• Screening is a unit operation that separates
materials in and/or on water (found in different
sizes) from water and from entering water
treatment facilities and mains.
• The unit involved is called a screen.
Where are Screens Located Within Treatment
Plants or Systems
Belt Filter Press
Classification of Screens
– Opening size [Coarse, Medium and Fine]
– Configuration [Bar Screens and Mesh
– Method used to clean the entrapped materials
(manually, mechanically, raked or water-jet
– Fixed or moving screen surface.
• Coarse Bar Racks
– remove coarse debris (twigs, branches, rags,
- Spacing coarse 2 – 6 in
medium 0.8 – 2in
• Fine Screens
- 3/8 to ½ in. (up to 10 mm or less,
- book < 6 mm (1/4 in.))
Types of Screens
- hand clean coarse screens •
- mechanically cleaned bar screens •
Types of Screens
1. Moving •
2. Fixed •
Types of Screens
There are many types of screens that can be used in water and
wastewater treatment processes of which:
1 - Bar or rack screens: Bar screens composed of parallel bars. Bars
usually vertical or inclined
2 - Band screens: Consists of a perforated belt passes over an upper and
3 - Perforated plate screen: Consists of a fixed band of perforated
4 - Wing screens: It has radial vanes which rotate on a horizontal axis
5 - Disk screens: Circular perforated disk with or without supporting bars
6 - Grating screens: Consists of two sets of parallel bars.
7 - Mesh screens: Mesh screens composed of a fabric with mesh size
depend on floating and suspending matter.
• Design Criteria of a Bar Screen :
• Approach Velocity
• Optimum Velocity : 0.6 m/s (through the screen opening)
• Maximum Velocity : 0.75 - 1.0 m/s (to prevent entrapped
materials being forced through the bars).
• Minimum Velocity : 0.4 m/s to prevent deposition of
• Typical Range : 0.6 - 1.0 m/s
• Headloss :
• hL = 0.05 - 0.15 m for drinking water.
hL = 0.10 - 0.40 m for sewage (wastewater)
• Cleaning Mechanism: Manual Mechanical
• Angle of Inclination (10-60 )
• Maximum head loss (cm)
Band or Belt Screens
• Flexible woven wire mesh screens normally
installed for a river supply.
– Consists of sections of perforated mild steel plates
connected together in a form of a band which is
revolved by an electric motor.
– Water passes inward through the screens and
solid matter is washed off by high pressure water
jets directed from inside of the screen.
Disk Screens and Drum Screens
Similar in principle to band screens, differing only
in the form of the moving screen.
– Rotating metallic disc - partially immersed.
– Solid caught in the screen are taken to the top, where
they are scrapped by the moving screen.
– Diameter : 2 to 5 m
– Speed 0.05 m/s
– Hollow drum.
– One end of the drum is closed.
– Water enters through the other end and passes out
through the perforation.
– Water jet is used for cleaning
Disposal of Screenings
• Screenings is the waste materials collected from screens.
Screenings should be properly disposed. Various methods of
screening disposal were used such as:
- dumping into large bodies of water,
- and shredding and returning it to wastewater collection or treatment
• Inland burying is efficient in small treatment plants, while
burning is best for medium and large treatment plants. Other
methods cause problems and may need subsequent
treatment. Digestion is used for large systems and in
combination with the treatment of the organic portion of
municipal solid waste.
Design of Coarse screens
• - factors to consider
• - clear openings between bars: opening needed - typically
less than 2 ins., at 22 - 45 o incline
• - location :installed ahead of grit chambers, may also be
installed ahead of equalization tanks, if present.
• - approach velocity:
• at least 0.4 m/s to prevent deposition of solids, and should
not exceed 0.9 m/s at peak flow rate
• - head loss through screens - limited to 150 mm (6 in.)
• - screens handling and disposal - quantity of screenings
depends on the type of waste water, geographic location,
screen size and weather
• - screenings - vary from 3.5 - 80 m3/106 m3,
about 80% moisture and density of 960 kg/m3
• - controls - operation cycle about 15 minutes for
mechanically raked screens
Fixed Screens: Bar Screens
• Bar racks (also called bar screens) are composed of
larger bars spaced at 25 to 80 mm apart. The
arrangement shown in the figure is normally used for
shoreline intakes of water by a treatment plant.
• The rack is used to exclude large objects;
• the traveling screen following it is used to remove
smaller objects such as leaves, twigs, small fish, and
other materials that pass through the rack.
• The arrangement then protects the pumping station
that lifts this water to the treatment plant.
• Coarse screens or bar racks (< 2.5 inch openings) :
• (1) removes large objects, rags, debris ;
• (2) protects downstream pumps, valves, pipelines ;
• (3) cleaning may be accomplished manually or
• (4) mechanically cleaned bar racks (5/8 inch - 1-3/4 inch)
typically used instead of coarse manually cleaned screens
• (5) bar rack is inclined to facilitate , cleaning ;
• (6) approach velocities should ensure self-cleaning, but not
dislodge solids ;
• (7) typical design : maximum velocity of 2.5 ft/sec through
bar rack opening.
Microstrainer (Fine Filter)
Microstrainer have been used to remove
suspended solids from raw water
containing high concentrations of algae .
It consists of a fine fabric or screen wound around
a metal drum
HEAD LOSSES IN SCREENS
AND BAR RACKS
• Head loss calculations determine the hydraulic head requirements for screen
H H L
C =0.7 for clean and 0.6 for clogged : empirical discharge coefficient to account
for turbulence and eddy losses
V : velocity of flow through openings in rack (m/s)
v : approach velocity in upstream channel (m/s)
g : acceleration due to gravity (m/s2)
Applying Bernuli before and after
V12=V22 + 2g(h2 – h1)
HEAD Loss IN MICROSTRAINERS
• A bar screen measuring 2 m by 5 m of surficial
flow area is used to protect the pump in a
shoreline intake of a water treatment plant. The
plant is drawing raw water from the river at a
rate of 8 m3/sec . The bar width is 20 mm and
the bar spacing is 70 mm. If the screen is 30%
clogged, calculate the head loss through the
screen. Assume Cd = 0.60.
• For screens used in
shoreline intakes, the
velocity of approach is
practically zero. Thus,
• From the previous figure,
the number of spacings is
equal to one more than
the number of bars. Let x
number of bars,
• Design a bar screen chamber through
which maximum, average, and minimum
rates of flow are respectively 15, 7.5, and
3.0 cfs; the screen is such that there is
one more space than there are bars; the
outlet is controlled by a proportional weir
such that depth of flow is directly
proportional to rate of flow ?.
• A schematic diagram of the proposed screen is given
in previous slide. A number of assumption need to be
made by the designer engineer concerning screen
incline angel, a, flow velocity through screen, Vs,
number of bars, Nb, bar's diameter, db, and space
between bars, Sb. In this case we assume;
α = 30o, Vs = 1.0 fps, Nb = 20
db = 0.25 inch, Sb = 1.0 in
• Width of the screen, Ws, is equal to
Ws = Number of Spaces x Space Distance
= (20 +1) x 1 = 21 inch = 1.75 ft
Width of screen chamber, Wc, is equal to
Wc = Width of screen + Bars thickness
= 21 + (20 x 0.25) = 26 inch = 2.17 ft
Length of the screen, Ls, is equal to
Depth of the screen chamber, dc
Sinus on angle α
Flow through screen = Area available for flow x flow
Q = A x Vc
A = Wc x dc
Q = Qmax = 15 cfs
Vc = 1.5 fps
dc = = 4.61 ft
Wc x Vc
Maximum depth of the screen chamber, dc, is
maximum dc = dc + (0.67 to 1.00) = 5.28 ft
Length of the screen Ls = dc/sin α = 9.22 ft
Maximum flow velocity through the screen chamber, Vc,
Maximum Vc = = 1.62 fps
Wc x maxdc
Shredding devices (communitor
or grinder) : shreds material to
1/4 inch - 3/8 inch.
• Comminutors are devices used in water and
wastewater treatment either in combination with
screens or independently with the aim of
chopping the oversized suspended and/or
floating material found in water and wastewater
or escaping the screens before entering the
treatment facilities and altering its operation.
• Comminutors consist of two sets of cutters one
is fixed while the other is moving. The distance
between the two sets equal to the size of
chopped material required.
• Comminution technology has been evolving quite
rapidly in response to the increasing burden entrained
solids have placed on treatment facilities.
• More advanced devices have been developed in rapid
succession. The result has been an exciting and fluid
race between the leading manufacturers to develop
the best size reduction device.
• The latest grinder innovations to be introduced have
coupled the power of twin shaft grinding with higher
flow capabilities and screw screening systems. Here’s
a rundown on the past and present state of the art in
wastewater solids reduction
For comminutor design, environmental engineer
or designer need to supply manufacturer with
the size of suspended and floating materials
present in water to be treated and that after
treatment along with its density and hydraulic
and organic loadings. Accordingly manufacturer
decides on the equipment needed to achieve