PHYSICAL - CHEMICAL TREATMENT OF WATER AND WASTEWATER - CHAPTER 12 ppsx

Coagulation Colloids are agglomerates of atoms or molecules whose sizes are so small that gravity has no effect on settling them but, instead, they stay in suspension. Because they stay in suspension, they are said to be stable. The reason for this stability is the mutual repulsion between colloid particles. They may, however, be destabilized by application of chemicals. Coagulation is the unit process of applying these chemicals for the purpose of destabilizing the mutual repulsion of the particles, thus causing the particles to bind together. This process is normally applied in conjunction with the unit operation of flocculation. The colloid particles are the cause of the turbidity and color that make waters objectionable, thus, should, at least, be partially removed. This chapter applies the techniques of the unit process of coagulation to the treatment of water and wastewater for the removal of colloids that cause turbidity and color. It also discusses prerequisite topics necessary for the understanding of coagulation such as the behavior of colloids, zeta potential, and colloid stability. It then treats the coagulation process, in general, and the unit process of the use of alum and the iron salts, in particular. It also discusses chemical requirements and sludge production. 12.1 COLLOID BEHAVIOR Much of the suspended matter in natural waters is composed of silica, or similar materials, with specific gravity of 2.65. In sizes of 0.1 to 2 mm, they settle rapidly; however, in the range of the order of 10 − 5 mm, it takes them a year, in the overall, to settle a distance of only 1 mm. And, yet, it is the particle of this size range that causes the turbidity and color of water, making the water objectionable. The removal of particles by settling is practical only if they settle rapidly in the order of several hundreds of millimeters per hour. This is where coagulation can perform its function, by destabilizing the mutual repulsions of colloidal particles causing them to bind together and grow in size for effective settling. Colloidal particles fall in the size range of 10 − 6 mm to 10 − 3 mm. They are aggregates of several hundreds of atoms or molecules, although a single molecule such as those of proteins is enough to be become a colloid. The term colloid comes from the two Greek words kolla , meaning glue, and eidos , meaning like. A colloid system is composed of two phases: the dispersed phase , or the solute , and the dispersion medium , or the solvent. Both of these phases can have all three states of matter which are solid, liquid, and gas. For example, the dispersion medium may be a liquid and the dispersed phase may be a solid. This system is called a liquid sol , an example of which is the turbidity in water. The dispersion medium may be a 12 TX249_frame_C12.fm Page 545 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 546 gas and dispersed phase may be solid. This system is called a gaseous sol , and examples are dust and smoke. Table 12.1 shows the different types of colloidal systems. Note that it is not possible to have a colloidal system of gas in a gas, because gases are completely soluble in each other. In the coagulation treatment of water and wastewater, we will be mainly interested in the solid being dispersed in water, the liquid sol or simply sol. Unless required for clarity, we will use the word ‘‘sol’’ to mean liquid sol. Sols are either lyophilic or lyophobic . Lyophilic sols are those that bind the solvent, while the lyophobic sols are those that do not bind the solvent. When the solvent is water, lyophilic and lyophobic sols are, respectively, called hydrophilic and hydrophobic sols . The affinity of the hydrophilic sols for water is due to polar functional groups that exist on their surfaces. These groups include such polar groups as − OH, − COOH, and − NH 2 . They are, respectively, called the hydroxyl, carboxylic , and amine groups. Figure 12.1a shows the schematic of a hydrophilic colloid. As portrayed, the functional polar groups are shown sticking out from the surface of the particle. Because of the affinity of these groups for water, the water is held tight on the surface. This water is called bound water and is fixed on the surface and moves with the particle. The hydrophobic colloids do not have affinity for water; thus, they do not contain any bound water. In general, inorganic colloids are hydrophobic, while organic colloids are hydrophilic. An example of an inorganic colloid is the clay particles that cause turbidity in natural water, and an example of an organic colloid is the colloidal particles in domestic sewage. 12.2 ZETA POTENTIAL The repulsive property of colloid particles is due to electrical forces that they possess. The characteristic of these forces is indicated in the upper half of Figure 12.1b. At a short distance from the surface of the particle, the force is very high. It dwindles down to zero at infinite distance from the surface. TABLE 12.1 Types of Colloidal Systems Dispersion Medium Dispersed Phase Common Name Example Solid Solid Solid sol Colored glass and gems, some alloys Solid Liquid Solid emulsion Jelly, gel, opal (SiO 2 and H 2 O), pearl (CaCO 3 and H 2 O) Solid Gas None Pumice, floating soap Liquid Solid Liquid sol Turbidity in water, starch suspension, ink, paint, milk of magnesia Liquid Liquid Liquid emulsion Oil in water, milk, mayonnaise, butter Liquid Gas Foam Whipped cream, beaten egg whites Gas Solid Gaseous sol Dust, smoke Gas Liquid Gaseous emulsion Mist, fog, cloud, spray Gas Gas Not applicable None TX249_frame_C12.fm Page 546 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 547 The electrical forces are produced due to the charges that the particles possess at their surfaces. These charges called primary charges are, in turn, produced from one or both of two phenomena: the dissociation of the polar groups and preferential adsorption of ions from the dispersion medium. The primary charges on hydrophobic colloids are due to preferential adsorption of ions from the dispersion medium. The primary charges on hydrophilic colloids are due chiefly to the polar groups such as the carboxylic and amine groups. The process by which the charges on these types of colloids are produced is indicated in Figure 12.2. The symbol R represents the colloid body. First, the colloid is represented at the top of the drawing, without the effect of pH. Then by a proper combination of the H + and OH − being added to the solution, the colloid attains ionization of both carboxylic and the amine groups. At this point, both ionized groups neutralize each other and the particle is neutral. This point is called the isoelectric point , and the corresponding ion of the colloid is called the zwitter ion . Increasing the pH by adding a base cause the added OH − to neutralize the acid end of the zwitter ion (the ); the zwitter ion disappears, and the whole particle becomes negatively charged. The reverse is true when the pH is reduced by the addition of an acid. The added H + neutralizes the base end of the zwitter ion (the COO − ); the zwitter ion disappears, and the whole particle becomes positively charged. From this discussion, a hydrophilic colloid can attain a primary charge of either negative or positive depending upon the pH. The primary charges on a colloid which, as we have seen, could either be positive or negative, attract ions of opposite charges from the solution. These opposite charges are called counterions . This is indicated in Figure 12.3. If the primary charges are FIGURE 12.1 (a) Hydrophilic colloid encased in bound water; (b) interparticle forces as a function of interparticle distance. (a) (b) Bound water Attraction Attraction Force Repulsion Repulsion Resultant Distance COOH COOH COOH HOOC HOOC NH 2 NH 2 NH 2 H 2 N H 2 N Colloid body NH 3 + TX249_frame_C12.fm Page 547 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 548 sufficiently large, the attracted counterions can form a compact layer around the primary charges. This layer is called the Stern layer . The counterions, in turn, can attract their own counterions, the coions of the primary charges, forming another layer. Since these coions form a continuous distribution of ions into the bulk of the solution, they tend to be diffused and form a diffused layer. The second layer is called the Gouy layer . Thus, the Stern and Gouy layers form an envelope of electric double layer around the primary charges. All of the charges in the Stern layer move with the colloid; thus, this layer is a fixed layer. In the Gouy layer, part of the layer may move with the colloid particle by shearing at a shear plane . This layer may shear off beyond the boundary of the fixed Stern layer measured from the surface of the colloid. Thus, some of the charges in the layer move with the particle, while others do not. This plane is indicated in Figure 12.3. The charges are electric, so they possess electrostatic potential. As indicated on the right-hand side of Figure 12.3, this potential is greatest at the surface and decreases to zero at the bulk of the solution. The potential at a distance from the surface at the location of the shear plane is called the zeta potential . Zeta potential meters are calibrated to read the value of this potential. The greater this potential, the greater is the force of repulsion and the more stable the colloid. 12.3 COLLOID DESTABILIZATION Colloid stability may further be investigated by the use Figure 12.1b. This figure portrays the competition between two forces at the surface of the colloid particle: the van der Waal’s force of attraction , represented by the lower dashed curve, and the force of repulsion, represented by the upper dashed curve. The solid curve represents the resultant of these two forces. As shown, this resultant becomes zero at a − a ′ and becomes fully an attractive force to the left of the line. When the resultant force becomes fully attractive, two colloid particles can bind themselves together. FIGURE 12.2 Primary charges of a hydrophilic colloid as a function of pH. COOH COOH NH 2 NH 3 + NH 3 + NH 3 OH COO – OH – COO – R pH RR R H + H, + OH – Isoelectric point TX249_frame_C12.fm Page 548 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 549 FIGURE 12.3 Charged double layer around a negatively charged colloid particle (left) and variation of electrostatic potential with distance from particle surface (right). Bulk of solution Diffuse layer Fixed layer Electronegative particle Shear plane Electrostatic potential Plane of shear Zeta potential Fixed layer Diffuse layer Solution bulk Distance from particle surface TX249_frame_C12.fm Page 549 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 550 Physical–Chemical Treatment of Water and Wastewater The force of repulsion, as we have seen, is due to the charges on the surface. Inherent in any body is a natural force that tends to bind particles together. This force is exactly the same as the force that causes adsorption of particles to an adsorbing surface. This is caused by the imbalance of atomic forces on the surface. Whereas atoms below the surface of a particle are balanced with respect to forces of neighboring atoms, those at the surface are not. Thus, the unbalanced force at the surface becomes the van der Waal’s force of attraction. By the presence of the primary charges that exert the repulsive force, however, the van der Waal’s force of attraction is nullified until a certain distance designated by a − a ′ is reached. The distance can be shortened by destabilizing the colloid particle. The use of chemicals to reduce the distance to a − a ′ from the surface of the colloid is portrayed in Figure 12.4. The zeta potential is the measure of the stability of colloids. To destabilize a colloid, its zeta potential must be reduced; this reduction is equivalent to the shortening of the distance to a − a ′ and can be accomplished through the addition of chemicals. The chemicals to be added should be the counterions of the primary charges. Upon addition, these counterions will neutralize the primary charges reducing the zeta potential. This process of reduction is indicated in Figures 12.4a and 12.4b; the potential is reduced in going from Figure 12.4a to 12.4b. Note that destabilization is simply the neutralization of the primary charges, thus reducing the force of repulsion between particles. The process is not yet the coagulation of the colloid. FIGURE 12.4 Reduction of zeta potential to cause destabilization of colloids. Fixed layer Fixed layer Diffuse layer Diffuse layer Zeta potential Zeta potential Electrostatic potential Plane of shear Distance from particle surface (a) Prior to addition of counter ions Distance from particle surface (b) After addition of counter ions TX249_frame_C12.fm Page 550 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 551 12.4 COAGULATION PROCESS The destabilization of colloids through the addition of counterions should be done in conjunction with the application of the complete coagulation process. Four meth- ods are used to bring about this process: double-layer compression, charge neutral- ization, entrapment in a precipitate, and intraparticle bridging. When the concentration of counterions in the dispersion medium is smaller, the thickness of the electric double layer is larger. Two approaching colloid particles cannot come close to each other because of the thicker electric double layer, there- fore, the colloid is stable. Now, visualize adding more counterions. When the con- centration is increased, the attracting force between the primary charges and the added counterions increases causing the double layer to shrink. The layer is then said to be compressed. As the layer is compressed sufficiently by the continued addition of more counterions, a time will come when the van der Waals force exceeds the force of repulsion and coagulation results. The charge of a colloid can also be directly neutralized by the addition of ions of opposite charges that have the ability to directly adsorb to the colloid surface. For example, the positively charged dodecylammoniun, C 12 H 25 , tends to be hydrophobic and, as such, penetrates directly to the colloid surface and neutralize it. This is said to be a direct charge neutralization, since the counterion has penetrated directly into the primary charges. Another direct charge neutralization method would be the use of a colloid of opposite charge. Direct charge neutralization and the compression of the double layer may compliment each other. A characteristic of some cations of metal salts such as Al(III) and Fe(III) is that of forming a precipitate when added to water. For this precipitation to occur, a colloidal particle may provide as the seed for a nucleation site, thus, entrapping the colloid as the precipitate forms. Moreover, if several of this particles are entrapped and are close to each other, coagulation can result by direct binding because of the proximity. The last method of coagulation is intraparticle bridging. A bridging molecule may attach a colloid particle to one active site and a second colloid particle to another site. An active site is a point in the molecule where particles may attach either by chemical bonding or by mere physical attachment. If the two sites are close to each other, coagulation of the colloids may occur; or, the kinetic movement may loop the bridge assembly around causing the attached colloids to bind because for now they are hitting each other, thus bringing out coagulation. 12.4.1 C OAGULANTS FOR THE C OAGULATION P ROCESS Electrolytes and polyelectrolytes are used to coagulate colloids. Electrolytes are materials which when placed in solution cause the solution to be conductive to electricity because of charges they possess. Polyelectrolytes are polymers possessing more than one electrolytic site in the molecule, and polymers are molecules joined together to form larger molecules. Because of the charges, electrolytes and poly- electrolytes coagulate and precipitate colloids. The coagulating power of electrolytes is summed up in the Schulze–Hardy rule that states: the coagulation of a colloid is affected by that ion of an added electrolyte that has a charge opposite in sign to NH 3 + TX249_frame_C12.fm Page 551 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 552 that of the colloidal particle; the effect of such an ion increases markedly with the number of charges carried. Thus, comparing the effect of AlCl 3 and Al 2 (SO 4 ) 3 in coagulating positive colloids, the latter is 30 times more effective than the former, since sulfate has two negative charges while the chloride has only one. In coagulating negative colloids, however, the two have about the same power of coagulation. The most important coagulants used in water and wastewater treatment are alum, copperas (ferrous sulfate), ferric sulfate, and ferric chloride. Later, we will specifi- cally discuss the chemical reactions of these coagulants at greater lengths. Other coagulants have also been used but, owing to high cost, their use is restricted only to small installations. Examples of these are sodium aluminate, NaAlO 2 ; ammonia alum, ; and potash alum, The reactions of sodium aluminate with aluminum sulfate and carbon dioxide are: (12.1) (12.2) 12.4.2 C OAGULANT A IDS 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. Acids and alkalis are used to adjust the pH to the optimum range. Typical acids used to lower the pH are sulfuric and phosphoric acids. Typical alkalis used to raise the pH are lime and soda ash. Polyelectrolytes are also used as coagulant aids. The cationic form has been used successfully in some waters not only as a coagulant aid but also as the primary coagulant. In comparison with alum sludges that are gelatinous and voluminous, sludges produced by using cationic polyelectrolytes are dense and easy to dewater for subsequent treatment and disposal. Anionic and nonionic polyelectrolytes are often used with primary metal coagulants to provide the particle bridging for effective coagulation. Generally, the use of poly- electrolyte coagulant aids produces tougher and good settling flocs. Activated silica and clays have also been used as coagulant aids. Activated silica is sodium silicate that has been treated with sulfuric acid, aluminum sulfate, carbon dioxide, or chlorine. When the activated silica is applied, a stable negative sol is produced. This sol unites with the positively charged primary-metal coagulant to produce tougher, denser, and faster settling flocs. Bentonite clays 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. Low turbidity waters are often hard to coagulate. Bentonite clay serves as a weighting agent that improves the settleability of the resulting flocs. 12.4.3 R APID M IX FOR C OMPLETE C OAGULATION Coagulation will not be as efficient if the chemicals are not dispersed rapidly throughout the mixing tank. This process of rapidly mixing the coagulant in the volume of the tank is called rapid or flash mix . Rapid mixing distributes the chemicals immediately throughout the volume of the mixing tank. Also, coagulation should Al 2 (SO 4 ) 3 (NH 4 ) 2 24H 2 O⋅⋅ Al 2 (SO 4 ) 3 K 2 SO 4 24H 2 O.⋅⋅ 6NaAlO 2 Al 2 + SO 4 () 3 14.3H 2 O 8Al OH() 3 3Na 2 SO 4 2.3H 2 O++→⋅ 2NaAlO 2 CO 2 3H 2 O++ 2Al OH() 3 ↓ Na 2 CO 3 +→ TX249_frame_C12.fm Page 552 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero be followed by flocculation to agglomerate the tiny particles formed from the coag- ulation process. If the coagulant reaction is simply allowed to take place in one portion of the tank because of the absence of the rapid mix rather than being spread throughout the volume, all four mechanisms for a complete coagulation discussed above will not be utilized. For example, charge neutralization will not be utilized in all portions of the tank because, by the time the coagulant arrives at the point in question, the reaction of charge neutralization will already have taken place somewhere. Interparticle bridging will not be as effective, since the force to loop the bridge around will not be as strong without the force of the rapid mix. Colloid particles will not effectively be utilized as seeds for nucleation sites because, without rapid mix, the coagulant may simply stay in one place. Finally, the compression of the double layer will not be as effective if unaided by the force due to the rapid mix. The force of the rapid mix helps push two colloids toward each other, thus enhancing coagulation. Hence, because of all these stated reasons, coagulation should take place in a rapidly mixed tank. 12.4.4 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. 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 coag- ulated 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. See Figure 12.5 for a picture of a jar testing apparatus. 12.5 CHEMICAL REACTIONS OF ALUM The alum used in water and wastewater treatment is Al 2 (SO 4 ) 3 ⋅ 14H 2 O. (The ‘‘14’’ actually varies from 13 to 18.) For brevity, this will simply be written without the water of hydration as Al 2 (SO 4 ) 3 . When alum is dissolved in water, it dissociates according to the following equation (Sincero, 1968): (12.3) By rapid mix, the ions must be rapidly dispersed throughout the tank in order to effect the complete coagulation process. Al 2 (SO 4 ) 3 2Al 3+ → 3SO 4 2− + TX249_frame_C12.fm Page 553 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero Because the water molecule is polar, it attracts Al 3+ forming a complex ion according to the following: (12.4) In the complex ion , Al is called the central atom and the molecules of H 2 O are called ligands. The subscript 6 is the coordination number, the number of ligands attached to the central atom; the superscript 3+ is the charge of the complex ion. The whole assembly of the complex forms what is called a coordination sphere. As indicated in Equation (12.4), aluminum has a coordination number of 6 with the water molecule. This means that no more water molecules can bind with the central atom but that any interaction would not be a mere insertion into the coordination sphere. In fact, further reaction with the water molecule involves hydrolysis of the water molecule and exchanging of the resulting OH − ion with the H 2 O ligand inside the coordination sphere. This type of reaction is called ligand exchange reaction. Some of the hydrolysis products of the ligand exchange reaction are mononu- clear, which means that only one central atom of aluminum is in the complex; and some are polynuclear, which means that more than one central atom of aluminum exists in the complex. Because the water molecule is not charged, may simply be written as Al 3+ . This is the symbol to be used in the complex reactions that follow. Without going into details, we will simply write at once all the complex ligand exchange equilibrium reactions. (12.5) (12.6) (12.7) FIGURE 12.5 A Phipps and Bird jar testing apparatus. (Courtesy of Phipps & Bird, Richmond, VA. © 2002 Phipps & Bird.) Al 3+ 6H 2 O+ Al(H 2 O) 6 3+ → Al(H 2 O) 6 3+ Al(H 2 O) 6 3+ Al 3+ H 2 O  Al(OH) 2+ H + ++ K Al(OH)c 10 −5 = 7Al 3+ 17H 2 O  Al 7 (OH) 17 4+ 17H + ++ K Al 7 (OH) 17 c 10 −48.8 = 13Al 3+ 34H 2 O  Al 13 (OH) 34 5+ 34H + ++ K Al 13 (OH) 34 c 10 −97.4 = TX249_frame_C12.fm Page 554 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero [...]... [Alopt]mg and [Alopt]geq be the milligrams per liter and gram equivalents per liter of optimum alum dose, respectively and let V be the cubic meters of water or wastewater treated Also, let MCaOkgeqAl and MCaOAl be the kilogram equivalents and kilogram mass of lime, respectively, used at a fractional purity of PCaO In the case of Ca(HCO3)2, the respective symbols are M Ca(HCO 3 )2 kgeqAl and M Ca(HCO... stand unrelated to Eqs (12. 55a), (12. 55b) As in the case of alum, it is impossible to determine the optimum dose using chemical reaction This value must be obtained through the jar test Let [FeIIopt]mg and [FeIIopt]geq be the milligrams per liter and gram equivalents per liter of optimum copperas dose, respectively, and let V be the cubic meters of water or wastewater treated Also, let MCaOkgeqFeII and. .. equivalent mass of lime = 3CaO/6 = 28.05 As determined by a jar test, let [FeIIIopt]mg and [FeIIIopt]geq be the milligrams per liter and gram equivalents per liter of optimum ferric salt dose, respectively, and let V be the cubic meters of water or wastewater treated Also, let MCaOkgeqFeIII and MCaOFeIII be the kilogram equivalents and kilogram mass of lime, respectively, used at a fractional purity of PCaO... w  γ Fe(OH) 3 c γ H  γ FeII K w   2 (12. 30) The value of [Hopt] may be solved by trial error 12. 7 CHEMICAL REACTIONS OF THE FERRIC ION The ferric salts used as coagulant in water and wastewater treatment are FeCl3 and Fe2(SO4)3 They have essentially the same chemical reactions in that both form the Fe(OH)3(s) solid When these coagulants are dissolved in water, they dissociate according to the following... [4.81(10 )] = 5.32 © 2003 by A P Sincero and G A Sincero –6 Ans 6 TX249_frame_C12.fm Page 560 Friday, June 14, 2002 2:29 PM 12. 6 CHEMICAL REACTIONS OF THE FERROUS ION The ferrous salt used as coagulant in water and wastewater treatment is copperas, FeSO4 ⋅ 7H2O For brevity, this will simply be written without the water of hydration as FeSO4 When copperas dissolves in water, it dissociates according to the... Maria and Jose are also friends Therefore, Pedro and Jose are friends Is this correct? No, because Pedro and Jose are, in reality, irreconcilable enemies Nonetheless, equivalent mass of CaCO3 = 50, no questions asked 12. 12 SLUDGE PRODUCTION Sludge is composed of solids and water in such a mixture that the physical appearance looks more of being composed of wet solids than being a concentrated water. .. way of expressing the alkalinity requirement is through the use of equivalents Using this method, the number of equivalents of the alum used is equal to the number of equivalents of the alkalinity required; in fact, it is equal to the same number of equivalents of any species participating in the chemical reaction All that is needed, therefore, is to find the number of equivalent masses of the alum and. .. same number of molecules of alum in the balanced chemical reaction as used in Equation (12. 46) should be used in Eqs (12. 47) and (12. 48); otherwise, the equivalent masses obtained are equivalent to each other From the reactions, the equivalent mass of lime (CaO) is 3CaO/6 = 28.05 and that of calcium carbonate is 3CaCO 3 /6 = 50 It is impossible to determine the optimum dose of alum using chemical reaction... of the ferric and the hydrogen ions and the complexes FeOH , Fe(OH) 2 , − 4+ Fe(OH) 4 , and Fe 2 (OH) 2 K sp, Fe(OH) 3 is the solubility product constant of the solid Fe(OH)3(s) and Kw is the ion product of water KFeOHc, K Fe(OH) 2 c , K Fe(OH) 4 c , and 2+ K Fe2 (OH) 2 c are, respectively, the equilibrium constants of the complexes FeOH , − 4+ Fe(OH) + , Fe(OH) 4 , and Fe 2 (OH) 2 2 Equations (12. 39)... 10 − − Fe ( OH ) 3 + – 14.5 (12. 22) – 9.4 K Fe ( OH )3 c = 10 (12. 23) – 5.1 (12. 24) − − The complexes are FeOH and Fe(OH) 3 Also note that the OH ion is a participant in these reactions This means that the concentrations of each of these complex ions are determined by the pH of the solution In the application of the above equations in an actual coagulation treatment of water, conditions must be adjusted . surface TX249_frame_C12.fm Page 549 Friday, June 14, 2002 2:29 PM © 2003 by A. P. Sincero and G. A. Sincero 550 Physical Chemical Treatment of Water and Wastewater The force of repulsion, as. in the actual operation of the plant. See Figure 12. 5 for a picture of a jar testing apparatus. 12. 5 CHEMICAL REACTIONS OF ALUM The alum used in water and wastewater treatment is Al 2 (SO 4 ) 3 . partially removed. This chapter applies the techniques of the unit process of coagulation to the treatment of water and wastewater for the removal of colloids that cause turbidity and color. It also discusses prerequisite