SEMINAR

SEDIMANTATION AIDED WITH COAGULATION METHODS

OF

PURIFICATION OF WATER

SUBJECT-BUILDING SERVICES

BY:-

AMIL KUMAR

IVTH SEMESTER

2013BAR002

Sedimentation Aided with Coagulation1

Some of the colloidal impurities will not settle even if the water is detained for long periods in the sedimentation tanks as the same charge on the clay particles repel each other and do not allow them to settle down. So the sedimentation is aided with coagulation. Coagulation is a process in which some chemical like alum or ferrous sulphate is mixed in water resulting in particle destabilization. Flocculation is the second stage of the formation of settleable particles (or flocs) from destabilized (neutral) colloidal particles and is achieved by gentle (slow) mixing. So in flocculation the alum is first mixed rapidly for dispersion and then slow mixing produces flocks. Both these stages of flocculation are greatly influenced by physical and chemical forces such as electrical charge on particles, exchange capacity, particle size and concentration, pH, water temperature and electrolyte concentration.

A large portion of particles suspended in water can be sufficiently small that their removal by sedimentation or filtration is not practicable. Most of these small particles are negatively charged, which is the major cause of the stability of suspended soil particles. Particles which might otherwise settle are mutually repelled by these charges and remain in suspension. Coagulation is a chemical technique directed toward destabilization of particle suspension. The most commonly used coagulant is alum (aluminum sulfate). Coagulation is usually followed by flocculation, which is a slow mixing technique promoting the aggregation of the destabilized (coagulated) particles. Coagulation followed by flocculation as an aid to sedimentation and filtration has been practiced for centuries. It is by far the most widely used process for the removal of substances producing turbidity in water. If water has high turbidity, flocculation followed by sedimentation is often used to reduce the quantity of material prior to entering the filter.

Filters for suspended particle removal can be made of graded sand, granular synthetic material, screens of various materials, and fabrics. The most widely used are rapid-sand filters in tanks. In these units, gravity

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holds the material in place and the flow is downwards. The filter is periodically cleaned by a reversal of flow and the discharge of backflushed water into a drain. Cartridge filters made of fabric, paper, or plastic material are also common and are often much smaller and cheaper and are disposable. Filters are available in several ratings depending on the size of particles to be removed. Activated carbon filters, described earlier, will also remove turbidity, but would not be recommended for that purpose only.

SEDIMENTATION

● Sedimentation is the separation from water, by gravitational settling, of suspended particles that are heavier than water.

● Sedimentation is used for separation of grit and particulate matter in the primary settling basin, separation of biological-floc in the activated-sludge settling basin, and separation of chemical-floc when the chemical coagulation process is used. It is also used for solids concentration in sludge thickeners.

● Sedimentation basins are constructed in a variety of shapes and sizes, circular tanks or rectangular tanks.

● The basin is comprised of four zones according to function:

1-The inlet zone. 2-The settling zone.

3-The sludge zone. 4-The outlet zone.

● The inlet zone is a region where the incoming suspension is distributed uniformly over the cross-section of the tank.

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● In the settling zone, the particles settle at the same rate as they would in a quiescent.

● In the outlet zone, the clarified liquid is collected uniformly over the cross-section of the basin.

● The solids collect in a sludge zone at the bottom of the tank.

RECTANGULAR BASIN WORKING

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RECTANGULAR BASIN VIEW

CIRCULAR BASIN WORKING

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CIRCULAR BASIN VIEW

Purpose

• The primary purpose of the coagulation/flocculation process is the removal of turbidity from the water.

•Turbidity is a cloudy appearance of water caused by small particles suspended therein.

• Water with little or no turbidity will be clear.

•Water with a high turbidity can be very difficult to properly disinfect.

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Why coagulation

Various sizes of particles in raw water

Particle diameter (mm)

TYPESettling velocity

10Pebble 0.73 m/s

1Course sand 0.23 m/s

0.1Fine sand 0.6 m/min

0.01Silt 8.6 m/d

0.0001 (10 micron) Large colloids 0.3 m/y

0.000001 (1 nano) Small colloids 3 m/million y

Colloids – so small in size: gravity settling not possible.

Coagulant aided sedimentation required to remove following impurities 1.Miscellaneous fragments of animal and vegetable matter

2. plankton mainly phytoplankton (microscopic plants).

3.Finely divided colloidal matter and clay.

4.Organic colouring matter partly in solution and partly in colloidal form.

5.Bacteria and viruses in small extent.

6.Complex mixture of organic compounds.

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Coagulation-Flocculation •In conventional coagulation-flocculation-sedimentation, a coagulant is added to the source water to create an attraction among the suspended particles.

•The mixture is slowly stirred to induce particles to clump together into “flocs.”

•The water is then moved into a quiet sedimentation basin to settle out the solids.

In the flash mixer, coagulant chemicals are added to the water and the water is mixed quickly and violently.

•The purpose of flash mixing is to evenly distribute the chemicals through the water.

• Flash mixing typically lasts a minute or less. If the water is mixed for less than thirty seconds, then the chemicals will not be properly mixed into the water.

• However, if the water is mixed for more than sixty seconds, then the mixer blades will shear the newly forming floc back into small particles.

After flash mixing, coagulation occurs. During coagulation, the coagulant chemicals neutralize the electrical charges of the fine particles in the water, allowing the particles to come closer together and form large clumps.

•The final step is flocculation. During flocculation, a process of gentle mixing brings the fine particles formed by coagulation into contact with each other.

•Flocculation typically lasts for about thirty to forty-five minutes.

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To summarize, •Coagulation Is •The addition and rapid mixing of coagulants

•The destabilization of colloidal and fine particles

•The initial aggregation of destabilized particles

•Flocculation Is •The gentle agitation to aggregate destabilized particles to form rapid-settling.

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Coagulation can be caused by any of the following: 1.Double layer compression

2.Charge neutralization.

3.Bridging.

4.Colloid entrapment.

1. DOUBLE LAYER COMPRESSION THEORY

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An Electric Double Layer consists of three parts:

•Surface charge - charged ions (commonly negative) adsorbed on the particle surface.

•Stern layer - counter ions (charged opposite to the surface charge) attracted to the particle surface and closely attached to it by the electrostatic force.

•Diffuse layer - a film of the dispersion medium (solvent) adjacent to the particle. Diffuse layer contains free ions with a higher concentration of the counter ions. The ions of the diffuse layer are affected by the electrostatic force of the charged particle.

•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.

•The zeta potential as the amount of repulsive force which keeps the particles in the suspension.

• If the zeta potential is large, then more coagulants will be needed.

When coagulants(Electrolytes) are added into the water it changes ionic concentration.

•Which compresses double layer and weakens repulsive forces.

•The basic goal of coagulation is to reduce the net repulsive force.

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2. Charge Neutralization

3. Bridging

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Bridging occurs when a coagulant forms threads or fibers which attach to several colloids, capturing and binding them together.

• Inorganic primary coagulants and organic polyelectrolytes both have then capability of bridging.

• Higher molecular weights mean longer molecules and more effective bridging.

4. Colloidal entrapment

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