Grit chambers are sedimentation basins designed to remove inorganic particles such as sand and gravel before sewage enters finer screening processes. They can be mechanically or manually cleaned and include horizontal and aerated types, with specific design criteria for settling velocity, overflow rate, and dimensions. The collected grit is often disposed of through washing, filling low-lying areas, or incineration, and requires a defined detention period for effective operation.

1. L-13
Grit Chamber
Environmental Engineering-II

2. Preliminary Treatment

3. Introduction and Objective
• Grit removal basins are the sedimentation
basins placed in front of the fine screen to
remove the inorganic particles having
specific gravity of 2.65 such as sand,
gravel, grit, egg shells, glass pieces,gravel, grit, egg shells, glass pieces,
metal fragments etc. and other non-
putrescible materials that may clog
channels or damage pumps due to
abrasion and to prevent their
accumulation in sludge digesters.

4. Settling Categories
• Type I - discrete settling in dilute suspensions,
grit chamber good example
• Type II – flocculent materials in dilute
suspensions, primary settling basin good example
• Type III (Zone or hindered settling) – high• Type III (Zone or hindered settling) – high
concentrations (>1000 mg/L), particles interfere
with each other’s settling, secondary settling
basin
• Type IV (Compression settling) – weight of
particles cause more settling, sludge zone in
clarifiers

5. Types of Grit Chambers
• Grit chambers are of two types:
mechanically cleaned and manually
cleaned.
• In mechanically cleaned grit chamber,• In mechanically cleaned grit chamber,
scraper blades collect the grit settled on
the floor of the grit chamber.
• The grit so collected is elevated to the
ground level by several mechanisms such
as bucket elevators, jet pump and air lift.

6. • The grit washing mechanisms are also
of several designs most of which are
agitation devices using either water or
air to produce washing action.
• Manually cleaned grit chambers• Manually cleaned grit chambers
should be cleaned at least once a
week.
• The simplest method of cleaning is by
means of shovel.

7. • There are two general types of grit
chambers
1. Horizontal flow grit chamber and
2. Aerated grit chamber

8. Horizontal flow grit chamber
• Velocity close to 0.3 m/s is maintained.
• Such velocity will carry most of the organic
matter through the channel and will tend to re-
suspend any that settle, but permit heavy grit
to settle.to settle.
• Therefore velocity in the grit chamber shall be
neither too low as to cause settling of lighter
organic particles nor should be so high as not
to cause the settlement of entire silt and grit
present in the sewage.

10. • Length – 10 to 18 m
• Depth of liquid – 1 to 1.3 m
• Velocity is controlled by means of
velocity control device.

11. Aerated Grit chamber

13. Vortex type grit
chamber

14. Design criteria
1. Settling Velocity:- is given by
Stoke’s Law for laminar flow:
vs = g(ρp – ρw)dp
2/18µ
v = g(G – 1)d 2/18υvs = g(Gs– 1)dp
2/18υ
Vs = settling velocity
dp= size of particle
Gs = Specific Gravity

15. ρp = density of particles
ρw = density of water
µ = dynamic viscosity
υ = µ/ ρw = kinematic viscosity
g = Gravitational accelerationg = Gravitational acceleration

16. 2. Overflow rate
SOR is given by
SOR = Q / As
Q = flow rate
A = Surface areaAs = Surface area

17. 3. Detention period
• Detention period may vary from 45
to 90 seconds.
• Generally it is taken as 60 sec.• Generally it is taken as 60 sec.

18. 4. Bottom scour and flow through
velocity

19. 5. Velocity Control
Devices

20. 6. Number of units
• For manually cleaned grit chambers at
least two units shall be provided..
• For mechanically cleaned units one• For mechanically cleaned units one
additional manually cleaned unit as by-
pass shall be provided

21. 7. Dimensions
• Free board of 150 to 300 mm shall be
provided.
• Length can be calculated as
L = vh x t
• Depth can be calculated as• Depth can be calculated as
D = vs x t
• Bottom slopes shall be provided.
• Breadth depends upon velocity control
device adopted.

22. 8. Head loss
• Head loss – 0.06 to 0.6 m
• Depends upon velocity control device
adoptedadopted

23. Disposal of Grit
• The moisture content of grit varies
from 14 to 34 percent.
• Grit is sometimes washed after
collection to reduce the organiccollection to reduce the organic
concentration which may be as much
as 50 percent of the total solids.

24. • Filling Low lying area.
• Incineration.
• Well-washed grit has been used on
sludge drying beds, as a cover forsludge drying beds, as a cover for
screenings, and as a surfacing
material for walks and roadways.

25. Partial flume

26. Aerated Grit chamber

27. Aerated grit chamber

28. Drained grit chamber with sediments

29. Objective Questions
1. In Grit chamber ______________ settling is
observed.
2. For grit chambers detention period of __ sec is
usually adopted.
3. Grit chamber removes particles having Sp. Gr. In3. Grit chamber removes particles having Sp. Gr. In
the range of ____ to _____.
4. Pick out odd one
a. Silt, Broken glass, card board, egg shells, metal
fragments
b. Parshall flume, sutro weir, V notch, Proportional
flow weir

30. Theory Questions
Q1. Explain necessity and working of grit
chamber with neat sketch.
Q2. Write a detailed note on ‘ design of
grit chamber’.grit chamber’.