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?