Chapt6 Preliminary Gritremoval[1]

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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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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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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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)


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

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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


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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

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Fig. 5 Typical cross section of a parabolic grit chamber



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


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Ratio of maximum flow to minimum flow, R = 0.188/0.0313 R = 6.0 3 >3 >3 >3


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.


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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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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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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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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


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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

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Chapt6 Preliminary Gritremoval[1]