The document provides design details for a manual grit chamber, including: 1) Dimensions are calculated based on an average flow of 3 MLD and peak flow of 7.5 MLD. 2) Settling velocity and reynolds number calculations are used to determine the diameter is 0.00015 m. 3) Based on an efficiency of 75% and other parameters, the overflow rate is calculated as 1010.3260330988 m3/m2/day.

1. DESIGN OF MANUAL GRIT CHAMBER
Detail Value Units Formula
Design Average Flow 3 MLD
3000 m3/d
0.0347 m3/s
Peak factor 2.5 As per CPHEEO pg. 3-3,Table 3.2 - population for 3 MLD - 27000
Peak flow 7.5 MLD
7500 m3/d
0.0868 m3/s
S.G 2.65
Diameter 0.00015 m
Kinematic Viscosity of sewage 0.00000114 m2/s (at 15 deg C - considering the minimum possible temperature for design - since
settling is temperature dependent and requires most volume at lowest
temperature - design for worst-case)
Settling Velocity 0.0177 m/s Vs=0.545(Ss-1)d*d/v
Reynold’s Number 2.3289 Vs*d/v
If Reynold’s number Greater than 0.5 If Reynold’s No. is greater than 0.5 the stroke’s law does not apply
Settling Velocity 0.0168018697487138 m/s Vs=[0.707(Ss-1)(d^1.6)(V^-0.6)]^0.714
Surface Overflow Rate
Settling Velocity 1529.28 m/d For 100% removal efficiency - theoretical ideal condition
1529.28 m3/m2/d (Rewriting as Overflow rate)
Efficiency of Removal η 0.75 Average Efficiency
Measure of Settling Basin Performance
'n'
1/8 For very good Performance
Overflow Rate (Q/A) 1010.3260330988 m^3/m^2/day η=1-{1+[n*(Vs/Q/A)]^(-1/n)
Dimensions of Grit Chambers
Number of Channels 1
Plan Area per channel 2.96933851224105 m^2
Kc Constant 4 Range 3-4.5 CPHEEO Manual Page 5-42
Ss Specific Gravity 2.65
Acceleration due to gravity g 9.81 m/s^2
Diameter of Particle 15 mm
Critical Displacement Velocity 0.160929798359409 m/s Vc=(8k*(Ss-1)gd/f)^0.5
Assuming Width of Channel 0.5 m As per nptel assume depth to width ratio 1:1.2
Length 6 m
Hydraulic Retention Time 60 seconds As per CPHEEO, HRT at avg. flow should not exceed 60 seconds
Therefore Volume of Channel 2.082 m3
Arriving at Liquid depth 0.7 m (Volume = Discharge x HRT)
Horizontal velocity of avg flow 0.10 m/s
Check for Vh < Vc: Hence Ok Vh should be less than Vc, to prevent Scouring or resuspension of grit
Providing grit storage depth 0.25 m give explanation - refer net if reqd
Providing free board 0.5 m
Total depth 0.95 m
Depth at peak flow 1.76 m Q = AV; Cross sectional area of flow = W x LD
1.80 m
Providing total height 1.80 m
Size of manual grit chamber 6m L x 0.5m W x 1.80m Hd + 0.5 FB
Proportional Flow Weir
Peak Flow 0.0868 m^3/sec
C 0.61 Coefficient which is assumed 0.61 for symmetrical sharp-edge weirs.
a 0.035 m Dimension of weir usually assumed between 25 mm and 50 mm
Base Width 0.10 m Base width of the weir
Liquid Depth Weir Width
0.035 0.05 x=(b/2)(1-((2/⫪)(tan^-1((y/a)-1)^1/2)
0.35 0.01
0.7 0.01
1.05 0.01
1.4 0.01
1

2. WeirWidth,Y
0.00
0.01
0.03
0.04
0.05
Liquid Depth,X
0.035 0.35 0.7 1.05 1.4
Units
2