Screening in Water Treatment Prcocess- Types, Methods and Procedure

Screening in water treatment is the first process for the water treatment process. There are few steps in the screening of water.

Methods of screening in water treatment process

1.Bar screens

Practically all intakes are screened, even though the screens may be of the simplest type of bar grille. The bars must be quite substantial in size (of about 25 mm diameter) and are normally spaced at 75-100 mm centres.

If the bars are inclined it is easier to clean them with a rake which may be a hand rake for occasional manual cleaning or mechanical rakes for continuous cleaning. Should a smaller mesh be necessary, it is best to group bars into frames so that each frame can be lifted out of the water, cleaned, and lowered back into position.

Screening in Water Treatment

To prevent unscreened water from passing through the intake when the screens are lifted, they should be provided in duplicate or provision should be made for stop log insertion upstream for temporary stoppage of the flow.

At river intakes a great deal of trash may collect; this is often seasonal with a spring and autumn abundance of water weed and foliage. Screens may be provided with automatically operated raking mechanisms to assist the removal of captured debris. Bar screens generally having larger spacing will not arrest smaller size debris. In such cases it is advisable to install a robust band or cup screen downstream of the protective bar screen. This is screening in water treatment process

2.Band and drum screens

If fine screening is adopted, some means must be found of continuously cleaning the screens or they rapidly become clogged. For this reason fine screens are usually arranged as endless bands or rotating drums of material perforated with holes of about 6 mm diameter. Plate 13 shows a drum or cup screen and a band screen.

Screening in Water Treatment

The screening element is in continuous motion and having captured and lifted debris from the intake water passes over water jets which wash off the screened material into a trough. A pressure supply of clean water is needed for the wash water jets, and this may have to be pumped from the strained water. The total amount of water required for washing may be of the order of 1% of the throughput. Fine screening must always be preceded by a coarse screen.

3.Microstrainers in screening of water treatment process

These are revolving drums mounted in open tanks with a straining medium which is usually a stainless steel wire fabric of a very fine mesh, fitted to the periphery of the drum. The drum is submerged for about 75% of its diameter (66% of area) and rotates at about 0.5-5 rpm (peripheral drum speeds of 3-50 m/min). Water to be treated enters the drum axially under gravity and flows out radially through the fabric, depositing particulate matter.

Screening in Water Treatment

Cleaning is accomplished by a row of water jets along the full length of the drum operating at about 2.5 bar maximum pressure. Particulate matter intercepted by the fabric rotates to the top of the drum where they are backwashed into a hopper running the full length of the drum and conveyed by a pipe which also acts as the axle for the drum assembly, to a point outside.

Water jets use about 1-1.5% of the total quantity of water strained and this washwater should be filtered and chlorinated. Total headloss through a microstrainer unit including inlets and outlets varies from about 150-200 mm. Single units have capacities of 10 m3/h to a maximum of 4000 m3/h for a 3.2 m diameter x 5 m wide drum. This is screening in water treatment process.

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