):

• High molecular-weight anionic or nonionic polymers
• Activated silica and clays
• Bentonite
• Acids and alkalis

These additives have been used as coagulant aids in conjunction with iron and alum primary coagulants in treating waters containing high color, low turbidity, and low mineral content (Sincero and Sincero, 2003c).

4.1.5.6. The jar test

In practice, irrespective of what coagulant or coagulant aid is used, the optimum dose and pH are determined by a jar test. This consists of four to six beakers (such as 1000 ml in volume) filled with the raw water into which varying amounts of dose are administered. Each beaker is provided with a variable-speed stirrer capable of operating from 0 to 100 rpm. (Sincero and Sincero, 2003c). Upon introduction of the dose, the contents are rapidly mixed at a speed of about 60 to 80 rpm for a period of one minute and then allowed to flocculate at a speed of 30 rpm for a period of 15 minutes. After the stirring is stopped, the nature and settling characteristics of the flocs are observed and recorded qualitatively as poor, fair, good, or excellent. A hazy sample denotes poor coagulation; a properly coagulated sample is manifested by well-formed flocs that settle rapidly with clear water between flocs. The lowest dose of chemicals and pH that produce the desired flocs and clarity represents the optimum. This optimum is then used as the dose in the actual operation of the plant . (Sincero and Sincero, 2003c).

The jar test is used to identify the most adapted mix of chemical compounds and concentrations for coagulation-flocculation. It is a batch test consisting of using several identical jars containing the same volume and concentration of feed, which are charged simultaneously with six different doses of a potentially effective coagulant. The six jars can be stirred simultaneously at known speeds. The treated feed samples are mixed rapidly and then slowly and then allowed to settle. These 3 stages are an approximation of the sequences based on the large scale plants of rapid mix, coagulation-flocculation, and settling basins. At the end of the settling period, test samples are drawn from the jars and turbidity of supernatant liquid is measured. A plot of turbidity against coagulant dose gives an indication of the optimum dosage (i.e. the minimum amount required to give acceptable clarification). The criteria thus obtained from a bench jar test are the quality of resultant floc and the clarity of the supernatant liquid after settling. The design of the full-scale plant process is then done based on the bench-scale selection of chemicals and their concentrations (URL 7).

4.1.6. Chemical Oxidation

Oxidation-reduction (redox) reactions form the basis for many water treatment processes addressing a wide range of water quality objectives. These may include removal of iron, manganese, sulphur, color, tastes, odor, and synthetic organics (herbicides and pesticides) (Hesby, 2005). A redox reaction consists of two half-reactions: One of the half-reactions involves a loss of electrons, and it is defined as oxidation. The other half-reaction, involving the gain of electrons, is defined as reduction. Chemical species serving as potential electron acceptors are regarded as oxidants. Those functioning as potential electron donors are known as reductants (Shammas et al., 2005). An oxidation and a reduction reaction must always be coupled because free electrons can not exist in solution and electrons must be conserved (Hesby, 2005). Oxidizing agents, or oxidants, used in water treatment include chlorine, chlorine dioxide, permanganate, oxygen, and ozone. The appropriate oxidant for achieving a specific water quality objective depends on a number of factors, including raw water quality, specific contaminants, and local chemical and power costs (Hesby, 2005). Chemical oxidation is a process involving the transfer of electrons from an oxidizing reagent to the chemical species being oxidized. In water and wastewater engineering, chemical oxidation serves the purpose of converting putrescible pollutant substances to innocuous or stabilized products. Chemical oxidation processes take place in natural waters and serve as an important mechanism in the natural self-purification of surface waters. Oxidative removal of dissolved iron and sulfide pollutants in aerated waters is a prominent example. The degradation of organic waste materials represents an even more important phenomenon associated with natural water self-purification. It is well known that the efficacy of natural water organic oxidations is due to the presence of microorganisms, which serve to catalyze a highly effective utilization of dissolved oxygen as an oxidant. In fact, such microorganism-catalyzed processes have been optimized and developed into the various forms of so-called “biological processes” in high concentration organic waste treatment applications (Shammas et al., 2005).

4.1.6.1. The concept of oxidation

Putrescible substances are known to comprise the most frequently occurring classes of pollutants in natural water systems. These substances have a most objectionable effect on water quality in that their decomposition often causes a depletion of dissolved oxygen in water. Dissolved oxygen is, in turn, essential to the existence of upper trophic aquatic organisms and is widely accepted as a most important indicator of the quality of a water system or its state of pollution. An analysis of oxygen balance in the aquatic environment shows that oxygen transfer from the atmosphere normally constitutes the most important oxygen source, whereas pollutional material consumption constitutes the major sinks. Chemical reactions giving rise to such consumption of oxygen are known as oxidation processes (Shammas et al., 2005). To account for proper materials balance, every chemical change of a specific nature must be accompanied by a process of opposite effect. Thus, the actual consumption of molecular oxygen is more properly termed as reduction, while the accompanying degradation of putrescible pollutants is defined as oxidation. In other words, oxidation and reduction must occur as coupled processes. The most functionally acceptable definition of oxidation–reduction is given in terms of electron transfer between reacting species (Shammas et al., 2005).

4.1.6.2. Hydrogen sulfide removal

Hydrogen sulfide is commonly found in well water, where it results in a distinctive rotten egg odor. As the water passes through the ground, it comes in contact with sulfates. If the water is highly mineralized or contains products of decomposition, these minerals and other substances will react with the sulfates and change them to hydrogen sulfide (H₂S). Surface waters rarely have hydrogen sulfide problems since the water is naturally aerated as it runs through streams (URL 3). Hydrogen sulfide gas turns into hydrosulfuric acid when it dissolves in water. The acid is weak but highly corrosive, eating up electrical contacts, causing a slight odor, and resulting in black water complaints. Water containing hydosulfuric acid will become very dark after remaining in the water lines for a few hours. The black water is most often noticed when flushing a fire hydrant (URL 3). The presence of larger quantities of hydrogen sulfide can be readily noted by odor. The disagreeable rotten egg odor is very characteristic of this gas and unless it is removed or reduced, the smell results in many complaints. As a result, even though H₂S gas in water is not injurious to people, it is usually removed when present (URL 3). There are three methods used for the removal of hydrogen sulfide. If there is a heavy concentration of the gas, the water should be aerated, allowing most of the gas to escape into the air. Aeration of hydrogen sulfide requires that the pH of the water first be lowered to 6 or less, and then the gas can be scrubbed away by aeration. The remaining gas (or lower concentrations of the gas) can be oxidized by chlorine. Alternatively, ozone can be used to convert hydrogen sulfide to sulfurous acid, but ozone is also corrosive so it may cause as many problems as it solves (URL 3).

4.1.6.3. Iron and manganese removal

Both iron (Fe) and manganese (Mn) are minerals, which can be found in water supplies. The minerals cause stains on on porcelain plumbing fixtures and laundry and cause coffee or tea to be cloudy and unpalatable. In addition, they can cause diarrhea (URL 3). Water containing iron and manganese will be clear when first discharged from a well. Upon exposure to air for several hours, the minerals oxidize and colored water results. The presence of oxidized iron causes water to be red and results in stains of the same color. Mn is a dark brown mineral and the resulting stains are dark brown or black (URL 3). The usual treatment to remove Fe and Mn from water is to oxidize the minerals as rapidly as possible and then to remove the oxidized material through filtration. Mn oxidizes and discolors water at a slower rate than iron, which affects the treatment method used for each mineral. In addition, pH affects the rate of oxidation for both minerals, so it is often necessary to change the pH of the water during treatment (URL 3). Dissolved Fe and Mn are normally in the reduced state (Fe II, Mn II) and can be removed by oxidizing to Fe III and Mn IV, where they will precipitate as Fe(OH)₃ and Mn(OH)₂. Precipitates are subsequently removed in sedimentation and/or filtration steps. Several oxidants are available for this process, namely, chlorine dioxide, ozone and potassium permanganate. They are also removed through conventional lime softening treatment (Hesby, 2005) In some cases the oxidation is accomplished entirely by the addition of chemicals. In other cases the water is first aerated, then an alkali is added to complete oxidation. The alkali optimizes the pH and uses the oxygen in the air to oxidize the iron and manganese. At the same time, the alkali reduces the carbon dioxide concentration in the water (URL 3).

4.1.6.4. Taste and odor removal

Most objectionable tastes and odors that occur in raw water, which particulary those of organic nature, can be mitigated by judicious application of a preoxidant. Surface waters in particular are prone to taste odor problems from presence of algae, other odor causing organisms and decaying vegetation (Hesby, 2005). The most well known and common odor causing compounds associated with algae, methyllisoborneol (MIB) and geosmin. Both are produced by actinomycetes and various blue-green algae and are particularly resistant to oxidation. Particulary tough applications may require both oxidation and adsorption step to lower tastes and odors to acceptable levels (Hesby, 2005).

4.1.7. Water Softening

Softening is the term given to the process of removing ions that interfere with the use of soap. These ions are called hardness ions due to the presence of multivalent cations, mostly calcium and magnesium. In natural waters, other ions that may be present to cause hardness but not insignificant amounts are Fe²⁺, Mn²⁺, Sr²⁺, Ba²⁺ and Al³⁺ (Sincero and Sincero, 2003d). Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water (URL 9). Hard waters are those waters that contain these hardness ions in excessive amounts (Sincero and Sincero, 2003d). The resulting soft water is more compatible with soap and extends the lifetime of plumbing. Water softening is usually achieved using lime softening or ion-exchange resins (URL 9).

4.1.7.1. Types of hardness

Hardness is generally expresses in terms of equivalent miligrams per liter of calcium carbonate. Total hardness is usually defined as simply the sum of magnesium and calcium hardness in mg CaCO₃/L. Total hardness can also be differentiated into carbonate and noncarbonate hardness. Carbonate hardness is the portion of total hardness present in the form of bicarbonate salts [Ca(HCO₃)₂ and Mg(CO₃)₂] and carbonate compounds (CaCO₃ and MgCO₃). Noncarbonate hardness is the portion of calcium and magnesium present as noncarbonate salts, such as calcium sulfate (CaSO₄), calcium chloride (CaCl₂), magnesium sulfate (MgSO₄), and magnesium chloride(MgCl₂) (Horsley et al., 2005). The following lists the general classification of hard waters (Sincero and Sincero, 2003c; Gottlieb, 2005): Soft <50 mg/L as CaCO₃ Moderately hard 75–150 mg/L as CaCO₃ Hard 150–300 mg/L as CaCO₃ Very hard > 300 mg/L as CaCO₃ A very soft water has a slimy feel. For example, rainwater, which is exceedingly soft, is slimy when used with soap. For this reason, hardness in water used for domestic purposes is not completely removed. Hardness is normally removed to the level of 75 to 120 mg/L as CaCO₃ (Sincero and Sincero, 2003d).

4.1.7.2. Problems with hard water

The presence of certain metal ions like Ca and Mg principally as bicarbonates, chlorides, and sulfates in water causes a variety of problems (URL 9).

• Limescale formation Hard water leads to the buildup of limescale, which can foul plumbing, and promote galvanic corrosion.^{[ 2]} In industrial scale water softening plants, the effluent flow from the re-generation process can precipitate scale that can interfere with sewage systems.

• Soap scum The slippery feeling experienced when using soap with soft water occurs because soaps tend to bind to fats in the surface layers of skin, making soap molecules difficult to remove by simple dilution. In contrast, in hard-water areas the rinse water contains calcium or magnesium ions which form insoluble salts, effectively removing the residual soap from the skin but potentially leaving a coating of insoluble stearates on tub and shower surfaces, commonly called soap scum. Which of these effects is considered more or less desirable varies from person to person, and those who dislike the sliminess and difficulty of washing off soap caused by soft water may harden the water by adding chemicals such as baking soda, calcium chloride or magnesium sulphate (URL 9).

4.1.7.3. Benefits of softening

Potential benefits of softening water at a central treatment plant include the following (Horsley et al., 2005):

• Reducing dissolved minerals and scale-forming tendencies
• Reducing consumption of household cleaning agents
• Removing radium 226 and 228
• Removing heavy metals, including arsenic
• Removing silica and fluoride
• Removing certain organic compounds and reducing total organic carbon (TOC)
• Removing iron and manganese
• Reducing turbidity of surface waters in conjunction with the hardness precipitation process The degree of removal of constituents usually depends on the treatment process used. Benefits to the consumer depend on source water quality and user requirements.

4.1.7.4. Water softening methods

The most common means for removing water hardness rely on ion-exchange polymers or reverse osmosis. Other approaches include precipitation methods and sequestration by the addition of chelating agents. Devices, which supposedly use magnetism or electrolysis as a water softening technique, claim to inhibit scale buildup without actually removing hardness ions from the water. Such devices have been marketed to consumers since the early 20th century, but are fraudulent (URL 9).

4.1.7.5. Plant types for hardness removal

In practice, two types of plants are generally used for chemical precipitation hardness removal: One type uses a sludge blanket contact mechanism to facilitate the precipitation reaction. The second type consists of a flash mix, a flocculation basin, and a sedimentation basin. The former is called a solids-contact clarifier. The latter arrangement of flash mix, flocculation, and sedimentation were discussed in previous chapters on unit operations. A solids-contact clarifier is shown in Figure 4.12. The chemicals are introduced into the primary mixing and reaction zone. Here, the fresh reactants are mixed by the swirling action generated by the rotor impeller and also mixed with a return sludge that are introduced under the hood from the clarification zone. The purpose of the return sludge is to provide nuclei that are important for the initiation of the chemical reaction. The mixture then flows up through the sludge blanket where secondary reaction and mixing occur. The reaction products then overflow into the clarification zone, where the clarified water is separated out by sedimentation of the reaction product solids. The clarified water finally overflows into the effluent discharge. The settled sludge from the clarification is drawn off through the sludge discharge pipe (Sincero and Sincero, 2003d).

Figure 4.12. Solids-contact clarifier (Courtesy of Infilco Degremont, Inc.) (URL 10).

4.1.7.5. Lime Soda Process

The chemicals soda ash and lime may be used for the removal of hardness caused by calcium and magnesium. Thus, the lime–soda process is used. This process, as mentioned, uses lime (CaO) and soda ash (Na₂CO₃). As the name of the process implies, two possible sets of chemical reactions are involved: the reactions of lime and the reactions of soda ash. To understand more fully what really is happening in the process, it is important to discuss these chemical reactions. Let us begin by discussing the lime reactions. CaO first reacts with water to form slaked lime, before reacting with the bicarbonate. (Sincero and Sincero, 2003d) Two types of solids are produced: Mg(OH)₂ and CaCO₃ and that the added calcium ion from the lime that would have produced an added hardness to the water has been removed as CaCO₃. Although the hardness ions have been precipitated out, the resulting solids, however, pose a problem of disposal in water softening plants. Magnesium, whether in the form of the carbonate or noncarbonate hardness, is always removed in the form of the hydroxide. Thus, to remove the total magnesium hardness, more lime is added to satisfy the overall stoichiometric requirements for both the carbonates and noncarbonates (Sincero and Sincero, 2003d). The implementation of the lime-soda process should be such that as much magnesium as possible is left unremoved and rely only on the removal of calcium to meet the desired treated water hardness. If the desired hardness level is not met by the removal of only the calcium ions, then removal of the magnesium may be initiated. This will entail the use of lime followed by the possible addition of soda ash to remove the resulting noncarbonate hardness of calcium. As noted before, the calcium ion is removed in the form of CaCO₃. This is the reason for the use of the second chemical known as soda ash for the removal of the noncarbonate hardness of calcium. It is worth repeating that soda ash is used for two purposes only: to remove the original calcium noncarbonate hardness in the raw water and to remove the by-product calcium noncarbonate hardness that results from the precipitation of the noncarbonate hardness of magnesium. It is also important to remember that by using lime the carbonate hardness of magnesium does not produce any calcium noncarbonate hardness. It is only the noncarbonate magnesium that is capable of producing the byproduct calcium noncarbonate hardness when lime is used (Sincero and Sincero, 2003d).