returned to the wastewater however. ground screenings may adversely affect operation and maintenance of down stream equipment such as clogging weir openings on sedimentation tanks or wrapping around air Fine screens he applications for fine screens range over a broad spectrum; uses (following coarse bar screens). primary treatment (as a substitute for primary clarifiers). and treatment of combined sewer overflows. Fine screens call also be used to remove solids from primary effluent that Screens for Preliminary and Primary Treatment. Fine screens used for preliminary treatment are of Examples of line screens are illustrated on Fig Sg De typically, the openings vary from 0.2 to 6 mm) m. or In many cases, application of fine screens is limited to plants where headloss through the screens is not a oblem ace primary wastewater-treatment plants, up to 0. 13 m3/s in esign capacity. Typical Fig. 5-4 Typical fin removal rates of bod and screens TSS are reported in Table wedge-wire, (drum, 5-2. Stainless-steel mesh or (c)step. In step screens, special wedge-shaped bars screenings are moved up are used as the screening the screen by means of medium. Provision is made movable and fixed vertical for the continuous removal plates. of the collected solids, supplemented bywater sprays to keep the screening medium clean. Headloss through the screens may range from about 0. 8 to 1. 4m Tab. 5-2 Typical removal data of bod and TSS with fine screens used to replace primary sedimentation Type of screen Size of openings rotary drum Static Wedge-wire Screens. Static wedgewire screens(see Fig 5-4a) customarily have 0.2 to 1.2 mm clear openings and are designed for flowrates of about 400 to 1200 L/m2. min of screen area. Headloss ranges from 1. 2 to 2 m. The wedge-wire medium consists of small stainless-steel wedge shaped bars with the flat part of the wedge facing the flow Appreciable floor area is required for installation and the screens must be cleaned once or twice daily with high-pressure hot water, steam, or degreaser to remove grease buildup. Static wedge-wire screens are generally applicable to smaller plants or for industrial installations Drum Screens. For the drum-type screen(see Fig. 5-4b), the screening or straining medium is mounted on a cylinder that rotates in a flow channel The wastewater flows either into one end of the drum and outward through the screen with the solids collection on the interior surface. or into the top of the unit and passing through to the interior with solids collection on the exterior. Internally fed screens with applicable for flow ranges of 0.03 to 0.8 m/s per screen, while externally fed screens are applicable for flowrates less than 0.13m'/s. drum screens are available n various sizes from o9 to 2 m in diameter and from 1.2 to 4 m Step Screens. Step screens, al though widely used in Europe, are a relatively new technology in fine reening in the United States. The design consists of two fixed and one movable(see Fig. 5-4c). The fixed and movable step plates alternate across the width of an landing. and are eventually transported to the top of the screen where they are discharged to a collection 5-55-5 returned to the wastewater, however, ground screenings may adversely affect operation and maintenance of down stream equipment such as clogging weir openings on sedimentation tanks or wrapping around air diffusers. Fine Screens The applications for fine screens range over a broad spectrum; uses include preliminary treatment (following coarse bar screens), primary treatment (as a substitute for primary clarifiers), and treatment of combined sewer overflows. Fine screens call also be used to remove solids from primary effluent that could cause clogging problems in trickling filters. Screens for Preliminary and Primary Treatment. Fine screens used for preliminary treatment are of the (l) static (fixed), (2) rotary drum, or (3) step type. Typically, the openings vary from 0.2 to 6 mm). Examples of line screens are illustrated on Fig. 5-4. In many cases, application of fine screens is limited to plants where headloss through the screens is not a problem. Fine screens may be used to replace primary treatment at small wastewater-treatment plants, up to 0.13 m3/s in design capacity. Typical removal rates of BOD and TSS are reported in Table 5-2. Stainless-steel mesh or special wedge-shaped bars are used as the screening medium. Provision is made for the continuous removal of the collected solids, supplemented by water sprays to keep the screening medium clean. Headloss through the screens may range from about 0.8 to 1.4 m. Tab. 5-2 Typical removal data of BOD and TSS with fine screens used to replace primary sedimentation Type of screen Size of openings (mm) Percent removed BOD TSS Fixed parabolic 1.6 5-20 5-30 Rotary drum 0.25 25-50 25-45 Static Wedge-wire Screens. Static wedgewire screens (see Fig. 5-4a) customarily have 0.2 to 1.2 mm clear openings and are designed for flowrates of about 400 to 1200 L/m2·min of screen area. Headloss ranges from 1.2 to 2 m. The wedge-wire medium consists of small stainless-steel wedge shaped bars with the flat part of the wedge facing the flow. Appreciable floor area is required for installation and the screens must be cleaned once or twice daily with high-pressure hot water, steam, or degreaser to remove grease buildup. Static wedge-wire screens are generally applicable to smaller plants or for industrial installations. Drum Screens. For the drum-type screen (see Fig. 5-4b), the screening or straining medium is mounted on a cylinder that rotates in a flow channel. The wastewater flows either into one end of the drum and outward through the screen with the solids collection on the interior surface, or into the top of the unit and passing through to the interior with solids collection on the exterior. Internally fed screens with applicable for flow ranges of 0.03 to 0.8 m3 /s per screen, while externally fed screens are applicable for flowrates less than 0.13 m3 /s. Drum screens are available n various sizes from 0.9 to 2 m in diameter and from 1.2 to 4 m in length. Step Screens. Step screens, although widely used in Europe, are a relatively new technology in fine screening in the United States. The design consists of two step-shaped sets of thin vertical plates, one fixed and one movable (see Fig. 5-4c). The fixed and movable step plates alternate across the width of an open channel and together form a single screen face. The movable plates rotate in a vertical motion. Through this motion solids captured on the screen face are automatically lifted up to the next fixed step landing, and are eventually transported to the top of the screen where they are discharged to a collection Fig. 5-4 Typical fine screens: (a)static wedge-wire, (b)drum, (c)step. In step screens, screenings are moved up the screen by means of movable and fixed vertical plates