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
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
Alum. When alum is added to wastewater containing calcium and magnesium bicarbonate alkalinity, a precipitate of aluminum hydroxide will form. The insoluble aluminum hydroxide is a gelatinous floc that settles slowly through the wastewater sweeping out sus reaction is exactly analogous when magnesium bicarbonate is substituted for the calcium salt Lime. A sufficient quantity of lime must therefore be added to combine with all the free carbonic acid and with the carbonic acid of the bicarbonates(half-bound carbonic acid) to produce calcium carbonate Much more lime is generally required when it is used alone than when sulfate of iron is also used where industrial wastes introduce mineral acids or acid salts into the wastewater Ferrous Sulfate and time. In most cases, ferrous sulfate cannot be used alone as a precipitant because lime must be added at the same time to form a precipitate. The formation of ferric hydroxide is dependent on the presence of dissolved oxygen, and, as a result, ferrous sulfate is not used commonly in wastewater Enhanced Removal of Suspended Solids in Primary Sedimentation he degree of clarification obtained when chemicals are added to untreated wastewater depends on the quantity of chemicals used, mixing times, and the care with which the process is monitored and controlled With chemical precipitation. it is possible to remove 80 to 90 percent of the total suspended solids (tss Comparable removal values for well-designed and well-operated primary sedimentation tanks without the ddition of chemicals are 50 to 70 percent of the TSS. 25 to 40 percent of the BOD, and 25 to 75 percent of the bacteria. Because of the variable characteristics of wastewater, the required chemical dosages nould be determined from bench-or pilot-scale tests Independent Physical-Chemical Treatment In some localities, industrial wastes have rendered municipal wastewater difficult to treat by biological means. In such situations. physical-chemical treatment may be an alternative approach. This method of treatment has met with limited success because of its lack of consistency in meeting discharge h costs for chemicals, handling and disposal of the great voli of sludge resulting from the addition of chemicals. and numerous operating problems. Based on typical performance results of full-scale plants using activated carbon, the activated-carbon columns removed only 50 to 60 percent of the applied total BOD, and the plants did not meet consistently the effluent standards for secondary treatment. In some instances, substantial process modifications have been required to reduce the operating problems and meet performance requirements, or the process has been replaced by biological treatment rare. Physical-chemical treatment is used more extensively for the treatment o Depending on the treatment objectives, the required chemical dosages and application rates should be determined from bench-or pilot-scale tests A flow diagram for the physical-chemical treatment of untreated wastewater is presented on Fig 6-5 first-stage prec pH adjustment by recarbonation(if required), the wastewater granular-medium filter to remove any residual floc and then through carbon columns to remove dissolved organic compounds. The filter is shown as optional, but its use is recommended to reduce the blinding and Caton treated effluent from the carbon column is usually chlorinated before discharge to the Centere Snubber receiving waters. cathan washin Fig. 6-5 Typical flow diagram ofan independent physical-chemical treatment pl resulting from chemical precipitation is one of the greatest difficulties associated with chemical treatment. Sludge is produced in great volume from most chen recipitation operations often reaching 0. 5 percent of the volume of wastewater treated when lime is us 6-4 Chemical Precipitation For Phosphorus Removal The removal of phosphorus from wastewater involves the incorporation of phosphate into TSS and the
6-8 Alum. When alum is added to wastewater containing calcium and magnesium bicarbonate alkalinity, a precipitate of aluminum hydroxide will form. The insoluble aluminum hydroxide is a gelatinous floc that settles slowly through the wastewater, sweeping out suspended material and producing other changes. The reaction is exactly analogous when magnesium bicarbonate is substituted for the calcium salt. If less than this amount of alkalinity is available, it must be added. Lime is commonly used for this purpose when necessary, but it is seldom required in the treatment of wastewater. Lime. A sufficient quantity of lime must therefore be added to combine with all the free carbonic acid and with the carbonic acid of the bicarbonates (half-bound carbonic acid) to produce calcium carbonate. Much more lime is generally required when it is used alone than when sulfate of iron is also used where industrial wastes introduce mineral acids or acid salts into the wastewater. Ferrous Sulfate and time. In most cases, ferrous sulfate cannot be used alone as a precipitant because lime must be added at the same time to form a precipitate. The formation of ferric hydroxide is dependent on the presence of dissolved oxygen, and, as a result, ferrous sulfate is not used commonly in wastewater. Enhanced Removal of Suspended Solids in Primary Sedimentation The degree of clarification obtained when chemicals are added to untreated wastewater depends on the quantity of chemicals used, mixing times, and the care with which the process is monitored and controlled. With chemical precipitation, it is possible to remove 80 to 90 percent of the total suspended solids (TSS) including some colloidal particles, 50 to 80 percent of the BOD, and 80 to 90 percent of the bacteria. Comparable removal values for well-designed and well-operated primary sedimentation tanks without the addition of chemicals are 50 to 70 percent of the TSS, 25 to 40 percent of the BOD, and 25 to 75 percent of the bacteria. Because of the variable characteristics of wastewater, the required chemical dosages should be determined from bench- or pilot-scale tests.. Independent Physical-Chemical Treatment In some localities, industrial wastes have rendered municipal wastewater difficult to treat by biological means. In such situations, physical-chemical treatment may be an alternative approach. This method of treatment has met with limited success because of its lack of consistency in meeting discharge requirements, high costs for chemicals, handling and disposal of the great volumes of sludge resulting from the addition of chemicals, and numerous operating problems. Based on typical performance results of full-scale plants using activated carbon, the activated-carbon columns removed only 50 to 60 percent of the applied total BOD, and the plants did not meet consistently the effluent standards for secondary treatment. In some instances, substantial process modifications have been required to reduce the operating problems and meet performance requirements, or the process has been replaced by biological treatment. Because of these reasons, new applications of physical-chemical treatment for municipal wastewater are rare. Physical-chemical treatment is used more extensively for the treatment of industrial wastewater. Depending on the treatment objectives, the required chemical dosages and application rates should be determined from bench- or pilot-scale tests. A flow diagram for the physical-chemical treatment of untreated wastewater is presented on Fig. 6-5. As shown, after first-stage precipitation and pH adjustment by recarbonation (if required), the wastewater is passed through a granular-medium filter to remove any residual floc and then through carbon columns to remove dissolved organic compounds. The filter is shown as optional, but its use is recommended to reduce the blinding and headloss buildup in the carbon columns. The treated effluent from the carbon column is usually chlorinated before discharge to the receiving waters. Fig. 6-5 Typical flow diagram of an independent physical-chemical treatment plant The handling and disposal of the sludge resulting from chemical precipitation is one of the greatest difficulties associated with chemical treatment. Sludge is produced in great volume from most chemical precipitation operations, often reaching 0.5 percent of the volume of wastewater treated when lime is used. 6-4 Chemical Precipitation For Phosphorus Removal The removal of phosphorus from wastewater involves the incorporation of phosphate into TSS and the
subsequent removal of those solids. Phosphorus can be incorporated into either biological solids(e.g, microorganisms) or chemical precipitates. The topics to be considered include (I)the chemistry of phosphate precipitation, (2)strategies for phosphorous removal, (3) phosphorus removal using metal salts and polymers, and (4) phosphorus removal using lime Chemistry of Phosphate Precipitation The chemical precipitation of phosphorus is brought about by the addition of the salts of multivalent metal ions that form precipitates of sparingly soluble phosphates. The multivalent metal ions used most commonly are calcium [ Ca(l)) aluminum [Al(llD), and iron Fe(ll) Polymers have been used effectively in conjunction with alum and lime as flocculant aids. Because the chemistry of phosphate recipitation with calcium is quite different than witl and iron. the two different precipitation are considered separately in the following discussion. Phosphate Precipitation with Calcium. Calcium is usually added in the form of lime Ca(OH). From the equations presented previously, it will be noted that te alkal unity to precipitate CaCO3. As the ph value of the waste t 10. excess calcium ions will then react with the phosphate to precipitate hydroxvlapa CaRol PO4)6(OH)2. Because of the reaction of lime with the alkalinity of the wastewater. the quantity of lime required will. in i pr the wastewater. The quantity of lime required to precipitate the phosphorus in wastewater is typically about 1. 4 to 1.5 times the total alkalinity expressed as CaCo3. Because a high pH value is required to precipitate phosphate, coprecipitation is usually not feasible. When lime is added to raw wastewater or to econdary effluent pH adiustment is usually required before subsequent treatment Recarbonation with carbon dioxide(CO,) is used to lower the pH vi Phosphate Precipitation with Aluminum and Iron. In the case of alum and iron I mole will precipitate I mole of phosphate; however, these reactions are deceptively simple and must be considered in light of the many competing reactions and their associated equilibrium constants, and the effects of alkalinity. pH. trace elements, and ligands found in wastewater. Because of the many competing reactions cannot be used to estimate the required chemical dosages directly. Therefore, dosages are generally established on the basis of bench-scale tests and occasionally by full-scale tests, especially if polymer used. Pure metal phosphates are precipitated within the shaded area, and mixed complex polynuclear species are formed outside toward higher and lower pH values Strategies for Phosphorus Removal The precipitation of phosphorus from wastewater can occur in a number of different locations within a process flow diagram(see Fig 6-6). The general locations where phosphorus can be removed may be classified as(1) pre-precipitation, (2)coprecipitation, and (3) postprecipitation 69
6-9 subsequent removal of those solids. Phosphorus can be incorporated into either biological solids (e.g., microorganisms) or chemical precipitates. The topics to be considered include (1) the chemistry of phosphate precipitation, (2) strategies for phosphorous removal, (3) phosphorus removal using metal salts and polymers, and (4) phosphorus removal using lime. Chemistry of Phosphate Precipitation The chemical precipitation of phosphorus is brought about by the addition of the salts of multivalent metal ions that form precipitates of sparingly soluble phosphates. The multivalent metal ions used most commonly are calcium [Ca(II)], aluminum [Al(III)], and iron [Fe(III)]. Polymers have been used effectively in conjunction with alum and lime as flocculant aids. Because the chemistry of phosphate precipitation with calcium is quite different than with aluminum and iron, the two different types of precipitation are considered separately in the following discussion. Phosphate Precipitation with Calcium. Calcium is usually added in the form of lime Ca(OH)2. From the equations presented previously, it will be noted that when lime is added to water it reacts with the natural bicarbonate alkalinity to precipitate CaCO3. As the pH value of the wastewater increases beyond about 10, excess calcium ions will then react with the phosphate to precipitate hydroxylapatite Ca10(PO4)6(OH)2. Because of the reaction of lime with the alkalinity of the wastewater, the quantity of lime required will, in general, be independent of the amount of phosphate present and will depend primarily on the alkalinity of the wastewater. The quantity of lime required to precipitate the phosphorus in wastewater is typically about 1.4 to 1.5 times the total alkalinity expressed as CaCO3. Because a high pH value is required to precipitate phosphate, coprecipitation is usually not feasible. When lime is added to raw wastewater or to secondary effluent, pH adjustment is usually required before subsequent treatment or disposal. Recarbonation with carbon dioxide (CO2) is used to lower the pH value. Phosphate Precipitation with Aluminum and Iron. In the case of alum and iron, 1 mole will precipitate 1 mole of phosphate; however, these reactions are deceptively simple and must be considered in light of the many competing reactions and their associated equilibrium constants, and the effects of alkalinity, pH, trace elements, and ligands found in wastewater. Because of the many competing reactions cannot be used to estimate the required chemical dosages directly. Therefore, dosages are generally established on the basis of bench-scale tests and occasionally by full-scale tests, especially if polymers are used. Pure metal phosphates are precipitated within the shaded area, and mixed complex polynuclear species are formed outside toward higher and lower pH values. Strategies for Phosphorus Removal The precipitation of phosphorus from wastewater can occur in a number of different locations within a process flow diagram (see Fig. 6-6). The general locations where phosphorus can be removed may be classified as (1) pre-precipitation, (2) coprecipitation, and (3) postprecipitation
phosphorus To further Insoluble processing phosphorus phosphorus phosphorus treatment ig.6-6 Alternative points of chemical addition for phosphorus removal: (a) before primary sedimentation, ( b) before and/or following biological treatment, (c) following secondary treatment, and (d-f) at several locations in a process (known as"split treatment Pre-precipitation. The addition of chemicals to raw wastewater for the precipitation of phosphorus primary sedimentation facilities is termed"pre-precipitation. "The precipitated phosphate is removed with the primary sludge Tab. 6-3 Factors affecting the choice of chemical for phosphorus removal st( including transportation) 5. Reliability of chemical suppl 8. Compatibility with other treatment processes Coprecipitation. The addition of ls to form precipitates that are removed biological sludge is defined as"coprecipitation. "Chemicals can be added to(D) the effluent from primary sedimentation facilities. (2) the mixed liquor(in the activated-sludge process). or( 3) the effluent from a Postprecipitation. Postprecipitation involves the addition of chemicals to the effluent from secondary sedimentation facilities and the subsequent removal of chemical precipitates. In this process, the chemical precipitates are usually removed in separate sedimentation facilities or in effluent filters( see Fig 6-6) Phosphorus Removal Using Metal Salts and Polymers As noted above, iron or aluminum salts can be added at a variety of different points in the treatment process(s orthophosphorous, adding aluminum 6-10
6-10 Pre-precipitation. The addition of chemicals to raw wastewater for the precipitation of phosphorus in primary sedimentation facilities is termed "pre-precipitation." The precipitated phosphate is removed with the primary sludge. Tab. 6-3 Factors affecting the choice of chemical for phosphorus removal 1. Influent phosphorus level 2. Wastewater suspended solids 3. Alkalinity 4. Chemical cost(including transportation) 5. Reliability of chemical supply 6. Sludge handling facilities 7. Ultimate disposal methods 8. Compatibility with other treatment processes Coprecipitation. The addition of chemicals to form precipitates that are removed along with waste biological sludge is defined as "coprecipitation." Chemicals can be added to (1) the effluent from primary sedimentation facilities, (2) the mixed liquor (in the activated-sludge process), or (3) the effluent from a biological treatment process before secondary sedimentation. Postprecipitation. Postprecipitation involves the addition of chemicals to the effluent from secondary sedimentation facilities and the subsequent removal of chemical precipitates. In this process, the chemical precipitates are usually removed in separate sedimentation facilities or in effluent filters (see Fig. 6-6). Phosphorus Removal Using Metal Salts and Polymers As noted above, iron or aluminum salts can be added at a variety of different points in the treatment process (see Fig. 6-6), but because polyphosphates and organic phosphorus are less easily removed than orthophosphorus, adding aluminum or iron salts after secondary treatment (where organic phosphorus and Fig. 6-6
