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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-1
Chapter 6
PRELIMINARY TREATMENT:-GRIT REMOVAL
Grit removal Grit removal is accomplished in channels or
retention tanks. The objective is to settle out inorganic matter
(particles with diameter above 0.2mm) whilst avoiding deposition of
putrescible organic material and during this process, the organic
matter must be kept in suspension. The average specific gravity of
grit particles and organic matter are 2.5 and 1.2 respectively and
therefore grit particles have a higher settling velocity than
organic matter. The settling velocity of grit is approximately 0.03
m/s. The horizontal flow velocities in grit channels are normally
0.3m/s and higher horizontal velocities result in scouring of the
deposited material. Grit settlement can therefore be used as a
guide to the plant performance. If the quantity of grit reduces, it
may be an indication that the flow rate has increased above 0.3 m/s
or the grit is being retained in suspension. If the amount of grit
increases, there may be a new effluent discharge. Types of grit
channels
comminutors constant velocity grit channel aerated grit channel
(Fig. 1) vortex grit chambers (Fig. 1)
For wastewater stabilization ponds, grit chambers may be
alternatively incorporating with the ponds in the form of a sump
and this is achieved by deepening under the inlet point of about
1.0 m extra depth to contain the grit for a period of 2 years (Fig.
2). A constant velocity grit channel is commonly found on many
wastewater treatment plants because it is cheaper and simple. It is
also easier to operate, with very low operational costs. The grit
is either dredged from the channel by machine or removed by hand
(Fig. 3). Since there is diurnal variation in flow, to control the
velocity at all flow rates, the channel should be designed so that
the velocity remains constant at all depths of flow and this can be
achieved by the use of a parabolic cross-section. In this case flow
will be proportional to the cross-sectional area. A flume is
incorporated at the downstream end of the channel to maintain a
constant velocity at all depths and at the same time measuring flow
(Fig. 3). The Venturi should not be drowned so that it produces an
upstream depth that is independent of conditions downstream
conditions. Usually a trapezoidal section is used instead of a
parabolic for ease of construction. Chapter 8: Preliminary
treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-2
Fig. 1(a) Schematic illustration of a grit comminutor
Fig. 1(b) Schematic illustration of a aerated grit chamber
Chapter 8: Preliminary treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-3
Grit chamber
Inlet pipe
Fig. 2 A typical Grit Chamber incorporated with WSP
Fig. 3 Constant grit channel
Design of a constant velocity grit channel One operational and
standby grit channels are provided. The design formulae are derived
using Fig. 4 Section A-A is the cross-section of the grit chamber
and B is the throat width of the Venturi flume.
A Chapter 8: Preliminary treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-4
v B W
C
A
C
Section A-A Section C-C
H
W
H
h
v2/2g
Fig.4 The configuration of the grit chamber and the flume Flow,
(7) vAQ = Where; V = Velocity in channel A =Cross-sectional area
vBhQ =
gvhH2
2
+= (8) Substituting equation 8 into 7:
( ) BhhHgQ 21212 = )
Flow in a flume is maximum when depth at throat is 32 of the
total energy
Chapter 8: Preliminary treatment: grit removal
4(9(h=32 H)
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-5
Substituting h in equation 9, the maximum flow becomes:
23
max 71.1 BHQ = or 23
max KBHQ = (10) Differentiating equation 10:
dHKBHdQ 21
23= (11) vWdHdQ = (12)
Equating equation 11 and 12 :
21
23 H
vkBW = (13)
Taking the velocity in the grit chamber to be 0.3 m/s, the above
equation becomes:
21
)6.0(3 HkBW = (14)
Dividing equation 10 by equation 14, the width of the channel
can be computed from:
HQW 5= (15)
grit of velocity Settlingflow oflocity depth x ve Channellength
channel The =
The settling velocity of grit is taken as 0.03m/s. Therefore the
length of the grit channel is given as:
channel ofdepth maximum x 100.03m/s
0.3m/s x Channel ofdepth Maximum ==L Thus the top width of the
channel is simply determined from the maximum flow and
corresponding depth. In practice. L= 20 x maximum depth of channel
to allow for turbulence and variation in settling velocity. Grit
storage is provided in the channel to reduce the frequency of
manual cleaning. The grit storage space is provided by lowering the
floor of grit channel. A 1 or 2 week grit storage space may be
provided and the volume of the grit storage space may be estimated
from: Chapter 8: Preliminary treatment: grit removal
5
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-6
length width x base channelGrit eGrit volumchannelgrit in
required storage ofDepth =
The grit volume =DWF x storage time x grit concentration The
grit concentration 0.01m 3/1000m3 Hydraulic grit washing within the
grit chamber is provided by bottom scouring. The scouring velocity
will re-suspend the organic particles which might have settled in
the chamber while the grit remains at the bottom. The scouring
reduces the volume of grit, odour formation and flies. The critical
horizontal velocity for scouring is given by Shields equation: (
)
dRfg
v gc18 =
Where: f =Particle shape factor (0.04-0.06) g =Gravitational
constant =Friction factor (0.03) Rd =Relative density
p . The density for sand particles is 2650kg/m3 and for
suspended solids is 1150kg/m3 and is the liquid density. d
=Diameter of particle, m The critical horizontal velocity should be
0.1 m/s to ensure sufficient scouring to re-suspend the organic
solids.
Example: 1: Design a grit chamber for the flow calculated in
example The depth of flow is computed at multiples of DWF (from DWF
to peak flow as in Table 1). H 3/2 is calculated from equation
8.10, taking a throat width b=0.25m. The width of the channel is
calculated from equation 15. Using the results in Table 1, a
parabolic section is plotted and a best-fit trapezoidal section is
selected (Fig. 5). Channel length L =20 x maximum depth =20 x 1.6
=32m
Grit volume =01.07
100018723
=1.31m3
Chapter 8: Preliminary treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-7
Depth of storage, taking the base width as 0.4m,
Ds=324.0
31.1
=0.1m The grit channel should be lowered by 0.1m to provide the
grit storage space. Other specifications of the grit channel are in
Fig. 6. Table 1 Computation of the cross-sectional area Multiples
of Domestic Domestic Combined H3/2 H W Domestic DWF DWF flow
DWF (m3/day) (m3/s) (m3/s) (m) (m) 0.25 3930.75 0.045 0.080
0.313 0.461 0.871 0.5 7861.5 0.091 0.126 0.490 0.622 1.011 0.75
11792.25 0.136 0.171 0.667 0.764 1.121 1 15723 0.182 0.217 0.845
0.894 1.212 1.25 19653.75 0.227 0.262 1.022 1.015 1.292 1.5 23584.5
0.273 0.308 1.200 1.129 1.363 1.75 27515.25 0.318 0.353 1.377 1.238
1.427 2 31446 0.364 0.399 1.554 1.342 1.486 2.25 35376.75 0.409
0.444 1.732 1.442 1.540 2.5 39307.5 0.455 0.490 1.909 1.539 1.591
2.75 43238.25 0.500 0.535 2.086 1.633 1.639
7
Fig. 5 Typical cross section of a parabolic grit chamber
Chapter 8: Preliminary treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-8
Fig. 6 Specifications of a constant grit channel
Key
Bb32< Hmax = max depth at measuring device
max3HE = ( )bBD = 3 max5.1 HL = ( )bBR = 2 BF 10 Example: 2
Design data Design population = 75 000 capita Average dry weather
flow, ADWF = 6.50Ml/d Minimum dry weather flow, MDWF = 2.70Ml/d
Peak dry weather flow, PDWF = 15.8Ml/d Peak design flow, PDF =
16.25Ml/d Inlet pipe invert level, I.L = 71.293m Design data The
grit chamber is designed to cater for the following hydraulic
conditions: Maximum design flow, Qmax = 16.25Ml/d (0188m3s 1)
Minimum design flow, Qmin = 2.70Ml/d (0.0313m3s-1) Design velocity,
v = 0.3ms-1 Grit concentration = 10ml per liter of influent
Therefore grit load = 0.01m3/1000m3 of influent
Chapter 8: Preliminary treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-9
Ratio of maximum flow to minimum flow, R = 0.188/0.0313 R = 6.0
3 >3 >3 >3
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-10
From Table 3 a flume throat width of 0.225m is the optimal size
for it is the smallest size possible flume. It therefore has the
minimum construction cost. Therefore a 0.225m throat width is
adopted. Plotting channel parabolic curve From W = kH1/2 and where
k = 0.97, the equation of parabola is: W = 0.97H1/2 Using above
equation 7 and varying values of channel depth (H), Table 4 is
constructed. Table 4: Data for plotting parabolic curve Error! Not
a valid link. Shape of grit channel Allow a base width of 400mm,
thus half base width: W = 200mm Best fit curve equation in Fig 7: y
= mx + c By using the line of best fit the best fit curve is: y =
0.983x - 0.15 as shown in Fig 7. Thus full base width, Wg = 2 x
200mm = 400mm Equating equation W = 0.97H1/2 to the linear equation
above gives a quadratic equation:
y = 1.06x2 - 0.983x + 0.15
Solving for x provides the following solutions: x = 0.20 or
0.74
Substitute 0.74 into the linear equation:
y = 0.570m Therefore the trapezoid is 0.570m from the base.
Chapter 8: Preliminary treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-11
Fig 7 Parabolic curve for channel width of 0.225 Properties of
the 0.225m flume Width of throat , b = 0.225m Maximum head ,Hmax =
0.620m Minimum head, Hmin = 0.188m Inlet channel invert level, I.L
= 71.105m Maximum width of parabolic curve = 1.520m Length of flume
,L = 1.5 x Hmax = 1.5 x 0.620m
=0.930m For 1.0 > Hmax/b > 0.75 Chapter 8: Preliminary
treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-12
Set approach channel width at B = 0.480 This gives b/B=
0250/0.480 = 0.47 < 0.7 O.K Flume dimensions Dimension E = 3Hmax
= 3 x 0.620 = 1.860m Dimension F = 10B = 10 x 0.480 = 4.800m
Dimension D = 3(B-b) = 3(0.480-0.225) = 0.765m Dimension R = 2(B-b)
= 2(0.480-0.225) = 0.510m Flume configuration
Length of grit channel Assume a detention time of one minute, t
=1 min = 60 second. Flow velocity, v = 0.3 ms-1 ( to achieve grit
settlement) Length o grit channel, Lg = v x t = (60 x 0.3) ms-1 x s
Lg = 18 m Grit storage Allow a seven days grit storage space
Average daily flow, ADWF = 6500m3/d Grit concentration =
0.01m3/1000m3 inflow Therefore seven days grit volume = (6500 x
10-3 x 7 x 0.01) m3 = 0.455m3 Depth of storage required in grit
channel = Grit volume/(Grit channel base width x length) =
0.455/(0.400 x 18) m3/m2 Therefore depth = 0.063 m Chapter 8:
Preliminary treatment: grit removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-13
Take minimum depth of grit storage as 0.065m. The grit storage
is provided by lowering the floor of the grit channel. The floor of
the grit channel is made to fall along the floor line at a slope of
say 1:60. A fall of 1:60 for 18m provides a vertical fall of 18/60
= 0.30m. The fall is provided by a screed of minimum thickness
0.025m and a maximum thickness of 0.325m. Floor level of grit
channel = (71.105-(0.325 + 0.065)) m = 70.715 m Grit sump The lower
end of the floor should be provided with a grit sump 0.400 x 0.300
x 0.150 meters deep to collect the grit. Number of grit channels
Two grit channels are to be provided so that one may be isolated,
drained and cleaned while the other is in operation. Each channel
is isolated by means of two penstocks, one at each end of the
channels. Drainage of channels is through a 50mm diameter pipe with
valves at the inlet and outlet. The valve at the outlet is
essential to prevent entry of frogs in the pipe. Velocity control
To obtain an approximately constant velocity in the grit channel
the flume should be lowered a distance ( called the step height)
below the floor of grit channel. Maximum to minimum flow ratio, R =
6.0 Step height, Y = Cr x Hmax Where Cr = (R1/3-1)/(R - 1)= = (61/3
- 1)/(6-1) = 0.163 Therefore Y = 0163 x 0.620 m Y =0.101m Therefore
flume floor level = (70.715 - 0.101) m = 70.614 m Say = 70.615m
Downstream conditions The downstream channel discharges into the
primary settling tanks division box. The floor level of the box is
set at least half the PST inlet pipe diameter below channel floor
level to prevent drowning of the channel. Therefore invert level of
the collection chamber = 70.615 - 1/2, where =PST inlet pipe
diameter. Assume that =450mm Chapter 8: Preliminary treatment: grit
removal
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
Eng., NUST 9/26/2006 6-14
I.L =70.615 - (450) Collection chamber I.L =70.390m Top of
concrete level Top water level of inlet channel = Inlet channel I.L
+ Hmax = (71.105 + 0.620)m = 71.725m Depth of water in inlet pipe =
(71.725 - 71.293)m = 0.432 m To avoid drowning of the influent pipe
consider this depth (0.80m) as for a pipe flowing at 0.8 full.
Therefore pipe diameter: = 0.432m/0.8 = 0.5775m = 600mm diameter
pipe Height of channel walls, Hw = (Pipe I.L - Channel I.L) + (Pipe
diam + grit channel storage depth ) + freeboard Hw =
(71.293-71.105) +(0.600 + 0.065) + 0150 m Hw = 1.00m Therefore top
of concrete level = (71.105 + 1.00)m = 72.105m References 1. Mara
D., (1976), Sewage Treatment in Hot Climates, John Wiley, UK 2.
Mara D., (1997), Design of Waste Stabilization Ponds in India,
Lagoon
Technology, UK 3. Mutamba J. 1998), Design of Cowdary Park BNR,
Design Project, Dept. of Civil
and Water Engineering, NUST, Zimbabwe. 4. Smith .M., (1995),
Unpublished Lecture Notes in Wastewater Engineering,
Loughborough University, UK
Chapter 8: Preliminary treatment: grit removal
14
Chapter 6PRELIMINARY TREATMENT:-GRIT REMOVALGrit removalFig. 2 A
typical Grit Chamber incorporated with WSPDesign of a constant
velocity grit channelFig.4 The configuration of the grit chamber
and the flumeFig. 5 Typical cross section of a parabolic grit
chamber