Pretreatment of water

Water having different types of impurities e.g., soluble, insoluble, micro-organisms and gases. This presentation helps to understand how "insoluble impurities" can removes from water.

1. PRETREATMENT OF WATER BY Dilip Kumar Kahalgaon Super Thermal Power Station NTPC Limited

2. PRETREATMENT OF WATER 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

3. INSOLUBLE IMPURITIES IN WATER Mud/Clay Sand Silt Protozoan Bacteria Rotifers Algae Diatoms

4. COAGULATION 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.

5. COAGULATION 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

6. COAGULATION 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.

7. COAGULATION 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. A1 2 (SO 4 ) 3 + 3 Ca(HCO 3 ) 2 ------------ > 2 Al(OH) 3 + 3CaSO 4 + 6 CO 2 Alum already in the water to treat

8. COAGULATION 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 lime or caustic must be added to counteract the pH depression of the primary coagulant. Alum and lime dosing

9. COAGULATION POLYMERS Low-to-medium weight, positively charged (cationic) polymers may be used alone or in combination with the aluminum and iron type coagulants to attract the suspended solids and neutralize their surface charge . Polymers are effective over a wider pH range than inorganic coagulants. They can be applied at lower doses, and they do not consume alkalinity. They produce smaller volumes of more concentrated, rapidly settling floc. The floc formed from use of a properly selected polymer will be more resistant to shear, resulting in less carryover and a cleaner effluent. Poly-aluminum chloride (Aln (OH)mCl3n-m)x

10. FLOCCULATION Following the first step of coagulation, a second process called flocculation 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 from submicroscopic microfloc to visible suspended particles. Flocculation Zone of clarifier

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

12. FLOCCULATION 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.

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

14. DESIGN CONSIDERATIONS REACTOR CLARIFIER DESIGN GEAR DRIVE PERIMETER LAUNDERS RADIAL LAUNDERS EFFLUENT STEEL HOOD ACCESS SLUDGE SCRAPPER

15. DESIGN CONSIDERATIONS REACTOR CLARIFIER DESIGN

16. FILTERATION .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)

17. DISINFECTION The process of killing disease-causing microorganisms. Commonly Chlorine is used as a disinfectant in pre-treatment of water. Liquid chlorine in tonners Evaporators converts liq. Cl 2 to gaseous Cl 2 Booster pump Gaseous chlorine Chlorinated water Water under treatment

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