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Design fundamentals of sedimentation tanks

Design fundamentals of sedimentation tanks. Submitted by: Keerti Upadhyay 4 th yr. Civil engg. Sedimentation :

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Design fundamentals of sedimentation tanks

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  1. Design fundamentals of sedimentation tanks Submitted by: Keerti Upadhyay 4th yr. Civil engg.

  2. Sedimentation : Sedimentation is a process used for various stages within a wastewater treatment plant to reduce the concentration of settleable and suspended matter. Sedimentation tanks, commonly referred to as clarifiers, are described as primary, intermediate and final clarifiers dependent upon the process stage. Primary clarifiers are installed to reduce organic loadings and floatables to the downstream treatment processes. Intermediate and/or final clarifiers are utilized after biological treatment processes to separate chemical and/or biological floc from the treatment process.

  3. Types of settling patterns: • Discrete settling • Flocculent settling • Hindered or zone settling • Compression settling

  4. General design consideration 1. Multiple units capable of independent operation shall be provided for plants having an average design capacity greater than 0.1 MGD unless temporary removal of a single unit from service for repairs will not result in an adverse effect to the quality of the receiving stream. 2. Clarifiers shall be arranged to facilitate operating flexibility and maintenance, assure continuity of treatment, and ease of installation of future units.

  5. Provision shall be made for dewatering and bypassing each unit independently process units. Effective flow measurement devices and controls shall be provided to permit proper flow distribution to each unit. The anticipated flow pattern should be considered in the selection of clarifier configuration and location and type of inlets and outlets Due consideration shall be given to the possible need for hydrostatic pressure relief devices to prevent structure flotation.

  6. Performance Unless laboratory data are available, primary settling shall be assumed to remove one- third of the influent BOD and 55 percent of the influent suspended solids. It is not recommended to return waste activated sludge to the primary clarifier.

  7. Types of sedimentation tanks: • Classification based on nature of working: • Fill and draw type • Continuous flow type • Classification based on location • Primary tank • Secondary tank • Classification based on shape • Rectangular tank • Circular tank • Hopper bottom type

  8. Design considerations Detention time: Flocculation proceeds through the coalescence of fine particles at a rate which is function of concentration and its ability to coalesce on collisions. Detention time: 1.5 to 2.5 hrs(primary) 1.5 to 2 hrs(sec) For cold climate safety factor is taken proper care. Generally 1.38 times of detention time required at 20 degree celcius. T=c/q where c= capacity of tank;(rec) T=d^2(.011d+.785h)/q (circular) T=h/sor

  9. Detention time(factors affecting it) Actual settling basins are affected by the dead spaces in the basins, eddy currents, wind currents and thermal currents. Dead spaces and eddy currents have rotational flow and do very little sedimentation since the inflow and outflow from these spaces is very small. As a result, the net volume available for settling is reduced and the mean flow-through time for the fluid element is decreased. Also, wind and thermal currents create flows that pass directly from the inlet to the outlet of the basin, which decreases the mean flow-through time

  10. Flow velocity: (camp formula) V=(8Bg/f(s-1)d)^.5 S specific gravity of particles B constant depending on type of material scoured F Darcy Weisbach friction factor V=k(g(s-1)d)^.5 , k=3 to 4.5 (Shield for)

  11. Surface loading rate and depth consideration: It represents hydraulic loading per unit surface area of tank in unit time. Overflow rates must be checked for average and peak flow Surface-Loading Rates a. Primary Clarifiers Surface-loading rates for primary clarifiers should not exceed 1,000 gallons per day per square foot at average design flow and shall not exceed 1,500 gallons per day per square foot at peak hourly flow.

  12. Surface-loading rates for secondary clarifiers following fixed film reactors should not exceed 800 gallons per day per square foot at average design flow and shall not exceed 1,200 gallons per day per square foot at peak hourly flow. 2) Surface-loading rates for final clarifiers following activated sludge processes, such as: conventional, step aeration, contact stabilization shall not exceed 1,200 gallons per day per square foot at peak hourly flow. 3) Surface-loading rates for final clarifiers following an extended aeration process shall not exceed 1,000 gallons per day per square foot at peak hourly flow.

  13. Inlets Inlets should be designed to dissipate the inlet velocity, to distribute the flow uniformly and to prevent short-circuiting. Provisions shall be made for removal of floating materials in inlet structures having submerged ports. Orifices placed in walls at the inlets should be sized to produce velocities from 0.5 to 1.0 fps. Orifices passing wastewater containing floc should not be smaller than 0.3 to 0.5 inches, to minimize floc breakup.

  14. Weir loading: Outlet weirs draw effluent without disturbing quiescent condition of tank particularly secondary ones.for all tanks except secondary tank of activated sludge process it is 100 m3/d/m otherwise it is 150m3/d/m. To achieve these parameters long weirs are avoided,regular indentation are made to have distributed flow.a free fall of .05 to.15 is arranged on total head available.

  15. Proportioning of tank The minimum distance from the influent inlet to effluent weirs shall be at least 10 feet for all clarifier configurations unless special provisions are made to prevent short-circuiting. Circular tanks: Dia varies from 3 to 60 m common being 12 to 30m. Depth is 2.5 for primary and 3.5 for secondary tank.floors are sloped from periphery to centre @7.5 to 10%.

  16. It is preferable that the sludge collector scraping mechanism be at least 6 feet below the water level. The length-to-width ratio for rectangular clarifiers should be equal to or greater than 4 to 1. Length to depth ratio 5 to 1, 25 to 1 The maximum horizontal velocity in a rectangular clarifier shall not exceed 1.5mph near the sludge layer in a primary clarifier and 2 feet per minute in a final clarifier. The side water depth (SWD) of mechanically cleaned clarifiers shall not be less than 7 feet for primary clarifiers and those following fixed film reactors.

  17. Free board The outer walls of clarifiers shall extend at least 6 inches above the surrounding terrain and shall provide at least 12 inches of freeboard to the water surface. Where clarifier walls do not extend 4 feet above the surrounding terrain, a cover, fence, wind screen or suitable barrier shall be provided to prevent high wind currents and debris from entering the clarifier and ice buildup which inhibits scum removal and settling

  18. Removal efficiency of tank Eddy currents Surface currents Vertical convection currents Density currents Y/y0=1-(1+nV/(q/a))^-(1/n) n=0 best,n=1 very poor Y/y0= efficiency of removal n= tank performance coefficient V= sor for ideal tank q/a=required sor to get needed efficiency

  19. SLUDGE HANDLING AND WITHDRAWAL 1. Mechanical sludge collection and withdrawal equipment is required and shall provide complete and continuous removal of settled sludge for intermediate and final clarifiers. The sludge collection equipment and the drive assembly shall be designed to withstand the maximum anticipated loads of transporting sludge to a hopper. 2.The floors of circular clarifiers shall be sloped at one-inch per foot (1:12) to form an inverted cone to a central sludge hopper. 3.The floor of a rectangular clarifier should be sloped at approximately 1 percent (1:100) toward the cross collectors or sludge hopper(s) located at the influent end.

  20. 4.Sludge draw-off pipes located on the side or bottom, shall be flush with the hopper bottom. Each sludge hopper shall have an individually valved sludge draw-off line that is at least 6 inches in diameter, if gravity flow, and, if pumped, 4 inches in diameter. The static head available for withdrawal of sludge shall be at least 30 inches to maintain a three foot per second (3 fps) velocity when removal is dependent upon gravity flow.

  21. Thank u…

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