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 6-26-2 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