6 Chemical Unit Processes 6-1 Role of Chemical Unit Processes in Wastewater Treatment The principal chemical unit processes used for wastewater treatment include(1)chemical coagulation, (2) chemical precipitation,(3)chemical disinfection,(4)chemical oxidation,(5)advanced oxidation processes,(6)ion exchange, and(7)chemical neutralization, scale control, and stabilization application of Chemical Unit Processes Applications of chemical unit processes for the management and treatment of wastewater are reported in Table 6-1 Tab. 6-1 Applications of chemical unit process in wastewater treatment Application Advanced oxidation processes Removal of refractory organic compounds Chemical coagulation The chemical destabilization of p an wastewater to bring about their aggregation during perikinetic and orthokinetic Flocculation Chemical disinfection Disinfection with chlorine, chlorine compounds, bromine, and。zone Control of slime growths in sewers Control of odors Chemical neutralization Control of pH Chemical oxidation Removal of BOD, grease, etc. Removal of ammonia(NH:) estruction of microorganisms pump stations, and treatment plants Removal of resistant organic compounds Chemical precipitation Enhancement removal of total suspended solids and BOD i rimary sedimentation facilities emoval of phosphorus Removal of heavy metals Physical-chemical treatment Corrosion control in sewers due to has Chemical scale control Chemical stabilization Stabilization of treated effluents lon exchang emoval of ammonia (NH4), heavy metals, tot solved solids emoval of organic compound Chemical processes, in conjunction with various physical operations, have been developed for the complete secondary treatment of untreated(raw)wastewater, including the removal of either nitrogen phosphorus or both. Chemical Fig. 6-1 have also beer Typical lime clarification developed remove facilities following phosphorus by chemical secondary treatment used precipitation, and are designed pretreatment step for advanced treatment of to be used in conjunction with wastewater using reverse biological treatment Other osmosis. Lime storage is chemical processes have been in the silo shown behind developed for the removal of building that is used eavy metals and for ific to house the lime slaking acilities and the reverse organic compounds and for the mosis units, used for advanced treatment odvanced treatment wastewater. Currently the most Granular medium-depth ant filters are shown to ight of the lime clarifier wastewater treatment are for(1) the disinfection of wastewater. 2)the precipitation of phosphorus and (3) the coagulation of particulate matter found in wastewater at 6-1
6-1 6 Chemical Unit Processes 6-1 Role Of Chemical Unit Processes In Wastewater Treatment The principal chemical unit processes used for wastewater treatment include (1) chemical coagulation, (2) chemical precipitation, (3) chemical disinfection, (4) chemical oxidation, (5) advanced oxidation processes, (6) ion exchange, and (7) chemical neutralization, scale control, and stabilization. Application of Chemical Unit Processes Applications of chemical unit processes for the management and treatment of wastewater are reported in Table 6-1. Tab. 6-1 Applications of chemical unit process in wastewater treatment Chemical processes, in conjunction with various physical operations, have been developed for the complete secondary treatment of untreated (raw) wastewater, including the removal of either nitrogen or phosphorus or both. Chemical processes have also been developed to remove phosphorus by chemical precipitation, and are designed to be used in conjunction with biological treatment. Other chemical processes have been developed for the removal of heavy metals and for specific organic compounds and for the advanced treatment of wastewater. Currently the most important applications of chemical unit processes in wastewater treatment are for (1) the disinfection of wastewater, (2) the precipitation of phosphorus, and (3) the coagulation of particulate matter found in wastewater at Fig. 6-1
varIous st stages in the treatment process(see Fig. 6-1) Considerations in the Use of chemical Unit processes In considering the application of the chemical unit processes to be discussed in this chapter. it is important to remember that one of the inherent disadvantages associated with most chemical unit processes. as compared with the physical unit operations. is that they are additive processes(i.e. something is added to the wastewater to achieve the removal of something else). As a result, there is usually a net increase in the dissolved constituents in the wastewater. For example, where chemicals are added to enhance the removal he total dissolved solids(TDS)concentration of the is added to the treated wastewater is to be reused the increase in dissolved constituents can be a significant factor This additive aspect is in contrast to the physical unit operations and the biological unit processes. which may be described as being subtractive. in that wastewater constituents are removed from the wastewater. A on processes is the unit processes is that the cost of 6-2 Fundamentals Of Chemical Coagulation about 0. 01 to l u m and is such that the attractive body forces between particles are considerably less than the repelling forces of the electrical charge. Under these stable conditions, Brownian motion keeps the particles in suspension. Brownian motion (i.e. random movement) is brought about by the constant thermal bombardment of the colloidal particles by the relatively small water molecules that surround them. Coagulation is the process of destabilizing colloidal particles so that n occur as a I of particle collisions. Coagulation reactions are often incomplete, and numerous side reactions with other substances in wastewater may take place depending on the characteristics of the waste water which will vary throughout the day as well as seasonally. To introduce the subject of chemical coagulation the following topics are discussed in this section: (1) basic definitions for coagulation and flocculation, (2)the nature of particles in wastewater, (3) the development and measurement of surface charge, (4)consideration of particle-particle interaction, (5)particle destabilization with potential determinations and electrolytes, (6) particle destabilization and aggregation with polyelectrolytes, and(7) particle destabilization and removal with hydrolyzed metal ions Basic definitions The term "chemical involved in the chemical destabilization of particles and in the formation of larger particles through flocculent are terms that will also be encountered in the literature on coagulation In general. a coagulant is at is added to destabilize flocculent is a chemical, typically organic added to enhance coagulants and flocculants include natural and synthetic organic polymers. metal salts such as alum or ferric sulfate and prehydrolized metal salts such as polyaluminum chloride(Pacd and polviron chloride (PICD. Flocculants, especially organic polvmers. are also used to enhance the performance of granular medium filters and in the dewatering of digested biosolids. In these applications. the flocculant chemicals are often identified as filter aids The term"flocculation"is used to describe the process whereby the size of particles increases as a result of particle collisions. There are two types of flocculation: (1)microflocculation(also known as perikinetic flocculation), in which particle aggregation is brought about by the random thermal motion of fluid molecules known as Brownian motion or movement and(2)macroflocculation (also known as and mixing in the fluid containing the particles to be flocculated. Another form of macroflocculation is brought about by differential settling in which large particles overtake small particles to form larger particles. The purpose of flocculation is to produce particles. by means of aggregation, that can be removed by inexpensive particle-separation procedures such as gravity sedimentation and filtration. Macro-flocculation is ineffectual until the colloidal particles reach a size of l to 10um through contacts produced by Brownian motion and gentle mixing Nature of Particles in Wastewater The particles in wastewater may, for practical purposes, be classified as suspended and colloidal Suspended particles are generally larger than 1.0 u m and can be removed by gravity sedimentation. In 6-2
6-2 various stages in the treatment process (see Fig. 6-1). Considerations in the Use of Chemical Unit Processes In considering the application of the chemical unit processes to be discussed in this chapter, it is important to remember that one of the inherent disadvantages associated with most chemical unit processes, as compared with the physical unit operations, is that they are additive processes (i.e., something is added to the wastewater to achieve the removal of something else). As a result, there is usually a net increase in the dissolved constituents in the wastewater. For example, where chemicals are added to enhance the removal efficiency of particulate sedimentation, the total dissolved solids (TDS) concentration of the wastewater is always increased. Similarly, when chlorine is added to wastewater, the TDS of the effluent is increased. If the treated wastewater is to be reused, the increase in dissolved constituents can be a significant factor. This additive aspect is in contrast to the physical unit operations and the biological unit processes, which may be described as being subtractive, in that wastewater constituents are removed from the wastewater. A significant disadvantage of chemical precipitation processes is the handling, treatment, and disposal of the large volumes of sludge that is produced. Another disadvantage of chemical unit processes is that the cost of most chemicals is related to the cost of energy. 6-2 Fundamentals Of Chemical Coagulation Colloidal particles found in wastewater typically have a net negative surface charge. The size of colloids (about 0.01 to 1μm and is such that the attractive body forces between particles are considerably less than the repelling forces of the electrical charge. Under these stable conditions, Brownian motion keeps the particles in suspension. Brownian motion (i.e., random movement) is brought about by the constant thermal bombardment of the colloidal particles by the relatively small water molecules that surround them. Coagulation is the process of destabilizing colloidal particles so that particle growth can occur as a result of particle collisions. Coagulation reactions are often incomplete, and numerous side reactions with other substances in wastewater may take place depending on the characteristics of the wastewater which will vary throughout the day as well as seasonally. To introduce the subject of chemical coagulation the following topics are discussed in this section: (1) basic definitions for coagulation and flocculation, (2) the nature of particles in wastewater, (3) the development and measurement of surface charge, (4) consideration of particle-particle interaction, (5) particle destabilization with potential determinations and electrolytes, (6) particle destabilization and aggregation with polyelectrolytes, and (7) particle destabilization and removal with hydrolyzed metal ions. Basic Definitions The term "chemical coagulation" as used in this text includes all of the reactions and mechanisms involved in the chemical destabilization of particles and in the formation of larger particles through perikinetic flocculation (aggregation of particles in the size range from 0.01 to 1μm). Coagulant and flocculent are terms that will also be encountered in the literature on coagulation. In general, a coagulant is the chemical that is added to destabilize the colloidal particles in wastewater so that floc formation can result. A flocculent is a chemical, typically organic, added to enhance the flocculation process. Typical coagulants and flocculants include natural and synthetic organic polymers, metal salts such as alum or ferric sulfate, and prehydrolized metal salts such as polyaluminum chloride (PACl) and polyiron chloride (PIC1). Flocculants, especially organic polymers, are also used to enhance the performance of granular medium filters and in the dewatering of digested biosolids. In these applications, the flocculant chemicals are often identified as filter aids. The term "flocculation" is used to describe the process whereby the size of particles increases as a result of particle collisions.There are two types of flocculation: (1) microflocculation (also known as perikinetic flocculation), in which particle aggregation is brought about by the random thermal motion of fluid molecules known as Brownian motion or movement and (2) macroflocculation (also known as orthokinetic flocculation), in which particle aggregation is brought about by inducing velocity gradients and mixing in the fluid containing the particles to be flocculated. Another form of macroflocculation is brought about by differential settling in which large particles overtake small particles to form larger particles. The purpose of flocculation is to produce particles, by means of aggregation, that can be removed by inexpensive particle-separation procedures such as gravity sedimentation and filtration. Macro-flocculation is ineffectual until the colloidal particles reach a size of 1 to 10μm through contacts produced by Brownian motion and gentle mixing. Nature of Particles in Wastewater The particles in wastewater may, for practical purposes, be classified as suspended and colloidal. Suspended particles are generally larger than 1.0 μm and can be removed by gravity sedimentation. In
practice, the distinction between colloidal and suspended particles is blurred because the particles removed by gravity settling will depend on the design of the sedimentation facilities. Because colloidal use of chemical coagulants and flocculant aids) must be used to help bring about the removal of these nderstand the role that chemical coagulants and flocculant aids play in bringing about the removal of colloidal particles, it is important to understand the characteristics of the colloidal particles found in wastewater, important factors that contribute to the characteristics of colloidal particles in wastewater include(1) particle size and number, (2) particle shape and flexibility, (3)surface properties including electrical characteristics, (4)particle-particle interactions, and (5) particle-solvent interactions. Particle size, particle shape and flexibilitv. and particle-solvent interactions are considered below. Because of their importance, the development and measurement of surface charge and particle-particle interactions are considered separately Particle Size and Number. The size of colloidal particles in wastewater considered in this text is pically in the range from 0.01 to 1.0 W m. As noted in Chap. 2, some researchers have classified the size range for colloidal particles as varying from 0.001 to l u m. The number of colloidal particles in untreated wastewater and after primary sedimentation is typically in the range from 10 to 10 2/mL. It is important to within a treatment plant The number of particles, as will be discussed later, is of importance with respect to the method to be used for their removal Particle Shape and Flexibility. Particle shapes found in wastewater can be described as spherical. semispherical. ellipsoids of various shapes(e. g. prolate and oblate) rods of v d diameter (e.g. E. coli. disk and disklike. strings of various lengths, and random coils. Large organic molecules are often found in the form of coils which may be compressed, uncoiled, or almost linear. The shape of some larger floc particles is often described as fractal. The particle shape will vary depending on the location within the treatment process that is being evaluated. The shape of the particles will affect the electrical properties. the particle-particle interactions, and particle. of particles encountered in wastewater. the theoretical treatment of particle-particle interactions is an Particle-Solvent Interactions. There are three general types of colloidal particles in liquids hydrophobic or"water-hating "hydrophilic or"water-loving "and association colloids. The first two types are based on the attraction of the particle surface for water. Hydrophobic particles have relatively little attraction for water; while hydrophilic particles have a great attraction for water. It should be noted, owever,that water can interact to some extent even with hydrophobic particles. Some water molecules will generally adsorb on the typical hydrophobic surface, but the reaction between water and hydrophilic colloids occurs to a much greater extent. The third type of colloid is known as an association colloidal. typically made up of surface-active agents such as soaps, svnthetic detergents. and dyestuffs which form organized aggregates Development and Measurement of Surface Charge An important factor in the stability of colloids is the presence of a surface charge. It develops in a number of different ways, depending on the chemical composition of the medium(wastewater in this case) and the nature of the colloid. Surface charge develops most commonly through(D) isomorphous replacement. (2 structural imperfections.(3)preferential adsorption, and( 4) ionization, as defined below. Regardless of ow it develops, the surface charge, which promotes stability, must be overcome if these particles are to be aggregated(flocculated into larger particles with enough mass to settle easily Isomorphous Replacement. Charge development through isomorphous replacement occurs in clay and other soil particles. in which ions in the lattice structure are replace replacement of Si with Al Structural Imperfections. In clay and similar particles, charge development can occur because of broken bonds on the crystal edge and imperfections in the formation of the crystal Preferential Adsorption. dispersed in water they will acquire a negative charge th (particularly hydroxyl ions lonization. In the case of substances such as proteins or microorganisms. surface charge is acquired through the ionization of carboxyl and amino groups. The Electrical Double Layer. When the colloid or particle surface becomes charged, some ions of the opposite charge(known as counterions) become attached to the surface. They are held there through electrostatic and van der Waals forces of attraction strongly enough to overcome thermal agitation
6-3 practice, the distinction between colloidal and suspended particles is blurred because the particles removed by gravity settling will depend on the design of the sedimentation facilities. Because colloidal particles cannot be removed by sedimentation in a reasonable period of time, chemical methods (i.e., the use of chemical coagulants and flocculant aids) must be used to help bring about the removal of these particles. To understand the role that chemical coagulants and flocculant aids play in bringing about the removal of colloidal particles, it is important to understand the characteristics of the colloidal particles found in wastewater, important factors that contribute to the characteristics of colloidal particles in wastewater include (1) particle size and number, (2) particle shape and flexibility, (3) surface properties including electrical characteristics, (4) particle-particle interactions, and (5) particle-solvent interactions. Particle size, particle shape and flexibility, and particle-solvent interactions are considered below. Because of their importance, the development and measurement of surface charge and particle-particle interactions are considered separately. Particle Size and Number. The size of colloidal particles in wastewater considered in this text is typically in the range from 0.01 to 1.0 μm. As noted in Chap. 2, some researchers have classified the size range for colloidal particles as varying from 0.001 to 1μm. The number of colloidal particles in untreated wastewater and after primary sedimentation is typically in the range from 106 to 1012/mL. It is important to note that the number of colloidal particles will vary depending on the location where the sample is taken within a treatment plant. The number of particles, as will be discussed later, is of importance with respect to the method to be used for their removal. Particle Shape and Flexibility. Particle shapes found in wastewater can be described as spherical, semispherical, ellipsoids of various shapes (e.g., prolate and oblate), rods of various length and diameter (e.g., E. coli), disk and disklike, strings of various lengths, and random coils. Large organic molecules are often found in the form of coils which may be compressed, uncoiled, or almost linear. The shape of some larger floc particles is often described as fractal. The particle shape will vary depending on the location within the treatment process that is being evaluated. The shape of the particles will affect the electrical properties, the particle-particle interactions, and particle-solvent interactions. Because of the many shapes of particles encountered in wastewater, the theoretical treatment of particle-particle interactions is an approximation at best. Particle-Solvent Interactions. There are three general types of colloidal particles in liquids: hydrophobic or "water-hating," hydrophilic or "water-loving," and association colloids. The first two types are based on the attraction of the particle surface for water. Hydrophobic particles have relatively little attraction for water; while hydrophilic particles have a great attraction for water. It should be noted, however, that water can interact to some extent even with hydrophobic particles. Some water molecules will generally adsorb on the typical hydrophobic surface, but the reaction between water and hydrophilic colloids occurs to a much greater extent. The third type of colloid is known as an association colloidal, typically made up of surface-active agents such as soaps, synthetic detergents, and dyestuffs which form organized aggregates known as micelles. Development and Measurement of Surface Charge An important factor in the stability of colloids is the presence of a surface charge. It develops in a number of different ways, depending on the chemical composition of the medium (wastewater in this case) and the nature of the colloid. Surface charge develops most commonly through (1) isomorphous replacement, (2) structural imperfections, (3) preferential adsorption, and (4) ionization, as defined below. Regardless of how it develops, the surface charge, which promotes stability, must be overcome if these particles are to be aggregated (flocculated) into larger particles with enough mass to settle easily. Isomorphous Replacement. Charge development through isomorphous replacement occurs in clay and other soil particles, in which ions in the lattice structure are replaced with ions from solution (e.g., the replacement of Si4+ with Al3+). Structural Imperfections. In clay and similar particles, charge development can occur because of broken bonds on the crystal edge and imperfections in the formation of the crystal. Preferential Adsorption. When oil droplets, gas bubbles, or other chemically inert substances are dispersed in water, they will acquire a negative charge through the preferential adsorption of anions (particularly hydroxyl ions). Ionization. In the case of substances such as proteins or microorganisms, surface charge is acquired through the ionization of carboxyl and amino groups. The Electrical Double Layer. When the colloid or particle surface becomes charged, some ions of the opposite charge (known as counterions) become attached to the surface. They are held there through electrostatic and van der Waals forces of attraction strongly enough to overcome thermal agitation
Surrounding this fixed layer of ions is a diffuse layer of ions Measurement of Surface Potential. If a particle is placed in an electrolyte solution, and an electr current is passed through the solution, the particle, depending on its surface charge, will be attracted to one or the other of the electrodes, dragging with it a cloud of ions. The potential at the surface of the cloud (called the surface of shear) is sometimes measured in wastewater-treatment operations. The measured value is often called the zeta potential. Theoretically, however, the zeta potential should correspond to the potential measured at the surface enclosing the fixed layer of ions attached to the particle. The use of the measured zeta potential value is limited because it will vary with the nature of the solution components Particle-Particle Interactions Particle-particle interactions are extremely important in bringing about aggregation by means of Brownian motion. The two principal forces involved are the forces of repulsion, due to the electrical properties of the charged plates, and the van der Waals forces of attraction. It should be noted that the van der Waals forces of attraction do not come into play until the two plates are brought together in close proximity to each The net total energy shown is the difference between the for he for attraction will predominate at short and long distances. The net energy curve contains a repulsive maximum that must be overcome if the particles, represented as the two plates, are to be held together by the van der Waals force of attraction. There is no energy barrier to overcome. Clearlv. if colloidal particles are to be removed by microflocculation. the repulsive force must be reduced. Although floc particles can form at long distances as shown by the net energy curve for condition 1, the net force holding these particles together is weak and the floc particles that are formed can be raptured easily Particle Destabilization with Potential-Determining lons and Electrolytes To bring about particle aggregation through microflocculation, steps must be taken to reduce particle charge or to overcome the effect of this charge. The effect of the charge can be overcome by(1) the addition of potential-determining ions, which will be taken up by or will react with the colloid surface to lessen the surface charge and(2)the addition of electrolytes, which have the effect of reducing the thickness of the diffuse electric laver and, thereby reduce the zeta potential Use of Potential-Determining lons. The addition of potential-determining ions to promote coagulation can be illustrated by the addition of strong acids or bases to reduce the charge of metal oxides hydroxides to near zero so that coagulation can occur. The magnitude of the effect will depend on the concentration of potential-determining ions added. It is interesting to note that depending on the concentration and nature of the counterions added it is possible to reverse the charge of the double laver he use of potential determining ions is not feasible in either water or wastewater treatment because of the massive concentration of ions that must be added to bring about sufficient compression of the electrical double laver to effect perikinetic flocculation. Use of Electrolytes. Electrolytes can also be added to coagulate colloidal suspensions. Increased concentration of an electrolyte that is needed to destabilize a colloidal suspension is nown as the critical coagulation concentration(CCC). Increasing the concentration of an indifferent electrolyte will not result in the restabilization of the colloidal particles Particle Destabilization and Aggregation with Polyelectrolytes Polyelectrolytes may be divided into two categories: natural and synthetic. Important natural polvelectrolvtes include polvmers of biological origin and those derived from starch products such as cellulose derivatives and alginates. Synthetic polyelectrolytes consist of simple monomers that are polymerized into high-molecular-weight substances, Depending on whether their charge. when placed in ater. is negative, positive, or neutral. these polyelectrolytes are classified as anionic, cationic, and Charge Neutralization In the first category polyelectrolytes act as coagulants that neutralize or lower the charge of the wastewater particles. Because wastewater particles normally are charged negatively. cationic polyelectrolytes are used for this purpose. In this application, the cationic polyelectrolytes are considered to be primary coagulants. To effect charge neutralization, the polyelectrolyte must be adsorbed to the particle. Because of the large number of particles found in wastewater. the mixing intensity mus sufficient to bring about the adsorption of the polvmer onto the colloidal particles. With inadequate mixing the polymer will eventually fold back on itself and its effectiveness in reducing the surface charge will be if the number of colloidal particles is limited. it will be difficult to remove them with Polymer Bridge Formation. The second mode of action of polyelectrolytes is interparticle bridging. In
6-4 Surrounding this fixed layer of ions is a diffuse layer of ions. Measurement of Surface Potential. If a particle is placed in an electrolyte solution, and an electric current is passed through the solution, the particle, depending on its surface charge, will be attracted to one or the other of the electrodes, dragging with it a cloud of ions. The potential at the surface of the cloud (called the surface of shear) is sometimes measured in wastewater-treatment operations. The measured value is often called the zeta potential. Theoretically, however, the zeta potential should correspond to the potential measured at the surface enclosing the fixed layer of ions attached to the particle. The use of the measured zeta potential value is limited because it will vary with the nature of the solution components. Particle-Particle Interactions Particle-particle interactions are extremely important in bringing about aggregation by means of Brownian motion. The two principal forces involved are the forces of repulsion, due to the electrical properties of the charged plates, and the van der Waals forces of attraction. It should be noted that the van der Waals forces of attraction do not come into play until the two plates are brought together in close proximity to each other. The net total energy shown is the difference between the forces of repulsion and attraction. The forces of attraction will predominate at short and long distances. The net energy curve contains a repulsive maximum that must be overcome if the particles, represented as the two plates, are to be held together by the van der Waals force of attraction. There is no energy barrier to overcome. Clearly, if colloidal particles are to be removed by microflocculation, the repulsive force must be reduced. Although floc particles can form at long distances as shown by the net energy curve for condition 1, the net force holding these particles together is weak and the floc particles that are formed can be raptured easily. Particle Destabilization with Potential-Determining Ions and Electrolytes To bring about particle aggregation through microflocculation, steps must be taken to reduce particle charge or to overcome the effect of this charge. The effect of the charge can be overcome by (1) the addition of potential-determining ions, which will be taken up by or will react with the colloid surface to lessen the surface charge and (2) the addition of electrolytes, which have the effect of reducing the thickness of the diffuse electric layer and, thereby, reduce the zeta potential. Use of Potential-Determining Ions. The addition of potential-determining ions to promote coagulation can be illustrated by the addition of strong acids or bases to reduce the charge of metal oxides or hydroxides to near zero so that coagulation can occur. The magnitude of the effect will depend on the concentration of potential-determining ions added. It is interesting to note that depending on the concentration and nature of the counterions added, it is possible to reverse the charge of the double layer and develop a new stable particle. The use of potential determining ions is not feasible in either water or wastewater treatment because of the massive concentration of ions that must be added to bring about sufficient compression of the electrical double layer to effect perikinetic flocculation. Use of Electrolytes. Electrolytes can also be added to coagulate colloidal suspensions. Increased concentration of a given electrolyte will cause a decrease in zeta potential and a corresponding decrease in repulsive forces. The concentration of an electrolyte that is needed to destabilize a colloidal suspension is known as the critical coagulation concentration (CCC). Increasing the concentration of an indifferent electrolyte will not result in the restabilization of the colloidal particles. Particle Destabilization and Aggregation with Polyelectrolytes Polyelectrolytes may be divided into two categories: natural and synthetic. Important natural polyelectrolytes include polymers of biological origin and those derived from starch products such as cellulose derivatives and alginates. Synthetic polyelectrolytes consist of simple monomers that are polymerized into high-molecular-weight substances. Depending on whether their charge, when placed in water, is negative, positive, or neutral, these polyelectrolytes are classified as anionic, cationic, and nonionic, respectively. Charge Neutralization. In the first category, polyelectrolytes act as coagulants that neutralize or lower the charge of the wastewater particles. Because wastewater particles normally are charged negatively, cationic polyelectrolytes are used for this purpose. In this application, the cationic polyelectrolytes are considered to be primary coagulants. To effect charge neutralization, the polyelectrolyte must be adsorbed to the particle. Because of the large number of particles found in wastewater, the mixing intensity must be sufficient to bring about the adsorption of the polymer onto the colloidal particles. With inadequate mixing, the polymer will eventually fold back on itself and its effectiveness in reducing the surface charge will be diminished. Further, if the number of colloidal particles is limited, it will be difficult to remove them with low polyelectrolyte dosages. Polymer Bridge Formation. The second mode of action of polyelectrolytes is interparticle bridging. In
this case, polymers that are anionic and nonionic(usually anionic to a slight extent when placed in water) become attached at a number of adsorption sites to the surface of the particles found in the wastewater. A articles become intertwined with other bridged particles during the flocculation process. The si nsional particles grows un particle removal is to be achieved by the formation of particle-polvmer bridges. the initial mixing of the olvmer and the wastewater containing the particles to be removed must be accomplished in a matter of second Fig. 6-2 Particles in wastewa Panicle with adsorbed polymer efinition sketch for interparticle bridging about by perikinetic or orthokinetic fiocculation Charge Neutralization and Polymer Bridge Formation. The third type of polyelectrolyte action may be classified as a charge neutral ization and bridging phe oolvelectrolvtes of extremely high molecular weight. Besides lowering the surface charge on the particle, these polyelectrolytes also form particle bridges as described above Particle Destabilization and Removal with Hydrolyzed Metal lons In contrast with the aggregation brought about by the addition of chemicals that act as counterions, electrolytes, and polymers, aggregation brought about by the addition of alum or ferric sulfate is a more complex process. To understand particle destabilization and the removals achieved with hydrolyzed metal ions, it will be instructive to consider first the formation of metal ion hydrolysis products. Operating ranges for action of metal salts and the importance of initial mixing are also considered in light of the formation of these particles Formation of Hydrolysis Products. In the was thought that free al products are responsible. Although the effect of these hydrolysis products is only now appreciated, it is interesting to note that their chemistry was first elucidated in the early 1900s by Pfeiffer(1902-1907 Bjerrum(1906-1920), and Wemer(1907)(Thomas, 1934). It should be noted that the complex compounds group of surrounding molecules or ions by coordinate co known as ligands and the atoms attached directly to the metal ion are called ligand donor atoms Ligand compounds of interest in wastewater treatment include carbonate(CO32). chloride(C1). hydroxide (OH). ammonia(NH3) and water(H2O). In addition, a number of the coordination compounds are also amphoteric in that they can exist both in strong acids and in strong bases Over the past 50 vears. it has been observed that the intermediate hydrolysis reactions of Al(llD) are much more complex than would be predicted on the basis of a model in which a base is added to the solution. At the present time the complete chemistry for the formation of hydrolysis reactions and products is not well understood. A hypothetical model, proposed by Stumm for Al(lD), is useful for the purpose of illustrating the complex reactions involved. A number of alternative formation sequences have also been proposed Before the reaction proceeds to the point where a negative aluminate ion is produced, polymerization as depicted in the following formula will usually take place The possible combinations of the various hydrolysis products are endless, and their enumeration is not the purpose here. What is important, however, is the realization that one or more of the hydrolysis products and/or polymers may be responsible for the observed action of aluminum or iron Further. because the hydrolysis reactions follow a stepwise process. the effectiveness of alumin iron will vary with time. For example, an alum slurry that has been prepared and stored will it is added to a wastewater Action of Hydrolyzed Metal lons. The action of hydrolyzed metal ions in bringing about the destabilization and removal of colloidal particles may be divided into the following three categories
6-5 this case, polymers that are anionic and nonionic (usually anionic to a slight extent when placed in water) become attached at a number of adsorption sites to the surface of the particles found in the wastewater. A bridge is formed when two or more particles become adsorbed along the length of the polymer. Bridged particles become intertwined with other bridged particles during the flocculation process. The size of the resulting three-dimensional particles grows until they can be removed easily by sedimentation. Where particle removal is to be achieved by the formation of particle-polymer bridges, the initial mixing of the polymer and the wastewater containing the particles to be removed must be accomplished in a matter of seconds. Charge Neutralization and Polymer Bridge Formation. The third type of polyelectrolyte action may be classified as a charge neutralization and bridging phenomenon, which results from using cationic polyelectrolytes of extremely high molecular weight. Besides lowering the surface charge on the particle, these polyelectrolytes also form particle bridges as described above. Particle Destabilization and Removal with Hydrolyzed Metal Ions In contrast with the aggregation brought about by the addition of chemicals that act as counterions, electrolytes, and polymers, aggregation brought about by the addition of alum or ferric sulfate is a more complex process. To understand particle destabilization and the removals achieved with hydrolyzed metal ions, it will be instructive to consider first the formation of metal ion hydrolysis products. Operating ranges for action of metal salts and the importance of initial mixing are also considered in light of the formation of these particles. Formation of Hydrolysis Products. In the past, it was thought that free A13+ and Fe3+ were responsible for the effects observed during particle aggregation; it is now known, however, that their hydrolysis products are responsible. Although the effect of these hydrolysis products is only now appreciated, it is interesting to note that their chemistry was first elucidated in the early 1900s by Pfeiffer (1902-1907), Bjerrum (1906-1920), and Wemer (1907) (Thomas, 1934). It should be noted that the complex compounds are known as coordination compounds, which are defined as a central metal ion (or atom) attached to a group of surrounding molecules or ions by coordinate covalent bonds. The surrounding molecules or ions are known as ligands, and the atoms attached directly to the metal ion are called ligand donor atoms. Ligand compounds of interest in wastewater treatment include carbonate (CO3 2- ), chloride (C1- ), hydroxide (OH), ammonia (NH3), and water (H2O). In addition, a number of the coordination compounds are also amphoteric in that they can exist both in strong acids and in strong bases. Over the past 50 years, it has been observed that the intermediate hydrolysis reactions of Al(III) are much more complex than would be predicted on the basis of a model in which a base is added to the solution. At the present time the complete chemistry for the formation of hydrolysis reactions and products is not well understood. A hypothetical model, proposed by Stumm for Al(III), is useful for the purpose of illustrating the complex reactions involved. A number of alternative formation sequences have 'also been proposed. Before the reaction proceeds to the point where a negative aluminate ion is produced, polymerization as depicted in the following formula will usually take place. The possible combinations of the various hydrolysis products are endless, and their enumeration is not the purpose here. What is important, however, is the realization that one or more of the hydrolysis products and/or polymers may be responsible for the observed action of aluminum or iron. Further, because the hydrolysis reactions follow a stepwise process, the effectiveness of aluminum and iron will vary with time. For example, an alum slurry that has been prepared and stored will behave differently from a freshly prepared solution when it is added to a wastewater. Action of Hydrolyzed Metal Ions. The action of hydrolyzed metal ions in bringing about the destabilization and removal of colloidal particles may be divided into the following three categories: Fig. 6-2