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sedimentation
- 2. Sedimentation • Water containsvariety 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 basinin 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 •PlainSedimentation: 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: Hinderedor 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)
- 10. • Types ofSedimentation 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
- 11. • Fill andDraw 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
- 12. • Continuous FlowType 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.
- 13. • 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.
- 14. • Hopper BottomTank • 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.
- 15. Rectangular sedimentation tankwith scraper arrangement
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- 17. Circular sedimentation tankwith plan and section
- 18. Circular Tank withMechanical scraper arrangement
- 19. Inlet and OutletArrangement for Settling Tank
- 20. Vertical cross-section througha 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
- 21. • 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.
- 22. • 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%
- 23. Tube Settler • Tubesettlers are commonly used in rectangular clarifiers to increase the settling capacity by reducing the vertical distance a suspended particle must travel. • High efficiency tube settlers use a stack of parallel tubes, rectangles or flat pieces separated by a few inches (several centimeters) and sloping upwards in the direction of flow. • This structure creates a large number of narrow parallel flow pathways encouraging uniform laminar flow as modeled by Stokes' law.
- 24. • Tube settlersystems are an inexpensive solution for drinking water and wastewater plants to increase treatment capacity, reduce new installation footprints, improve effluent water quality, and decrease operating costs. • Constructed of lightweight PVC. • tube settler modules can be easily supported with minimal structures that often incorporate effluent troughs and baffles. Modules are available in a variety of sizes to fit any tank geometry and tube lengths to accommodate a wide range of flows. • Tube settler modules consist of a series of sloped, tubular channels and structural wave sheets that reduce particle settling distance and increase the effective settling area in a clarifier or sedimentation basin.
- 27. • Very costeconomical since it uses 1/10th of space required by conventional clarifiers. • No moving parts, operates totally on Gravity. • No maintenance list. • Laminar flow ensures excellent treated water quality. • Wide choice of materials of construction such as steel, concrete/FRP. • Non-corrosive tubes can be lifted easily for cleaning without shutdown. • Flexible configurations • Required Very Less Space • Easy installation • No Power Consumption • Simple in Operation • Easily Mobility Advantages of Tube Settlers