الفصل الميكانيكى/Screening

Screening

by

technolab el-bahaa group

general.dr

bahaa badr

Operation conditioning:

Screening is the first treatment station, both for surface and wastewater. It's purpose to:

Protect the structure downstream against large objects which could create obstructions in some of the facility's units,

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)
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
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-1 at 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.

DIFFERENT TYPES OF SCREENS :

PARABOLIC SCREEN

CURVED SCREEN

INCLINED BAR SCREEN

incline.gif (5055 bytes)Inclined bar screen

In operation, sewage flows though the inclined screen, approaching from the upstream side and after passing through the screen, departing on the downstream side. The screen is periodically raked by a mechanized comb system which is actuated either by level switches mounted in the channel upstream of the screen or by time clock. A doctor blade at the top of the comb travel removes the screenings collected by the moving combs.

The moving combs are suspended between two endless side chains, which are driven through a head shaft, and sprockets. The gear motor and moving comb system is protected from damage caused by jamming by a torque overload coupling with a micro switch.

The screenings removed from the doctor blade drop onto a skid plate, which transports the screening down to a container, belt conveyor or sluice pipe.

CURVED BAR SCREEN

This type 2 bar screens uses hydraulics to ensure a simple kinematical operation, you can see it in the following picture

1. Rigid frame with bar screen rack 2

2. Moving frame

3. Moving frame jack

4. Rake carriage

5. Lifting jack

6. Ejector

7. Electrical cabinet

8. Latticed covers

9. Hydraulic unit

In operation, sewage flows though the curved screen, approaching from the upstream side and after passing through the screen, departing on the downstream side. The screen is periodically raked by a mechanized comb which is actuated either by level switches mounted in the channel upstream of the screen or by time clock. The screenings collected by the moving comb are cleaned from the comb by a doctor blade at the top of the comb travel.

The screenings removed from the doctor blade drop onto a skid plate, which transports the screening down to a container, belt conveyor or sluice pipe.

RADIAL BAR SCREEN

Features and operation are similar to the curved bar screen. The drive mechanism is simpler for small installations and will remove a larger quantity of screenings due to two cleanings per revolution.

radial.gif (3741 bytes)Radial Bar Screen

STEP SCREEN

Step screens have become popular with some clients due to their ability to remove smaller sized solids than the bar type screens. We have access to several makes of this type of machine.

BRUSH TYPE INCLINED SCREEN

These units are inclined screens with a rotary brush type belt cleansing system. This device is strongly favored by some clients instead of the step screen.

STATIC SCREEN

These devices are for smaller flows and require manual cleansing. They may be of the inclined bar type or the parabolic static sieve bend. This latter type can remove solids down to 0.5mm size.

SCREENINGS HANDLING

As a result of the increased volume of entrained sharp objects such as hypodermic syringes and the like, mechanical screenings handling equipment now has a higher profile in the process designers strategy.

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