Coagulation and Flocculation

Why coagulation and Flocculation is needed? Various sizes of particles in raw water Particle diameter Type Settling velocity mm 10 Pebble 0.73 m/s 1 Course sand 0.23 m/s 0.1 Fine sand 0.6 m/min 0.01 Silt 8.6 m/d 0.0001 (10 micron) Large colloids 0.3 m/yr 0.000001 (1 nano) Small colloids 3 m/million yr G r a v I t y s e t t l I n g Colloids - Impart color and turbidity to water – Aesthetically not acceptable Colloids – so small- gravity settling not possible

Colloids Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid.

Characteristics Solids dispersed in liquid, which will not settle by the force of gravity. When a solid colloid stays in suspension and does not settle in the system is in a stable condition Large specific surface area: large surface area per unit volume of the particles. It makes colloids can adsorb materials such as water molecular and ions. It opposes electrostatic comparing to water.

Colloids 3. Hydrophilic and Hydrophobic Colloids Focus on colloids in water. “Water loving” colloids: hydrophilic. “Water hating” colloids: hydrophobic. Molecules arrange themselves so that hydrophobic portions are oriented towards each other. If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e.g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water.

Colloids Hydrophilic and Hydrophobic Colloids

Hydrophilic Colloids Colloidal solid surface has water-soluble group, such as amino , carboxyl, sulfonic , hydroxyl. These water-soluble groups can promote water of hydration of colloidal particles. Hydrophilic colloids are general organic colloids, such as proteins and their degradation products. Hydrophobic Colloids Hydrophobic : There is not obvious water of hydration. Hydrophilic colloids are general inorganic colloids, such as clay.

- Colloid Stability Electrostatic force H2O Colloid Colloids have a net negative surface charge Electrostatic force prevents them from agglomeration Brownian motion keeps the colloids in suspension - Repulsion Colloid - A Colloid - B Impossible to remove colloids by gravity settling Electrostatic force The ionization of surface group. The adsorption of ions from the surrounding solution. The ion deficit within the mineral lattice.  for colloidal mineral

ζ, colloidal particles repulsion , colloidal particles more stable The stability of colloidal particles in water is dependence on the electrostatic force and van der waals force. electrostatic force and van der waals force can be represented by Zeta potential : q = charge per unit area d = thickness of the layer surrounding the shear surface through which the charge is effective D = dielectric constant of the liquid ζ, colloidal particles repulsion , colloidal particles more stable

Colloid Destabilization By charge neutralization, colloids can be destabilized: Positively charges ions (Na+, Mg2+, Al3+, Fe3+ etc.) neutralize the colloidal negative charges and thus destabilize them. With destalization, colloids aggregate in size and start to settle

Theory Reducing zeta potential to vander waal force level: When coagulant adding to water, it will hydrolysis to form positively charged hydroxo-metallic ion complexe, These positive charges hydroxo-metallic will adsorped on the negative charge of the colloids surface, to reduce the zeta potential to destabilization point, these destabilized colloids and hydroxo-metallic complex by van der waals force adsorption and flocculation. These process can be speeded by agitation. Particle bridge: enmeshment of particles: coagulant complex can be polymerized. As concentration is larger than Ksp, it will settle down.

What is Coagulation? Coagulation is the destabilization of colloids by addition of chemicals that neutralizes the negative charges The chemicals are known as coagulants, usually higher valence cationic salt (Al3+, Fe3+ etc.) Coagulation is essentially a chemical process - Rapid mixing is required to disperse the coagulant throughout the liquid

Coagulation has three forms: electrokinetic: zeta potential reduction perikinetic : Brownian movement to make interparticle contact orthokinetic : agitation to make interparticle contact

Aluminum sulfate (Al2(SO4)3) Ferrous sulfate (FeSO4) Coagulant Aluminum sulfate (Al2(SO4)3) Ferrous sulfate (FeSO4) Ferric sulfate (Fe2(SO4)3) Ferric chloride (FeCl3) Polyaluminium Chloride (PAC): Al2(OH)3Cl3 Lime Coagulant aids : to produce a quick forming, dense, rapid settling floc and to insure optimum coagulation. Three different kinds of coagulant aids: Alkalinity addition : CaO, Ca(OH)2, Na2CO3 Polyelectrolytes : anionic : negative charge cationic : positive charge polyampholites: both negative and positive charge Turbidity addition : Clay Relative coagulating power: Na+ = 1; Mg2+ = 30 Al3+ > 1000; Fe3+ > 1000

What is Flocculation? Flocculation is the agglomeration of destabilized particles into a large size particles known as flocs which can be effectively removed by sedimentation or floatation. The addition of another reagent called flocculant or a flocculant aid may promote the formation of the floc Thus, flocculation is essentially a physical process that describes the transport of destabilized particles.

Jar Tests The jar test – a laboratory procedure to determine the optimum pH and the optimum coagulant dose A jar test simulates the coagulation and flocculation processes Determination of optimum pH Fill the jars with raw water sample (500 or 1000 mL) – usually 6 jars Adjust pH of the jars while mixing using H2SO4 or NaOH/lime (pH: 5.0; 5.5; 6.0; 6.5; 7.0; 7.5) Add same dose of the selected coagulant (alum or iron) to each jar (Coagulant dose: 5 or 10 mg/L) Jar Test

Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid mix helps to disperse the coagulant throughout each container Reduce the stirring speed to 25 to 30 rpm and continue mixing for 15 to 20 mins This slower mixing speed helps promote floc formation by enhancing particle collisions which lead to larger flocs Turn off the mixers and allow flocs to settle for 30 to 45 mins Then measure the final residual turbidity in each jar Plot residual turbidity against pH Jar Test set-up

The pH with the lowest residual turbidity will be the optimum pH Residual turbidity Versus pH Optimum pH: 6.3

Determination of optimum coagulant dose Repeat all the previous steps. This time adjust pH of all jars at optimum (6.3 found from first test) while mixing using H2SO4 or NaOH/lime Add different doses of the selected coagulant (alum or iron) to each jar (Coagulant dose: 5; 7; 10; 12; 15; 20 mg/L) Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid mix helps to disperse the coagulant throughout each container Reduce the stirring speed to 25 to 30 rpm and continue mixing for 15 to 20 mins

This slower mixing speed helps promote floc formation by enhancing particle collisions which lead to larger flocs Turn off the mixers and allow flocs to settle for 30 to 45 mins Then measure the final residual turbidity in each jar Plot residual turbidity against coagulant dose Coagulant Dose mg/L Optimum coagulant dose: 12.5 mg/L The coagulant dose with the lowest residual turbidity will be the optimum coagulant dose

Rapid Mixing (Coagulation) Objective To uniformly mix the coagulant with colloidal matters present in raw water so as to bring about colloidal destabilization How to achieve rapid mixing? L Horizontal baffled tank The water flows in horizontal direction. The baffle walls help to create turbulence when the water hit the surface and thus facilitate mixing W Plan view (horizontal flow) Vertical baffled tank The water flows in vertical direction. The baffle walls help to create turbulence when the water hit the surface and thus facilitate mixing H L Isometric View (vertical flow)

Hydraulic Jump: Hydraulic Jump creates turbulence and thus help better mixing. Coagulant In-line flash mixing Mechanical mixing Back mix impeller flat-blade impeller Inflow Chemical feeding Chemical feeding Inflow

Design of Flocculator (Slow & Gentle mixing) Flocculator is designed mainly to provide enough interparticle contacts to achieve particles agglomeration so that they can be effectively removed by sedimentation or floatation Transport Mechanisms Brownian motion: for relatively small particles which follow random motion and collides with other particles (Perikinetic motion) Differential settling: Particles with different settling velocity in the vertical alignment collides when one overtakes the other (Orthokinetic motion)

Mechanical mixing: Using different types of mechanical mixers to promote particles contacts and thus their agglomeration (Orthokinetic motion) L H Transverse paddle Cross flow Flocculator (sectional view) W Plan (top view)

The degree of mixing is measured by Velocity Gradient (G) Mixing and Power The degree of mixing is measured by Velocity Gradient (G) Higher the G value, the intense the mixing and vice versa Velocity Gradient is the relative velocity of the two fluid particles at a given distance G = dv/dy = 1.0/0.1 = 10 S-1 0.1 m 1 m/s In mixer design, the following equation is more useful: G= velocity gradient, S-1; P = Power input, W V = Tank volume, m3;  = Dynamic viscosity, (Pa.s)

G value for coagulation: 700 to 1000 S-1; 3000 to 5000 S-1 for Mixing time: 30 to 60 S in-line blender; 1-2 sec G value for flocculation: 20 to 80 S-1; Mixing time: 20 to 60 min In the flocculator design, Gt (also known Camp No.); a product of G and t is commonly used as a design parameter Typical Gt for flocculation is 2 x 104 - 105 Large G and small T gives small but dense floc Small G and large T gives big but light flocs We need big as well as dense flocs which can be obtained by designing flocculator with different G values 1 2 3 G1:40 G2:30 G3:20

Power Calculation How many horsepower do we need to supply to a flocculation basin to provide a G value of 100s-1 and a Gt of 100,000 for 10 MGD flow? (Given:  = 0.89 x 10-3 Pa.s; 1 hp = 745.7 watts) Solution: Retention time, t = Gt/G = 100,000/100 = 1000 secs Volume of Flocculation basin, V = (0.438 m3/sec) x (1000 sec) = 438 m3 P = G2 V x  = 1002 x 438 x 0.89 x10-3 = 3898 watts = 3898/745.7 = 5.22 hp