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 Wastewater TreatmentScreening, Grit Removal,

Primary Sedimentation Draft Lecture Notes

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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… 

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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… 

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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