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 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 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 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 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 (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 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: 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 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 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 (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. 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 polyphosphorus are transformed into orthophosphorus) usually results in the best removal. Some additional nitrogen removal occurs because of better settling, but essentially no ammonia is removed unless chemical additions to primary treatment reduce BOD loadings to the point where nitrification can occur. A number of the important features of adding metal salts and polymers at different points in the treatment process are discussed in this section. Metal Salt Addition to Primary Sedimentation Tanks. When aluminum or iron salts are added to untreated wastewater, they react with the soluble orthophosphate to produce a precipitate. Organic phosphorus and polyphosphate are removed by more complex reactions and by adsorption onto floc particles. The insolubilized phosphorus, as well as considerable quantities of BOD and TSS, are removed from the system as primary sludge. Adequate initial mixing and flocculation are necessary upstream of primary facilities, whether separate basins are provided or existing facilities are modified to provide these functions. Polymer addition may be required to aid in settling. In low-alkalinity waters, the addition of a base is sometimes necessary to keep pH in the 5 to 7 range. Alum generally is applied in a molar ratio in the range of a 1.4 to 2.5 mole Al/mole P. Metal Salt Addition to Secondary Treatment. Metal salts can be added to the untreated wastewater, in the activated-sludge aeration tank, or the final clarifier influent channel. In trickling filter systems, the salts are added to the untreated wastewater or to the filter effluent. Multipoint additions have also been used. Phosphorus is removed from the liquid phase through a combination of precipitation, adsorption, exchange, and agglomeration, and removed from the process with either the primary or secondary sludges, or both. Theoretically, the minimum solubility of AlPO4 occurs at about pH 6.3, and that of FePO4 occurs at about pH 5.3; however, practical applications have yielded good phosphorus removal anywhere in the range of pH 6.5 to 7.0, which is compatible with most biological treatment processes. The use of ferrous salts is limited because they produce low phosphorus levels only at high pH values. In low-alkalinity waters, either sodium aluminate and alum or ferric plus lime, or both, can be used to maintain the pH higher than 5.5. Improved settling and lower effluent BOD result from chemical addition, particularly if polymer is also added to the final clarifier. Dosages generally fall in the range of a 1 to 3 metal ion-phosphorus molar ratio. Metal Salt and Polymer Addition to Secondary Clarifiers. In certain cases, such as trickling filtration and extended aeration activated-sludge processes, solids may not flocculate and settle well in the secondary clarifier. This settling problem may become acute in plants that are overloaded. The addition of aluminum or iron salts will cause the precipitation of metallic hydroxides or phosphates, or both. Aluminum and iron salts, along with certain organic polymers, can also be used to coagulate colloidal particles and to improve removals on filters. The resultant coagulated colloids and precipitates will settle readily in the secondary clarifier, reducing the TSS in the effluent and effecting phosphorus removal. Dosages of aluminum and iron salts usually fall in the range of 1 to 3 metal ion/phosphorus on a molar ratio basis if the residual phosphorus in the secondary effluent is greater than 0.5 mg/L. To achieve phosphorus levels below 0.5 mg/L, significantly higher metal salt dosages and filtration will be required. Polymers may be added (1) to the mixing zone of a highly mixed or internally recirculated clarifier, (2) preceding a static or dynamic mixer, or (3) to an aerated channel. Although mixing times of 10 to 30 seconds have been used for polymers, shorter mixing times are favored . Polymers should not be subjected to insufficient or excessive mixing, as noted previously, because the process efficiency will diminish, resulting in poor settling and thickening characteristics. Phosphorus Removal Using Lime The use of lime for phosphorus removal is declining because of (1) the substantial increase in the mass of sludge to be handled compared to metal salts and (2) the operation and maintenance problems associated with the handling, storage, and feeding of lime. When lime is used, the principal variables controlling the dosage are the degree of removal required and the alkalinity of the wastewater. The operating dosage must usually be determined by onsite testing. Lime has been used customarily either as a precipitant in the primary sedimentation tanks or following secondary treatment clarification. Although lime recalcination lowers chemical costs, it is a feasible alternative only for large plants. Where a lime recovery system is required for a cost-effective operation, it includes a thermal regeneration facility, which converts the calcium carbonate in the sludge to lime by heating to 980。C. The carbon dioxide from this process or other onsite stack gas (containing 10 to 15 percent carbon dioxide) is generally used as the source of recarbonation for pH adjustment of the wastewater. Lime Addition to Primary Sedimentation Tanks. Both low and high lime treatment can be used to precipitate a portion of the phosphorus (usually about 65 to 80 percent). When lime is used, both the calcium and the hydroxide react with the orthophosphorus to form an insoluble hydroxyapatite [Ca5(OH)(PO4)3]. A residual phosphorus level of 1.0 mg/L can be achieved with the addition of effluent filtration facilities to which chemicals can be added. In the high lime system, sufficient lime is added to raise the pH to about 11. After precipitation, the effluent must be recarbonated before biological treatment. In activated-sludge systems, the pH of the primary effluent should not exceed 9.5 or 10; higher pH values can result in biological process upsets. In the trickling filter process, the carbon dioxide generated during treatment is usually sufficient to lower the pH without recarbonation. The dosage for low lime treatment is usually in the range of 75 to 250 mg/L as Ca(OH)2 at pH values of 8.5 to 9.5. In low lime systems, however, the conditions required for precipitation are more specialized; the Ca2+/Mg2+ mole ratio is ≤5/1. Lime Addition Following Secondary Treatment. Lime can be added to the waste stream after biological treatment to reduce the level of phosphorus and TSS. Single-stage process and two-stage process flow diagrams for lime addition are shown on Fig. 6-7. On Fig. 6-7a, a single-stage lime precipitation process is used for the treatment of secondary effluent. In the first-stage clarifier of the two-stage process shown on Fig. 6-7b, sufficient lime is added to raise the pH above 11 to precipitate the soluble phosphorus as basic calcium phosphate (apatite). The calcium carbonate precipitate formed in the process acts as a coagulant for TSS removal. An example of a large lime precipitation unit is shown on Fig. 6-8. The excess soluble calcium is removed in the second-stage clarifier as a calcium carbonate precipitate by adding carbon dioxide gas to reduce the pH to about 10. Generally, there is a second injection of carbon dioxide to the second-stage effluent to reduce the formation of scale. To remove the residual levels of TSS and phosphorus, the secondary clarifier effluent is passed through a multimedia filter or a membrane filter. Care should be taken to limit excess calcium in the filter feed to ensure cementing of the filter media will not occur. Fig. 6-7 Typical lime treatment process flow diagram for phosphorus removal: (a)single-stage system; (b)two-stage system Phosphorus Removal with Effluent Filtration Depending on the quality of the settled secondary effluent, chemical addition has been used to improve the performance of effluent filters. Chemical addition has also been used to achieve specific treatment objectives including the removal of specific contaminants such as phosphorus, metal ions, and humic substances. The removal of phosphorus by chemical addition to the contact filtration process is used in many parts of the country to remove phosphorus from wastewater treatment plant effluents which are discharged to sensitive water bodies. A two-stage filtration process has proved to be very effective for the removal of phosphorus. Based on the performance data from full-scale installations, phosphorus levels equal to or less than 0.02 mg/L have been achieved in the filtered effluent. To achieve such low levels of phosphorus removal, the backwash water from the second filter which contains small particles and residual coagulant is recycled to the first filter to improve floc formation within the first-stage filter and the influent to waste ratio. Comparison of Chemical Phosphorus Removal Processes The advantages and disadvantages of the removal of phosphorus by the addition of chemicals at various points in a treatment system are summarized in Table 6-4. It is recommended that each alternative point of application be evaluated carefully. Tab. 6-4 Advantages and disadvantages of chemical addition in various sections of a treatment plant for phosphorus removal Estimation of Sludge Quantities from Phosphorus Precipitation The additional BOD and TSS removals afforded by chemical addition to primary treatment may also solve overloading problems on downstream biological systems, or may allow seasonal or year-round nitrification, depending on biological system designs. The BOD removal in the primary sedimentation operation is on the order of 50 to 60 percent at a pH of 9.5. The amount of primary sludge will also increase significantly. 6-5 Chemical Precipitation For Removal Of Heavy Metals And Dissolved Inorganic Substances The technologies available for the removal of heavy metals from wastewater include chemical precipitation, carbon adsorption, ion exchange, and reverse osmosis. Of these technologies, chemical precipitation is most commonly employed for most of the metals. Common precipitants include hydroxide (OH) and sulfide (S2-). Carbonate (CO32-) has also been used in some special cases. Metal may be removed separately or coprecipitated with phosphorus. Precipitation Reactions Metals of interest include arsenic (As), barium (Ba), cadmium (Cd), copper (Cu), mercury (Hg), nickel (Ni), selenium (Se), and zinc (Zn). Most of these metals can be precipitated as hydroxides or sulfides. Solubility products for free metal concentrations in equilibrium with hydroxide and sulfide precipitates are reported in Table 6-5. In wastewater treatment facilities, metals are precipitated most commonly as metal hydroxides through the addition of lime or caustic to a pH of minimum solubility. However, several of these compounds are, as discussed previously, amphoteric (i.e., capable of either accepting or donating a proton) and exhibit a point of minimum solubility. The minimum effluent concentration levels that can be achieved in the chemical precipitation of heavy metals are reported in Table 6-6. In practice, the minimum achievable residual metal concentrations will also depend on the nature and concentration of the organic matter in the wastewater as well as the temperature. Because of the many uncertainties associated with the precipitation of metals, laboratory bench-scale or pilot-plant testing should be conducted. Tab. 6-5 Solubility products for free metal ion concentrations in equilibrium with hydroxides and sulfides Tab. 6-6 Practical effluent concentration levels achievable in heavy metals removal by precipitation Coprecipitation with Phosphorus As discussed previously, precipitation of phosphorus in wastewater is usually accomplished by the addition of coagulants, such as alum, lime or iron salts, and polyelectrolytes. Coincidentally with the addition of these chemicals for the removal of phosphorus is the removal that occurs of various inorganic ions, principally some of the heavy metals. Where both industrial and domestic wastes are treated together, it may be necessary to add chemicals to the primary settling facilities, especially if onsite pretreatment measures prove to be ineffective. When chemical precipitation is used, anaerobic digestion for sludge stabilization may not be possible because of the toxicity of the precipitated heavy metals. As noted previously, one of the disadvantages of chemical precipitation is that it usually results in a net increase in the total dissolved solids of the wastewater that is being treated. 6-6 Chemical Oxidation Chemical oxidation in wastewater treatment typically involves the use of oxidizing agents such as ozone (O3), hydrogen peroxide (H202), permanganate (MnO4), chloride dioxide (ClO2), chlorine (C12) or (HOC1), and oxygen (O2), to bring about change in the chemical composition of a compound or a group of compounds. Included in the following discussion is an introduction of the fundamental concepts involved in chemical oxidation, an overview of the uses of chemical oxidation in wastewater treatment, and a discussion of the use of chemical oxidation for the reduction of BOD and COD, the oxidation of ammonia, and oxidation of nonbiodegradable organic compounds. Advanced oxidation process (AOPs) in which the free hydroxyl radical (HO.) is used as a strong oxidant to destroy specific organic constituents and compounds that cannot be oxidized by conventional oxidants such as ozone and chlorine are discussed in later chapters. Fundamentals of Chemical Oxidation Oxidation-Reduction Reactions. Oxidation-reduction reactions (known as redox equations) take place between an oxidizing agent and a reducing agent. In oxidation-reduction reactions both electrons are exchanged as are the oxidation states of the constituents involved in the reaction. While an oxidizing agent causes the oxidation to occur, it is reduced in the process. Half-Reaction Potentials. Because of the almost infinite number of possible reactions, there are no summary tables of equilibrium constants for oxidation-reduction reactions. What is done instead is the chemical and thermodynamic characteristics of the half reactions are determined and tabulated so that any combination of reactions can be studied. Representative half reactions are given in Table 6-8. Of the many properties that can be used to characterize oxidation-reduction reactions, the electrical potential (i.e., voltage) or emf of the half reaction is used most commonly. Thus, every half reaction involving an oxidation or reduction has a standard potential E。associated with it. Tab. 6-7 Standard electrode potentials for oxidation half reactions for chemical disinfection The half-reaction potential is a measure of the tendency of a reaction to proceed to the right. Half reactions with large positive potential, E。, tend to proceed to the right as written. Conversely, half reactions with large negative potential, E。, tend to proceed to the left. Rate of Oxidation-Reduction Reactions. As noted previously, the half-reaction potentials can be used to predict whether a reaction will proceed as written. Unfortunately, the reaction potential provides no information about the rate at which the reaction will proceed. Chemical oxidation reactions often require the presence of one or more catalysts for the reaction to proceed or to increase the rate of reaction. Transition metal cations, enzymes, pH adjustment, and a variety of proprietary substances have been used as catalysts. Applications Some of the more important applications of chemical oxidation in wastewater management are summarized in Table 6-8. In the past, chemical oxidation was used most commonly to (I) reduce the concentration of residual organics, (2) control odors, (3) remove ammonia, and (4) reduce the bacterial and viral content of wastewaters. Chemical oxidation is especially effective for the elimination of odorous compounds (e.g., oxidation of sulfides and mercaptans). In addition to the applications reported in Table 6-8, chemical oxidation is now commonly used to (1) improve the treatability of nonbiodegradable (refractory) organic compounds, (2) eliminate the inhibitory effects of certain organic and inorganic compounds to microbial growth, and (3) reduce or eliminate the toxicity of certain organic and inorganic compounds to microbial growth and aquatic flora. Tab. 6-8 Typical applications of chemical oxidation in wastewater collection, treatment, and disposal Chemical Oxidation of Nonbiodegradable Organic Compounds. Typical chemical dosages for both chlorine and ozone for the oxidation of the organics in wastewater are reported in Table 6-10. As shown in Table 6-10, the dosages increase with the degree of treatment, which is reasonable when it is considered that the organic compounds that remain after biological treatment are typically composed of low-molecular-weight polar organic compounds and complex organic compounds built around the benzene ring structure. Because of the complexities associated with composition of wastewater, chemical dosages for the removal of refractory organic compounds cannot be derived from the chemical stoichiometry, assuming that it is known. Pilot-plant studies must be conducted when either chlorine, chlorine dioxide, or ozone is to be used for the oxidation of refractory organics to assess both the efficacy and required dosages. Chemical Oxidation of Ammonia The chemical process in which chlorine is used to oxidize the ammonia nitrogen in solution to nitrogen gas and other stable compounds is known as breakpoint chlorination. Perhaps the most important advantage of this process is that, with proper control, all the ammonia nitrogen in the wastewater can be oxidized. However, because the process has a number of disadvantages including the buildup of acid (HCl) which will react with the alkalinity, the buildup of total dissolved solids, and the formation of unwanted chloro-organic compounds, ammonia oxidation is seldom used today. 6-7 Chemical Neutralization, Scale Control, And Stabilization The removal of excess acidity or alkalinity by treatment with a chemical of the opposite composition is termed neutralization. In general, all treated wastewaters with excessively low or high pH will require neutralization before they can be dispersed to the environment. Scaling control is required for nanofiltration and reverse osmosis treatment to control the formation of scale, which can severely impact performance. Chemical stabilization is often required for highly treated wastewaters to control their aggressiveness with respect to corrosion. pH Adjustment In a variety of wastewater-treatment operations and processes, there is often a need for pH adjustment. Because a number of chemicals are available that can be used, the choice will depend on the suitability of a given chemical for a particular application and prevailing economics. General information on the chemicals used most commonly for pH adjustment is given in Table 6-9. Wastewater that is acidic can be neutralized with any number of basic chemicals, as reported in Table 6-9. Sodium hydroxide (NaOH, also known as caustic soda) and sodium carbonate, although somewhat expensive, are convenient and are used widely by small plants or for treatment where small quantities are adequate. Lime, which is cheaper but somewhat less convenient, is the most widely used chemical. Lime can be purchased as quicklime or slaked hydrated lime, high-calcium or dolomitic lime, and in several physical forms. Limestone and dolomitic limestone are cheaper but less convenient to use and slower in reaction rate. Because they can become coated in certain waste-treatment applications, their use is limited. Calcium and magnesium chemicals often form sludges that require disposal. Tab. 6-9 Chemicals used most commonly for the control of pH Alkaline wastes are less of a problem than acid wastes but nevertheless often require treatment. If acidic waste streams are not available or are not adequate to neutralize alkaline wastes, sulfuric acid is commonly employed. Based on the chlorine dose used for disinfection, the pH of the disinfected effluent will be lower than that allowed for reuse applications and for dispersal to the environment. In such cases, neutralization is controlled by automatic instruments using a feedback loop, and the final effluent pH is recorded. Depending on the sensitivity of the environment, two-stage neutralization may be required. The reagent chemicals can be fed automatically, in the form of solutions, slurries, or dry materials. If the reaction rate is slow, instrumentation and control design must take this factor into account. Analysis of Scaling Potential With the increasing use that is being made of nanofiltration, reverse osmosis, and electrodialysis in wastewater reuse applications, adjustment of the scaling characteristics of the effluent to be treated is important to avoid calcium carbonate and sulfate scale formation. Depending on the recovery rate, the concentration of salts can increase by a factor of up to 10 within the treatment module. When such a salt concentration increase occurs, it is often possible to exceed the solubility product of calcium carbonate and other scale-forming compounds. The formation of scale within the treatment module will cause a deterioration in the performance, ultimately leading to the failure of the membrane module. The tendency to develop calcium carbonate (CaCO3) scale during the advanced treatment of treated effluent can be approximated by calculating the Langelier saturation index (LSI) of the concentrate stream. Scaling Control Usually, CaCO3 scale control can be achieved using one or more of the following methods: . Acidifying to reduce pH and alkalinity . Reducing calcium concentration by ion exchange or lime softening . Adding a scale inhibitor chemical (antiscalant) to increase the apparent solubility of CaCO3 in the concentrate stream . Lowering the product recovery rate Because it is not possible to predict a priori the value of pH in water treated with reverse osmosis, it is usually necessary to conduct pilot-scale studies using the same modules that will be used in the full-scale installation. Stabilization Wastewater effluent that is demineralized with reverse osmosis will generally require pH and calcium carbonate adjustment (stabilization) to prevent metallic corrosion, due to the contact of the demineralized water with metallic pipes and equipment. Corrosion occurs because material from the solid is removed (solubilized) to satisfy the various solubility products. Demineralized water typically is stabilized by adding lime to adjust the LSI, using the procedure outlined above. 6-8 Chemical Storage, Feeding, Piping, And Control Systems The design of chemical precipitation operations involves not only the sizing of the various trait operations and processes but also the necessary appurtenances. Because of the corrosive nature of many of the chemicals used and the different forms in which they are available, special attention must be given to the design of chemical storage, feeding, piping, and control systems. The following discussion is intended to serve as an introduction to this subject. In domestic wastewater-treatment systems, the chemicals employed can be in a solid, liquid, or gaseous form. Coagulants in the dry solid form generally are converted to solution or slurry form prior to introduction into the wastewater. Coagulants in the liquid form are usually delivered to the plant in a concentrated form and have to be diluted prior to introduction into the wastewater. Chemicals in the gas form (generally stored as a liquid), typically used for disinfection purposes, are either dissolved in water before injection or are injected directly into the wastewater. Fig. 6-9 Classification of chemical-feed system The various types of feeders are classified on Fig. 6-9. Chemical feeders are generally designed to be (1) proportioning, feeding chemical in proportion to the influent wastewater flowrate, and (2) constant feed, designed to deliver chemical at a fixed rate regardless of the influent flowrate. Chemical Storage and Handling General information on the handling, storage, and feeding requirements for various chemicals is presented in Table 6-10. The specific storage facilities required will depend on the form in which the chemical is available locally. For small treatment plants the available forms are usually limited. A typical storage facility for chemicals for a small treatment facility is shown on Fig. 6-10. Dry Chemical-Feed Systems A dry chemical-feed system typically consists of a storage hopper, dry chemical feeder, a dissolving tank, and a pumped or gravity distribution system. The units are sized according to the volume of wastewater, treatment rate, and optimum length of time for chemical feeding and dissolving. Hoppers used with powdered chemicals that are compressible and can form an arch such as lime are equipped with positive agitators and a dust-collection system. Dry chemical feeders are of either the volumetric or the gravimetric type. In the volumetric type the volume of the dry chemical fed is measured whereas in the gravimetric type the weight of chemical fed is measured. Of the dry chemical-feed systems, the loss in weight feed system is used most commonly in wastewater-treatment operations. With a dry feed system, the dissolving operation is critical. The capacity of the dissolving tank is based on the detention time, which is directly related to the wettability or rate of solution of the chemical. When the water supply is controlled for the purpose of forming a constant-strength solution, mechanical mixers should be used. Depending on the flow pattern within the mixing tank, it may be necessary to add baffles for effective mixing. In smaller mixing tanks, the mixer can be set at an angle to avoid the use of baffles. Solutions or slurries are often stored after dissolving and discharged to the application point at metered rates by chemical-feed pumps. Liquid Chemical-Feed Systems Liquid chemical-feed systems typically include a solution storage tank, transfer pump, day tank for diluting the concentrated solution, and chemical-feed pump for distribution to the application point. In general, liquid feed systems provide for better initial contact and dispersion of the chemical and the wastewater. In systems where the liquid chemical does not require dilution, the chemical-feed pumps draw liquid directly from the solution storage tank. The storage tank is sized based upon the stability of the chemical, feed rate requirements, delivery constraints (cost, size of tank truck, etc.), and availability of the supply. Solution feed pumps are usually of the positive-displacement type for accurate metering of the chemical feed. Gas Chemical-Feed Systems Chemicals that are used as a gas include ammonia, chlorine, oxygen, ozone, and sulfur dioxide. Gas feed systems are used mostly for feeding chemicals used for disinfection and dechlorination. Chlorine, the most commonly used chemical for disinfection, is supplied in a liquid form within the storage container and evaporates continuously as the gas is drawn from the headspace above the liquid in the storage container. Initial Chemical Mixing Perhaps the least appreciated fact about chemical addition is the importance of both the initial and uniform mixing of the chemical with the wastewater to be treated. The optimal time for mixing can, as discussed in this section, vary from a fraction of a second to several seconds or more. Because of the difficulties in achieving extremely low mixing times in large treatment plants with a single mixing device, the use of multiple mixing devices is recommended. The particular mixing device selected for a given application must be based on a consideration of the reaction times and operative mechanisms for the chemicals that are being used.