Screening is normally the first unit operation used at wastewater treatment plants. The general purpose of screens is to remove large objects such as rags, paper, plastics, metals, and the like. These objects, if not removed, may damage the pumping and sludge removal equipment; hang over weirs; and block valves, nozzles, channels, pipelines, and appurtenances, thus creating serious plant operation and maintenance problems.


Screening devices can be broadly classified as coarse or fine. Screens may be manually or mechanically cleaned. Various types of coarse and fine screens are discussed below.

Coarse Screen

Coarse screens are used primarily as protective devices and therefore are used as the first treatment unit. Common type of protective devices include bar racks (or bar screens). A screen composed of parallel bars or rods is called a bar rack.

Bar racks or bar screens:

-Ahead of pumps and grit removal facilities
-Bar racks may be manually cleaned or mechanically cleaned. Manually cleaned racks are provided at small wastewater treatment facilities. The clear spacing’s of the bars may range from 1.5 cm to 4.0 cm

Fine Screen

The fine screens are now available in openings ranging from 0.035 to 6 mm. They are usually mechanically cleaned. The purpose of fine screens is to provide pretreatment or primary treatment. Various types of microscreens have been developed in recent years that are used for upgrading effluent quality from secondary treatment plants.

Drum screen or strainer

The screens consist of a rotating cylinder that has screen covering the circumferential area of the drum. The liquid enters the drum and moves radially out. The solids deposited are removed by a jet of water from the top and discharged into a trough. The screen openings may range in size giving coarse medium and fine screens. The micro strainers have very fine screens and are used to polish the secondary effluent or remove algae from the effluent of stabilization ponds.

Step screen

The step screens have step-shaped laminae, where every other lamina is connected to one fixed and one moveable part on which a carpet of solids is formed. This gives a high sieving efficiency. The rotating step-shaped laminae convey the solids upwards, step-by-step. The circular movement is self-cleaning, and the solids are automatically off-loaded at the discharge point.

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