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

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

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.

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.

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?

Horizontal Settling Basins

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

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

scouring.

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?

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

D

pwp

s C

gdv

)(34

34.0324

RR

DNN

C

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

Type II – Dilute suspension of flocculating particles

• 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 - 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

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]

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).

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.

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