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Review of Settling

Review of Settling. AKA sedimentation – suspended solids removed from liquid phase by gravity Common applications in Wastewater Treatment – grit chamber, primary settling basin, secondary settling basin

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Review of Settling

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  1. Review of Settling • AKA sedimentation – suspended solids removed from liquid phase by gravity • Common applications in Wastewater Treatment – grit chamber, primary settling basin, secondary settling basin • Common applications in Water Treatment – settling after coagulation/flocculation, settling after lime-softening precipitation, settling after iron and manganese removal

  2. 4 Settling Categories • Type I - discrete settling in dilute suspensions, grit chamber good example • Type II – flocculent materials in dilute suspensions, primary settling basin good example • Type III (Zone or hindered settling) – high concentrations (>1000 mg/L), particles interfere with each other’s settling, secondary settling basin • Type IV (Compression settling) – weight of particles cause more settling, sludge zone in 1o or 2o clarifiers

  3. What is overflow rate, vo,vL? • This is our design parameter. • Overflow rate, vo, is also the velocity of the liquid, vL. • Units (gal/ft2•d or m3/m2•d), Q/A, but this is a velocity (m/d). • All particles with vs > vo will settle (be removed). • Particles in water are a range of sizes with a range of vs. Our objective is to design a system to settle as many particles as we can in a reasonable time.

  4. Upflow Clarifiers • For vs > vo all particles will settle. • For vs < vo no particles will be removed (settle). • For vs = vo all particles suspended (fluidized bed). • Note various zones of a clarifier.

  5. Note the influent comes into the center ring, flows under a skirt and then upwards to outer weirs. How is this different from Essex Junction?

  6. Horizontal Settling Basins

  7. Horizontal Settling Basin • H is depth of settling zone • For vs > vL all particles will settle • For vs < vL particles will be removed at ratio (vs/vL) • The reason is that some particles enter at a depth below the water level so settle in a height < H

  8. Note the configuration of the weirs. This provides more weir length to minimize scouring.

  9. Nonideal Basins • This is like the one as Essex Junction. Wiers in the center. Why? • Somewhat like an upflow but not exactly. • Essex Junction had short-circuiting problems in their clarifiers. Why?

  10. Type I – Discrete Settling • Force balance applied to particle (theoretical analysis). Assume spherical particles. • Terminal settling velocity (vs) is constant. • Stoke’s Law for laminar flow: vs = g(ρp – ρw)dp2/18μ • Check NR, if not laminar use: • NR=dpv/μ

  11. Grit Chambers – Type I • Typical configurations are horizontal, aerated, or vortex type. • Design based on removal of grit particles (ρp = 2.65 g/cm3) • Typical dimensions range from 2-5 m in depth, 7.5-20 m in length, 2.5-7 m in width. • Width:depth ratios, 2:1 typical • Detention times, 3 min typical. Use peak hourly flow for design. • See Metcalf and Eddy for more design information

  12. Particle size changes due to flocculation (sticking together) of particles, therefore vs changes. Need to use empirical data or perform column experiment. Typical column experiment and data on right. Primary Settling Basin good example. Particles are sticky. Type II – Dilute suspension of flocculating particles

  13. Type II - Use Tables for typical WW and Water Treatment Floc particles instead of column tests • Typical detention times, 2 hours. • Overflow rate, vo, for average flow use range (32-48 m3/m2•d) for peak flow use (80-120 m3/m2•d). • Weir loading (125-500 m3/m•d). • Typical depths, 3-5 m • Typical diameter, 12-45m • Typical length, 25-40 m

  14. Type III – Hindered Settling • Concentrated suspension settles as a zone • Secondary clarifiers • To determine the rate of settling of the zone, measure the height of the interface at different times in a column [dh/dt = settling velocity of the blanket]

  15. Type III Design Considerations • Overflow rates based on a) area needed for clarification, b) area for sludge thickening, c) rate of sludge withdrawal. • Often use tables of empirical data for design of known systems. • Typical values for conventional activate sludge (16-33 m3/m2•d). • Alum or iron floc (14.5-22 m3/m2•d). • Lime-softening floc (22-82 m3/m2•d).

  16. Type IV – Compression Settling • Bottom of clarifier, sludge zone is good example. • Stirring serves to break up floc, and allows water to escape. Use sludge rakes. • Weight of sludge allows for compaction. • Sloped bottom of clarifiers allows for collection of sludge.

  17. Weir Loading Rates • Common design parameter but not as critical as overflow rate. • Avoid high velocities of water at outlet which can cause carry over of solids at outlet • Use tables for typical loading rates • Small WWTP (<0.04 m3/s) weir loading < 120 m3/m•d • Light alum floc weir OFR 143-179 m3/m•d • Heavy floc (lime softening) 268-322 m3/m•d

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