The first unit operation generally encountered in wastewater treatment plants is screening. A screen is a device with openings, generally of uniform size, that is used to retain solids found in the influent wastewater to the treatment pant. The principal role of screening is to remove coarse materials (pieces of wood, plastics, rags, papers, leaves, roots etc.) from the flow stream that could:

• Damage subsequent process equipment e.g. pumps, valves, pipe lines, impellers.
• Reduce overall treatment process reliability & effectiveness, or
• Contaminate waste way

The screen chamber is a big filteration screen that helps in eliminating big objects such as plastics that can stop the flow of sewer system.

Uses	1st step of sewage water solids removal
Instrument used	screens
Screen Uses	protecting valves, drains, pumps, unblocking sewage flow
Screen maintenance	manual
Protective gear for hygiene	required
Screen wire spacing	distanced uniformly
Design	straight wires
Channel	rectangular
Channel floor	7-15cm < arriving sewer’s invert.
Channel bed	flat / intended slope
Channel use	trap grit
Minimum Bar racks required	2
Bar rack use	manage sever flow
Flow stoppage system	required
Channel draining systems	essential
Flow entrance angle	mooth diversion
Discharge structure	proper

The mechanically maintained screens are much widely used in the industry; the bypass passage with another screen is added just to neutralise the malfunctioning of the mechanical variant. The screens are required to be cleared via the front side or back side.

The speed at which the water current flows can impact the screen, if the current is at low speed, there are number of objects getting trapped in the screen. The industrial activities utilize the screens but some of them do

ABRASIVE SCREEN
Uses	protection of sewer system components
Examples	bar racks, wire design
Cleaning methods	by hand, mechanical
Wire spacing	15-40mm

The smaller plants utilize the screens that require the cleaning via hand, but the large plants need a mechanical variant of this screen.

THE GRINDER
Bypass mode	yes
Shredding method	teeth

These grinders make use of the sharp tools that chop through the screenings

FINDER SCREENS
Variants	fixed / static / wedge wire / drum / step
Static screen position	vertical / horizontal / angular
Cleaning method	mechanical via rakes and brushes
Size	0.035-6mm
Uses	finer processing post abrasive screens application,
Other uses	unclogging trickling filters.

These are not best suited for the sewage collections as their small size can easily trap a lot of dirt that will result in cluttering of the superfine screens, the portable screens are cleaned while they are being used or are in the mid of functioning , the centrifugal version of the screen makes use of the revolving screen

Screenings are disposed with municipal trash or burnt.