Screening is a unit operation that separates materials into different sizes. The unit involved is called a screen. As far as water and wastewater treatment is concerned, only two "sizes" of objects are involved in screening: the water or wastewater and the objects to be separated out. Settling is a unit operation in which solids are drawn toward a source of attraction. In gravitational settling, solids are drawn toward gravity; in centrifugal settling, solids are drawn toward the sides of cyclones as a result of the centrifugal field; and in electric-field settling, as in electrostatic precipitators, solids are drawn to charge plates. Flotation is a unit operation in which solids are made to float to the surface on account of their adhering to minute bubbles of gases (air) that rises to the surface. On account of the solids adhering to the rising bubbles, they are separated out from the water. This chapter discusses these three types of unit operations as applied to the physical treatment of water and wastewater.
Figure 5.1 shows a bar rack and a traveling screen. Bar racks (also called bar screens) are composed of larger bars spaced at 25 to 80 mm apart. The arrangement shown in the figure is normally used for shoreline intakes of water by a treatment plant. The rack is used to exclude large objects; the traveling screen following it is used to remove smaller objects such as leaves, twigs, small fish, and other materials that pass through the rack. The arrangement then protects the pumping station that lifts this water to the treatment plant. Figure 5.2 shows a bar screen installed in a detritus tank. Detritus tanks are used to remove grits and organic materials in the treatment of raw sewage. Bar screens are either hand cleaned or mechanically cleaned. The bar rack of Figure 5.1 is mechanically cleaned, as shown by the cable system hoisting the scraper; the one in Figure 5.2 is manually cleaned. Note that this screen is removable. Table 5.1 shows some design parameters and criteria for mechanically and hand-cleaned screens.
Figure 5.3 shows a microstrainer. As shown, this type of microstrainer consists of a straining material made of a very fine fabric or screen wound around a drum. The drum is about 75% submerged as it is rotated; speeds of rotation are normally about from 5 to 45 rpm. The influent is introduced from the underside of the wound fabric and exits into the outside. The materials thus strained is retained in the interior of the drum. These materials are then removed by water jets that directs the loosened strainings into a screening trough located inside the drum. In some designs, the flow is from outside to the inside.
Microstrainers have been used to remove suspended solids from raw water containing high concentrations of algae. In the treatment of wastewater using oxidation ponds, a large concentration of algae normally results. Microstrainers can be used for this purpose in order to reduce the suspended solids content of the effluent
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