sedimentation and its types

Water supply

1. sedimentation BY.M.Z .J

2. sedimentation Sedimentation is water treatment process, during which water has little or no movement, and suspended solids sink to the bottom under the force of gravity and form a sediment, this process is called sedimentation.

3. Purpose of sedimentation 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.

4. Sedimentation Principle Suspended solids present in water having specific gravity greater than that of water tend to settle down by gravity as soon as the turbulence is retarded by offering storage. Inorganic suspended solids having specific gravity of about 2.65; and   Organic suspended solids having specific gravity of about 1.04 The particles having specific gravity of about 1.20 or so readily settle down at the bottom of tank. But it is difficult to cause the settlement of lighter particles.  This phenomenon of settling down of particles at the bottom of sedimentation tank is known as hydraulic subsidence and every particle has its own hydraulic settling value which will cause its hydraulic subsidence . Basin in which the flow is retarded is called  sedimentation basin. Theoretical average time for which the water is detained in the settling tank is called the  detention period

5. Types of sedimentation Plain sedimentation : It is process of settling down of solids and impurities in the raw water to the bottom of sedimentation basin by natural gravity force alone, no chemical is added. This method is very cheaper and mostly used in all filtration and purification system of water. Sedimentation by using clarifier and contact : In this method chemicals are mixed in water and that water is rotated by help of pumps for period of two hours per day, and suspended solids are settled down in the bottom of reservoir or tank etc.

6. Types of sedimentation Chemically Assisted sedimentation or clarification: this is process in which chemicals are added to water and through mixing the suspended solids and other impurities are stick together and form floc , which settles to the bottom of basin. In general most used process is chemically assisted horizontal sedimentation and following assumptions are based on. water generally flows through a tank as an irregular flow thus the intention of sedimentation is to create condition in which the flow is uniform as for long enough period, to permit the maximum practical amount of floc to be settled before the water reaches at the end of tank.

7. Parameters influencing sedimentation Size, shape and weight of particles. Viscosity and temperature of water. Detention period of tank. Effective depth of tank. Area of tank. Surface over flow rate. Velocity of flow and outlet design.

8. Type of sedimentation basin Sedimentation tank may function either intermittent or continuously. The intermittent type tank are those which store water for a certain period and keep it on completely rest. In continuous type tank , the flow velocity is only reduced and water is not brought to complete rest . Sedimentation tank or basin may be either circular or rectangular in plan. Long narrow rectangular tanks with horizontal flow are generally preferred to the circular tank of radial or spiral flow.

9. Rectangular sedimentation basin Rectangular basins are the simplest design, allowing water to flow horizontally through a long tank  This type of basin is usually found in large-scale water treatment plants. Rectangular basins have a variety of advantages such as predictability, cost-effectiveness, and low maintenance. They are the least likely to short-circuit, especially if the length is at least twice the width. The inlet and outlet arrangements of rectangular basis are shown below. A disadvantage of rectangular basins is the large extent of land area required.

10. circular sedimentation basin Square and circular sedimentation basins with horizontal flow are often known as clarifiers . This type of basin  faces short-circuiting problems. below Figure shows the inlet and outlet arrangements .

11. Design detail Detention period for plain sedimentation 4-8 hours and for coagulated sedimentation 3-4 hours. Velocity of flow not greater than 30cm/ mint( 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%.

12. Detention period The theoretical (calculated)  time required for a given amount of water or wastewater to pass through a tank at a given rate of flow. In other words it is average time for which the water is detained in tank. t= = = = Where t is detention period, Q is flow rate.  

13. Detention period Detention time is inversely proportional to the incoming flow rate - as flow rate increases, the detention time decreases . Example Calculate the detention time for a rectangular sedimentation tank with the following dimensions, length is 20m, width is 10m, depth is 4m, and the flow rate is 0.5 per sec. detention time =  tank volume                          incoming water flow rate  

14. Detention period Note units of measurement are all consistent. ie time is given in seconds and length is given in meters . detention time =  (20 x 10 x 4)                                 0.5 detention time =  800                           0.5 detention time = 1600 seconds detention time = 26.7 minutes The term  detention  is often used interchangeably with  retention  and  hydraulic retention time .

15. http://nptel.ac.in/courses/105104102/Domestic%20water%20treat.htm http://www.thewatertreatments.com/wastewater-sewage-treatment/types-sedimentation-basin/