Raw water intake, screening, and aeration
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Introduction Raw water intake structures Types of intake structures Intake site selection Intake-design consideration Screening Types of screening Aeration Purpose of aeration Types of aerators –
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Introduction Process flow chart in water supply project
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Raw water intakes withdraw water from a river, lake, or reservoir over a predetermined range of pool levels. Screens remove large floating objects from the water – to protect pumping equipment. Aeration removes gases and volatile compounds and also to oxidize certain dissolved metals. 4
Raw water intake structures •
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Control withdrawal of raw water from a surface water source. Selectively withdraw the best quality water while excluding fish, floating debris, coarse sediment, and other objectionable suspended matter. Intake contains gates, screens, control valves, pumps, chemical feeders, flow meters, offices and machine shop. 5
Types of intake structures 1. Floating intakes
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2. Submerged intakes
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3. Exposed or tower intakes 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 10
Intake-design consideration 2. Intake-port location
1. Intake velocities •
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High velocities – head loss, entrain suspended matter, trap fish, and other aquatic animals. Velocity below 8 cm/s allows aquatic animals to escape, and minimize the suspended matter.
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Water quality in each stratum may vary. To achieve, multiple intake ports set at various levels are generally provided. Top intake – less than 2 m below normal level. Bottom intake – least 1 m above the bottom 11
3. Gates •
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Usually sluice gates . Large cast iron gates that slide vertically on a guide track.
4. Control of ice •
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In cold regions, gates covered with ice. To avoid, intake ports are kept below water surface below 8m . Space heaters, Compressed air is used to remove ice.
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Screening •
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It is a unit operation that removes suspended matter from water. Screens may be classified as coarse, fine, or microstrainer, microstrainer, depending on the size of material removed. Located at intake structure, raw water pump station, or water treatment plant. 13
1. Coarse screen or trash rack To fprevent large objects rom entering the conveyance system. Consists of vertical flat bars, or, in some cases, round pipes spaced with 5 to 8 cm of clear opening. Installed outside of any sluice gate. The velocity through the coarse screen is generally less than 8 cm/s. cm/s.
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2. Fine screen •
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To remove smaller objects that may damage pumps or other equipment. Screens consists of heavy wire mesh with 0.5 cm square opening. The typical design velocity through the effective area is in the range of 0.4 to 0.8 m/s. There are two types: Traveling types: Traveling screens and passive screen installation
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Typical fine screens Traveling screen
Passive screen
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3. Microstrainer •
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Mainly to remove plankton and algae from impounded waters. Microstrainer is installed before chemical coagulation will improve the performance of clarifiers. Consists of rotating cylindrical frame covered with fine wire mesh fabric. Water enters the cylinder and moves radially out. Damaged by abrasive material – metal , sand. Problem – build up of slime on the 17 fabric.
Aeration Aeration involves bringing air or other gases in contact with water. The purpose of aeration are 1. Redu Reduce ce the the conc conc.. of tast taste e and and odor odor cau causi sing ng substance by volatilization 2. To oxidiz oxidize e iron iron and mangan manganese ese,, rend renderi ering ng them them insoluble. 3. To di diss ssol olve ve a gas gas in the the wat water er ( O2 and CO2) 4. To remo remove ve comp compou ound nds s for for bett better er water water treatment ( H2S removal before chlorination and CO2 removal before softening) 18
Types of aerators 1. Gravity aerators •
Utilize weirs, waterfalls, cascades, inclined planes with riffle plates, perforated tray towers
2. Spray aerators •
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This spray droplets of water into the air from orifices or nozzles. To produce an atomized jet, large power required & water must be free of large solids.
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3. Diffused-air aerators •
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Aerated in tanks using compressed air . Aeration period 10 to 30 mins. Air supply 0.1 to 1 m3/min.
4. Mechanical aerators •
Motor driven impellers or combination of impeller with air injection device.
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Water works engineering – planning, design, and operation By syed R. QASIM, Edward M. Motley and guang zhu Prentice-hall , india (2006) And photos from internet.
REFERENCE
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