hopper. The circular pattern of the moving plates provides a self-cleaning feature for each step. Normal ranges of openings between the screen plates are 3 to 6 mm; however, openings as small as 1 mm are available. Solids trapped on the screen also create a"filter mat"that enhances solids removal performance. In addition to wastewater screening, step screens can be used for removal of solids from septage, primary sludge or digested biosolids Design of Fine-Screen Installations. Mechanically cleaned coarse screens should precede some types of fine screens. Newer designs of internally fed rotary screens that use wedge-wire instead of screen fabric are structurally more rugged. These designs can handle coarse solids that are transported through wastewater pumps; thus upstream protective devices may not be required flowrates. Flushing water should be provided nearby so that the buildup of grease and other solids on creen can be removed periodically. In colder climates. hot water or steam is more effective for grease The important determination is the headloss during operation; headloss depends on the size and amount of solids in the wastewater. the size of the apertures. and the method and frequency of cleanin Microscreen Microscreening involves the use of variable low-speed(up to 4 r/min). continuously backwashed. The filtering fabrics have openings of 10 to 35 m and are fitted on the drum periphery. The wastewater enters the open end of the drum and flows outward through the rotating-drum screening cloth. The collected solids are backwashed by high-pressure iets into a trough located within the drum at the highest point of the drum. The principal applications for microscreen are to remove suspended solids from secondary effluent and from stabilization-pond effluent Typical suspended solids removal achieved with microscreen ranges from 10 to 80 percent, with an average of 55 percent Problems encountered with microscreen include incomplete solids removal and inability to handle solids fluctuations. Reducing the rotating speed of the drum and less frequent flushing of the screen have resulted in increased removal efficiencies but reduced capacity The functional design of a microscreen involves(1)characterizing the suspended solids with respect to the concentration and degree of flocculation,(2) selecting design parameters that will not only assure sufficient capacity to meet maximum hydraulic loadings with critical solids characteristics but also meet operating performance requirements over the expected range of hydraulic and solids loadings, and (3) providing backwash and cleaning facilities to maintain the capacity of the screen. Typical design information for microscreens is presented in Table 5-3. Because of the variable performance of microscreen, pilot-plant studies are recommended, especially if the units are to be used to remove solids from stabilization-pond effluent, which may contain significant amounts of algae Tab. 5-3 Typical design information for microscreen used for screening secondany settled effluent Screen size 20-35pum Stainless steel or polyester screen cloth are available in size Hydraulic loading rate -6m m min Based on submerged surface area of drum Head loss 75-150mm bypass should be provided when headloss exceed 200mm 70-75%of height Varies depending on screen design 25-5m 3m is most used size. Drum speed I 4.5m/min at 75mm Maximum rotating speed is limited to 45 m/min Backwash requirements Screenings Characteristics and Quantities Screenings are the material retained on bar racks and screens. The smaller the screen opening, the greater will be the quantity of collected screenings. While no precise definition of screenable material exists, and no recognized method of measuring quantities of screenings is available, screenings exhibit some common Screenings Retained on Coarse Screens. Coarse screenings, collected on coarse screens of about 12 mm or greater spacing. consist of debris such as rocks. branches. pieces of lumber. leaves. paper. tree roots. plastics, and rags. The accumulation of oil and grease can be a serious problem, especially in cold climates The quantity and characteristics of screenings collected for disposal vary, depending on the type of bar screen, the size of the bar screen opening, the type of sewer system, and the geographic location. Typical data on the characteristics and quantities of coarse screenings to be expected at wastewater-treatment plants served by conventional gravity sewers are reported in Table 5-4 5-65-6 hopper. The circular pattern of the moving plates provides a self-cleaning feature for each step. Normal ranges of openings between the screen plates are 3 to 6 mm; however, openings as small as 1 mm are available. Solids trapped on the screen also create a "filter mat" that enhances solids removal performance. In addition to wastewater screening, step screens can be used for removal of solids from septage, primary sludge, or digested biosolids. Design of Fine-Screen Installations. Mechanically cleaned coarse screens should precede some types of fine screens. Newer designs of internally fed rotary screens that use wedge-wire instead of screen fabric are structurally more rugged. These designs can handle coarse solids that are transported through wastewater pumps; thus upstream protective devices may not be required. An installation should have a minimum of two screens, each with the capability of handling peak flowrates. Flushing water should be provided nearby so that the buildup of grease and other solids on the screen can be removed periodically. In colder climates, hot water or steam is more effective for grease removal. The important determination is the headloss during operation; headloss depends on the size and amount of solids in the wastewater, the size of the apertures, and the method and frequency of cleaning. Microscreens Microscreening involves the use of variable low-speed (up to 4 r/min), continuously backwashed, rotating-drum screens operating under gravity-flow conditions. The filtering fabrics have openings of 10 to 35 m and are fitted on the drum periphery. The wastewater enters the open end of the drum and flows outward through the rotating-drum screening cloth. The collected solids are backwashed by high-pressure jets into a trough located within the drum at the highest point of the drum. The principal applications for microscreens are to remove suspended solids from secondary effluent and from stabilization-pond effluent. Typical suspended solids removal achieved with microscreens ranges from 10 to 80 percent, with an average of 55 percent. Problems encountered with microscreens include incomplete solids removal and inability to handle solids fluctuations. Reducing the rotating speed of the drum and less frequent flushing of the screen have resulted in increased removal efficiencies but reduced capacity. The functional design of a microscreen involves (1) characterizing the suspended solids with respect to the concentration and degree of flocculation, (2) selecting design parameters that will not only assure sufficient capacity to meet maximum hydraulic loadings with critical solids characteristics but also meet operating performance requirements over the expected range of hydraulic and solids loadings, and (3) providing backwash and cleaning facilities to maintain the capacity of the screen. Typical design information for microscreens is presented in Table 5-3. Because of the variable performance of microscreens, pilot-plant studies are recommended, especially if the units are to be used to remove solids from stabilization-pond effluent, which may contain significant amounts of algae. Tab. 5-3 Typical design information for microscreens used for screening secondary settled effluent Item Typical value Remarks Screen size 20-35µm Stainless steel or polyester screen cloth are available in size ranging from 15-60µm Hydraulic loading rate 3-6m3 /m2·min Based on submerged surface area of drum Head loss 75-150mm Bypass should be provided when headloss exceed 200mm Drum submergence 70-75% of height Varies depending on screen design Drum diameter 2.5-5m 3 m is most used size, Drum speed 4.5m/min at 75mm headloss Maximum rotating speed is limited to 45 m/min Backwash requirements 2% of throughput at 350kPa Screenings Characteristics and Quantities Screenings are the material retained on bar racks and screens. The smaller the screen opening, the greater will be the quantity of collected screenings. While no precise definition of screenable material exists, and no recognized method of measuring quantities of screenings is available, screenings exhibit some common properties. Screenings Retained on Coarse Screens. Coarse screenings, collected on coarse screens of about 12 mm or greater spacing, consist of debris such as rocks, branches, pieces of lumber, leaves, paper, tree roots, plastics, and rags. The accumulation of oil and grease can be a serious problem, especially in cold climates. The quantity and characteristics of screenings collected for disposal vary, depending on the type of bar screen, the size of the bar screen opening, the type of sewer system, and the geographic location. Typical data on the characteristics and quantities of coarse screenings to be expected at wastewater-treatment plants served by conventional gravity sewers are reported in Table 5-4