This presentation includes certain basic calculations related to rectangular clarifiers.

1. RECTANGULAR CLARIFIER
DESIGNING OF INTERNALS

2. INTERNALS OF A RECTANGULAR CLARIFIER

3. • Inlet Baffle – Used to prevent the fluid velocities at the entrance of the inlet
extending into the tank. Also to direct solids.
• Drive Unit - Clarifier and thickener drive units provide the rotational force
necessary to turn the rake arms in a circular basin.
• Effluent Weir and launder – As the water level flows over the edge of the clarifier,
the excess water flows into the effluent lauder and then flows out of the weirs and
notches.
• Scum Trough – Removes grease scum and floating matter on the water surface.
• Idler pockets –
• Sludge hopper – used to dehydrate the collected sludge and to store for a small
time.

4. DESIGN
PARAMETERS
Flow Flux: 3000/3.5 x 5.6 = 153.1 m/day of wastewater
Flow velocity =
3000 𝑚3
24∗60∗60 𝑠
= 0.035 𝑚3/𝑠
Design flow = 0.035 m3
Horizontal velocity =
0.035
3.5∗5.6
= 1.786* 10-3 m/s
𝑭𝒓 =
𝑽 𝒇
𝟐
𝒈∗𝑹𝒉
𝑅ℎ =
𝑎𝑟𝑒𝑎
𝑝𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟
= (3.5*5.6)/[2*(3.5+5.6)] = 1.08
Fr =
1.786∗10−3 2
9.81∗1.08
= 3.00 * 10-7 Flow is laminar.

5. • Surface over flowrate
Q/A = 0.035/(3.5*20) = 5*10-4 m/s (m3/s.m2)
• Detention Time
Tank Volume = 3.5*5.6*20 = 392 m3
Detention Time = 392/0.035 = 11 200 s= 3.11 h
• Weir Loading Rate (WOR)
Weir = 2 times from the width
WOR =
3000 𝑚3/𝑑𝑎𝑦
2∗3.5𝑚
= 428.57 m2/day
= 428.57/(24*60*60)= 4.96*10-3 m2/s

6. V – NOTCH DESIGN
Notch length = B
Total perimeter where V notches are installed = 20+20+3.5 = 43.5 m
Total number of V notches = 43.5/B
Flow rate thorough one V notch = 4.96*10-3 / (43.5/B)
Assume 𝜃 = 90° and for this 𝜃,
Assume that the weir is fully contracted,
b = 2H , B = 6H
Q = 4.28 * Ce*tan
𝜽
𝟐
*(H+k)5/2
4.96∗10−3
43.5
∗ 6𝐻 = 4.28 ∗ 𝐶𝑒 ∗ tan 45° ∗ (𝐻 + 𝐾)
5
2
Ce = 0.5779048
K = 2.90251*10-3
4.96∗10−3
43.5
∗ 6𝐻 = 4.28 ∗ 0.5779048 ∗ tan 45° ∗ (𝐻 + 2.90251 ∗ 10
− 3)
5
2
H = 0.0111738712 m = 1.2 cm
B = 7.2 cm
No of v notches around the perimeter = 43.5/0.072 =604.1667 = 605

7. RECTANGULAR CLARIFIER SLOPE
• Normally the slope is in between 1:100 to 1:300 range.
• Slope is present to direct the collected sludge towards the sludge hopper and to
the sludge outlet pipe.
• The angle varies from 0.191 to 0.573 Degrees

8. SCUM BAFFLE DESIGN
• Typical height of scum baffles are
300 mm (30 cm).
• One third of the height should be
above water level.
• Like mud flaps(optional).

9. TECHNICAL DRAWINGS

10. Overall Technical Drawing