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 components and configuration
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.
|1st step of sewage water solids removal
|protecting valves, drains, pumps, unblocking sewage flow
|Protective gear for hygiene
|Screen wire spacing
|7-15cm < arriving sewer’s invert.
|flat / intended slope
|Minimum Bar racks required
|Bar rack use
|manage sever flow
|Flow stoppage system
|Channel draining systems
|Flow entrance angle
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
Types of screen
|protection of sewer system components
|bar racks, wire design
|by hand, mechanical
The smaller plants utilize the screens that require the cleaning via hand, but the large plants need a mechanical variant of this screen.
These grinders make use of the sharp tools that chop through the screenings
|fixed / static / wedge wire / drum / step
|Static screen position
|vertical / horizontal / angular
|mechanical via rakes and brushes
|finer processing post abrasive screens application,
|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.