About Trash Racks

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Trash racks are simply bar screens (this includes manual bar screens and mechanical bar screens) with larger space openings, the openings are typically between 38 mm (or 1.5 in [metric system conversion]) through 150 mm (or 6 in). Trash racks are installed to prevent stumps, bricks, logs, 2"x4"s, and traffic cones from penetrating the treatment processes. Trash racks, preceding a bar screen with more narrow openings, are primarily used in combined sewer overflow (CSO) conduits. Sometimes, the trash rack and bar screen will be installed in tandem, the trash rack reduces potential damage to the bar screen that may be caused by very bulky debris that enter the storm sewers.

Trash Racks

Manual Bar Screens

Mechanical Bar Screens

Chain Bar Screens

At the influent of a wastewater treatment plant, there are usually two channels. A primary channel for screening where some type of bar screen will be installed, manual, mechanical, or other. And then there is the bypass channel, occasionally trash racks will be installed in the bypass channel. Screenings collected by the trash rack may be cleaned manually or mechanically. Screenings that are removed from the bar screen are usually conveyed to a trash receptacle, where it is stored until it hauled away. Screenings are either digested, incinerated, or taken to a landfill. Some screenings are not so easily removed from the trash racks. Fibrous materials, such as hair and bottom balls that become tangled between the bars may present some challenges. The rakes used to remove these materials may lodge the debris further into the bar screen increasing the headloss through the screen. If possible, the head loss through the bar screen should be kept to a minimal.

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To get an idea of how much head loss there will be through the bar screen, a quick "back of the envelope calculation" can be performed.

h = 1 ((V²- v²)/2g)

0.7

where:   h = head loss, m (ft)
    v = approach velocity, m/s (ft/sec)
               V = velocity through bar opening, m/s (ft/sec)
            g = acceleration from gravity
0.7 = head loss coefficient

In some cases trash racks are installed as a retrofit, therefore the available space may be limited. In such situations treatment plants and/or CSO systems have basket type trash racks that are hoisted and cleaned manually.

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Always consult with the manufacturer or your local equipment representative if you have specific 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.