Screening is the first unit operation in water treatment plants. A screen is a device with openings generally of uniform size that is used to retain the coarse solids found in raw water such as streams, lakes, ponds etc. the coarse solids usually consist of sticks, racks, boards, dead animals and other large objects that often find their way into a water body.
Bar Screen (Image credit: Boeep, China)
The primary purpose of screens is to protect pumps and other mechanical equipment and to prevent clogging of valves and other appurtenances in the treatment plant. As such, the screen is normally the first unit operation performed on the oncoming water.
Types of screens
Water screens are classified into two namely: fine and coarse screen, depending on their construction.
Coarse screens: They usually consist of vertical bars spaced one or more centimeters apart and inclined away from the incoming flow. Solids retained by the bars are usually removed by manual raking in small plants, while mechanically cleaned units are used in larger plants. A typical coarse screen is called the bar screen.
Fine screens: They usually consist of woven wire cloth or perforated plates mounted on a rotating disk or drum partially submerged in the flow or on a travelling belt. Fine screens should be mechanically cleaned on a continual basis. The clear water head loss through fine screens may be obtained from Manufacturer’s rating tables or it may be calculated by means of the common orifice formula:
hL = (1/2g) (Q/CA)2 (1)
C = Crest of discharge
Q = Actual discharge from screens (m3/s)
A = Effective submerged open area, m2
g = Acceleration due to gravity (m/sec2)
hL = head loss (m)
Design of bar screen
The design associated with the use of screen involves the determination of the head loss through them. The approach used for the coarse screens (racks) differ from that used for the fine screens (formula given in Eqn (1)).
Hydraulic head loss through bar racks is function of bar shape and the velocity head of flow between the bars. Kirshmer has proposed the following equation for the head loss:
hL = β (w/b)4/3 hv sin θ (2)
hL = head loss (m)
β = a bar shape factor
w = maximum cross-sectional width of bars facing direction of flow (m)
b = minimum clear spacing of bars (m)
hv = velocity head of flow approaching rack, m = v2/2g
θ = angle of inclination of rack with the horizontal.
Kirshmer’s values of several shapes of bars are given below:
|S/No||Bar type||Bar shape factor (β – values)|
|2||Rectangular with semi-circular upstream face||1.83|
|4||Rectangular with semi-circular upstream and downstream face||1.67|
The head loss calculated from Eqn (1) applies only when the bars are clean. Head loss increase with the increase in clogging.
Determine the head loss for a rack provided with rectangular bars of thickness 1.5 cm. the minimum clear distance between bars is 5 cm. the velocity of the flow through the screens is 0.3 m/sec. Angle of rack is 30o with horizontal.
The head loss through the rack is given by:
hL = β (w/b)4/3 hv sin θ
Substituting the values of β = 2.42; w = 15 cm = 0.015 m; b = 5 cm = 0.05 m;
hv = v2/2g = 0.32/ (2 x 9.81) = 0.004587 m; θ = 30o
Substituting these values into the head loss equation,
hL = 2.42 (0.015/0.05)4/3 x 0.004587 sin 30o ≈ 1.114 x 10-3 m.