This document provides information about grit removal in wastewater treatment. It discusses that grit such as sand and eggshells can be easily removed from wastewater by reducing the velocity in a grit channel. Grit chambers are used to remove these particles to prevent damage to equipment and clogging. There are two main types of grit chambers - horizontal flow and aerated. The document provides design criteria for both types and works through an example design for a grit chamber for a town with a population of 200,000.

1. Physical Unit Operations
Grit Chamber
Unit-III

2. Grit Removal in Grit Chamber
• Sand, ash, Cinder, Bone Chip, egg shells,
etc., of size less than 0.2 mm are included in
grit. It is not putrescible and possesses higher
subsidence value than the organic matter. It is
therefore possible to remove grit from the
waste water easily by reducing the
wastewater velocity in long channel called
as grit channel. The velocity is reduced to
about 0.3 m/sec. The settled grit is washed
before its disposal.

3. Grit Removal in Grit Chamber

4. Grit Removal in Grit Chamber
• Grit Chamber is provided for the purpose
of removal of silt and sand particles mainly
so that the same will not cause, the wear and
tear of vanes of pumps, clogging of pipes, as
well as, valve operation difficult. Removal of
grit also reduces accumulation of inert material
in subsequent treatment units. Cementing
effects are also prevented in settling tanks
and digester by removal of grit.

5. Grit Removal in Grit Chamber

6. Grit Removal in Grit Chamber

7. Grit Removal in Grit Chamber
• Grit Removal Unit may be a grit Channel,
Grit Chamber or a Grit Basin. The Word grit
chamber shall be used in subsequent
discussion.
• There are two types of grit chambers
• (i) Horizontal flow Grit Chambers
• (ii) Aerated Grit Chamber

8. Grit Removal in Grit Chamber
• Horizontal flow grit chamber are designed to
maintain a velocity of around 0.3 m/sec. Such a
velocity falls then the organic particles also settle
down and if velocity becomes high grit particles
will not settle. So to cause only settlement of
grit, it is necessary to maintain constant
velocity of around 0.3 m/sec. The Waste water
flow varies and therefore it is required to
maintain the constant velocity by providing
proportional flow weirs, partial flumes and
palmer- Bowlup flumes.

9. Horizontal Flow Grit Chamber

10. Grit Removal in Grit Chamber

11. Grit Removal in Grit Chamber
• Aerated Grit Chambers are Used for
Selective Removal of Grit in medium and
large sized wastewater treatment plants.

12. Aerated Grit Chambers

13. Aerated Grit Chambers

14. Grit Collection & Removal
• Mechanical Grit Collection in velocity
controlled horizontal flow grit chambers and
aerated grit chambers is achieved by the
conventional equipment with scrapers screws
buckets plows or some combinations of these.
In some cases steep bottom slope is provided
which will collect the grit at Central Point of
Removal.

15. Grit Collection & Removal

16. Grit Collection & Removal
• Grit Removal is achieved by air pumps for
small aerated grit chambers. Grit can also be
removed by tubular conveyors, buckets type
collectors, elevators screws conveyors, grit
pumps and clam shell buckets.

17. Clam shell buckets

18. Quantity of Grit
• Grit Quantity varies greatly. It depends upon the
following factors:
• Type of Sewerage System i.e. Separate or Combined
Sewerage System
• Climate Condition
• Soil Type
• Sewer Grades
• Type of Industrial Wastewaters
• Relative Use of Garbage and grinders
• The grit quantity may range from 5 to 200 m3/ Million
Cubic Meter of Wastewater. The typical Value can be
Considered as 30 m3/ million Cubic meter of
wastewater.

19. Quantity of Grit

20. Grit Disposal
• Various methods are Used for Grit Disposal
• They are
• Sanitary Landfill: In low lying areas or large natural
pits the grit is disposed. Such a method of disposal is
preferred when site of disposal is far away from city
or town.
• Land Spreading
• Incineration with Sludge: Incineration is burning at
very high temperature in excess Oxygen. City solid
waste can be incinerated. Sludge and grit can be
taken to the incinerator for burning. Grit can also
be buried when its quantity is small.

21. Sanitary Landfill

22. Land Spreading

23. Incineration with Sludge

24. Design of Grit Chambers
Following Information should be collected for designing of the
grit chambers
• Wastewater Characteristics and Size of Grit particles to be
removed.
• Design average, peak and lowest flow.
• Information about existing plant if it is to be expanded.
• Type of Grit Chamber to be provided i.e. Horizontal flow,
aerated etc.
• Influent pipe data and static head force main and hydraulic
grade line in case grit removal preceded pumping station.
• Head loss constraints for Grit Removal Efficiency.
• Treatment plant design criteria by Bureau of Indian Standards.

25. Design Criteria for Horizontal Flow Grit
Chambers
• Detention Time = 40 to 60 Sec
• Horizontal flow velocity = 15 to 30 cm/sec
approximately
• = 4 𝑔 𝑆𝑠 − 1 𝑑
• Where g= Acceleration due to Gravity= 9.81
m/sec2
• Ss= Sp Gravity of Grit= 2.65
• D= dia of Grit in m.

26. Design Criteria for Horizontal Flow Grit
Chambers
• (iii) Surface Overflow Rate (SOR)
• = 500-1500 m3/m2/day
• Length to Breadth Ratio = 6 to 15
• Length to depth Ratio = 10 to 30
• Depth= (1.5 to 2) + free board
• Free Board = 0.75 to 1.0 m

27. Design Criteria for Aerated Grit
Chamber
No. Design Parameter Range of Variation Remarks
1 Dimension
Depth
Length
Width
Width/ Depth Ratio
2-5 m
7.5 to 20
2.5 to 7.0
1:1 to 5:1
Width of Basin is
limited to prevent
turbulence in the
bank.
2 Transverse Velocity
at Surface
0.6 to 8 m/sec
3 Detention Time at
Peak flow, minutes
2 to 5 If Grit Chamber is
Used to remove Grit
less than 0.2 mm a
longer detention
time may be
provided.
4 Air Supply L/S m 4.6 to 12.4
Litres/Sec/m length
of the Tank.

28. Design Example
• Design a Suitable Grit Chamber to Cater a town
of 2 Lakh population with 150 l/capita/day
Sewage Contribution
• Assume Peak Factor = 1.5
• Maximum Sewage Contribution per day
• = Average Contribution x Peak factor
• = 150 x 200000 x 1.5 litres
• = 150 x 200000 x 1.5 m 3
1000
= 45000 m3

29. Design Example
• 45000 m3 sewage produced daily i.e. in
• 24 x 60 x 60= 86400 Seconds
• Therefore Maximum Sewage Discharge
• = 45000 m3/ Sec
86400
= 0.52 m3/Sec
Assume horizontal velocity 0.3 m/sec and detention time 50
sec
Therefore Length of the tank Required
= Velocity x Detention Time
= 0.3 x 50
= 15 m

30. Design Example
• Now Settling Velocity of Grit Particles are governed by
Hazen’s Modified Equation as Given Below:
• Vs= 60.6 (Ss-1) d (3t + 70)
100
Where, t= temp of Waste water 0 C
d= dia of particle in cm
Vs= Settling Velocity in cm/sec
Ss= Sp Gravity of Grit, 2.65
If Ss= 2.65 above equation becomes
Vs= d (3t + 70)
Taking t= 27 0C
Vs= 3 m/sec

31. Design Example
• We want to remove 0.2 mm particle so Settling
Velocity = 3 cm/sec
• Therefore Depth of Tank= 3.0 cm/sec x 50
sec.3
• = 150 cm
• = 1.5 m
• Taking Length to Width Ratio as 10 : 1
• Width of the tank = 1.5m

32. Design Example
• Now Check for SOR Surface Overflow Rate
• Width= 1.5 m
• Length=15 m
• Plan Area= 1.5 x 15 = 22.5 m2
• Max Sewage flow = 45000 m3/day
• Max SOR = 45000 m3/day
22.5
= 2000 m3/m2/day
Which is higher than permitted as per Criteria

33. Design Example
So, take length: Width ratio as 6:1
Therefore Width= 2.5 m
Therefore Plan Area= 2.5 x 15
= 37.5 m2
Therefore Max SOR= 45000 m3/ day
37.5 m2
= 1200 m3/m2/day
Which is less than 1500 therefore O.K.

34. Grit Chamber

35. References
• Sewage Disposal & Air Pollution Engineering:
S.K. Garg
• Environmental Engineering : By Prof B.R.Shah
Prof A M Malek
• Internet Websites

36. Thanks !