Review of Settling
• AKA sedimentation – suspended solids removed
from liquid phase by gravity
• Common applications in Wastewater Treatment –
grit chamber, primary settling basin, secondary
• 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 1 o or
What is overflow rate, vo,vL?
• This is our design parameter.
• Overflow rate, vo, is also the velocity of the liquid,
• Units (gal/ft2•d or m3/m2•d), Q/A, but this is a
• 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
• For vs > vo all particles
• For vs < vo no particles
will be removed (settle).
• For vs = vo all particles
suspended (fluidized bed).
• Note various zones of a
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
• This is like the one as
Essex Junction. Wiers
in the center. Why?
• Somewhat like an
upflow but not
• Essex Junction had
problems in their
Type I – Discrete Settling
• Force balance applied to particle
(theoretical analysis). Assume
• Terminal settling velocity (vs) is
• Stoke’s Law for laminar flow:
vs = g(ρp – ρw)dp2/18μ
• Check NR, if not laminar use:
4 ( p w ) gd p
• NR=dpv/μ CD 0.34
Grit Chambers – Type I
• Typical configurations are horizontal, aerated, or
• Design based on removal of grit particles (ρp =
• 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
Type II – Dilute suspension of
• Particle size changes due to
together) of particles,
therefore vs changes.
• Need to use empirical data
or perform column
• Typical column experiment
and data on right.
• Primary Settling Basin good
example. Particles are
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
• 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
suspension settles as a
• 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
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
• Typical values for conventional activate sludge
• Alum or iron floc (14.5-22 m3/m2•d).
• Lime-softening floc (22-82 m3/m2•d).
Type IV – Compression
• Bottom of clarifier, sludge zone is good
• 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
• 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
• Light alum floc weir OFR 143-179 m3/m•d
• Heavy floc (lime softening) 268-322 m3/m•d