Many bar screens have no mechanical cleaning devices and are cleaned periodically by hand rakes (see Figure 7.14.2). Manually cleaned screens, except those used for emergency, should be placed on a slope of 30° to 45° with the horizontal. This positioning increases screening surface by 40 to 100%, facilitates cleaning, and prevents excessive head loss by clogging. The clear opening between bars should be 1 to 13/4 in, and operators should remove screening as often as necessary (from two to five times a day) to secure free flow in the sewer.
When excessive head loss is expected, an overflow bypass channel equipped with a trash rack (vertical bars set 3 to 4in apart) can be used.
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