CE 428 Water and Wastewater Design
Grit Chambers
Dr. S.K. Ong
Grit Removal
Characteristics of grit
- sand, gravel, cinders, eggshells, bone chips, seeds, coffee grounds and other heavy materials
- predominantly inert, composition variable
- moisture content 13 - 65%, volatile organic content - 1 - 56%
- specific gravity - clean grit particles - 2.65, for material with substantial organic material
attached to inerts - approx. 1.3
- bulk density in the range of 1600 kg/m3
- most grits are retained on a No. 100 mesh sieve (0.15 mm or larger)
- typical settling velocity for 100 mesh grit is 1.3 cm/s or 2.6 ft/min
Purpose of Grit removal
- to protect mechanical equipment from abrasion and abnormal wear
- to reduce conduit clogging caused by deposition of grit particles in pipes and channels
- to prevent loading the treatment plant with inert matter that might interfere with the operation of
treatment units such as siltation of anaerobic digester and aeration tanks
Type of Grit Removal
- horizontal flow (square or rectangular configuration)
- aerated (rectangular)
- vortex-type
Horizontal flow type
- open channels with sufficient detention time to allow particles to settle and to maintain constant
velocity to scour organics
- the velocity of flow controlled by
(i) dimensions of unit
(ii) special influent distribution gates
(iii) special weir sections at the effluent end (proportional weir)
- designed to remove grit > 0.21 mm dia. to as low as 0.15 mm dia.
- grit removal is accomplished by a conveyor with a scraper, buckets or plows
- may require grit washing equipment to remove organics
Typical Design Information for Horizontal Grit Removal
Parameters
Range
Typical
Detention time (s)
45 - 90
60
Horizontal Velocity (ft/s)
0.8 - 1.3
1.0 - 1.25
Headloss in a control section
as percent of depth in channel
30 - 40%
36%
Allowance for inlet and outlet
turbulence
2Dm - 0.5 L
(at least 50% increase in theoretical length is
recommended, Dm - max. depth, L - length)
Grit quantities
0.004 - 0.037 m3/1000 m3
Aerated Grit Chambers
- popularity of aerated grit chamber
- less wear on grit-handling equipment
- in many cases, no need for separate grit washing equipment
- normally designed to remove particles 65 mesh (0.2 mm) or larger
- velocity of roll or agitation governs the size of particles of a given specific gravity to be removed
- quantity of air is adjusted to provide the roll and washing of the grit to remove organic
matter
- grit removed by using grab buckets on monorails centered over the grit collection and
storage trough or by a flushing through a drain
Typical Design information for Aerated Grit Chambers
Range
Detention time (mins) at max. flow
2- 5
Dimensions
Depth (ft)
7 -16
Length (ft)
25 - 65
Width (ft)
8 - 23
Width -depth ratio
1:1 - 5:1
Length-width ratio
3:1 - 5:1
Air supply (ft3/min/ft of length)
2-5
Grit quantities (ft3/mgal)
0.5 - 27
Typical
3
1.5 : 1
4:1
2.0
Vortex-type Grit Chambers
- vortex created
- grit move to the outside of the unit and collected
- air scour to remove organics
- grit removed by air lift pump or by bottom drain
- typically designed to remove 95% of 100 micron (150 mesh) grit
- head required to achieve this removal is a few meters, for example, 95% removal of 25 micron head requirement - 5 - 7 m
- typical units can handle peak flows up to 0.31 m3/s (7 mgd)
Typical Design Information for Vortex Grit Chambers
Range
Typical
Detention time (s)
30
Dimensions
Diameter (ft)
4 - 24
Upper
Lower
3-6
height (ft)
9 - 16
Removal Rates %
50 mesh (0.3 mm)
95+
70 mesh (0.24 mm)
85+
100 mesh (0.15 mm)
65+
Design Example for horizontal aerated grit chamber
Design an aerated grit chamber with an average flow of 11.4 mgd (0.5 m3/s) with a peaking factor of 2.75
1.
Peak hourly flow rate for design
= 11.4 x 2.75
2.
Grit chamber volume
Assume 2.0 minutes for peak hourly flow
Assume two chambers in use all the time
= 31.35 mgd
31 .35
mgd ) x10 6
2
x 2 min  2,910 ft 3
=
gal
hr
min
(7.48 3 )( 24
)( 60
)
day
hr
ft
(
4.
Dimensions
Use width to depth ratio of 1.1:1 and a depth of 8 ft
Width
Length
5.
= 1.1 x 8 = 8.8 ft
= 2,910 ft2/(8 x 8.8)
= 41.3 ft
Use 41 ft.
Check horizontal velocity
31 .35
mgd
2
(
) x10 6
gal
hr
min
sec
(7.48 3 )( 24
)( 60
)( 60
)
day
hr
min
ft
vh 
 0.34 ft / s
8x8.8
(on the low side, but acceptable)
6.
7.
Air supply
use 5 ft3/m. ft of length
air required
= 41 ft x 5 ft3/m. ft of length
= 205 ft3/min
assume 7 ft3/mgd
= 11.4 mgd x 7 ft3/mgd
= 79.8 ft3/day
Average grit removal
volume of grit
Primary Sedimentation
 purpose – to remove settable organic solids and to reduce the solids load on the biological treatment unit
 primary sedimentation or clarification is achieved in large basins under relatively quiescent conditions
Two types of design available:
horizontal flow and circular
Design Criteria
 overflow rate (gpm/ft2 or m3/m2/d)
 weir overflow rate (gpm/ft or m3/m/d)
 detention time (hours)
 solids loading rate (lbs/ft2/d or kg/m2/d) (more important for secondary sedimentation tanks)
Design Data
Overflow rates
Average flow
Range
(gal/ft2/d)
(m3/m2/d)
Typical
800 – 1200
32 – 48
2,000 – 3,000
80 – 120
2,500
100
Weir overflow rate
Peak hourly flow (gal/ft2/d)
(m3/m2/d)
10,000 – 40,000
125 – 500
20,000
250
Detention time (hrs) (average flow)
1.5 - 2.5
2.0
Peak hourly flow (gal/ft2/d)
(m3/m2/d)
Typical dimensions
Rectangular
Depth (ft)
10 – 15
12
Length (ft)
50 – 300
80 – 130
Width (ft)
10 – 80
16 – 32
Bottom slope
1 – 2%
Speed of scraper (ft/min)
2–4
3
Circular
Depth (ft)
10 – 15
12
Diameter (ft)
10 – 200
40 – 150
Bottom slope (in/ft)
¾-2
Speed of scraper (rev/min)
0.02 = 0.05
0.03
_______________________________________________________________________________
Expected BOD and suspended solids removal between 30 – 40% and 50 – 70%, respectively (see Figure
12- 4, may be different for different wastewater).
Design example:
Design a sedimentation tank for a municipal wastewater with an average flow of 5.000 m3/day and a
peak hourly flow = 12,500 m3/day
Use Figure, assume 60% SS removal, overflow rate = 35 m3/m2/day
Required surface area
2.
Circular tank diameter
d = (4x 143/3.142)1/2
(have to select size appropriate for circular scraper)
3.
Assume 15 m diameter (to fit a 15 m diameter scraper)
surface area needed = 3.142 (15)2/4
= 176.7 m2
4.
Assume side wall depth = 3 m
5.
Detention time
6.
For peak hourly flow requirements, find overflow rate = 12,500/176.7 = 70.7 m3/m2/d
(ok)
7.
Check weir overflow rate
= 12,500/(3.142 x 15)
= 265 m3/m2/day
(ok)
SS removal at peak flow rate
 40%
8.
= 5,000/35
= 143 m2
1.
= 13.5 m
Volume of tank = 176.7 x 3
= 530.1/5000
= 0.106 days
= 530.1 m3
= 2.54 hrs
(ok)