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Raw water intake,
screening, and aeration
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Contents
Introduction
Raw water intakestructures
Types of intake
structures
Intake site selection
Intake-designconsideration
Screening Types of screening
Aeration
Purpose of aeration
Types of aerators 2
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Introduction
Process flow chart in water supplyproject
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Raw water intake structures
Control withdrawal of raw water froma surface water source.
Selectively withdraw the best quality
water while excluding fish, floatingdebris, coarse sediment, and otherobjectionable suspended matter.
Intake contains gates, screens,control valves, pumps, chemicalfeeders, flow meters, offices andmachine shop.
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Types of intake structures
1. Floating intakes
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2. Submergedintakes
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3. Exposed or towerintakes
3.a. wet-intake
towers 3.b. dry intake tower
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4. Shore-intake
structures 5. Pier intakes
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Intake site selection
Water quality Water depth Stream or current velocities
Foundation stability Access Power availability Proximity to water treatment plant Environmental impact Hazard to navigate
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Intake-design consideration
1. Intake velocities
High velocities headloss, entrain
suspended matter,trap fish, and otheraquatic animals.
Velocity below 8 cm/s
allows aquatic animalsto escape, andminimize thesuspended matter.
2. Intake-portlocation
Water quality in eachstratum may vary.
To achieve, multipleintake ports set atvarious levels aregenerally provided.
Top intake less than2 m below normallevel.
Bottom intake least
1 m above the bottom11
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3. Gates
Usually sluice gates .
Large cast iron gates
that slide vertically ona guide track.
4. Control of ice
In cold regions, gatescovered with ice.
To avoid, intake portsare kept below watersurface below 8m .
Space heaters,
Compressed air isused to remove ice.
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Screening
It is a unit operation that removessuspended matter from water.
Screens may be classified as coarse,fine, or microstrainer, depending onthe size of material removed.
Located at intake structure, rawwater pump station, or watertreatment plant.
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1. Coarse screen or trashrack
To fprevent large objects rom enteringthe conveyance system.
Consists of vertical flat bars, or, in some
cases, round pipes spaced with 5 to 8cm of clear opening.
Installed outside of any sluice gate.
The velocity through the coarse screenis generally less than 8 cm/s.
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2. Fine screen
To remove smaller objects that maydamage pumps or other equipment.
Screens consists of heavy wire meshwith 0.5 cm square opening.
The typical design velocity throughthe effective area is in the range of0.4 to 0.8 m/s.
There are two types:Travelingscreens and passive screen
installation. 15
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Typical fine screens
Traveling screen Passive screen
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3. Microstrainer
Mainly to remove plankton and algaefrom impounded waters.
Microstrainer is installed beforechemical coagulation will improve theperformance of clarifiers.
Consists of rotating cylindrical framecovered with fine wire mesh fabric.Water enters the cylinder and movesradially out.
Damaged by abrasive material metal ,sand.
Problem build up of slime on thefabric. 17
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Aeration
Aeration involves bringing air orother gases in contact with water.
The purpose of aeration are
1. Reduce the conc. of taste and odor causingsubstance by volatilization
2. To oxidize iron and manganese, rendering theminsoluble.
3. To dissolve a gas in the water ( O2 and CO2)
4. To remove compounds for better watertreatment ( H2S removal before chlorination andCO2 removal before softening)
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Types of aerators
1. Gravity aerators
Utilize weirs,waterfalls, cascades,
inclined planes withriffle plates,perforated tray towers
2. Spray aerators
This spray droplets ofwater into the air from
orifices or nozzles. To produce an
atomized jet, largepower required &
water must be free oflarge solids.
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3. Diffused-airaerators
Aerated in tanks usingcompressed air .
Aeration period 10 to30 mins.
Air supply 0.1 to 1m3/min.
4. Mechanicalaerators
Motor driven impellersor combination of
impeller with airinjection device.
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REFERENCE
Water works engineering planning,design, and operation
By syed R. QASIM, Edward M. Motley and
guang zhuPrentice-hall , india (2006)
And photos from internet.
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