SCREENS

Description

Screens separate solid material from liquid streams. Wastewater is passed through a series of constrictions (the screen) and objects that are too large to pass through (screenings) are retained. Screens can be constructed from metal bars, metal mesh, wedge wire, perforated metal or plastic or polypropylene sacks.

The variety of screens reflects the wide range of applications and of mechanisms required to keep them free of screenings. Static screens have no moving parts and are kept clean by the flow of liquid, as in run-down screens, or manually, as in bar screens and screening sacks. Bar and wire screens can be vertical, inclined or curved depending on the application.

Some screens are agitated to keep them clear of debris, as in agitated sacs, or to separate different solids from each other, as in vibratory screens. Raked bar screens and brushed screens use devices to keep the screening surface free, either periodically or continuously. A drum screen rotates in the waste stream and screenings collect on the inside or the outside. Sprays, brushes, chevron bars or screw conveyors are used to remove screenings of different sizes. Band screens lift the screenings out of the wastewater channel and allow the screened water to pass through.

Collected screenings must be disposed of and some drum screens incorporate rollers to press the screenings into a cake for easy handling.

Application

Screens remove material at an early stage that would cause problems or be more expensive to treat at later stages. Screens are used in almost all effluent treatment facilities, but particularly for the treatment of sewage and food industry effluents. Screens remove visible and unsightly material including plastics.

Agitated sacs remove debris from municipal waste streams without becoming blinded with faecal material. Run down screens are used for fibrous and nonfatty waste, in vegetable processing and the textile industries. Brushed screens are used to separate fatty wastes and effluents that would blind nonmechanical screens. Drum screens are used to remove food industry organic wastes.

Sizing

Most screens require either no power or a small supply. A typical rotating screen sized for an effluent flow of about 750 m3/hour may require a 1-25 kW electrical motor.