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 agitation6-3 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