Adsorption and charge neutralization Adsorption and interparticle bridging dsorption and charge neutralization involves the adsorption of mononuclear and polynuclear metal hydrolysis species on the colloidal particles found in wastewater. It should be noted that it is also possible to get charge reversal with metal salts, as described previously with the addition of counterions Adsorption and interparticle bridging involves the adsorption of polynuclear metal hydrolysis species and olymer species which, in turn, will ultimately form particle-polymer bridges, as described previously. As the coagulant requirement for adsorption and charge neutralization is satisfied, metal hydroxide precipitates and soluble metal hydrolysis products will form. If a sufficient concentration of metal salt is added. large amounts of metal hydroxide floc will form. Following macroflocculation, large floc particles will be formed that will settle readily. In turn. as these floc particles settle, they sweep through the water ed from the wastewater. In most wastewater applications, the sweep floc mode of operation is used most commonly where particles are to be removed by sedimentation The sequence of reactions and events that occur in the one 1\Zon Definition sketch for the 80 coagulation and removal of ffects of the continued particles can be illustrated addition of a coagulant F (e.g, alum) on the 6-3. In zone 1. sufficient destabilization and Flocculation of colloidal added to destabilize the colloidal particles. even though some reduction ir surface charge may occur due to the presence of Fet and some mor ydrolysis species. In zone 2, the colloidal particles ave been destabilized by the adsorption of mono- and polynuclear hydrolysis species, and, if allowed to flocculate and settle, the residual turbidity would be lowered as shown. In zone 3. as more coagulant is added, the surface charge of the particles has reversed due to the continued adsorption of mono- and polynuclear hydrolySIs S ecies. As the colloidal particles are now positively charged. they cannot be d by perikinetic flocculation ched. where large amounts ill be removed sweep action of the settling floc particles. and the residual turbidity will be lowered as shown. The coagulant dosage required to reach any of the zones will depend on the nature of the colloidal particles and the pH and temperature of the wastewater. Specific constituents (e. g, organic matter) will also have an effect on the nt dose It is also very important to note that the example reaction sequence given and the coagulation process illustrated on Fig. 6-3 are time-dependent. For example, if it is desired to destabilize the colloidal particles in wastewater with mono- and polynuclear species, then rapid and intense initial mixing of the metal salt and the wastewater containing the particles to be destabilized is of critical importance. If the reaction is allowed to proceed to the formation of metal hydroxide floc, it will be difficult to contact the chemical and the particles. As discussed below, it has been estimated that the formation of the mono- and polynuclear and polymer hydroxide species occurs in a fraction of a second Solubility of Metal Salts. To further appreciate the action of the hydrolyzed metal ions, it will be useful to consider the solubility of the metal salts. The operating region for alum precipitation is from a ph range for ron precipitation with minimum solubilIty occurring at a ph of 8.0 Operating Regions for Action of Metal Salts. so p Because the chemistry of the various reactions Optimum there is no complete theory explain the action of hydrolyzed metal ions s: To quantify qualitatively the application of um as a tion of alum
6-6 1. Adsorption and charge neutralization 2. Adsorption and interparticle bridging 3. Enmeshment in sweep floc Adsorption and charge neutralization involves the adsorption of mononuclear and polynuclear metal hydrolysis species on the colloidal particles found in wastewater. It should be noted that it is also possible to get charge reversal with metal salts, as described previously with the addition of counterions. Adsorption and interparticle bridging involves the adsorption of polynuclear metal hydrolysis species and polymer species which, in turn, will ultimately form particle-polymer bridges, as described previously. As the coagulant requirement for adsorption and charge neutralization is satisfied, metal hydroxide precipitates and soluble metal hydrolysis products will form. If a sufficient concentration of metal salt is added, large amounts of metal hydroxide floc will form. Following macroflocculation, large floc particles will be formed that will settle readily. In turn, as these floc particles settle, they sweep through the water containing colloidal particles. The colloidal particles that become enmeshed in the floc will thus be removed from the wastewater. In most wastewater applications, the sweep floc mode of operation is used most commonly where particles are to be removed by sedimentation. The sequence of reactions and events that occur in the coagulation and removal of particles can be illustrated pictorially as shown on Fig. 6-3. In zone 1, sufficient coagulant has not been added to destabilize the colloidal particles, even though some reduction in surface charge may occur due to the presence of Fe3+ and some mononuclear hydrolysis species. In zone 2, the colloidal particles have been destabilized by the adsorption of mono- and polynuclear hydrolysis species, and, if allowed to flocculate and settle, the residual turbidity would be lowered as shown. In zone 3, as more coagulant is added, the surface charge of the particles has reversed due to the continued adsorption of mono- and polynuclear hydrolysis species. As the colloidal particles are now positively charged, they cannot be removed by perikinetic flocculation. As more coagulant is added, zone 4 is reached, where large amounts of hydroxide floc will form. As the floc particles settle, the colloidal particles will be removed by the sweep action of the settling floc particles, and the residual turbidity will be lowered as shown. The coagulant dosage required to reach any of the zones will depend on the nature of the colloidal particles and the pH and temperature of the wastewater. Specific constituents (e.g., organic matter) will also have an effect on the coagulant dose. It is also very important to note that the example reaction sequence given and the coagulation process illustrated on Fig. 6-3 are time-dependent. For example, if it is desired to destabilize the colloidal particles in wastewater with mono- and polynuclear species, then rapid and intense initial mixing of the metal salt and the wastewater containing the particles to be destabilized is of critical importance. If the reaction is allowed to proceed to the formation of metal hydroxide floc, it will be difficult to contact the chemical and the particles. As discussed below, it has been estimated that the formation of the mono- and polynuclear and polymer hydroxide species occurs in a fraction of a second. Solubility of Metal Salts. To further appreciate the action of the hydrolyzed metal ions, it will be useful to consider the solubility of the metal salts. The operating region for alum precipitation is from a pH range of 5 to about 7, with minimum solubility occurring at a pH of 6.0, and from about 7 to 9 for iron precipitation, with minimum solubility occurring at a pH of 8.0. Operating Regions for Action of Metal Salts. Because the chemistry of the various reactions is so complex, there is no complete theory to explain the action of hydrolyzed metal ions. To quantify qualitatively the application of alum as a function of pH, taking into account the action of alum as described above, Amirtharajah and Mills (1982) developed the Fig. 6-3
diagram shown on Fig. 6-4. Although Fig 6-4 was developed for water treatment applications. it has been found to apply reasonably well to most wastewater applications, with minor variations. As shown on Fig 6-4, the approximate regions in which the different phenomena associated with particle removal in conventional sedimentation and filtration processes are operative are plotted as a function of the alum dose and the ph of the treated effluent after alum has been added. For example. optimum particle removal sweep floc occurs in the pH range of 7 to 8 with an alum dose of 20 to 60 mg/L Fig. 6-4 Typical operating ranges for alum coagulation Generally. for many wastewater effluents that have high pH values(e, g. 7.3 to 8.5 extremely low alum dosages. Because the characteristics of wastewater will vary from treatment plant to treatment plant, bench-scale and pilot-plant tests must be conducted to establish the appropriate chemical Importance of Initial Chemical Mixing with Metal Salts. Perhaps the least appreciated fact about chemical addition of metal salts is the importance of the rapid initial mixing of the chemicals with the wastewater to be treated. They found that the rate-limiting step in the coagulation process was the time required for the colloidal transport step brought about by Brownian motion(ie, perikinetic flocculation) which was estimated to be on the order of 1.5 to 3. 3 10-s. Clearly, based on the literature and actual field evaluations, the instantaneous rapid and intense mixing of metal salts is of critical importance, especially where the metal salts are to be used as coagulants to lower the surface charge of the colloidal particles. It should be noted that although achieving extremely low mixing times in large treatment plants is often difficult, low mixing times can be achieved by using multiple mixers 6-3 Chemical Precipitation For Improved Plant Performance Chemical precipitation. as noted previously, involves the addition of chemicals to alter the physical state of dissolved and suspended solids and facilitate their removal by sedimentation. In the past, chemical precipitation was often used to enhance the degree of Tss and BOD removal: (1)where there were seasonal variations in the concentration of the wastewater(such as in cannery wastewater), (2)where an intermediate degree of treatment was required, and (3)as an aid to the sedimentation process. Since about 1970. the need to provide more complete removal of the organic compounds and nutrients(nitrogen and hosphorus) contained in wastewater has brought about renewed interest in chemical precipitation. In current practice, chemical precipitation is used (1) as a means of improving the performance of primary (2)as a basic step in the independent physical-chemical treatment of wastewater.(3)for Aside from the determination of the required chemical dosages, the principal design considerations related facilities and the selection and design of the chemical storage, feeding. piping and control systems essing to the use of chemical precipitation involve the analysis and design of the necessary sludge pro Chemical Reactions in Wastewater Precipitation Applications Over the years a number of different substances have been used as precipitants. The degree of clarification obtained depends on the quantity of chemicals used and the care with which the process is controlled. It is possible by chemical precipitation to obtain a clear effluent, substantially free from matter in suspension or in the colloidal state. The chemicals added to wastewater interact with substances that are either normally present in the wastewater or added for this purpose. The most common chemicals are listed in Table 6-2. The reactions involved with(1)alum,(2) lime, (3)ferrous sulfate(copperas)and lime, (4) ferric chloride, (5)ferric chloride and lime, and(6) ferric sulfate and lime are considered in the following discussion(Metcalf Eddy, 1935) Tab. 6-2 Inorganic chemicals used most commonly for coagulation and precipitation processes in wastewater treatment Chemical Formule Form Percent 17(A2O3 Al2SO314H° 594 17A2O Aluminum chloride 133.3 Liquid 63-73。sco 8599 Ferric chloride 162.2 20 (Fe) Ferric sulfate 515 Granular Ferrous sulfate Granular Sodium aluminate No Al,O. 163.9 100
6-7 diagram shown on Fig. 6-4. Although Fig. 6-4 was developed for water treatment applications, it has been found to apply reasonably well to most wastewater applications, with minor variations. As shown on Fig. 6-4, the approximate regions in which the different phenomena associated with particle removal in conventional sedimentation and filtration processes are operative are plotted as a function of the alum dose and the pH of the treated effluent after alum has been added. For example, optimum particle removal by sweep floc occurs in the pH range of 7 to 8 with an alum dose of 20 to 60 mg/L. Fig. 6-4 Typical operating ranges for alum coagulation Generally, for many wastewater effluents that have high pH values (e.g., 7.3 to 8.5), low alum dosages in the range of 5 to 10 mg/L will not be effective. With proper pH control it is possible to operate with extremely low alum dosages. Because the characteristics of wastewater will vary from treatment plant to treatment plant, bench-scale and pilot-plant tests must be conducted to establish the appropriate chemical dosages. Importance of Initial Chemical Mixing with Metal Salts. Perhaps the least appreciated fact about chemical addition of metal salts is the importance of the rapid initial mixing of the chemicals with the wastewater to be treated.They found that the rate-limiting step in the coagulation process was the time required for the colloidal transport step brought about by Brownian motion (i.e., perikinetic flocculation) which was estimated to be on the order of 1.5 to 3.3×10-3 s. Clearly, based on the literature and actual field evaluations, the instantaneous rapid and intense mixing of metal salts is of critical importance, especially where the metal salts are to be used as coagulants to lower the surface charge of the colloidal particles. It should be noted that although achieving extremely low mixing times in large treatment plants is often difficult, low mixing times can be achieved by using multiple mixers. 6-3 Chemical Precipitation For Improved Plant Performance Chemical precipitation, as noted previously, involves the addition of chemicals to alter the physical state of dissolved and suspended solids and facilitate their removal by sedimentation. In the past, chemical precipitation was often used to enhance the degree of TSS and BOD removal: (1) where there were seasonal variations in the concentration of the wastewater (such as in cannery wastewater), (2) where an intermediate degree of treatment was required, and (3) as an aid to the sedimentation process. Since about 1970, the need to provide more complete removal of the organic compounds and nutrients (nitrogen and phosphorus) contained in wastewater has brought about renewed interest in chemical precipitation. In current practice, chemical precipitation is used (1) as a means of improving the performance of primary settling facilities, (2) as a basic step in the independent physical-chemical treatment of wastewater, (3) for the removal of phosphorus, and (4) for the removal of heavy metals. Aside from the determination of the required chemical dosages, the principal design considerations related to the use of chemical precipitation involve the analysis and design of the necessary sludge processing facilities, and the selection and design of the chemical storage, feeding, piping, and control systems. Chemical Reactions in Wastewater Precipitation Applications Over the years a number of different substances have been used as precipitants. The degree of clarification obtained depends on the quantity of chemicals used and the care with which the process is controlled. It is possible by chemical precipitation to obtain a clear effluent, substantially free from matter in suspension or in the colloidal state. The chemicals added to wastewater interact with substances that are either normally present in the wastewater or added for this purpose. The most common chemicals are listed in Table 6-2. The reactions involved with (1) alum, (2) lime, (3) ferrous sulfate (copperas) and lime, (4) ferric chloride, (5) ferric chloride and lime, and (6) ferric sulfate and lime are considered in the following discussion (Metcalf & Eddy, 1935). Tab. 6-2 Inorganic chemicals used most commonly for coagulation and precipitation processes in wastewater treatment