Batch sedimentation What is sedimentation…? Goals of gravity s sedimentation Applications of sedimentation zone settling velocity Factors affecting zone settling velocity Design of Zone Settling Tanks What is Thickener and Clarifiers…? Thickener Area Calculation Types of clarifier 

1. Mechanical Operation (2150502)
GUJARAT
TECHNOLOGICAL
UNIVERSITY
Batch sedimentation, Clarifiers
and Thickeners

2. Batch sedimentation,
Clarifiers and
Thickeners

3. Contents
 What is sedimentation…?
 Goals of gravity s sedimentation
 Applications of sedimentation
 zone settling velocity
 Factors affecting zone settling velocity
 Design of Zone Settling Tanks
 What is Thickener and Clarifiers…?
 Thickener Area Calculation
 Types of clarifier

4. What is sedimentation…?
Most wastewaters and waters contain solids, and in
many treatment processes solids are generated e.g.,
phosphate precipitation, coagulation and activated
sludge bioxidation. Particles in water and wastewater
that will settle by gravity within a reasonable period of
time can be removed by "sedimentation" in
sedimentation basins (also known as "clarifiers").
Gravity separation can obviously be applied only to
those particles which have density greater than water.

6. Fig.1 Fig.2 Fig.3

8. Goals of Gravity Sedimentation
1) Produce a clarified (free of suspended
solids) effluent.
2) Produce a highly concentrated solid sludge
stream.

9. Applications Of Sedimentation
Applications in Water Treatment:
1. settling of coagulated and flocculated waters prior to
filtration
2. settling of coagulated and flocculated waters in a
softening plant
3. settling of treated waters in an iron and manganese
removal plant
Applications in Wastewater Treatment:
1. grit removal
2. suspended solids removal in primary clarifier
3. biological floc removal in activated sludge

10. Zone settling velocity
 The height of the interface (between the
clarified zone and the zone settling zone)
versus time is plotted in the figure below to
determine the "zone settling velocity"
(ZSV). Velocity of this interface is steady
after some induction period but changes
with time as compression begins. The
slope of the steady interface subsidence
rate represents zone settling velocity.

12. Factors affecting zone settling
velocity
1. Suspended solids concentration
2. Depth of settling column (or tank)
3. Stirring ( 0.5 – 2 rpm to prevent “arching”)
4. Temperature
5. Polymer addition ( affects matrix structure)

13. Design of Zone Settling Tanks
• Two important functions
of these sedimentation
tanks are : clarification
and thickening.
• For a continuous flow
clarifier, operated at
steady-state, mass flow of
suspended solids can
schematically represented
as follows:

14. What is Thickener and Clarifiers..?
• Thickeners and clarifiers are both used to separate
liquids and solids by settling.
• Thickeners are used to concentrate solids, while
clarifiers are used to purify liquids.

15. Thickener Area Calculation
• where A = area (m2)
Q0 = feed rate of suspension (m3/s)
Y = mass ratio liquid to solid in feed
U = mass ratio liquid to solid in
underflow
C = particle volume fraction (1-ε)
ρs = density of solid (kg/m3)
uT = terminal velocity at conc. C (m/s)
ρf = density of liquid (kg/m3)
 
fT
s
u
CUYQ
A


 0

16. Types of
clarifier
Circular
clarifier
Parallel plate
clarifier

17. Circular Clarifiers
 Thickeners and clarifiers use
slowly rotating rake arms to
separate solid particulate. A
liquid feed with suspended
solids is fed into a tank with a
diameter of 5 to 500 feet. As the
particles settle, angled rake arms
move the concentrated slurry
toward the center of the tank,
where it is removed. Clear
liquid overflows the top of the
tank and is collected in a trough.

18.  In the thickener shown below, cables controls the
height of the rake arms. When heavy sludge builds up
at the bottom, the rakes are raised slightly. Once the
blockage has been dispersed, the rake arms are
lowered back to normal position. Adjustable rake
arms increase operating efficiency by reducing
maintenance downtime.

19. Application of circular clarifier:
• Thickeners and clarifiers are often used in
water and wastewater treatment plants to
remove solids, chemicals, microbes and
other impurities.
• Thickeners and clarifiers are also used in the
paper industry, the uranium industry, and in
alumina, coal, copper, and iron ore
production.

20. • Advantages
• Effective solid-liquid
separation.
• Can be used for a
variety of liquid-solid
separations.
• Disadvantages
• Inefficient for small
scale operation.
• Large initial investment.
• Requires large
installation area.
circular clarifier

21. Parallel plate clarifier
• Parallel plate
clarifiers are
designed to create
laminar flow,
allowing gravity to
separate heavier
elements from the
rest of the fluid, as
shown in figure.

22. Applications:
• Parallel plate clarifiers are often used to
separate solids and low density liquids, such
as oils.
• They can be suitable for many applications,
including chemical processing, coolant
systems, food processing, oil refinery, paper
making, and industrial waste management.
• The horizontal clarifier operates at 40 GPM
and is used to treat plating wastewater.

23. • Advantages
• Low space
requirement.
• No moving parts
in settling area.
• No filter media
required.
• Disadvantages
• Cannot be used
operations larger than
2000 GPM.

25. Gravity Thickener
Thickening is the process by which bio solids are condensed to
produce a concentrated solids products.
Thickening wastewater solids reduces the volume of residuals,
improves operation, and reduces costs for subsequent storage,
processing, transfer, end use, or disposal.
Gravity thickening uses the natural tendency of higher-density
solids to settle out of liquid to concentrate the solids.
Gravity thickeners consist of a circular tank (usually with a
conical bottom) that is fitted with collectors or scrapers at the
bottom.

26.  Primary and/or secondary solids are fed into the tank through a
center well, which releases the solids at a low velocity near the
surface of the tank.
 The solids settle to the bottom of the tank by gravity, and the
scrapers slowly move the settled, thickened solids to a discharge
pipe at the bottom of the tank.
 A v- notch weir located at the top of the tank allows the supernatant
to return to a clarifier.

28. Gravity settling occurs when solid particles
travel downward due to their weight.
 Settlement continues as solids begin to
concentrate near the bottom of the tank, but the
settlement rate decreases as the solids
concentrations increase.
Solids at the bottom of the tank can reach as
high as 15 percent total solids (TS). A more
typical result is 4 to 6 percent TS.
Liquid at the surface of the tank is nearly clear,
with suspended solids concentrations as low as
200 mg/L.

30. Industrial thickeners