theoretical_design_of_sedimentation_basins

Crispen Mutsvangwa: Water Engineering, Education & Applications, www.weea.co.za of 7

Water Engineering Education & Applications Email: [email protected] Website www.weea.co.za or [email protected] Paper No. WT3/11: Theoretical Design of Sedimentation Basins Crispen Mutsvangwa MSc (Eng.,); MSc Water & Environmental Management © Copyright 2011. Water Engineering, Education & Applications

THEORETICAL DESIGN OF SEDIMENTATION BASINS

Theoretical design of sedimentation process is generally based on the concept of the ideal settling basin. A particle entering the basin will have a horizontal velocity equal to the velocity of fluid. Rectangular basins

Fig. 1: Rectangular horizontal flow sedimentation tank

Settling zone

Outlet zone

Inlet zone

vs

D vH


Fig. 3 Typical inboard weir arrangement to increase the weir length


Speed of horizontal flow, DB

QareaQvH ×

==

Time of horizontal flow, ocityve

travelofceDisTH .tan

=

QDBL

DBQLTH

××=

⎟⎠⎞

⎜⎝⎛

×

=

Time for falling the depth, D

sD v

D

veloctysettling

DepthT ==

But tine of fall = horizontal flow time: DH TT =

⇒ Q

DBLVD

s

××=

⇒ AQ

BLQVs =×

= =surface loading rate (m3/m2.day)

BLA ×= =surface area of the tank

Area

B

D


Therefore the depth of a basin is not a factor in determining the size particle that can be removed completely in the settling zone. The determining factor is the surface loading, which correspond to the terminal settling velocity of particles that are 100% removed in the basin.

Design principles

• Rectangular tanks should be long and narrow • Incoming flocs should be distributed uniformly over the width and depth of tank • Outer weir should be wide enough to reduce high velocities. Rectangular tanks are

Hydraulically more stable and control of large volumes of water is easier • More compact than radial flow tanks (circular tanks) • Limited length of outflow weir available • Complicated weir arrangements e.g. 1/3 of tank length • They are commonly used for large treatment plants with large flows • Particle removal independent of tank depth • They save space since walls can be share

• Efficient use of the basin volume may be obtained by subdividing it vertically by

addition of horizontal trays. This increases the surface are of basin and reduces surface overflow rate (tube setters or plate settlers).

• Provide at least two tanks Design parameters

• Retention time It is equal to the volume of water in tank divided by the flow rate

( )flowofrate

kofvolumecapacityQ

DBLT tan=

××=

2 to 4 hrs for discrete particles 4 to 6hrs for flocculent particles

• Surface loading Incoming flow divided by the plan area (effective surface area)

Area

L

B


AQ

BLQ

=×

; m3/m2.day

• Weir overflow rate maximum rate of flow per unit length of outlet weir

( )( )mweirsoflengthtotal

daymflowimumWOR /max 3

= , (m3/m.day)

Rectangular tanks have the disadvantage of having a small weir length. If the weir overflow rate is very high, this may result in excessive velocities at the outlet. These velocities extend back into the settling zone, causing particles and flocs which would have otherwise be removed as sludge to be drawn into the outlet. Recommended values are in Table 1, but generally range from 140m3/m.day to 340m3/m.day. It maybe necessary to install inboard weir designs. Surface dimensions WL : L=2 to 4 times width or 10 to 20 times the depth

Lmax =48m Lmin =12m for ease use of equipment

Depth for discrete particles =2.5 to 5m Depth for flocculate particle =3 to 4m Horizontal velocities vH <9m/hr for Flocculant particles

vH <36m/hr for discrete particles Circular tanks

• flow is radial, and it enters at the centre and is baffled to the periphery • horizontal velocity continually decreases towards the perimeter, resulting in the

change of the absolute settling velocity of a particle. Therefore the particles follow a parabolic path as opposed to the straight line in rectangular tank

• short circuit result also from the uneven distribution of velocities and also as result of wind currents

• flow control is difficult compared to rectangular tanks • weir overflow rates not a problem since the entire circumference is used for

overflow • to prevent extremely thin sheets of water from being drawn off, overflow weirs on

circular tanks are installed. They consists of V-notches pf metal plates, which reduce the effective overflow area. The plates should be level to prevent short circuiting

• sludge removal simpler and requires less maintenance Table 2; Design parameters and operating standards Rectangular Circular Upward flow


Settling Velocity (mm/ sec) 0.1- 0.5 ----- ------- Horizontal velocity 14-15m/hr Surface loading (m3/ m2. day) 10- 50 10-45 28-43 Retention time (hours) 1.5- 4 1.5-4 2-3 Outflow weir loading m3/m. day 100-450 100-450 100-450 Average depth (m) 1.75 – 3.0 1.5 – 2.5 4.25 – 7.75 Plan dimensions (metres) Up to 100 long; length: width

from 4:1 to 5:1 3.3 – 30 diameter. 5 – 9 square

Base slope 1:25 to 1:100; 22.3 deg to 0.6 deg. (inlet end lowest).

1:6 to 1:8. 7.5 deg to 10 deg. (centre lowest)

1:1 to1:2 45 deg to 63.5 deg. (centre lowest)

Fig. 2: Circular radial flow sedimentation tank

Example 1 Design a rectangular sedimentation tank(s) for a city to treat 5 000m3/day. A column test indicated that a surface loading of 20m/day produce better results in the removal of the flocculent particles at a depth 0f 3.5m. Solution Allow a minimum of 2 tanks for flexibility of operation and maintenance.

Therefore flow to each tank = daym /25002

5000 3=

Desludging

Influent

Settled effluent


Surface loading, SL =BL×

=250020

Therefore L x B =125m2 Assume L : B ratio of 4:1 Hence L= 4W2 Giving W= 5.59m Say W=6m Therefore L=6 x4 =24m Checks

i. Weir overflow rate, WOR

= daymmdaymmirLenghtofwe

QWOR ./450./4154

2500 33 <=== OK

ii. A depth of 3m assumed (Range 1.75 to 3m).

Retention time, T 2500

346 ××=

××=

QDBLT

=0.69hrs<1 to 1.5 hrs not OK Increase the tank dimensions and redo the checks until all conditions are satisfied.

theoretical_design_of_sedimentation_basins

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