Water Quality Engineering Gao Jingfeng Professor College of Environmental and Energy Engineering Beijing University of Technology
Coagulation Outline • Screening • Settling Theory • Grit Chamber • Settling Basin • Filtration • Neutralization • Ion Exchange
Section 8:Coagulation Outline Basic Definition of Chemical Coagulation Mechanics of Chemical Coagulation Characteristics of the Colloidal Particles Surface Charges of Colloidal Particles The Electrical Double Layer Particle Destabilization Coagulation Steps and Coagulants Design Considerations
Information related to Coagulation • Colloidal particles found in wastewater typically have a net negative surface charge. • The size of colloids (about 1~500 nm) 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 motionkeeps 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 • Coagulation is the process of destabilizing colloidal particles so that particles growth can occur as a result of particle collisions. • Theprocess is employed for the Removal of waste materials in suspended or colloidal form.
Basic Definition of Chemical Coagulation • Chemical Coagulation includes all of the reactions and mechanisms involved in the chemical destabilization of particles and in the formation of larger particles through perikinetic flocculation （异向混凝，又称电动混凝）
Basic Definition of Chemical Coagulation-cont’d • Acoagulantis the chemical that is added to destabilize the colloidal particles in wastewater so that floc formation can result.(凝聚) • Aflocculentis a chemical, typically organic, added to enhance the flocculation process. .(絮凝) • Typical coagulants and flocculants includenatural and synthetic organic polymers, metal salts such as alum or ferric sulfate, and metal salts such as PolyAluminum chloride (PACL) and PolyIron chloride (PICL).
Mechanism of Coagulation In perikinetic coagulation, the zeta potential is reduced by ions or colloids of opposite charge to a level below the Van der Waals attractive forces. Perikinetic Coagulation (异向混凝) Coagulation In orthokinetic coagulation, the micelles aggregate and form Clumps（块团）that agglomerate（成团） the colloidal particles. Orthokinetic Coagulation (同向混凝)
Mechanism of Coagulation 又称电动混凝，是通过带相反电荷的离子或胶体使zeta电位值降低，从而使静电斥力减少至范德华引力之下 Perikinetic Coagulation (异向混凝) Coagulation （混凝） Orthokinetic Coagulation (同向混凝) 胶束的聚集，并且形成胶体颗粒聚团
Particles in Water • There are three types of objects which can be found in water. In order from smallest to largest, these objects are: • Coagulation / flocculation will remove colloidal and suspended solids from water. Chemicals in Solution Colloidal Solids Suspended Solids
Chemicals in solution • Chemicals in solution have been completely dissolved in the water. They are electrically charged and can interact with the water, so they are completely stable and will never settle out of the water. • Chemicals in solution are not visible, either using the naked eye or using a microscope, and are less than 1 nm in size. • An example of a chemical in solution is sugar in water.
Colloidal solids • Colloidal solids, also known as nonsettleable solids, do not dissolve in water although they are electrically charged. • Still, the particles are so small that they will not settle out of the water even after several years and they cannot be removed by filtration alone. Colloidal solids range between 1 and 500 nm in size and can be seen only with a high-powered microscope. • Examples include bacteria, fine clays, and silts. • Colloidal solids often cause colored water, such as the "tea color" of swamp water.
Suspended Solids • Suspended, orsettleable, solids will settle out of water over time, though this may be so slow that it is impractical to merely allow the particles to settle out in a water treatment plant. • The particles are more than 1,000 nm in size and can be seen with a microscope or, sometimes, with the naked eye. • Examples of suspended solids include sand and heavy silts.
Characteristics of the Colloidal Particles • Important factors that contribute to the characteristics of colloidal particles in wastewater include: • Particle size and number • Particle shape and flexibility • Particle-particle interactions • Particle-solvent interactions • Surface properties including electrical characteristics
Particle Size and Number • The size of colloidal particles in wastewater considered is typically in the range from 0.01 to 1.0 mm. Some researchers have classified the size range for colloidal particles as varying from 0.001 to 1 mm. • The number of colloidal particles in untreated wastewater and after primary sedimentation is typically in the range from 106 to 1012/mL . • The number of colloidal particles will very depending on the location where the sample is taken within a treatment plant.
Particle Shape and Flexibility Particle shapes found in wastewater: • Spherical; Semispherical • Ellipsoids椭圆体of various shapes (e.g. prolate扁长的and oblate 扁圆的) • Rods of various length diameter (e.g. E.Coli) • Disk and disklike • Strings of various lengths • Random coils (e.g. large organic molecules) • Fractal (e.g. some larger floc particles) • 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.
Particles-Solvent Interactions Colloidal Particles in Liquids Hydrophobic (憎水的) Hydrophilic (亲水的) Association(交联) Colloids
Particles-Solvent Interactions Hydrophobic particles have relatively little attraction for water. Hydrophobic Hydrophilic particles have a great attraction for water. Hydrophilic Typically made up of surface-active Agents(表面活性剂) such as soaps, synthetic detergents, and dyestuffs(染料) which form organized (胶态离子). Association
Particle-Particle Interactions • 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 don not come into play until the two plates are brought together in close proximity to each other. • Particle-particle interactions are extremely important in bringing about aggregation by means of Brownian motion. • The theory that has been developed to describe particle-particle interaction is based on the consideration of interaction between two charged flat plates and between two charged spheres.
Particle-Particle Interactions-1 • The net interaction curve is formed by subtracting the attraction curve from the repulsion curve.
Electrical Charges • The chemistry of coagulation and flocculation is primarily based on electricity. • Electricity is the behavior of negative and positively charged particles due to their attraction and repulsion. • Like charges (two negatively charged particles or two positively charged particles) repel each other while opposite charges (a positively charged particle and a negatively charged particle) attract. Negatively charged particles repel each other due to electricity.
Surface Charge Development Ways of Surface Charge • Isomorphous replacement(同离子置换) • Structural imperfections(结构不完整性) • Preferential adsorption(选择性吸附) • Ionization(电离) 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.
Development Ways ofSurface Charge Charges development through isomorphous replacement occurs in clay and other soil particles, in which ions in the lattice（晶格结构）structure are replaced with ions from solution (e.g. the replacement or Si4+ with Al3+ ). Isomorphous replacement (同离子置换) 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. Structural Imperfections (结构不完整性)
Development Ways ofSurface Charge 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 hydroxy ions). Preferential Adsorption (选择性吸附) In the case of substances such as proteins or microorganisms, surface charge is acquired through the ionization of carboxyl and amino groups. Ionization (电离)
Introduction Electrical to Charges • Most particles dissolved in water have a negative charge, so they tend to repel each other. As a result, they stay dispersed and dissolved or colloidal in the water. • The purpose of most coagulant chemicals is to neutralize the negative charges on the turbidity particles to prevent those particles from repelling each other.
Introduction Electrical to Charges • The amount of coagulant which should be added to the water will depend on the zeta potential, a measurement of the magnitude of electrical charge surrounding the colloidal particles. • Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the negative particles in the water, as shown in below figure: Positively charged coagulants attract to negatively charged particles due to electricity.
Electrical Charges • The combination of positive and negative charge results in a neutral, or lack of charge. As a result, the particles no longer repel each other. • The next force which will affect the particles is known as van der Waal's forces.Van der Waal's forces refer to the tendency of particles in nature to attract each other weakly if they have no charge. Neutrally charged particles attract due tovan der Waal's forces.
Electrical Charges • Once the particles in water are not repelling each other, van der Waal's forces make the particles drift toward each other and join together into a group. When enough particles have joined together, they become floc and will settle out of the water. Particles and coagulants join together into floc.
About Floc • The end product of a well-regulated coagulation / flocculation process is water in which the majority of the turbidity has been collected into floc, clumps of bacteria and particulate impurities that have come together and formed a cluster. • The floc will then settle out in the sedimentation basin, with remaining floc being removed in the filter. • The best floc size is 0.1 to 3 mm. Larger floc does not settle as well and is more subject to breakup in the flocculation basin. Smaller floc also may not settle.
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 overcomethermal agitation. • Surrounding this fixed layer of ions is a diffuse layer of ions, which is prevented from forming a compact double layer by thermal agitation. • The electrical double layer consists of a compact layer (Stern) in which the potential drops from ψ0 to ψs, and a diffuser layer in which the potential drops from ψs to 0 in the bulk solution.
Two Ways to Visualize the Double Layer • The left view shows the change in charge density around the colloid. • The light shows the distribution of positive and negative ions around the charged colloid.
总电位或ψ电位：胶核表面的电位离子与溶液主体之间的电位差则称为总电位或ψ电位。总电位或ψ电位：胶核表面的电位离子与溶液主体之间的电位差则称为总电位或ψ电位。 • 胶体的电动电位（ζ电位）：当胶体粒子运动时，扩散层中的大部分反离子就会脱离胶团，向溶液主体扩散。其结果必然使胶粒产生剩余电荷（其量等于脱离胶团的反离子所带电荷数值，符号与电位离子相同），使胶粒与扩散层之间形成一个电位差，此电位称为胶体的电动电位，常称为ζ电位。 • 在总电位一定时，扩散层愈厚，ζ电位愈高，反之，扩散层愈薄，ζ电位愈低。
Flocculation • Flocculationis used to describe the process whereby the size of particles increases as a result of particle collisions. Flocculation Microflocculation (Perikinetic flocculation) Macroflocculation (Orthokinetic flocculation)
Flocculation In microflocculation, particle aggregation is brought about by the random thermal motion of fluid molecules known as Brownian motion or movement. In macroflocculation, 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. Purpose of Flocculation To produce particles, by means of aggregation, that can be removed by inexpensive particle-separation procedures such as gravity sedimentation and filtration.
Particle Destabilization • To bring about 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: • Potential-determining ions • Electrolytes • Polyelectrolytes • Hydrolyzed Metal Ions
Use of Potential-Determining Ions • The addition of potential-determining ions, which will be taken up by or will react with the colloid surface to lessen the surface charge. • 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. • 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.
Use of Potential-Determining Ions • When the surface charge (either positive or negative) of particles is greater than the thermal kinetic energy of the particles, the particles will not flocculate. • 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 be added to coagulate colloidal suspensions. • The addition of electrolytes, which have the effect of reducing the thickness of the diffuse electric layer and reduce the potential. • Increased concentration of a given electrolyte will cause a decrease in potential and a corresponding decrease in repulsive forces. • The use of electrolytes is not feasible in either water or wastewater treatment.
Polyelectrolytes Polyelectrolyte Anionic, cationic, nonionic Natural Synthetic Include polymers of biological origin and those derived from starch products such as cellulose(纤维素) derivatives and alginates(藻酸盐). Consist of simple monomers that are polymerized into high-molecular-weight substances.
Use of Polyelectrolytes First Action: Charge Neutralization • Cationic polyelectrolytes act as coagulants that neutralize or lower the charge of the wastewater particles, because wastewater particles normally are charged negatively 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.
Use of Polyelectrolytes Second Action: Polymer Bridge Formation • Anionic or nonionic polymers here used to form interparticle bridging. • 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.