Domestic Wastewater Treatment Mobilization Construction - AskTOP

EM 1110-3-17211 May 847-1 . General considerations . Screening is a process necessary toprotect pumps and subsequent treatment units of a wastewater treatmentplant . Its main function is to remove sticks, stones, rags, trash, andother debris .7-2 . Bar screens .CHAPTER 7SCREENINGa . Description and function . The primary function of coarsescreening is protection of downstream facilities rather than effectiveremoval of solids from the plant influent . All screens used in sewagetreatment plants or in pumping stations may be divided into thefollowing classifications :(1) Trash racks, which have a clear opening between bars of1-1/2 to 4 inches and are usually cleaned by hand, by means of a hoist,or possibly by a power-operated rake .(2) Standard mechanically cleaned bar screen, with clearopenings from 1/2 to 1-1/2 inches (figure 7-1) .(3) Fine screen, with openings 1/4 inch wide or smaller .b . Design basis . Screens will be located where they are readilyaccessible . An approach velocity of 2 fps, based on average flow ofwastewater through the open area, is required for manually cleaned .barscreens . For mechanically cleaned screens, the approach velocity willnot exceed 3 .0 fps at maximum flows .(1) Bar spacing . Clear openings of 1 inch are usuallysatisfactory for bar spacing, but 1/2 to 1-1/2 inch openings may beused . The standard practice will be to use 5/16-inch by 2-inch bars upto 6 feet in length and 3/8-inch by 2-inch or 3/8-inch by 2-1/2-inchbars up to 12 feet in length . The bar will be long enough to extendabove the maximum sewage level by at least 9 inches .(2) Size of screen channel . The maximum velocity through thescreen bars, based on maximum normal daily flow, will be 2 .0 fps . Forwet weather flows or periods of emergency flow, a maximum velocity of3 .0 fps will be allowed . This velocity will be calculated on the basisof the screen being entirely free from debris . To select the properchannel size, knowing the maximum storm flow and the maximum dailynormal flow, the procedure is as follows : the sewage flow (mgd)multiplied by the factor 1 .547 will give the sewage flow (cfs) . Thisflow in cfs divided by the efficiency factor obtained from table 7-1will give the wet area required for the screen channel . The minimumwidth of the channel should be 2 feet, and the maximum width of the