5 Physical Unit Operation Operations used for the treatment of wastewater in which change is brought about by means of or through the application of physical forces are knov hysical unit operations. Because physical unit operations were derived originally from observations of the physical world. they were the first treatment methods to be used. Today, physical unit operations, as shown on Fig. 5-1, are a major part of most wastewater treatment systems ude(D) screening. (2) coarse solids reduction(comminution maceration, and screenings grinding)(3) flow equalization. (4)mixing and flocculation.(5) grit removal. (6) sedimentation,(7 high-rate clarification. (8)accelerated gravity separation(vortex separators).(9) flotation(10) oxvgen transfer(1)packed-bed filtration, membrane separation(12) aeration(12)biosolid dewatering and (13) volatilization and stripping of volatile organic compounds (vocs) Off-line flow equalization (used to dampen peak flows water storage Waste backwash water Prima Se Chlorine Ellen Screens and Effluent Chlorine Chlorine contact Recycled biosolids Waste biosolids Legend Thickener retum flow Waste biosolids ○ Unit operations M Unit processes Thickened biosolids Recycle or solids streams processing facilities Fig 5-1 Location of physical unit operations in wastewater treatment plant flow diagram 5-1 Screening The first unit operation generally encountered in wastewater-treatment plants is screening. A screen is a device with openings, generally of uniform size. that is used to retain solids found in the influent wastewater to the treatment plant or in combined wastewater collection systems subject to overflows, especially from stormwater. The coarse materials from the flow stream that could (D damage subsequent process equipment. (2) reduce overall treatment process reliability and effectiveness, or( 3)contaminate waterways. Fine screens are sometimes used in place of or following coarse screens where greater removals of solids are required to (1)protect process equipment or(2)eliminate materials that may inhibit the beneficial reuse of biosolids All aspects of screenings removal, transport, and disposal must be considered in the application of downstream processes.(2)health and safety of the oLE removal required because of potential effects on screenings contain pathogenic organisms and attract insects. ( 3) odor potential, and(4) requirements for handling, transport. and disposal. i.e., removal of organics(by washing) and reduced water cor pressing), and (5)disposal options. Thus, an integrated approach is required to achieve effective screenings management Classification of screens The types of screening devices commonly used in wastewater treatment are shown on Fig. 5-2. Two general types of screens, coarse screens and fine screens, are used in preliminary treatment of wastewater Coarse screens have clear openings ranging from 6 to 150 mm; fine screens have clear openings less than
5-1 5 Physical Unit Operation Operations used for the treatment of wastewater in which change is brought about by means of or through the application of physical forces are known as physical unit operations. Because physical unit operations were derived originally from observations of the physical world, they were the first treatment methods to be used. Today, physical unit operations, as shown on Fig. 5-1, are a major part of most wastewater treatment systems. The unit operations most commonly used in wastewater treatment include (1) screening, (2) coarse solids reduction (comminution, maceration, and screenings grinding), (3) flow equalization, (4) mixing and flocculation, (5) grit removal, (6) sedimentation, (7) high-rate clarification, (8) accelerated gravity separation (vortex separators), (9) flotation, (10) oxygen transfer, (11)packed-bed filtration, membrane separation, (12 ) aeration, (12)biosolid dewatering, and (13) volatilization and stripping of volatile organic compounds (VOCs). Fig. 5-1 Location of physical unit operations in wastewater treatment plant flow diagram 5-1 Screening The first unit operation generally encountered in wastewater-treatment plants is screening. A screen is a device with openings, generally of uniform size, that is used to retain solids found in the influent wastewater to the treatment plant or in combined wastewater collection systems subject to overflows, especially from stormwater. The coarse materials from the flow stream that could (1) damage subsequent process equipment, (2) reduce overall treatment process reliability and effectiveness, or (3) contaminate waterways. Fine screens are sometimes used in place of or following coarse screens where greater removals of solids are required to (1) protect process equipment or (2) eliminate materials that may inhibit the beneficial reuse of biosolids. All aspects of screenings removal, transport, and disposal must be considered in the application of screening devices, including (1) the degree of screenings removal required because of potential effects on downstream processes, (2) health and safety of the operators as screenings contain pathogenic organisms and attract insects, (3) odor potential, and (4) requirements for handling, transport, and disposal, i.e., removal of organics (by washing) and reduced water content (by pressing), and (5) disposal options. Thus, an integrated approach is required to achieve effective screenings management. Classification of Screens The types of screening devices commonly used in wastewater treatment are shown on Fig. 5-2. Two general types of screens, coarse screens and fine screens, are used in preliminary treatment of wastewater. Coarse screens have clear openings ranging from 6 to 150 mm; fine screens have clear openings less than
6 mm. Micro screens, which generally have screen openings less than 50 um, are used principally in removing fine solids from treated effluents Screening Fig. 5-2 Definition sketch for types of screen ed in wastewater treatment Microscreen c 6mm 6 to 150 mm The screening element may Mechanically Static Step perforate cleaned openings may be of any shape but generally are circular or ctangular slots. A screen Reciprocating Caten composed of parallel bars or rods is often called a"bar rack" or a coarse screen and is used for the removal of coarse solids Fine screens are devices consisting of perforated plates, wedgewire elements, and wire cloth that have smaller openings. The materials removed by these devices are known as screenings pipelines and other appurtenances from damage or clogging by rags and large obiects. Industrial waste-treatment plants may or may not need them, depending on the character of the wastes. According to the method used to clean them, coarse screens are designated as either hand-cleaned or mechanically Hand-Cleaned Coarse Screens. Hand-cleaned coarse screens are used frequently ahead of pumps in small wastewater pumping stations and sometimes used at the headworks of small- to medium-sized wastewater-treatment plants. Often they are used for r standby screening in bypass channels for service during high-flow periods, when mechanically cleaned screens are being repaired. nt of a ower failure. Normally, mechanically cleaned screens are provided in place of hand-cleaned screens to minimize manual labor required to clean the screens and to reduce flooding due to clogging Where used. the length of the hand cleaned bar rack should not exceed the distance that can be conveniently raked by hand. approximately 3 m. The screen bars are welded to spacing bars located at the rear face, out of the way of the tines of the rake. a perforated drainage plate should be provided at the top lation of grit and other heavy materials in the channel ahead of the screen and following it. The channel floor should be level or should slope downward through the screen without pockets to trap solids. The channel preferably should have a straight approach, perpendicular to the bar screen, to promote uniform distribution of screenable solids throughout the flow and on the screen. Typical design information for hand-cleaned bar screens is provided in Table Tab. 5-1 Typical design information for manually and mechanically cleaned bar racks Unit Manual mm 5-15 mm 25-38 25-5 Maximum m/s 0.3-0.6 0.6-1.0 m/s 0.3-0.5 Allowable headloss
5-2 6 mm. Micro screens, which generally have screen openings less than 50 μm, are used principally in removing fine solids from treated effluents. Fig. 5-2 Definition sketch for types of screens used in wastewater treatment The screening element may consist of parallel bars, rods or wires, grating, wire mesh, or perforated plate, and the openings may be of any shape but generally are circular or rectangular slots. A screen composed of parallel bars or rods is often called a "bar rack" or a coarse screen and is used for the removal of coarse solids. Fine screens are devices consisting of perforated plates, wedgewire elements, and wire cloth that have smaller openings. The materials removed by these devices are known as screenings. Coarse Screens (Bar Racks). In wastewater treatment, coarse screens are used to protect pumps, valves, pipelines and other appurtenances from damage or clogging by rags and large objects. Industrial waste-treatment plants may or may not need them, depending on the character of the wastes. According to the method used to clean them, coarse screens are designated as either hand-cleaned or mechanically cleaned. Hand-Cleaned Coarse Screens. Hand-cleaned coarse screens are used frequently ahead of pumps in small wastewater pumping stations and sometimes used at the headworks of small- to medium-sized wastewater-treatment plants. Often they are used for standby screening in bypass channels for service during high-flow periods, when mechanically cleaned screens are being repaired, or in the event of a power failure. Normally, mechanically cleaned screens are provided in place of hand-cleaned screens to minimize manual labor required to clean the screens and to reduce flooding due to clogging. Where used, the length of the hand cleaned bar rack should not exceed the distance that can be conveniently raked by hand, approximately 3 m. The screen bars are welded to spacing bars located at the rear face, out of the way of the tines of the rake. A perforated drainage plate should be provided at the top of the rack where the raking may be stored temporarily for drainage. The screen channel should be designed to prevent the accumulation of grit and other heavy materials in the channel ahead of the screen and following it. The channel floor should be level or should slope downward through the screen without pockets to trap solids. The channel preferably should have a straight approach, perpendicular to the bar screen, to promote uniform distribution of screenable solids throughout the flow and on the screen. Typical design information for hand-cleaned bar screens is provided in Table 5-1. Tab. 5-1 Typical design information for manually and mechanically cleaned bar racks Parameter Unit Cleaning methods Manual Mechanical Bar size Width Depth mm mm 5-15 25-38 5-15 25-38 Clear space between bars mm 25-50 15-75 Slope from vertical ° 30-45 0-30 Approach velocity Maximum Minimum m/s m/s 0.3-0.6 0.6-1.0 0.3-0.5 Allowable headloss mm 150 150-600
Mechanically Cleaned Bar Screens. The design of mechanically cleaned bar screens has evolved over the years to reduce the operating and maintenance problems and to improve the screenings removal Many uns include extensive use of corrosion-resistant materials including stainless steel and plastics( ABS. etc). Me ally cleaned bar screens are divided into four principal types: (1)chain dri (2)reciprocating rake Continous (3)catenary, and (4) chain scraper continuous Raking tynes Bar reck Cable-driven screens were used extensively in the past but largely have been replaced in wastewater applications by the other types of screens re motor Examples of the necha ntlvent cover oc bar screens are shown on Fig 5-3 ng screens Guide tra (a)front clean, front (b)reciprocating rake (c)catenary, (continuous belt Chain-Driven Screens. Chain driven mechanically cleaned bar screens can be divided into categories based on whether the screen is raked to clean from the front(upstream) side or the back(downstream )side and whether the rakes return to the bottom of the bar screen from the front or back. Each type has its advantages and disadvantages, although the general mode of operation is similar. In general, front cleaned, front return screens(see Fig. 5-3a) are more efficient in terms of retaining captured solids, but they are less rugged and are susceptible to jamming by solids that collect at the base of the rake. Front cleaned, front return screens are seldom used for plants serving combined sewers where large objects can jam the rakes. In front cleaned, back return screens, the cleaning rakes return to the bottom of the bar screen on the downstream side of the screen, pass under the bottom of the screen, and clean the bar screen as the rake rises The potential for jamming is minimized, but a hinged plate, which is also subject to jamming is required to seal the pocket under the screen In back cleaned screens, the bars protect the rake from damage by the debris. However, a back cleaned screen is more susceptible to solids carryover to the down-stream side, particularly as rake wipers wear out. The bar rack of the back cleaned, back return screens is less rugged than the other types because the top of the rack is unsupported so the rake tines can pass through. Most of the chain-operated screens share the disadvantage of submerged sprockets that require frequent operator attention and are difficult to maintain. Additional disadvantages include the adjustment and repair of the heavy chains, and the need to dewater the channels for inspection and repair of submerged parts Reciprocating Rake(Climber)Screen. The reciprocating-rake-typo bar screen(see Fig 5-3b)imitates the movements of a person raking the screen. The rake moves to the base of the screen, engages the bars, and pulls the screenings to the top of the screen where they are removed. Most screen designs utilize a cogwheel drive mechanism for the rake. A major advantage is that all parts requiring m above the waterline and can be easily inspected and maintained without dewatering the channel. The front cleaned, front return feature minimizes solids carryover. The screen uses only one rake instead of multiple rakes that are used with other types of screens. As a result, the reciprocating rake screen may have limited capacity in handling heavy screenings loads, particularly in deep channels where a long"reach"is v. The nigh overhead clearance required to accommodate the rake mechanism can limit its use in retrofit applications Catenary Screen. A catenary screen is a type of front cleaned, front return chain driven screen, but it 5-3
5-3 Mechanically Cleaned Bar Screens. The design of mechanically cleaned bar screens has evolved over the years to reduce the operating and maintenance problems and to improve the screenings removal capabilities. Many of the newer designs include extensive use of corrosion-resistant materials including stainless steel and plastics(ABS, etc). Mechanically cleaned bar screens are divided into four principal types: (1) chain driven, (2) reciprocating rake, (3) catenary, and (4) continuous belt. Cable-driven bar screens were used extensively in the past but largely have been replaced in wastewater applications by the other types of screens. Examples of the different types of mechanically cleaned bar screens are shown on Fig. 5-3 Fig 5-3 Typical mechanically cleaned coarse screens: (a)front clean, front return chain-driven; (b)reciprocating rake, (c)catenary, (d)continuous belt Chain-Driven Screens. Chain driven mechanically cleaned bar screens can be divided into categories based on whether the screen is raked to clean from the front (upstream) side or the back (downstream) side and whether the rakes return to the bottom of the bar screen from the front or back. Each type has its advantages and disadvantages, although the general mode of operation is similar. In general, front cleaned, front return screens (see Fig. 5-3a) are more efficient in terms of retaining captured solids, but they are less rugged and are susceptible to jamming by solids that collect at the base of the rake. Front cleaned, front return screens are seldom used for plants serving combined sewers where large objects can jam the rakes. In front cleaned, back return screens, the cleaning rakes return to the bottom of the bar screen on the downstream side of the screen, pass under the bottom of the screen, and clean the bar screen as the rake rises. The potential for jamming is minimized, but a hinged plate, which is also subject to jamming, is required to seal the pocket under the screen. In back cleaned screens, the bars protect the rake from damage by the debris. However, a back cleaned screen is more susceptible to solids carryover to the down-stream side, particularly as rake wipers wear out. The bar rack of the back cleaned, back return screens is less rugged than the other types because the top of the rack is unsupported so the rake tines can pass through. Most of the chain-operated screens share the disadvantage of submerged sprockets that require frequent operator attention and are difficult to maintain. Additional disadvantages include the adjustment and repair of the heavy chains, and the need to dewater the channels for inspection and repair of submerged parts. Reciprocating Rake (Climber) Screen. The reciprocating-rake-typo bar screen (see Fig. 5-3b) imitates the movements of a person raking the screen. The rake moves to the base of the screen, engages the bars, and pulls the screenings to the top of the screen where they are removed. Most screen designs utilize a cogwheel drive mechanism for the rake. A major advantage is that all parts requiring maintenance are above the waterline and can be easily inspected and maintained without dewatering the channel. The front cleaned, front return feature minimizes solids carryover. The screen uses only one rake instead of multiple rakes that are used with other types of screens. As a result, the reciprocating rake screen may have limited capacity in handling heavy screenings loads, particularly in deep channels where a long "reach" is necessary. The nigh overhead clearance required to accommodate the rake mechanism can limit its use in retrofit applications. Catenary Screen. A catenary screen is a type of front cleaned, front return chain driven screen, but it
has no submerged sprockets. In the catenary screen(see Fig. 5-3c), the rake is held against the rack by the weight of the chain. If heavy obiects become jammed in the bars. the rakes pass over them instead of has a relatively large"footprint"and thus re Continuous Belt Screen. The continuous belt screen is a relatively new development for use in screening applications in the United States. It is a continuous, self-cleaning screening belt that removes fine and coarse solids(see Fig. 5-3d). A large number of screening elements(rakes) are attached to the drive chains the number of screening elements depends on the depth of the screen channel. Because the creen openings can range from 0.5 to 30 mm, it can be used as either a coarse or a fine screen. Hooks n the belt el Design of Coarse Screen Installations. Considerations in the design of screening installations include(D) location:(2)approach velocity: (3)clear openings between bars or mesh size: (4) headloss through the Because the purpose of coarse screens is to remove large objects that may damage or clog downstream equipment, in nearly all cases. they should be installed ahead of the grit chambers. If grit chambers are placed before screens, rags a terial could foul the grit cham ber collector mechanisms rap around air piping and settle with the grit. If grit is pumped, further fouling or clogging of the pumps will likely occur In hand-cleaned installations, it is essential that the velocity of approach be limited to approximately 0. 45 s at average flow to provide adequate screen area for accumulation of screenings between raking operations. Additional area to limit the velocity may be obtained by widening the channel at the screen gging the screen, the upstream head will increase. submerging new areas for the flow to pass through. The structural design of the screen should be adequate to prevent collapse if it becomes plugged completely For most mechanically cleaned coarse screen installations, two or more units should be installed so that one unit may be taken out of service for maintenance. Slide gates or recesses in the channel walls for the insertion of stop logs should be provided ahead of, and behind, each screen so that the unit can dewatered for screen maintenance and repair If only one unit is installed, it is absolutely essential that a bypass channel with a manually cleaned bar screen be provided for emergency use. Sometimes the manually cleaned bar screen is arranged as an overflow device if the mechanical screen should become inoperative, especially during unattended hours. An appI mize solids To prevent the pass-through of debris at peak flowrate screen should not exceed 0.9 m/s Headloss throu cleaned coarse screens is typically limited to about 150 mm by operational controls. Hydraulic losses through bar screens are a function of approach velocity and the velocity through the bars. The headloss through coarse screens can be estimated using the following equation where hr= headloss. m C= an empirical discharge coefficient to account for turbulence and eddy losses, typically 0.7 for a clean screen and 0.6 for a clogged screen V= velocity of flow through the openings of the bar screen, m/s v=approach velocity in upstream channel, m/s g=acceleration due to gravity, 9 18 m/s The headloss calculated using above equation applies only when the bars are clean. Headloss increases with the degree of clogging. The buildup of headloss can be estimated by assuming that a part of the open space in the upper portion of the bars in the flow path is clogged Although most screens use rectangular bars, optional shapes, i.e., "teardrop"and trapezoidal, are available For the optional shapes, the wider width dimension is located on the upstream side of the bar rack to make it easier to dislodge materials trapped between the bars. The alternative shapes also reduce headloss Screenings from the rake mechanism are usually discharged directly into a hopper or container or into a screenings press. For installations with multiple units, the screenings may be discharged onto a conveyor or into a pneumatic eiector system and transported to a common screenings storage hopper. As an alterative, screenings grinders may be used to grind and shred the screenings. Ground screenings are then
5-4 has no submerged sprockets. In the catenary screen (see Fig. 5-3c), the rake is held against the rack by the weight of the chain. If heavy objects become jammed in the bars, the rakes pass over them instead of jamming. The screen, however, has a relatively large "footprint" and thus requires greater space for installation. Continuous Belt Screen. The continuous belt screen is a relatively new development for use in screening applications in the United States. It is a continuous, self-cleaning screening belt that removes fine and coarse solids (see Fig. 5-3d). A large number of screening elements (rakes) are attached to the drive chains; the number of screening elements depends on the depth of the screen channel. Because the screen openings can range from 0.5 to 30 mm, it can be used as either a coarse or a fine screen. Hooks protruding from the belt elements are provided to capture large solids such as cans, sticks, and rags. Design of Coarse Screen Installations. Considerations in the design of screening installations include (1) location; (2)approach velocity;(3)clear openings between bars or mesh size; (4) headloss through the screens; (5) screenings handling processing, and disposal; and (6) controls. Because the purpose of coarse screens is to remove large objects that may damage or clog downstream equipment, in nearly all cases, they should be installed ahead of the grit chambers. If grit chambers are placed before screens, rags and other stringy material could foul the grit chamber collector mechanisms, wrap around air piping, and settle with the grit. If grit is pumped, further fouling or clogging of the pumps will likely occur. In hand-cleaned installations, it is essential that the velocity of approach be limited to approximately 0.45 m/s at average flow to provide adequate screen area for accumulation of screenings between raking operations. Additional area to limit the velocity may be obtained by widening the channel at the screen and by placing the screen at a flatter angle to increase the submerged area. As screenings accumulate, partially plugging the screen, the upstream head will increase, submerging new areas for the flow to pass through. The structural design of the screen should be adequate to prevent collapse if it becomes plugged completely. For most mechanically cleaned coarse screen installations, two or more units should be installed so that one unit may be taken out of service for maintenance. Slide gates or recesses in the channel walls for the insertion of stop logs should be provided ahead of, and behind, each screen so that the unit can be dewatered for screen maintenance and repair. If only one unit is installed, it is absolutely essential that a bypass channel with a manually cleaned bar screen be provided for emergency use. Sometimes the manually cleaned bar screen is arranged as an overflow device if the mechanical screen should become inoperative, especially during unattended hours. An approach velocity of at least 0.4 m/s is recommended to minimize solids deposition in the channel. To prevent the pass-through of debris at peak flowrates, the velocity through the bar screen should not exceed 0.9 m/s.Headloss through mechanically cleaned coarse screens is typically limited to about 150 mm by operational controls. Hydraulic losses through bar screens are a function of approach velocity and the velocity through the bars. The headloss through coarse screens can be estimated using the following equation: 2 2 1 ( ) 2 L V v h C g − = where hL = headloss, m C = an empirical discharge coefficient to account for turbulence and eddy losses, typically 0.7 for a clean screen and 0.6 for a clogged screen V = velocity of flow through the openings of the bar screen, m/s v = approach velocity in upstream channel, m/s g = acceleration due to gravity, 9.18 m/s2 The headloss calculated using above equation applies only when the bars are clean. Headloss increases with the degree of clogging. The buildup of headloss can be estimated by assuming that a part of the open space in the upper portion of the bars in the flow path is clogged. Although most screens use rectangular bars, optional shapes, i.e., "teardrop" and trapezoidal, are available. For the optional shapes, the wider width dimension is located on the upstream side of the bar rack to make it easier to dislodge materials trapped between the bars. The alternative shapes also reduce headloss through the rack. Screenings from the rake mechanism are usually discharged directly into a hopper or container or into a screenings press. For installations with multiple units, the screenings may be discharged onto a conveyor or into a pneumatic ejector system and transported to a common screenings storage hopper. As an alterative, screenings grinders may be used to grind and shred the screenings. Ground screenings are then
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 include preliminary treatmen (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. x De Typically, the openings vary from 0.2 to 6 mm) the o static(fixed).(2) rotary drum or(3)step 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 design capacity. Typical Fig. 5-4 Typical fine 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, 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 eep 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 BOD 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 5-5
5-5 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