Sanitary Engineering. Coagulation and Flocculation. Week 3

Transcription

1 Sanitary Engineering Coagulation and Flocculation Week 3 1

2 Coagulation and Flocculation Colloidal particles are too small to be removed by sedimentation or by sand filtration processes. Coagulation: Destabilization colloids by Flocculation: Aggregation of destabilized colloids To turn the small particles into larger particles removed in subsequent processes such as settling or filtration. When combined with subsequent physical removal it achieves Removal of turbidity Remove infectious agents, Remove toxic compounds that have adsorbed to the surface of particles, Remove precursors to the formation of disinfection byproducts, and Make the water palatable. 2

3 Characteristics of Particles Electrical Properties: surface charge.. causes the particles to remain in suspension without aggregating for long periods of time. Particle suspensions are thermodynamically unstable and. aggregation process is very slow, the particles cannot be removed by sedimentation in a reasonable amount of time. Negative charge arises in four principal ways (Ionization, Adsorption, Isomorphous replacement and Structural imperfections) Electrical Double Layer: In natural waters, the processes described above nearly always result in a negative surface charge on particles. Negatively charged particles accumulate positive counterions on and near the particle s surface As shown on (Fig.1) a layer of cations will bind negatively charged particle to form a fixed adsorption layer known as the Helmholtz layer or Stern layer. The layer of cations and anions that extends from the Helmholtz layer to the bulk solution where the charge is zero and electroneutrality is satisfied is known as the diffuse layer. Taken together the adsorbed Stern diffuse layer are known as the electric double layer. 3

4 Fig.1 4

5 Zeta Potential: When a charged particle is placed in an electric field, it will migrate to the pole of opposite charge. This movement is called electrophoresis. As the particle moves, a portion of the water near the surface moves with it. This movement displaces the ion cloud and gives it the shape shown in (Fig.2). The electric potential between the shear plane and the bulk solution is called the zeta potential. Fig.2 5

6 Particle Stability: Particles in natural waters remain stable when there is a balance between the electrostatic force of the charged particles and attractive forces known as van der Waals forces. Because the particles have a net negative charge, the principal mechanism controlling stability is electrostatic repulsion. Van der Waals forces arise from magnetic and electronic resonance when two particles approach one another. Because the double layer extends further into solution than the van der Waals forces, an energy barrier is formed that prevents particles from aggregating. Coagulation Coagulants : Inorganic coagulants used for the treatment of potable water exhibit the following characteristics They are nontoxic at the working dosage. They have a high charge density. They are insoluble in the neutral ph range. 6

7 Physics of Coagulation: There are four mechanisms employed to destabilize natural water suspensions: Compression of the Double Layer: If the electric double layer is compressed, the repulsive force is reduced and particles will come together as a result of Brownian motion and remain attached due to van der Waals forces of attraction. Both the ionic strength and the charge of counterions are important in the compression of the double layer. 7

8 Adsorption and Charge Neutralization: Hydrolyzed metal salts, prehydrolyzed metal salts, and cationic polymers have a positive charge. They destabilize particles through charge neutralization. Enmeshment in a Precipitate. With doses exceeding saturation for the metal hydroxide, aluminum and iron salts form insoluble precipitates and particulate matter is entrapped in the precipitate. This type of destabilization has been described as sweep coagulation Chemistry of Coagulation: Aluminum: Aluminum can be purchased as either dry or liquid alum [Al 2 (SO 4 ) 3 14H 2 O]. When alum is added to water and aluminum hydroxide precipitates, the overall reaction is: Iron. Iron can be purchased as either the sulfate salt (Fe 2 (SO 4 ) 3 x H 2 O) or the chloride salt (FeCl 3 x H 2 O). It is available in various forms, Dry and liquid forms are available. the overall precipitation reactions for ferric sulfate and ferric chloride are as follows. 8

9 Ferric sulfate: Ferric chloride: Flocculation The basis for describing the mechanisms of flocculation is that stirring water containing particles created velocity gradients that brought about particle collisions. Microscale Flocculation: The floccula on of small par cles (less than 0.1 μm in diameter) is caused by diffusion. The rate of flocculation is relative to the rate at which the particles diffuse. Thus, the primary mechanism of aggregation is through Brownian motion. After a period of seconds, the microflocs range in size from 1 to about 100 μm in diameter. Macroscale Flocculation: Mixing is the major flocculation mechanism for particles greater Mechanical mixing causes unequal shearing forces on the floc, and some of the floc are 9

10 broken up. After some period of mixing, a steady state distribution of floc sizes is achieved and formation and breakup become nearly equal. Differential Settling: Because the floc particles are of different size, they settle at different rates. Differences in the settling velocities cause the particles to collide and flocculate. Chemical Sequence: The addition of multiple chemicals to improve flocculation is common practice. The order of addition is important to achieve optimum results at minimum cost. Typically, the addition of a polymer after the addition of hydrolyzing metal salts is most effective. Ideally, the polymer addi on should be made 5 to 10 minutes a er the addi on of the hydrolyzing metal salt. This allows for the formation of pinpoint floc that is then bridged by polymer. In conventional water treatment plant design this is rarely possible because of space limitations. 10