Various physical operation for wastewater treatment plant

1. Physical Unit Operations screening
By
R M Bhadaraka

2. Screening
 First operation employed in wastewater treatment.
 It is necessary to retain floating matter and coarse solids as a pieces of
cloths garbage so as to protect pumps and other units from clogging.
 The retained material at screens is called as screenings.
 For proper functioning it required to clean the screens.
 A good designer shall provided two screen units 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
opening.
 Screens are kept Grit Chamber.

3. Classification of Screen
Sr No. Classify Spacing
1 Coarse Screen ≥ 50mm
2 Medium Screen 20 to 50mm
 Bar Screens are inclined at 30 to 60 to direction of flow to increase a the area of
Screening.
 Screens are fixed permanently and may either be manually cleaned or mechanically
cleaned.
 The amount of screening may be 0.03 to 0.08 𝑚3/million litre of wastewater.

4. Screening
 If Screening formed are 0.05 𝑚3
/ML for a wastewater treatment plant
with 50 MLD flow of wastewater the totally daily amount of screening
may be around 50 X 0.05 = 2.5 𝑚3
 Screenings can be disintegrated digested incinerated or buried.
Sr No. Design Parameter 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 150 150

5. Head Loss
 Head loss through the bar rack is calculated form following formulae.
𝑯 =
𝑽 𝟐
− 𝒗 𝟐
𝟐𝒈
𝟏
𝟎. 𝟕
Where,
H = 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
𝑠2
𝑯 =0.0728 (𝑽 𝟐
− 𝒗 𝟐
)
Note : this equation used to find out head loss in clean or partially clogged bar screen

6. Head Loss
H = β
𝑾
𝒃
𝟒
𝟑
h sinθ
Where,
W = Maximum cross sectional width of bars facing the wastewater flow, meters
b = Minimum clear bar spacing
h = velocity head of wastewater
β = Bar shape factor value of bar shape factor for clean bar rack.
Sr no. Type of bar β
1 Sharp edge rectangular bar 2.42
2 Rectangular with semi circular face facing upstream 1.79
3 Rectangular with both ends semi circular 1.67
4 Circular bars 1.83
Note : this equation used to find out head loss of clean bar screen

7. Quantity of Screening
 Quantity of screening depend upon type of screen, weather and
characteristic of wastewater.
 Narrow opening will collect more screening.
 Hot weather shall increase water use and waste water generation shall be
more.
 Quantity of screening may be 0.03 to 0.08 𝑚3
𝑀𝐿
 It may be taken average 0.05 𝑚3
𝑀𝐿

8. Characteristic of Screening
 Screening contain approximately 80% moisture and has a normal Wt-
density of 960 kg/𝑚3