6 mm. Micro screens, which generally have screen openings less than 50 um, are used principally in removing fine solids from treated effluents Screening Fig. 5-2 Definition sketch for types of screen ed in wastewater treatment Microscreen c 6mm 6 to 150 mm The screening element may Mechanically Static Step perforate cleaned openings may be of any shape but generally are circular or ctangular slots. A screen Reciprocating Caten composed of parallel bars or rods is often called a"bar rack" or a coarse screen and is used for the removal of coarse solids Fine screens are devices consisting of perforated plates, wedgewire elements, and wire cloth that have smaller openings. The materials removed by these devices are known as screenings pipelines and other appurtenances from damage or clogging by rags and large obiects. Industrial waste-treatment plants may or may not need them, depending on the character of the wastes. According to the method used to clean them, coarse screens are designated as either hand-cleaned or mechanically Hand-Cleaned Coarse Screens. Hand-cleaned coarse screens are used frequently ahead of pumps in small wastewater pumping stations and sometimes used at the headworks of small- to medium-sized wastewater-treatment plants. Often they are used for r standby screening in bypass channels for service during high-flow periods, when mechanically cleaned screens are being repaired. nt of a ower failure. Normally, mechanically cleaned screens are provided in place of hand-cleaned screens to minimize manual labor required to clean the screens and to reduce flooding due to clogging Where used. the length of the hand cleaned bar rack should not exceed the distance that can be conveniently raked by hand. approximately 3 m. The screen bars are welded to spacing bars located at the rear face, out of the way of the tines of the rake. a perforated drainage plate should be provided at the top lation of grit and other heavy materials in the channel ahead of the screen and following it. The channel floor should be level or should slope downward through the screen without pockets to trap solids. The channel preferably should have a straight approach, perpendicular to the bar screen, to promote uniform distribution of screenable solids throughout the flow and on the screen. Typical design information for hand-cleaned bar screens is provided in Table Tab. 5-1 Typical design information for manually and mechanically cleaned bar racks Unit Manual mm 5-15 mm 25-38 25-5 Maximum m/s 0.3-0.6 0.6-1.0 m/s 0.3-0.5 Allowable headloss5-2 6 mm. Micro screens, which generally have screen openings less than 50 μm, are used principally in removing fine solids from treated effluents. Fig. 5-2 Definition sketch for types of screens used in wastewater treatment The screening element may consist of parallel bars, rods or wires, grating, wire mesh, or perforated plate, and the openings may be of any shape but generally are circular or rectangular slots. A screen composed of parallel bars or rods is often called a "bar rack" or a coarse screen and is used for the removal of coarse solids. Fine screens are devices consisting of perforated plates, wedgewire elements, and wire cloth that have smaller openings. The materials removed by these devices are known as screenings. Coarse Screens (Bar Racks). In wastewater treatment, coarse screens are used to protect pumps, valves, pipelines and other appurtenances from damage or clogging by rags and large objects. Industrial waste-treatment plants may or may not need them, depending on the character of the wastes. According to the method used to clean them, coarse screens are designated as either hand-cleaned or mechanically cleaned. Hand-Cleaned Coarse Screens. Hand-cleaned coarse screens are used frequently ahead of pumps in small wastewater pumping stations and sometimes used at the headworks of small- to medium-sized wastewater-treatment plants. Often they are used for standby screening in bypass channels for service during high-flow periods, when mechanically cleaned screens are being repaired, or in the event of a power failure. Normally, mechanically cleaned screens are provided in place of hand-cleaned screens to minimize manual labor required to clean the screens and to reduce flooding due to clogging. Where used, the length of the hand cleaned bar rack should not exceed the distance that can be conveniently raked by hand, approximately 3 m. The screen bars are welded to spacing bars located at the rear face, out of the way of the tines of the rake. A perforated drainage plate should be provided at the top of the rack where the raking may be stored temporarily for drainage. The screen channel should be designed to prevent the accumulation of grit and other heavy materials in the channel ahead of the screen and following it. The channel floor should be level or should slope downward through the screen without pockets to trap solids. The channel preferably should have a straight approach, perpendicular to the bar screen, to promote uniform distribution of screenable solids throughout the flow and on the screen. Typical design information for hand-cleaned bar screens is provided in Table 5-1. Tab. 5-1 Typical design information for manually and mechanically cleaned bar racks Parameter Unit Cleaning methods Manual Mechanical Bar size Width Depth mm mm 5-15 25-38 5-15 25-38 Clear space between bars mm 25-50 15-75 Slope from vertical ° 30-45 0-30 Approach velocity Maximum Minimum m/s m/s 0.3-0.6 0.6-1.0 0.3-0.5 Allowable headloss mm 150 150-600