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Prelim 2 Review Office Hour Special! 5:30 pm on Tuesday in 366. With a focus on Sedimentation and Filtration. Sedimentation: How do these velocities connect to design?. Settle capture – determines the slowest terminal velocity floc that will settle to a surface - PowerPoint PPT Presentation
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With a focus on Sedimentation and Filtration
Prelim 2 ReviewOffice Hour
5:30 pm on Tuesday in 366
Exit Channel
Entrance Channel
Launder
Inlet Manifold
Plate Settlers
Diffusers
Sed Tank Drain
Floc Weir
AguaClara Sedimentation Tank
Floc Hopper
Floc Hopper Drain (to remove sludge)
Dump poorly flocculated water channelExit Weir that controls water levels all the way back to the next free fall (LFOM!)
How can you drain and then refill a sedimentation tank?
Sedimentation: How do these velocities connect
to design?• Settle capture – determines the slowest
terminal velocity floc that will settle to a surface
• Slide capture – determines the slowest terminal velocity floc that will slide down the plate. Function of velocity gradient at plate settler surface, angle, and floc density
• Upflow – set to obtain good performance of the floc blanket
• Terminal – floc property set by maximum energy dissipation rate of the environment it is in AND by the rate at which flocs can aggregate
Settle Capture Velocity• Which provides better removal of
particles, a low settle capture velocity or a high settle capture velocity?• Suppose you keep plate length
constant and decrease spacing between plates. What happens to performance and cost?
Performance ratio (conventional to plate/tube
settlers)• Compare the area on
which a particle can be removed
• Use a single plate settler to simplify the comparison
a
SL
cosL asinSa
Conventional capture area
sinconventionalSA Wa
Plate/tube capture area
cossinplateSA W WL aa
1 cos sinPlateratio
c
V LAV S
a a
extra
Quiz 11/5/14
a)Decrease the amount of plastic needed for plates
b)Decrease the velocity gradient at the plate
c)Decrease the slide capture velocityd)Decrease the height of the plate
settlerse)Decrease the head loss through the
plate settlersf)Decrease the Reynolds number for
flow between the plates
List ALL that are true if you decrease the plate settlers spacing (and maintain the same up flow in the sed tank and the same capture
velocity)
• Suppose the max energy dissipation rates on the way into the sed tank are…• 10 mW/kg leaving the
flocculator• 100 mW/kg in the inlet
manifold• 1000 mW/kg leaving the
jet reverser• 0.1 mW/kg in the floc
blanket
What is the sedimentation velocity of the flocs entering the sedimentation
tank?
10 100 10000.1
1
Energy dissipation rate (mW/kg)Te
rmin
al v
eloc
ity (m
m/s
)
Flow Distribution• Does high velocity in a sed tank inlet
manifold help or hurt flow distribution?• Why?
• If you increase velocity in a manifold what else must you do to maintain flow distribution?
2
2
14
1 ParallelPath
QManifold L
Q
V gH
What would the horizontal velocity be above the plate settlers if we removed the launder?
Assume sed tank is 5 m long, 1 m wide, upflow velocity is 1 mm/s and the water depth above the plates is 20 cm.
Plate Settler Head Loss
0 10 20 30 40 500
1
2
3
4
5
Plate spacing (mm)
Hea
d lo
ss (μ
m)
0 10 20 30 40 502
4
6
8
10
Plate spacing (mm)Ve
loci
ty e
quiv
alen
t to
ener
gy lo
ss (m
m/s)
Head loss is tiny! We need some head loss to get reasonable flow distribution between (and within) plates. This lack of head loss may be one of the reasons for poor performance of full scale plate settlers. The velocity of any turbulent eddies or mean flow needs to be less than ______ to achieve uniform flow through plate settlers. The floc blanket will end up helping us here!
f 2
62 1
sin cosPlate Plate
c
V Vh
g S V a a
2V gh
4 mm/s
This represents an opportunity to improve design!
Sed Questions
• What determines the depth of the sedimentation tank?• What happens if you make the
sedimentation tank 2 m wide or 50 cm wide?• Why does the flow divide evenly
between the sed tanks?
Sed Tank as a Circuit: Flow Distribution
Challenge
Inlet channel
Settled water outlet channel
Inlet manifoldDiffuser exit loss
Launder orifice lossFloc blanket and plate settlers
Effluent manifold (Launder)
Pressure change from
Flow deceleration and head loss
Battery creates 5 to 10 cm of potential energy to drive the flow
Pressure change from flow acceleration and head loss
Launder exit losses
Flow distribution is a major hydraulic design constraint (see manifolds.pptx)
Floc Roll Up
• What relationship did we linearize to obtain a simpler analytical solution for the floc roll up failure mode?• What must be true for
this approximation to be reasonable?
a
Va
u
gsin(
a)
ga
Plate Settlers• If AguaClara adopted a design change and
doubled the upflow velocity in our sedimentation tanks, then what changes would we need to make in our plate settlers? • Approximately how much longer would they
need to be if we keep the spacing the same?• If we keep velocity gradient constant, then how
must spacing change?
1
sin cos
Active Active
c c
V VS T
V VL
a a
62
V dvS
aa
Floc Hopper Q's:• What is in the floc
hopper before it fills with flocs?• Why do flocs flow
over the floc hopper weir? • Why does a floc
blanket interface slowly rise?
Floc Blanket• Why would a floc blanket not work if
AguaClara didn’t use plate settlers?• What mechanisms could explain why
a floc blanket reduces the turbidity of the settled water?
Backwash Requirements –
Head Loss (force balance)
2WH
FiBwlh
FiSandH
1WHManometerH( )1 1
FiBw
Sandl FiSand FiSand
Water
h H
For FiSand=0.4 and Sand=2650 kg/m3
1 FiSand( )FiSandWater
1
0.99
=
Weight of water/Filter area Weight of sand/Filter area( ) ( )
1 21Manometer Water W W FiSand FiSand Sand FiSand FiSandP g H H H g H
( )1 2 FiBwManometer Water W W FiSand lP g H H H h
Big picture: Prelim question that didn’t make
the cut…An AguaClara plant that includes all of the latest in design innovations (flocculation, floc blanket, floc hopper, plate settlers, stacked rapid sand filter) is built for a community of 15,000 people with a per capita demand of 3 mL/s per person. The facility has 8 sed tanks and 2 filters. During a drought the flow rate through the plant is reduced by 50%. Which unit processes are affected negatively by a reduction in flow rate and what changes in operation would you advise?
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