one of the component for of water treatment plant

1. Sedimentation
BY
R M BHADARAKA

2. Sedimentation
• It is physical process
• In this process heavier organic and inorganic particles settle down at
bottom of the sedimentation tank.
• For achieve the removal of suspended settleble particles of desired size
the wastewater is retained in sedimentation tank for a fixed time which
is called detention period.
• The upper clear water is collected from the tank.
• The bottom of the tank formation of sludge because of settled
particles.
• The sludge is drawn out from the bottom through the pipe called
sludge pipe.

3. Type of sedimentation
• Plain Sedimentation
• Sediment with coagulation
The settlement of the particle largely depends upon their specific gravity and
their type.

4. Stoke’s Law
𝑣 =
𝑔
18
(𝑠 − 1)𝑑2
𝜗
Where v = settling velocity mm/sec
d = Diameter of particle, mm
μ = Dynamic viscosity of wastewater centistoke
ν =
𝜇
𝜌
= 𝑘𝑖𝑛𝑒𝑚𝑎𝑡𝑖𝑐 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑠𝑡𝑒𝑤𝑎𝑡𝑒𝑟, 𝑐𝑒𝑛𝑡𝑖𝑠𝑡𝑜𝑘𝑒
g = acceleration due to gravity 9810 mm/𝑠𝑒𝑐2
If taking g = 981 cm/𝑠𝑒𝑐2
and 𝑣= 1 centistoke at 20 C the Factor
𝑔
18𝑣
= 545
At 20 C V = 545 (s - 1) 𝒅 𝟐
At Other temp V =418 (s - 1) 𝒅 𝟐 𝟑𝒕+𝟕𝟎
𝟏𝟎𝟎

5. Newton’s Law
V = 𝟑. 𝟑𝟑𝒈𝒅(𝒔 − 𝟏)
Where v = settling velocity mm/sec
d = Diameter of particle, mm
g = acceleration due to gravity 9810 mm/𝑠𝑒𝑐2

6. Hazen’s Equation
V =418 (s-1)
𝟑𝒕+𝟕𝟎
𝟏𝟎𝟎
Where v = settling velocity mm/sec
d = Diameter of particle, mm
t = temperature
 Stoke’s law is valid for particles < 0.2mm and Newton’s equation is valid for
particles > 1mm.
 The grit particles of wastewater are having size between 0.1 to 1 mm.

7. Hazen’s Equation
V =60.6 (s-1)d
𝟑𝒕+𝟕𝟎
𝟏𝟎𝟎
Where v = settling velocity mm/sec
d = Diameter of particle, mm
t = temperature
 This equation is applicable for discrete spherical grit particle.

8. Type of settling tanks
 Based on nature of working
 Fill and Empty type
 Continuous flow type
 Based on location in treatment plant
 Preliminary settling tank
 Primary Settling tank
 Secondary Settling tank
 Based on shape of the tank in plant
 Rectangular settling tank
 Circular tanks

9. Fill and Empty type
This type of tank first of all wastewater is filled and allowed to
remains in still condition for about one hour.
The supernatant is drawn out and settled sludge is cleared.
The cleaning takes another 4 to 6 hours
There is wastage of time and labour
Generally not provided in wastewater treatment plant.

10. Continuous flow type
This type of tanks wastewater is continuously supplied at one point
called inlet and same after treatment leaves continuously from the
other point called outlet.
The wastewater entering the tank is called influent and leaving the
tank is called effluent.

11. Surface Overflow Rate
It is expressed in
𝑚3
𝑚2 /𝑑𝑎𝑦.
Sr No Type of sedimentation SOR Depth
Average flow Peak Flow
1 Primary settling only 25-30 50-60 3 to 3.5
2 Primary settling followed by
secondary treatment
35-50 80-125 3 to 3.5
3 Primary settling with activated
sludge returns
25 -35 50-60 3.5 to 4.5
4 Secondary settling with trickling
filter
10-25 40-50 3 to 3.5
5 Secondary settling with activated
sludge
15-35 40-50 3.5 to 4.5
6 Secondary settling with extended
aeration
8-15 35 3.5 to 4.5

12. Detention Period
It is theoretical time for which wastewater remains inside the tank.
It is calculated by dividing the volume of tank with the discharge.
It is normally kept as 1 to 1.5 hours for wastewater treatment.
The detention time in PST is not increase because of the
development of septic condition.

13. Velocity of flow
Velocity of flow in case of rectangular primary sedimentation tanks
can be taken between 15 to 30 cm/min.

14. Weir loading rate
Weir loading rates have some effect on removal efficiency of settling
tanks.
PST are generally designed for loading less than 370
𝑚3
𝑚
/𝑑

15. Dimension for PST
Clarifier Classification Range Typical
Rectangular
Length, m 10 to 100 25 to 60
Length to width ratio 1 to 7.5 4
Length to depth ratio 4.0 to 25.0 7 to 18
Width 3 – 24 6 to 10
Side water depth, m 2.5 to 3.0 3.5
Circular
Diameter, m 3 to 60 10 – 40
Side water depth, m 3 to 6 4

16. Example
Design a primarily settling tank of rectangular shape for a town having
population of 40000 with water supply of 135 l/c/d
 Assume 80% of water supply to the town is converted in to wastewater
waste water formed daily = 40000 * 135 * 0.8 litre
= 40 * 135 * 0.8 m3
= 4320 m3/d
Assume detention period of 2 hours.
capacity of tank = flow for 2 hour = 4320 * 2 / 24
= 360 m3
Again let us assume SOR of 30 m3/m2 day
surface area = water flow / SOR
= 4320 / 30
= 144 m2
Effective depth = volume / Surface area = 360/144 = 2.5 m

17. Example
Design a primarily settling tank of rectangular shape for a town having
population of 40000 with water supply of 135 l/c/d
 Take L/B = 4
L = 4B
L * B = 144
4B * B = 144
B = 6
L = 4B = 4 * 6 = 24 m
Provide 4 m additional for inlet and outlet arrangment
L = 24 + 4 = 28 m
Effective depth = 2.5 m
Provide 1 m for sludge accumulation and 0.5 m for free board
Total depth = 2.5 + 1 + 0.5 = 4.0 m
Hence dimension of tank will be 6 * 28 * 4 m.