CE 523 NOTES ON SCREENS
Wastewaters may contain coarse suspended and floating matter that may damage or
interfere with the operation of pumps and other equipment. This material is usually
removed by simple screening devices, which represents the most economical form of
In water supply systems, screens are usually located at intakes from rivers, lakes or open
storage reservoirs, where they serve to prevent the entry of logs and sticks or even fish
and animals (see introductory presentation). In sewerage systems, screens are sometimes
provided at the inlet to sewage pumping stations where rags, paper and other coarse
materials could cause damage by fouling pump impellers. lso, screens are placed on
sewer overflow structures where it is desirable to prevent coarse materials from fouling
the receiving environment. !creens are usually installed at the inlets to wastewater
treatment plants to remove materials that may otherwise become entangled in pumps,
scrapers and aeration devices, or may foul weirs and channels.
Classification of screens
!creens may be classified variously according to" their clear opening si#e (as coarse,
medium or fine)$ their configuration (racks, bar screens or mesh screens)$ the method
used to clean the entrapped material from the screen surface (manually%raked or
mechanically%raked, or water%&et cleaned)$ and also according to whether the screen
surface is fi'ed or moving. (ombined screening and maceration devices are sometimes
used at wastewater treatment works.
Racks and bar screens These are the simplest forms, and are widely used at inlets to
wastewater treatment plants, and on river intakes for water supplies.
Bar screens consist of parallel metal bars with )* to +* mm clear openings in coarse
screens, and ,* to )* mm clear openings in fine to medium screens. -ine screens are
usually preceded by coarse screens in order to prevent the risk of damage by large
ob&ects, or e'cessive head loss as screenings accumulate.
.ar screens may be satisfactorily cleaned by manual raking in small installations
(-igure,a%i). /arge installations, however, are generally provided with mechanical
devices for removing screenings at regular intervals (-igure ,a%ii). !uch devices may be
operated either by time clocks or by pressure%sensing probes, and are activated when the
head loss across the screen becomes e'cessive. large number of proprietary screens
with mechanical rakes is currently available. In most cases, manufacturers provide
design charts to facilitate selection of appropriate screen si#es.
Mesh screens These are used mainly in water supply and water treatment installations.
They have square openings ranging in si#e from , to )0 mm or greater, according to
design requirements. 1ost fine mesh screens are used in locations where relatively small
amounts of material are to be removed, and these usually are manually cleaned. !uch
screens, for e'ample, may follow bar screens on water intakes in order to prevent small
fish from being drawn into the intake.
Drum screen This is a continuous screening device consisting of a hollow drum, usually
) to 0 m in diameter, which rotates about its hori#ontal a'is (-igure ,b). 2ne end of the
drum is closed. Water enters the drum through the opposite end and passes out through
perforations or mesh openings in the drum3s periphery. s the drum rotates, screenings
caught on it are carried clear of the water surface and are deposited in a collecting trough.
Water &ets help to clean the entrapped material from the screen surface.
Disc screen This consists of a large rotating partly immersed metal disc, often set at an
angle of about 4*° to the vertical. !olids caught on the face of the disc are carried to the
top where they are removed by revolving brushes.
Fine belt screens These are made of fle'ible woven wire mesh. lthough only rarely
used in the treatment of water and municipal sewage, they are commonly used in
industrial wastes treatment. The rotation of the screen is an aid to cleaning. !ome belt
screens are vibrated to increase efficiency and prevent clogging when screening out fine
material, such as spent grain from a distillery or fine organic wastes from a canning
Medium belt screens These are used for continuous operation to remove floating and
suspended debris from water supply intakes. They consist of sections of mesh attached to
frames which are hinged together and mounted on a chain belt (-igure ,a%ii).
Microstrainers These are a development of the drum screen in which the screening fabric
consists of woven stainless steel micromesh (-igure ,c). 1icromeshes, with apertures
ranging from ,0 to +5 mm, are used to remove very fine suspended matter from feed
water. They are usually *.6 to 4 m in diameter and rotate at a ma'imum peripheral speed
of *.0 m7s. 8ffective cleaning of microstrainers requires high%pressure water &ets using
good quality water. To control the growth of slime bacteria, which would rapidly clog the
micromesh, the drum surface can be bathed in ultraviolet light.
The principal application of microstraining is the pre%treatment of some types of water
supply, prior to using other treatment processes. In the case of some good%quality upland
waters, it may be all the treatment necessary for removing some types of algae and other
planktonic organisms. In wastewater treatment, microstrainers are also used at a tertiary
stage to further improve a good%quality secondary effluent by removing suspended solids
escaping from the final sedimentation tanks. In these applications, microstrainers fulfil a
function more usually associated with filters than with conventional screens.
/oading rates of 40* to ) 4** m
.d have been claimed for microstrainers, but in any
particular application the design should be based on tests carried out on the water to be
treated. -or design purposes, conventional water analyses, such as those used for
suspended solids, provide inadequate information since the number, si#e and shape of
particles all affect the rate of removal by microstrainers .
Wedgewire screens !ee figure below and http"77www.wedgewire.com7 !ee also figure at end.
With sharp edges (almost no radius) and a
very smooth surface, the ‘V’ shape profile wires
ensure that the solid particles make only 2
points contact with the screen surface..
articles do not get trapped in the slot
opening and do not have any pro!lem
sliding on the surface. "s a result,clogging
of the slot opening is none, the !linding
effect gets minimi#ed and the open area
availa!le remains constant
Wedgewire screens are superior for retaining media, filtering, and si#ing. $n comparison with wire mesh and
perforated metal, wedge wire continuous slot screens have more open area, have very precise openings,
are stronger and more dura!le, are virtually non%clogging and reduce media a!rasion. Wire mesh and
perforated metal may !e less e&pensive initially, !ut wedgewire screens offer easier installation and long%
term operating and cost !enefits. Wedge wire screens have ma&imum design fle&i!ility, can !e constructed
in a wide variety of shapes and si#es from a variety of corrosion resistant alloys such as type '(), '*+,
'*+,, '2*, and )*(- stainless steels, as well as nickel alloys such as .%2/+.
Underdrains of filters
and packed columns
In order to retain finer
material within large
process units, there is a
need for distributors or
no##les or screens.
8'amples of these are
shown in the figure at
Disposal of screenings
!creenings removed at water supply intakes usually comprise leaves, sticks and other
vegetable matter which are relatively inoffensive and may be returned to the river
downstream of the intake. Wastewater screenings are often heavily contaminated with
faecal matter and need to be carefully disposed of as quickly as possible 9 usually by
burial, incineration, or maceration and returned to the flow.
Design of screens
The main factors to be considered in screen design are" the strength and durability of the
screening medium$ the clear screen area$ ma'imum flow velocity through the screen
apertures so that screenings will not be dislodged$ minimum velocity in the approach
channel to prevent settlement of suspended matter on the channel floor, and the head loss
through the screen. In most cases mild steel is satisfactory for making screens but other
materials may need to be considered in abnormally corrosive environments. The strength
of the screen media should be sufficient to withstand impact of large floating ob&ects and
the forces that may be generated by mechanical cleaning devices.
:enerally, the velocity through screen apertures should not e'ceed *.+ to ,.* m7s,
depending upon the nature of the materials to be removed and their susceptibility to being
forced through the screen at higher flow velocity. To prevent deposition of sand and other
suspended matter in approach channels, a minimum flow velocity is required. This is
related to both the si#e and density of the suspended impurities and to the flow velocity in
the channel from which the water or wastewater was drawn. minimum velocity of
*.4 to *.5 m7s is usually sufficient to prevent e'cessive accumulation of settled materials
in approach channels.
Head loss through screens
The minimum head loss through clean screens can be estimated by considering the screen
to appro'imate an orifice or a short conduit, or even by applying a minor loss e'pression.
widely quoted minor loss equation, useful for estimating minimum head loss through
clean bar screens is" θ β sin
ere h is the head loss across the screen (m)$ β is the shape factor (; ).5 for square%edged
rectangular bars)$ w is the ma'imum bar width (mm)$ b is the minimum aperture width
(mm)$ Va is the velocity in the approach channel (m7s)$ g is acceleration due to gravity,
and θ is the angle of inclination of the bars to the hori#ontal (commonly 4*°).
This e'pression is of use in determining the minimum hydraulic losses through screens at
the various rates of flow that occur, but is of no value in determining head loss once
material begins to accumulate on the screen surface. <esign should take into account the
ma'imum increase in head loss likely to occur under the critical condition of ma'imum
flow and minimum cleaning frequency. With mechanically%raked screens, this can be
readily accommodated by providing an automatic cleaning system. 1anually%raked
screens, however, should have liberal freeboard in the upstream channel to avoid the
danger of overtopping the channel at high flows.
Example on use of screens
Dirty water flows through a channel at 900 m
/h. A bar screen is to be used with 10 mm
openings, 16 mm bar width at an angle within the channel, which is 1 m deep and 60 cm
wide. The restriction to the design is that no more than a 15 cm increase in depth
upstream can be allowed. Available apertures will be reduced to 30% due to fouling in
the process. Suggest a suitable angle.
/h = 0.25 m
For a minimum velocity of 0.5 m/s, maximum depth would be x in 0.25/0.6x = 0.5, i.e. x
= 5/6 m or 0.833 m ∴ For a 15 cm increase in head, downstream depth will be 0.68 m.
Assumption If ∆h = 0.15 m for a screen in which apertures have been reduced to 30%,
then ∆h for a clean screen will be 0.15 x
= 0.045 m.
5 . )
6 . = )
0 . *
5 . ) *50 . *
∴ sin θ = 0.78 ∴ θ = 51.8
Another approach would be to increase w and decrease b, assuming a uniform deposit.
Because of the exponent larger than 1, the effect of this is more dramatic. This is
probably a less correct approach, as the dirt is more usually deposited as big blobs
obstructing parts of the whole width of the apertures, leaving the rest of the aperture
0e%Watering .ollectors .hemical Water 1reatment
2low 0istri!ution 2ilter 3lements etro%.hemical Waste 1reatment
4nderdrains .ar!on 5etention 6ineral rocessing -ugar
$ntake -creens .atalyst 5etention .ement .orn 6illing
5esin 1raps 0istri!utors 2ood rocessing harmaceuticals
2ilter -upport .ores 5eactor -creens 7everage ulp 8 aper
9utlet 7askets 1rap 2ilters etroleum Water 8 $rrigation
.enter ipes :;: -trainers
Normal screening direction
PPROFILE BAR SCREENS
1his construction is uni<ue to =endrick. 1he mechanically
interlocked design is perfect for applications that re<uire high
strength and a very flat surface. We !elieve it>s the strongest
construction availa!le on the market. "nd it is used heavily in
cylindrical applications. $t can !e manufactured in slot openings
as close as (.((?: and flatness to .(+(.
rofile 7ar screens are used in etrochemical, 2ish 0iversion,
ulp 8 aper, Waste Water, 2ood, and "rchitectural industries.
LOOPED WIRE SCREENS
,ooped Wire screens provide the perfect solution to vi!ratory
applications that re<uire high strength !oth laterally and
Wires are looped around a supporting cross rod and
compressed to fit snugly. -lots can !e manufactured to (.((?:.
,ooped Wire is used in the 7rewing, Waste Water, 6ining and
"ggregate, and .hemical industries.
RESISTANCE WELDED SCREENS
"re produced !y wrapping a wire helically around support rods
to create a cylinder with continuous slot openings. 1he wires are
automatically resistance welded to each support rod producing
a very strong wedgewire cylinder with e&cellent !eam, !urst and
collapse strength. 5esistance welded cylinders produce screens
with slot openings to .(() and very large percentage of open
area. 1he screens are widely used in food and !everage
processing, water intakes, fish diversion, architectural, and