TAILIEUCHUNG - Standard Methods for Examination of Water & Wastewater_4

Kiểm tra này là một hoạt động đơn vị phân tách vật liệu vào kích cỡ khác nhau. Các đơn vị liên quan được gọi là một màn hình. Như xử lý nước và nước thải là có liên quan, chỉ có hai "kích cỡ" của các đối tượng tham gia trong việc kiểm tra nước hoặc nước thải và các đối tượng được tách ra. | 5 Screening Settling and Flotation 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. SCREENING Figure 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 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 is mechanically cleaned as shown by the cable system hoisting the scraper the one in Figure is manually cleaned. Note that this screen is removable. .

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