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Wastewater TreatmentScreening, Grit Removal,
Primary Sedimentation Draft Lecture Notes
1
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Wastewater Treatment Plant Layout
http://www.saskatoon.ca/DEPARTMENTS/Utility%20Services/Water%20and%20Wastewater%20Treatment/Waste
water%20Treatment%20Plant/Pages/default.aspx
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Conventional Wastewater Treatment-Process
Flow Diagram
(1) Large Objects (2) Grit (3) SS (4) BOD5 (5) Pathogens3
1
2 3
4
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Screening
4ManuallyCleaned
MechanicallyCleaned
Objective of Screening: Remove Large Objects Protect Downstream Equipment and Conduits
Bar Screens
ManuallyCleaned
MechanicallyCleaned
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Screening, Cont’d…
5
Single Bar Rack Screen
Double Bar Rack Screen
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Design of Bar Screens
Design Parameter Recommended Design Value
Mounting Angle- θ 45- 90
For Mechanical Screens
30- 70 For Manual Screens
Velocity in Approach Channel 0.45 m/ sec (Minimum)
Velocity between Bars 0.60- 0.90 m/sec
1.2 m/sec (Max)
Screen Bars Shape Square
Round
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Assume Velocity through Screen @ Design Flow = 0.9 m/s
Area of Openings = Q/V= 0.54/0.9=0.6 m2
Assume Depth of Flow = 0.7 m
Clear Width of Openings = 0.6 m2/0.7 m = 0.86 m or 0.9 m
Assume Space between Bars = w = 2.5 cm
Total No. of Spacings = n*w = 0.9/0.025 = 36 Spacings Total Number of Bars = No. Spacings – 1 = 36-1 = 35 bars
Assume bar Width = 1 cm
Width of Channel = 35*1 + 36* 2.5 = 125 cm
Check Approach Velocity◦ Area of flow = 0.7*1.25 = 0.875 m2
◦ Approach Velocity = 0.54/0.875=0.62 m/s (O.K.)
Estimate Channel Width = 10*width◦
Area of flow = 10*1.25 = 12.5 m
Simple Bar Screen Design Example
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n*w w w w w w w
d
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Grit Removal
Grit includes: Sand, Silt, egg shells, Coffee Grounds
Design Principle: Remove Grit by Sedimentation.
Selective Removal: Remove Grit but not lighter
Wastewater particles. Grit particles are Larger and
denser than typical Organic particles in wastewater
Grit Removal Systems:
System with Controlled Horizontal Velocity-High enough to prevent setting of organic Solids
Systems that use Air to prevent organic solids fromsetting
Cyclones which allow larger and denser particles tosettle but not organic solids
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Grit Removal, Cont’d…
9
Aerated Grit Chamber Cyclone Grit Removal
Variable Depth/VelocityControlled Systems
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Aerated Grit Removal
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Design Parameter Recommended Value
Detention time 2- 5 minutesTypically ≈ 3 minutes
Depth 2-5 m
Width 2.5 - 7 m
Width/Depth (W/D) 2: 1
Length/Width (L/W) 3:1 - 5:1
Air Supply ≈ 0.30 3/m/min
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Grit Removal-Design Steps
Select detention time & estimate volumeSelect No. units and find volume of each
Proportion chamber: Assume depth : width :
length
Provide free board
Select air supply rate and estimate air supply
requirements
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Primary Sedimentation (Settling) Tanks
12(a) Rectangular and (b) Circular primary sedimentation tank
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Primary Sedimentation (Settling) Tanks
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Design of Primary Sedimentation Tanks
Parameter Recommended Design Value
Detention Time 90-180 min
Overflow Rate 32- 48 m3/m2/d @ Qavge
80-120 m3/m2/d @ Qmax
Removal Rate 50 -75% TSS25- 40% BOD5
Circular tanks 3-5 m, Up to 40 m
Rectangular Tank Depth = 3-5 m
Width : Length = 1: 3 to 1: 5 (Up to 40 m)Width : Depth = 2 : 1 to 3 : 1
Horizontal Velocity <0.3 m/min
• Weir Loading < 600 m3/m/d
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Grit Removal-Design Steps
Select OFR & estimate surface area
Select depth and find volume
Check detention time
Select No. units and find surface area of each
Decide shape and proportion tank i.e., assumedepth : width : length or estimate diameter
Provide free board
Check horizontal velocity
Select weir loading, find weir length then:
arrange weir for rectangular tanks
Estimate weir length for circular tanks and
check weir loading 15
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