Unit-III

1. Physical Unit Operations
Screening

2. Screening
• Screening is the first Unit Operation employed in wastewater
treatment. Screening is necessary to retain floating matter and
coarse solids as pieces of cloths garbage so as to protect pumps
and other units from clogging. Thus retained material at screens is
called as screenings. It is required to clean the screen for its proper
functioning. A good Designer shall provide two screen units one as
standby. Screens may consist of vertical or inclined bars (bar racks
or bar screens), wire mesh or perforated plates having either
circular or rectangular openings.

3. Screening

4. Screening

5. Screening
• Screens are Kept ahead of grit chambers, Bar Screens are
classified according to the spacing of bars as coarse screen (
Spacing ≥ 50 mm, Medium ( Opening 20 to 50 mm). Bar screen are
inclined at 30 0C to 60 0C to the direction of flow to increase the area
of screening.
• Screens are fixed permanently and may either be manually
cleaned or mechanically cleaned. The total amount of screening
may be 0.03 to 0.08 m3/million litre of wastewater.
• Screening can be disintegrated, digested, incinerated or buried.

6. Screening

7. Design Criteria for Bar Screens
( Bar Racks)
Design Parameters Manually Cleaned Mechanically Cleaned
1. Velocity through Rack ( m/s) 0.3- 0.6 0.6- 1.0
2. Bar Size
Width (mm)
Depth (mm)
4-8
25-50
8-10
50-75
3. Spacing Between Bars (mm) 25-75 10-50
4. Slope from Horizontal Degrees 45-60 75-85
5. Allowable head loss in clogged
condition mm
150 150

8. Design Criteria for Bar Screens ( Bar Racks)
Head Loss through the bar rack is calculated from the following formulae.
• HL=
𝑽 𝟐
−𝒗 𝟐
𝟐𝒈
𝟏
𝟎.𝟕
… … … … … … . . 𝟏
• = 0.0728 (V2- v2)
• HL= 𝜷
𝑾
𝒃
𝟒
𝟑
Hv. Sinθ………………..(2)
• Where,
• HL= Head Loss through bar rack in meters
• V= Velocity through Rack, m/s
• v= Velocity in upstream of bar Rack, m/s
• G= Acceleration due to Gravity
• = 9.81 m/sec2

9. Design Criteria for Bar Screens
( Bar Racks)
• W= Maximum Cross Sectional Width of bars facing the wastewater flow,
meters
• b= minimum clear bar spacing
• Hv= Velocity head of wastewater flow approaching the bar in meters
• β = Bar Shape factor Value of Bar Shape factor for Clean Bar Rack are
Given Below
• Sharp Edge Rectangular Bar- 2.42
• Rectangular with Semi circular faceing Upstream- 1.79
• Rectangular with both ends Semi-circular – 1.67
• Circular Bars – 1.83

10. Characteristics Quantity & Composition of
Screenings
• Quantity of Screenings depends upon the type of screen, weather,
& Characteristics of wastewater. Narrow opening will collect more
screenings & hot weather shall increase the use of water & Wastewater
formation shall be more thereby increasing screenings. Quantity of
screenings may be 0.03 to 0.08 m3/ ML with an average of around
0.05 m3/ML
• Screening contain approximately 80 % moisture and has a normal
wt. density of 960 kg/ m3. Screening are odorous and attract flies.
Often screenings are sent to grinders where they are ground and
returned to wastewater treatment plant.

11. Characteristics Quantity & Composition of
Screenings

12. Design of BAR Screens
Following Information are Required for design:
• Peak Wet Weather flow.
• Peak Dry Weather flow.
• Hydraulic Design of Influent Conduit.
• Treatment Plant Design Criteria Specified by Bureau of Indian Standards or other
Recognized agency.
• Equipment Manufacturers, their specifications for equipment.
• Velocity through bars
• Information on Existing facility if plant is to be expanded.
• Bar Spacing and head loss constrained through the rack & Through entire plant.
• Existing Site Plan with Contours.
• Velocities through Screen Chamber.

13. Design Example
• Design Bar Screen for a Peak flow of 50 MLD. Assume Other
data, Maximum Rate of flow in m3/Sec
• = 50 x 10 6 x 10-3 = 0.5787 m3/sec
24 x 60 x 60
Let Us Assume the velocity through the screen as 0.8 m/sec
Net Area of Screen= 0.5787
0.8
= 0.7234 m 2

14. Design Example
• Let us use bars of 10 mm x 50 mm with 10 mm dimensions facing the flow, at a spacing of 40 mm between the bars. So,
• Gross Area = s + t
Net Area s
= Clear Spacing + Bar Thickness
Clear Spacing
Therefore, Gross area in our Case
= Net Area x { Clear Spacing + Bar Thickness }
Clear Spacing
Gross Area = 0.7234 x 40 + 10
40
= 0.9043
Keep Screen at 45 0 Inclination with horizontal then Gross Area required
= 0.9043
Sin 45
= 1.2788 m 2

15. Design Example
• Velocity in the approach Channel i.e. Slightly Upstream of Bar
Rack
• = 0.8 x 40
50
= 0.64 m/sec
Thus we have V= 0.8 m/sec
v= 0.64 m/sec

16. Design Example
• Therefore,
• HL=
𝑽𝟐−𝒗𝟐
𝟐𝒈
𝟏
𝟎.𝟕
• = 0.0729 ( V2- v2)
• = 0.0729 ( 0.8 2 – 0.64 2)
• = 0.017 m
• = 1.7 cm
• This will be the head-loss when the screen is clean
• If it is half Clogged.

17. Design Example
• V= 2 x 0.8
• = 1.6 m/sec
&
• HL= 0.0729 ( 1.6 2 – 0.64 2)
• = 0.157 m
• = 15.7 cm
• So to reduce the head loss frequently cleaning of screen is required.

18. References
• Environmental Engineering : By Prof B.R.Shah
Prof A M Malek
• Internet Websites

19. Thanks …