About Manual Bar Screens

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Manual Bar screens were more widely used in the past at the headworks of most wastewater treatment plants. Today you will usually see manual bar screens at older, smaller (< 4000 m3/d [ or 1 mgd] [metric system conversion]) wastewater treatment plants, pump stations, or in bypass channels for comminutors and mechanically cleaned bar screens at larger WWTPs. Manual bar screens have been replaced with more robust mechanical bar screens, comminutors, and grinder pumps. The shift away from the manual bar screens can be attributed to the inefficiencies and manual labor requirements.

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Manual bar screens have little or no motorized cleaning equipment therefore must be periodically cleaned manually by hand. Not only is maintaining the manual bar screen lab intensive, but when removal of screenings from the bar screen is infrequent, flooding and overflow occurs due to clogging. If the bar screen does become clogged, the head accumulated from hair pinning and solids matting on the bar screen between cleanings, creates a massive surge when the screenings are removed.

The high velocity flow produced from the backed up head may compromise solids capture effectiveness of downstream equipment. When excessive head loss is anticipated, a bypass channel with a trash rack (bar spacing between 3 to 4 inches) may be utilzed. Sometimes cleaning the bar screen may produce some undersirable results, as minor as it may be you should be aware. When bar screens are cleaned, whether manual or mechanical, positively engaging the screen will cause some screenings to shear, resulting in breakthrough.

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As previously stated, some manual bar screens are major "work horses" at smaller plants. The manual bar screens that are still being utilized commonly have bar spacing between 25 mm to 50 mm (or 1 to 2 inches [metric to inches]), schedule cleaning as often as necessary (each situation may be different) to ensure unobstructed flow of the waste stream. Screenings removed manually from the bar screen are conveyed to a perforated plate to drain excess water before being disposed of. This may be very important depending on the method of disposal as it may reduce transportation cost. For these bar screens, it is important that the bar lenght not exceed a distance that can be manually cleaned conveniently, use 3 m (or 10 feet) as a "rule of thumb".

Typical bar screens are between 10 mm (or 3/8 inch) to 50 m (or 2 inches) thick, by about 50 mm (2 inches) deep. For these bar screens, the bars are positioned at a 30 to 75 degree angel from the horizontal, with the more common configuration being 35 to 45 degrees. This configuration will augment screening surface area up to 100%, it wil also facilitate cleaning and eliminate increased head from matting.

The bar screen channel should be designed in such a way as to reduce the build up of grit and other debris directly upstream and downstream of screen. For example, the floor of the channel should be flush with or sloped downward through the bar screens with a crevice where solids can be trapped.

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As with other structures at a wastewater treatment plant, the approach channel should be straight in order to promote evenly distributed screening over the area of the bar screen. With a minimum approach velocity of 0.4 m/s (or 1.25 ft/sec) to avoid grit deposition. The velocity through the bar screen not exceed 0.62 ms (or 2 ft/sec) at average flow. Exceeding this flow may minimize the amount of available screening surface area for collecting solids in between cleaning.

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Always consult with the manufacturer or your local equipment representative if you have specifica questions about bar screens. This site is done by a random individual to help get you informed, but is not to substitute for professional opinion. Please read our disclaimer.