Raw & Fine Screens (Water & Wastewater Treatment Plants, WWTP_CAMIX Vietnam)
DESCRIPTION
Operation conditioning:
Screening is the first treatment station, both for surface and wastewater. It's purpose to:
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Protect the structure downstream against large objects which could create obstructions in some of the facility's units. | |
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Easily separate and remove large matter carried along by the raw water, which might negatively affect the efficiency of later treatment procedures or make their implementation more difficult. |
The efficiency of the screening operation depends of the spacing between screen bars:
■ Fine screening, for a spacing under 10 mm
■ Medium screening, for spacing of 10 to 40 mm
■ Coarse screening, for spacing of over 40 mm
Usually the fine screening is preceded by a preliminary screening operation for purposes of protection.
Screening is carried to out by a manually cleaned bar screen (large in size, in order to reduce the frequency of screenings collection operations) or, preferably, by an automatically cleaned bar screen (essential in cases of high flow rates of for water with a high solids content). The automatic bar screen is usually protected by a sturdy preliminary bar screen, which should also be provided with an automatic cleaning systems in large facilities and in case of raw water containing a high volume of coarse matter.
To reduce manual operations as much as possible, screening procedures have become increasingly automated, even in small facilities. Automation is essential in situations where large amounts of plant matter are carried by the water and arrive all at once at the bar screen, tending to mat the bars and completely clogging the screen in a few minutes. Fine screens must be automated.
The collected refuse is stored in a container of given capacity, calculated according the acceptable frequency of refuse disposal operations.
Usual spacing are:
| ■ | For surface waters, between 20 and 40 mm (upstream the strainer). | |
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For municipal wastewater: for raw water, from 15 to 30 mm (but upstream from a straining and/ or lamellae settling process, fine screening is necessary); for sludge (if necessary), 10 mm or less. | |
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For some industrial effluents, especially agri-food effluents, fine bar screening ( or at times, medium screening followed by straining). |
HYDRAULIC SIZING-CLOGGING:
Under normal circumstances, the crossing velocity through the bar screen should be sufficient for matter to attach itself to the screen without producing an excessive loss of head or a complete clogging of the bars, or allowing matter to be carried by the flow; normally acceptable crossing velocities between bars average between 0.6 and 1.0 m.s-1 and 1.2 to 1.4 m.s-1at the maximum water flow.
These velocities apply to the area of the clogged bar screen that is still clear. The degree screen that is still clear. The degree of clogging depends on the water quality and on the system used to recover waste from the bar screen. For automatic bar screens it can be anywhere between 10% (surface water) and 30% wastewater with a high solids content). For manually cleaned bar screens, the area of immersed bar screen must be larger, so as to avoid frequent cleanings.
AUTOMATIC CONTROL AND THE PROTECTION OF BAR SCREENS:
In general, the bar screens cleaning systems works on an intermittent basis. It can be controlled in three ways:
a) by a cyclic system of controllable frequencies ( 1 min to 1 h) and lengths of time ( 1 to 15 min)
b) By a differential head loss indicator
c) by a combination of both systems. When the bar screen is located downstream from a pumping station, the control mechanism can be linked to start-up of the pumps, with a built-in timer to keep the screen in operation for 1 to 30 minutes.
Automatic bar screen must be equipped with torque limiter to prevent equipment damage in case of overloading or blocking.
Normally, reciprocating cleaning bar screens, both curved and straight, include a device to ensure that the rake automatically stops moving at a point outside of the screen area, so as to avoid jamming upon restarting.
REFERENCE PHOTOS
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