BY Dilip Kumar Kahalgaon Super Thermal Power StationNTPC Limited

In pretreatment of water insoluble/suspended

impurities and living organisms are removed from raw

water. Pretreatment of water includes the following

processes:

Coagulation

Flocculation

Sedimentation

Filtration &

Disinfection

Mud/Clay

Sand

Silt

Protozoan

Bacteria

Rotifers

Algae

Diatoms

Coagulation is the process of destabilization by charge neutralization . Once neutralized, particles no longer repel each other and can be brought together . Coagulation is necessary for the removal of colloidal sized suspended matter.

The process in which the negative charge on particles is neutralized, usually by addition of positive charges such as those provided by alum. The neutralization of particles allows them to clump together forming larger particles which are easier to settle

A high-energy, rapid-mix to properly disperse the coagulant and promote particle collisions is needed to achieve good coagulation. Over-mixing does not affect coagulation, but insufficient mixing will leave this step incomplete. Coagulants should be added where sufficient mixing will occur. Proper contact time in the rapid-mix chamber is typically 1 to 3 minutes.

COAGULANTS

Common coagulant chemicals used are alum, ferric sulfate, ferric chloride, ferrous sulfate, and sodium aluminate. The first four will lower the alkalinity and pH of the solution while the sodium aluminate will add alkalinity and raise the pH.

A12(SO4)3 + 3 Ca(HCO3)2------------> 2 Al(OH)3 + 3CaSO4 + 6 CO2

Alum already in the waterto treat

COAGULANTS

Typical iron and aluminum coagulants are acid salts that lower the pH of treated water by hydrolysis. Depending on initial raw water alkalinity and pH, an alkali such as limeor caustic must be added to counteract the pH depression of the primary coagulant.

Alum and lime dosing

POLYMERS

Low-to-medium weight, positively charged(cationic) polymers may be used alone or incombination with the aluminum and iron typecoagulants to attract the suspended solids andneutralize their surface charge . Polymers areeffective over a wider pH range than inorganiccoagulants. They can be applied at lowerdoses, and they do not consume alkalinity.They produce smaller volumes of moreconcentrated, rapidly settling floc. The flocformed from use of a properly selectedpolymer will be more resistant to shear,resulting in less carryover and a cleanereffluent. Poly-aluminum chloride

(Aln (OH)mCl3n-m)x

Following the first step of coagulation, a second process calledflocculation occurs.

Flocculation is the process of bringing together the destabilized or “

Coagulated” particles to form a larger agglomeration, or “floc”.

Flocculation, a gentle mixing stage, increases the particle size fromsubmicroscopic microfloc to visible suspended particles.

Flocculation Zone of clarifier

Design contact times for flocculation range from 15 or 20 minutes to an hour or more.

Flocculation Zone

Flocculation requires careful attention to the mixing velocity and amount of mix energy. To prevent the floc from tearing apart or shearing, the mixing velocity and energy input are usually tapered off as the size of the floc increases. Once flocs are torn apart, it is difficult to get them to reform to their optimum size and strength.

.S Sedimentation refers to the physical removal of suspension, or settling, that occurs once the particles has been coagulated and flocculated.

DESIGN CONSIDERATIONS

REACTOR CLARIFIER DESIGN

GEAR DRIVE

PERIMETER LAUNDERS

RADIAL LAUNDERS

EFFLUENT

STEEL HOOD

ACCESS

SLUDGE SCRAPPER

DESIGN CONSIDERATIONS

REACTOR CLARIFIER DESIGN

.S The process of passing water through a filter to remove particles. In water treatment, the filter material is typically sand, sometimes with a layer of anthracite (coal) above.

Clarified water in

Filtered water out

Gravity Sand Filter (GSF)

The process of killing disease-causing microorganisms. Commonly Chlorine is used as a disinfectant in pre-treatment of water.

Liquid chlorine in tonnersEvaporators converts liq. Cl2 to gaseous Cl2

Booster pump

Gaseous chlorine

Ch

lorin

ate

d w

ate

r

Water under treatment

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