Coagulation and flocculation

Coagulation and flocculation

1. Class : T.Y. B. Tech Structural Subject: Water Supply Engineering Unit-III Sedimentation, Coagulation, Flocculation Prepared by Prof. H. N. Wagh Assistant Professor, Department of Structural Engineering Sanjivani College of Engineering, Kopargaon, 423603 Mail- [email protected], 7588026776

2. Sedimentation Water contains variety of particles- mineral as well as organic. Some particles may be colloidal, very fine or coarse. Also, the sp. gravity of the particles may range from 1.01 to 2.65. This indicates that different particles requires different time for their settlement. Sedimentation is the process of removal of suspended particles of impurities by gravitational settling, it occurs when particles are heavier than water.

3. Principle of Sedimentation The very fundamental principle under lying the process of sedimentation is that, the suspended matter present in water, is having specific gravity greater than that of water. In still water these particles will therefore tend to settle down by gravity, where as in a flowing water, they kept in suspension because of turbulence. Hence as soon as turbulence is retarded by offering storage to the water or reducing the flow velocity, the impurities tend to settle at the bottom of the tank. This the principle of sedimentation.

4. The basin in which the flow of water is retarded is called as settling tank or sedimentation tank. Detention Time: The theoretical average time for which the water is detained in the tank is called the detention period. OR detention time of sedimentation tank may be defined as the average theoretical time required for the water to flow through the tank. Purpose of Settling: To remove coarse dispersed phase. To remove coagulated and flocculated impurities. To remove precipitated impurities after chemical treatment. To settle the sludge (biomass) after activated sludge process / tricking filters

5. Types of sedimentation Plain Sedimentation: When impurities are separated from water by action of natural forces alone i.e. gravitational and natural aggregation of settling particles, the process is called plain sedimentation. Suspended particles of size 0.01mm and above and having sp. gr. greater than 1 are removed by plain sedimentation. Sedimentation with coagulation: When chemicals or other substances are added to induce or hasten aggregation and settling of finely divided suspended matter, colloidal substances and large molecules, the process is called sedimentation with coagulation or simply clarification.

6. Factors Affecting Sedimentation Shape , Size, Density and Nature of particles Viscosity , density and temperature of water/wastewater Surface overflow rate Velocity of flow Inlet and outlet arrangements Detention period Effective depth of settling zone

7. TYPES OF SEDIMENTATION Type -I: Discrete particle settling No interaction between particles Settling velocity is constant for individual particles Dilute solid’s concentration Examples : pre-sedimentation in water treatment, grit removal in wastewater. Type –II: Flocculent settling Particles collide and adhere to each other resulting in particle growth Dilute solid’s concentration Examples : coagulation/flocculation settling in water treatment and primary sedimentation in wastewater treatment

8. Type III: Hindered or Zone Settling Particles are so close together movement is restricted Intermediate solids concentration Solids move as a block rather than individual particles Fluidic interference causes a reduction in settling velocity Distinguishable solids liquid interface Example : settling of secondary effluents Type IV: Compression Settling Particles physically in contact Water is squeezed out of interstitial spaces Volume of solids may decrease High concentration of solids (sludges)

9. Types of Sedimentation Tanks Depending upon various factors sedimentation tanks are classified as follows. 1. Based on methods of operation a. Fill and draw type tank b. Continuous flow type tank 2. Based on shape a. Circular tank b. Rectangular tank c. Hopper bottom tank 3. Based on location a. Primary tank b. Secondary tank

10. Fill and Draw Type of Sedimentation Tank: In case of fill and draw type sedimentation tank, water is stored for some time in the tank after it is filled completely. The time may be 24 hours. In that time, the suspended particles are settled at the bottom of the tank. After 24 hours, the water is discharged through outlet. Then settled particle are removed. This removal action requires 6-12 hours. So, one complete action of sedimentation requires 30-40 hours in case of fill and draw type sedimentation tank

11. Continuous Flow Type Sedimentation Tank In this type of thank, water is not allowed to rest. The flow always takes place but with a very small velocity. During this flow, suspended particles are settle at the bottom of the tank. The flow may be either in horizontal direction or vertical direction.

12. Circular Tank Circular sedimentation tanks are mostly preferred for continuous flow type sedimentation. In this case influent is sent through central pipe of the tank and radial flow takes place. Mechanical sludge scrappers are provided to collect the sludge and collected sludge is carried through sludge pipe provided at the bottom. But circular tanks are uneconomical as compared to rectangular tanks but they have high clarification efficiency.

13. Hopper Bottom Tank In case of hopper bottom tank, a deflector box is located at the top which deflects the influent coming from central pipe to downwards. Sludge is collected at the bottom and it is disposed through sludge pump.

14. Rectangular sedimentation tank with scraper arrangement

15. Circular sedimentation tank

16. Circular sedimentation tank with plan and section

17. Inlet and Outlet Arrangement for Settling Tan k

18. Vertical cross-section through a rectangular sedimentation tank H = depth of settling zone L = length of settling zone B = width of settling zone V = volume of settling zone Q = volumetric flow rate v = flow speed/velocity T = detention time = hydraulic retention time

19. Inlet Zone: The inlet distributes flow uniformly across the inlet to the tank. The normal design includes baffles that gently spread the flow across the total inlet of the tank and prevent short circuiting in the tank. Settling Zone: The settling zone is the largest portion of the sedimentation basin. This zone provides the calm area necessary for the suspended particles to settle. Sludge Zone: The sludge zone, located at the bottom of the tank, provides a storage area for the sludge before it is removed for additional treatment or disposal. Sludge is removed for further treatment from the sludge zone by scraper or vacuum devices which move along the bottom. Outlet Zone: Outlet zone (or launder) should provide a smooth transition from the sedimentation zone to the outlet from the tank. This area of the tank also controls the depth of water in the basin. Weirs set at the end of the tank control the overflow rate and prevent the solids from rising to the weirs and leaving the tank before they settle out.

20. Design Details Detention period: for plain sedimentation: 3 to 4 h, and for coagulated sedimentation: 2 to 2.5 h. Velocity of flow: Not greater than 30 cm/min (horizontal flow). Tank dimensions: L:B = 3 to 5:1. Generally L= 30 m (common) maximum 100 m. Breadth= 6 m to 10 m. Circular: Diameter not greater than 60 m. generally 20 to 40 m. Depth 2.5 to 5.0 m (3 m). Surface Overflow Rate: For plain sedimentation 12000 to 18000 L/d/m2 tank area; for thoroughly flocculated water 24000 to 30000 L/d/m2 tank area. Slopes: Rectangular 1% towards inlet and circular 8%

21. Coagulation with Flocculation Very fine Suspended clay particles, electrically charged colloidal particles and many more cannot settle down due to gravitational force in sedimentation tank. When water contains such fine impurities, it becomes necessary to apply such process which can easily remove them from water. Hence to remove such impurities certain chemicals are added to the water called as coagulant and the process is known as coagulation. A coagulant is the substance (chemical) that is added to the water to accomplish coagulation.

22. PARTICLES SETTLING VELOCITIES Particle size, mm Type Settling velocity 10 Pebble 0.73 m/s 1 0.1 Course sand Fine sand 0.23 m/s 0.6 m/min 0.01 Silt 8.3 m/day 0.0001 Large colloids 0.3 m/year 0.000001 Small colloids 3 m/million years

23. Coagulation: The process of mixing certain chemicals in water to neutralize the electrical charges on the particles and to form an insoluble, gelatinous flocculent precipitate for absorbing and entraining suspended and colloidal particles of impurities is called coagulation. Factors affecting the coagulation: i) Type of coagulant ii) Dose of coagulant iii)Time and method of mixing of the coagulant iv) Character of water – pH, Temperature, nature and quantity of impurities.

24. Coagulation process: i) feeding ii) mixing iii) flocculation iv) sedimentation or clarification

25. Flocculation: flocculation is essentially an operation designed to force agitation in the water and induce coagulation. Basically flocculation is a slow mixing process or agitation process in which destabilized colloidal particles are brought into intimate contact in order to promote their agglomeration. The operation , slow mixing is achieved in basin commonly know as the Flocculator.

26. The objective of the flocculation step is to cause the individual destabilised colloidal particles to collide with one another and with the precipitate formed by the coagulant in order to form larger floc particles. Flocculation involves the stirring of water to which a coagulant has been added at a slow rate, causing the individual particles to “collide” with each other and with the flocs formed by the coagulant. In this way the destabilised individual colloidal particles are agglomerated and incorporated into the larger floc particles. The rate of agglomeration or flocculation is depends on: Concentration of turbidity, type of coagulant added and its dose and mean velocity gradient (G) in the basin.

27. Flocculation is controlled through the introduction of energy into the water (through paddles or by means of baffles in the flocculation channel) to produce the right conditions (required velocity gradient) for floc to grow to the optimum size and strength. The velocity gradient “G” value is an extremely important factor that determines the probability of particles to collide and form floc. If the G-values are too low, the probability of collisions is low and poor floc formation results. If G values is too high, shear forces become large and this may result in floc break-up. Acceptable G-values for the coagulation process is between 400 and 100 per seconds. . For the flocculation process, it is in the order of 100 per seconds.

28. Paddle Flocculators

29. Commonly used coagulants: The commonly used metal coagulants fall into two general categories: those based on aluminum and those based on iron. The aluminum coagulants include aluminum sulfate, aluminum chloride and sodium aluminate. The iron coagulants include ferric sulfate, ferrous sulfate, ferric chloride and ferric chloride sulfate. Other chemicals used as coagulants include hydrated lime and magnesium carbonate. When metal coagulants are added to water the metal ions (Al and Fe) hydrolyze rapidly but in a somewhat uncontrolled manner, forming a series of metal hydrolysis species. The efficiency of rapid mixing, the pH, and the coagulant dosage determine which hydrolysis species is effective for treatment.

30. COAGULANT SELECTION: The choice of coagulant chemical depends upon the type of suspended solid to be removed, raw water conditions, facility design, and cost of chemical. Final selection of coagulant (or coagulants) should be made with jar testing and plant scale evaluation. Consideration must be given to required effluent quality, effect upon down stream treatment process performance, cost, method and cost of sludge handling and disposal, and cost of the dose required for effective treatment

31. Inorganic Coagulants: Inorganic coagulants such as aluminum and iron salts are the most commonly used. When added to water, these highly charged ions to neutralize the suspended particles. The inorganic hydroxides that are formed produce short polymer chains which enhance microfloc formation. Inorganic coagulants usually offer the lowest price per kg, are widely available, and, when properly applied, are effective in removing most suspended solids. They are also capable of removing a portion of the organic precursors which may combine with chlorine to form disinfection by-products.

32. Inorganic coagulants produce large volumes of floc which can also entrap bacteria as they settle. Inorganic coagulants may alter the pH of the water since they consume alkalinity. When applied in a lime soda ash softening process, alum and iron salts generate demand for lime and soda ash. They also require corrosion-resistant storage and feed equipment. It is important to note that large volumes of settled floc must be disposed of in an environmentally acceptable manner. Alum, ferric sulfate, and ferric chloride, lower the alkalinity, and pH of water.

33. Alum A12(SO4)3.18H2O+3Ca(HCO3)2---------> 2 Al(OH)3 + 3CaSO4 + 18H2O +6 CO2. Ferric Sulfate Fe2(SO4)3 + 3 Ca(HCO3)2 ------------> 2 Fe(OH)3 + 3CaSO4 + 6 CO2. Ferric Chloride 2 Fe Cl3 + 3 Ca(HCO3)2 ------------> 2 Fe(OH)3 + 3CaCl2 + 6CO2

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