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
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 ns 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 -6mim min Based on submerged surface area of drum 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 5-6
5-6 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
Tab. 5-4 Typical information on the characteristics and quantities of screenings remo ved from stewater with coarse screens Moisture content,% Volume of screenings(L/m) between bars. mm 700-100 37-74 -80 600-1000 15-37 600-1000 Combined storm and sanitary collection systems may produce volumes of so s several times the mounts produced by separate systems. The quantities of screenings have also been observed to vary widely, ranging from large quantities during the "first flush"to diminishing amounts as the wet weather flows persist. The quantities of screenings removed from combined sewer flows are reported to range 3.5to84 00 m' of flow Screenings Retained on Fine Screens. Fine screenings consist of materials that are retained on screens with openings less than 6 mm. The materials retained on fine screens include small rags. paper. plastic materials of various types razor blades. grit, undecomposed food waste, feces. etc. Compared to coarse screenings. the specific weight of the fine screenings is slightly lower and the moisture content is slightly higher. Because putrescible matter, including fecal material, is contained within screenings, they must be handled and disposed of properly. Fine screenings contain substantial grease and scum. which require similar care, especially if odors are to be avoided. Screenings Handling, Processing, and Disposal. In mechanically cleaned screen installations, screenings are discharged from the screening unit directly into a screenings grinder. a pneumatic eiector or a container for disposal; or onto a conveyor for transport to a screenings compactor or collection he Belt conveyors and pneumatic ejectors are generally the primary means of mechanically transporting screenings. Belt conveyors offer the advantages of simplicity of operation, low maintenance, freedom from clogging and low cost. Belt conveyors give off odors and may have to be provided with covers Pneumatic ejectors are less odorous and typically require less space; however, they are subject to clogging if large objects are present in the screenings Screenings compactors can be used to dewater and reduce the volume of screenings(see Fig 5-5) Such devices, including hydraulic ram and Discharge screw compactors, receive screenings directly from the bar screens and are capable of transporting the compacted screenings to a Feed receiving hopper. Compactors can reduce the water content of the screenings by up to 50 Screenings percent and the volume by up to 75 percent. As with automatic controls can sense jams, automatically reverse the mechanism. and actuate alarms and shut down equipment. Typical device used for compacting screenings Means of disposal of screenings include(1 removal by hauling to disposal areas (landfill solid wastes. (2)dispos onlv). and( 4) discharge to grinders or macerator where they are ground and returned to the wastewater. The first method of disposal is most commonly used. In some cases, screenings are required to be lime stabilized for the control of pathogenic organisms before disposal in landfills. 5-2 Coarse Solids reduction as an alternative to bar screens or fine screens. comminutors and macerator can be used to intercept coarse solids and grind or shred them in the screen channel. High-speed grinders are used in conjunction with mechanically cleaned screens to grind and shred screenings that are removed from the wastewater. The solids are cut up into a smaller, more uniform size for return to the flow stream for bsequent removal by downstream treatment operations and processes. Comminutors, macerator, and grinders can theoretically eliminate the messy and offensive task of screenings handling and disposal. The use of comminutors and macerator is particularly advantageous in a pumping station to protect the pumps 5-7
5-7 Tab. 5-4 Typical information on the characteristics and quantities of screenings removed from wastewater with coarse screens Size of openings between bars,mm Moisture content,% Specific weight kg/m3 Volume of screenings(L/m3 ) Range Typical 12.5 60-90 700-1100 37-74 50 25 50-80 600-1000 15-37 22 37.5 50-80 600-1000 7-15 11 50 50-80 600-1000 4-11 6 Combined storm and sanitary collection systems may produce volumes of screenings several times the amounts produced by separate systems. The quantities of screenings have also been observed to vary widely, ranging from large quantities during the "first flush" to diminishing amounts as the wet weather flows persist. The quantities of screenings removed from combined sewer flows are reported to range from 3.5 to 84 L/1000 m3 of flow. Screenings Retained on Fine Screens. Fine screenings consist of materials that are retained on screens with openings less than 6 mm. The materials retained on fine screens include small rags, paper, plastic materials of various types razor blades, grit, undecomposed food waste, feces, etc. Compared to coarse screenings, the specific weight of the fine screenings is slightly lower and the moisture content is slightly higher. Because putrescible matter, including fecal material, is contained within screenings, they must be handled and disposed of properly. Fine screenings contain substantial grease and scum, which require similar care, especially if odors are to be avoided. Screenings Handling, Processing, and Disposal. In mechanically cleaned screen installations, screenings are discharged from the screening unit directly into a screenings grinder, a pneumatic ejector, or a container for disposal; or onto a conveyor for transport to a screenings compactor or collection hopper. Belt conveyors and pneumatic ejectors are generally the primary means of mechanically transporting screenings. Belt conveyors offer the advantages of simplicity of operation, low maintenance, freedom from clogging, and low cost. Belt conveyors give off odors and may have to be provided with covers. Pneumatic ejectors are less odorous and typically require less space; however, they are subject to clogging if large objects are present in the screenings. Screenings compactors can be used to dewater and reduce the volume of screenings (see Fig. 5-5). Such devices, including hydraulic ram and screw compactors, receive screenings directly from the bar screens and are capable of transporting the compacted screenings to a receiving hopper. Compactors can reduce the water content of the screenings by up to 50 percent and the volume by up to 75 percent. As with pneumatic ejectors, large objects can cause jamming, but automatic controls can sense jams, automatically reverse the mechanism, and actuate alarms and shut down equipment. Fig. 5-5 Typical device used for compacting screenings Means of disposal of screenings include (1) removal by hauling to disposal areas (landfill) including co-disposal with municipal solid wastes, (2) disposal by burial on the plant site (small installations only), (3) incineration either alone or in combination with sludge and grit (large installations only), and (4) discharge to grinders or macerators where they are ground and returned to the wastewater. The first method of disposal is most commonly used. In some cases, screenings are required to be lime stabilized for the control of pathogenic organisms before disposal in landfills. 5-2 Coarse Solids Reduction As an alternative to coarse bar screens or fine screens, comminutors and macerators can be used to intercept coarse solids and grind or shred them in the screen channel. High-speed grinders are used in conjunction with mechanically cleaned screens to grind and shred screenings that are removed from the wastewater. The solids are cut up into a smaller, more uniform size for return to the flow stream for subsequent removal by downstream treatment operations and processes. Comminutors, macerators, and grinders can theoretically eliminate the messy and offensive task of screenings handling and disposal. The use of comminutors and macerators is particularly advantageous in a pumping station to protect the pumps
against clogging by rags and large objects and to eliminate the need to handle and dispose of screenings They are particularly useful in cold climates where collected screenings are subject to freezing screenings at wastewater-treatment plants. One school of thought maintains that once coarse solids have been removed from wastewater. they should not b thought maintains that once cut up the solids are ly handled in the downstream processes. Shredded solids often present downstream problems, particularly with rags and plastic bags. as they tend diffusers and clarifier Plastics and other non-biodegradable material may als so advers bio- solids that are to be beneficially reused Approaches to using comminutors, macerator, and grinders are applicable in many retrofit situations Examples of retrofit applications include plants where a spare channel has been provided for the future installation of a duplicate unit or in very deep influent pumping stations where the removal of screenings may be too difficult or costly to achieve. Alternative approaches may also be possible, such as using chopper pumps at pumping stations or installing grinders ahead of sludge pumps Comminutors Comminutors are used most commonly in small wastewater-treatment plant less than 0.2 m/s Comminutors are installed in a wastewater flow channel to screen and shred material to sizes from 6 to 20 mm without removing the shredded solids from the flow stream. a typical comminutor uses a stationary horizontal screen to intercept the flow(see Fig. 5-6)and a rotating or oscillating arm that contains cutting teeth to mesh with the screen. the cutting teeth and the shear bars cut coarse material. The small sheared particles pass through the material, namely, rags, that can collect on downstream treatment equipment screen and into the down-stream channel. Comminutors may create a string Fig. 5-6 Typical communitors used for particle size reduction of solids diverter screen Macerator DIMe Macerator are slow-speed grinders that typically consist of two sets of counter-rotating blies with blades( see F The assemblies are mounted vertically in the flow Counter channel. The blades or teeth on Counter- the rotating assemblies have a rotating close tolerance that effectively Patting chops material as it passes through the unit the F Fig. 57 Tpical mcerators: (alsdiemaic of i-dhanel tpe sbw-speed grinder/macerator; (b view of a maceratr ropes mounted in an open channeli(schematic of linked-screen macerator installations to shred solids, particularly ahead of wastewater and sludge pumps, or in channels at smaller wastewater-treatment plants. Sizes for pipeline applications typically range from 100 to 400 mm in diameter Another type of macerator used in channel applications is a moving, linked screen that allows wastewater to pass through the screen while diverting screenings to a grinder located at one side of the channel(see Fig. 5-7c). Standard sizes of this device are available for use in large channels ranging from widths of 750 to 1800 mm and depths of 750 to 2500 mm. The headloss is lower than that of the units with counter-rotating blades shown on Fig 5-7a Grinders High-speed grinders, typically referred to as hammer-mills, receive screened materials from bar screens The materials are pulverized by a high-speed rotating assembly that cuts the materials passing through the
5-8 against clogging by rags and large objects and to eliminate the need to handle and dispose of screenings. They are particularly useful in cold climates where collected screenings are subject to freezing. There is a wide divergence of views, however, on the suitability of using devices that grind and shred screenings at wastewater-treatment plants. One school of thought maintains that once coarse solids have been removed from wastewater, they should not be returned, regardless of the form. The other school of thought maintains that once cut up, the solids are more easily handled in the downstream processes. Shredded solids often present downstream problems, particularly with rags and plastic bags, as they tend to form ropelike strands. Rag and plastic strands can have a number of adverse impacts, such as clogging pump impellers, sludge pipelines, and heat exchangers, and accumulating on air diffusers and clarifier mechanisms. Plastics and other non-biodegradable material may also adversely affect the quality of bio-solids that are to be beneficially reused. Approaches to using comminutors, macerators, and grinders are applicable in many retrofit situations. Examples of retrofit applications include plants where a spare channel has been provided for the future installation of a duplicate unit or in very deep influent pumping stations where the removal of screenings may be too difficult or costly to achieve. Alternative approaches may also be possible, such as using chopper pumps at pumping stations or installing grinders ahead of sludge pumps. Comminutors Comminutors are used most commonly in small wastewater-treatment plants, less than 0.2 m3 /s. Comminutors are installed in a wastewater flow channel to screen and shred material to sizes from 6 to 20 mm without removing the shredded solids from the flow stream. A typical comminutor uses a stationary horizontal screen to intercept the flow (see Fig. 5-6) and a rotating or oscillating arm that contains cutting teeth to mesh with the screen. The cutting teeth and the shear bars cut coarse material. The small sheared particles pass through the screen and into the down-stream channel. Comminutors may create a string of material, namely, rags, that can collect on downstream treatment equipment. Fig.5-6 Typical communitors used for particle size reduction of solids Macerators Macerators are slow-speed grinders that typically consist of two sets of counter-rotating assemblies with blades (see Fig. 5-7a). The assemblies are mounted vertically in the flow channel. The blades or teeth on the rotating assemblies have a close tolerance that effectively chops material as it passes through the unit. The chopping action reduces the potential for producing ropes of rags or plastic that can collect on downstream equipment. Macerators can be used in pipeline installations to shred solids, particularly ahead of wastewater and sludge pumps, or in channels at smaller wastewater-treatment plants. Sizes for pipeline applications typically range from 100 to 400 mm in diameter. Another type of macerator used in channel applications is a moving, linked screen that allows wastewater to pass through the screen while diverting screenings to a grinder located at one side of the channel (see Fig. 5-7c). Standard sizes of this device are available for use in large channels ranging from widths of 750 to 1800 mm and depths of 750 to 2500 mm. The headloss is lower than that of the units with counter-rotating blades shown on Fig. 5-7a. Grinders High-speed grinders, typically referred to as hammer-mills, receive screened materials from bar screens. The materials are pulverized by a high-speed rotating assembly that cuts the materials passing through the Fig. 5-7 Typical macerators: (a)schematic of in-channel type slow-speed grinder/macerator;(b)view of a macerator mounted in an open channel;(c)schematic of linked-screen macerator Horizontal rotating diverter screen Counterrotating cutting assembly Counterrotating cutting assembly
unit. The cutting or knife blades force screenings through a stationary grid or louver that encloses the rotating assembly. Wash-water is typically used to keep the unit clean and to help transport materials back to the wastewater stream. Discharge from the grinder can be located either upstream or downstream of the bar screen Flow equalization is a method used to overcome the operational problems caused by flowrate variations. erformance of the downstream processes and to reduce the Description/Application Flow equalization simply is the damping of flowrate variations to achieve a constant or nearly constant flowrate and can be applied in a number of different situations, depending on the characteristics of the collection system. The principal applications are for the equalization of (1) dry-weather flows to reduce peak flows and loads, (2)wet-weather flows in sanitary collection systems experiencing inflow and infiltration, or (3)combined stormwater and sanitary system flows The application of flow equalization in wastewater treatment is illustrated in the two flow diagrams given on Fig. 5-8. In the in-line arrangement (Fig. 5-8a) Flowrate varies Flowrate is relatively const fle through the equalization Inner basin. This arrangement Equalization Secondary Emuer can be used to achieve a considerable amount of Controlled-tiow constituent concentration and flowrate damping In间 Flowrate va Flowrate is relatively constant the off-line arrangement Overflow ( Fig. 5-8b), only the flow above Secondary ELuent treatmont predetermined flow is diverted into qualization basin pumping Although pumping Fig. 5-8 Typical wastewater treatment plant flow diagram incorporating flow equalization:(ain-linn requirements equalization (b)offline equalization. Flow equaltation ca be applied after grit remoul afier primary are minimized sedimentation, and aftersecondary treatment where advanced treatment is used arrangement,the amount of constituent concentration damping is considerable reduced. Off-line equalization is sometimes used to capture the"first flush "from combined collection systems. The principal benefits that are cited as deriving from application of flow equalization are: (1 biological treatment is enhanced, because shock loadings are eliminated or can be minimized inhibiting substances can be diluted and pH can be stabilized (2) the effluent quality and thickening performance of secondly sedimentation tanks following biological treatment is improved through improved consistency in solids loading:(3)effluent filtration surface area requirements are reduced. filter performance is improved, and more uniform filter-backwash cycles are possible by lower hydraulic loading; and (4)in chemical mping of mass loading improves chemical feed control and process reliabilitv. Apart from improving the performance of most treatment operations and processes, flow equalization is an attractive option for upgrading the performance of overloaded treatment plants. Disadvantages of flow equalization include(1) relatively large land areas or sites are needed. (2) equalization facilities may have to be d maintenance is required, and 4)capital cost is increased. Design considerations The design of flow equalization facilities is concerned with the following questions Where in the treatment process flowsheet should the equalization facilities be located What type of equalization flowsheet should be used, in-line or off-lin v What is the required basin volume? 9
5-9 unit. The cutting or knife blades force screenings through a stationary grid or louver that encloses the rotating assembly. Wash-water is typically used to keep the unit clean and to help transport materials back to the wastewater stream. Discharge from the grinder can be located either upstream or downstream of the bar screen. 5-3 Flow Equalization Flow equalization is a method used to overcome the operational problems caused by flowrate variations, to improve the performance of the downstream processes, and to reduce the size and cost of down- stream treatment facilities. Description/Application Flow equalization simply is the damping of flowrate variations to achieve a constant or nearly constant flowrate and can be applied in a number of different situations, depending on the characteristics of the collection system. The principal applications are for the equalization of (1) dry-weather flows to reduce peak flows and loads, (2) wet-weather flows in sanitary collection systems experiencing inflow and infiltration, or (3) combined stormwater and sanitary system flows. The application of flow equalization in wastewater treatment is illustrated in the two flow diagrams given on Fig. 5-8. In the in-line arrangement (Fig. 5-8a), all of the flow passes through the equalization basin. This arrangement can be used to achieve a considerable amount of constituent concentration and flowrate damping. In the off-line arrangement (Fig. 5-8b), only the flow above some predetermined flow limit is diverted into the equalization basin. Although pumping requirements are minimized in this arrangement, the amount of constituent concentration damping is considerable reduced. Off-line equalization is sometimes used to capture the "first flush" from combined collection systems. The principal benefits that are cited as deriving from application of flow equalization are: (1) biological treatment is enhanced, because shock loadings are eliminated or can be minimized, inhibiting substances can be diluted and pH can be stabilized (2) the effluent quality and thickening performance of secondly sedimentation tanks following biological treatment is improved through improved consistency in solids loading; (3) effluent filtration surface area requirements are reduced, filter performance is improved, and more uniform filter-backwash cycles are possible by lower hydraulic loading; and (4) in chemical treatment, damping of mass loading improves chemical feed control and process reliability. Apart from improving the performance of most treatment operations and processes, flow equalization is an attractive option for upgrading the performance of overloaded treatment plants. Disadvantages of flow equalization include (1) relatively large land areas or sites are needed, (2) equalization facilities may have to be covered for odor control near residential areas, (3) additional operation and maintenance is required, and (4) capital cost is increased. Design Considerations The design of flow equalization facilities is concerned with the following questions: ✓ Where in the treatment process flowsheet should the equalization facilities be located? ✓ What type of equalization flowsheet should be used, in-line or off-line? ✓ What is the required basin volume? Fig. 5-8 Typical wastewater treatment plant flow diagram incorporating flow equalization: (a)in-line equalization ;(b)off-line equalization. Flow equalization can be applied after grit removal, after primary sedimentation ,and after secondary treatment where advanced treatment is used
What are the features that should be incorporated into design? How can the deposition of solids and potential odors be controlled? Location of equalization Facilities. The best location for equalization facilities must be determined for each system. Because the optimum location will vary with the characteristics of the collection system and the wastewater to be handled, land requirements and availability, and the type of treatment required detailed studies should be performed for several locations throughout the system. Where equalization facilities are considered for location adjacent to the wastewater-treatment plant, it is necessary to evaluate ow they could be integrated into the treatment process flowsheet. In son treatmen treatment causes fewer problems with solids deposits and scum accumulation. If flow-equalization systems are to be located ahead of primary settling and biological systems. the design must provide for sufficient mixing to prevent solids deposition and concentration variations. and aeration to prevent odor In-Line or Off-Line Equalization. As shown on Fig. 5-8. it is possible to achieve considerable amping of constituent mass loadings to the downstream processes with in-line equalization, but onl slight damping is achieved with off-line equal ization. Volume Requirements for the Equalization Basin. The volume required for flowrate equalization is plotted versus the time of day. The average daily flowrate. also plotted on the same diagram is the straight line drawn from the origin to the endpoint of the diagram. Diagrams for two typical flowrate patterns are shown on Fig 5-9 To determine the required volume, a line parallel to the coordinate axis, defined by the average daily flowrate, is drawn tangent to the mass inflow curve. The required volume is then equal to the vertica distance from the point of tangency to the straight line representing the average flowrate (see Fig. 5-9a). If the inflow mass curve goes above the line representing the average flowrate(see Fig. 5-9b), the inflow mass diagram must be bounded with two lines that are parallel to the average flowrate line and tangent to extremities of the inflow mass diagram. The distance between the two line The physical interpretation of the diagrams shown on Fig 5-9 is as follows. At the low point of tangency(flowrate pattern A)the storage basin is egins to fill because the slope of the inflow mass diagram is greater than that of the average daily flowrate. The basin continues to fill until it becomes full at midnight For flowrate pattern B, the basin is filled equalization at the upper point of tangency ig. 5-9 Schematic Tme ot determination of the required equalization basin a)Flowrate pattem A (b) Flowrate patter日 storage volume for two typical flowrate patterns In practice. the volume of the equalization basin will be larger than that theoretically determined to account for the following factors Continuous operation of aeration and mixing equipment will not allow complete drawdown. although ecial structures can be built v Volume must be provided to accommodate the concentrated plant recycle streams that are expected. if such flows are returned to the equalization basin (a practice that is not recommended unless the basin is covered because of the potential to create odors v Although no fixed value can be given, the additional volume will vary from 10 to 20 percent of the heoretical value, depending on the specific conditions Basin Configuration and Construction. In equal ization basin design, the principal factors that must be considered are (1)basin geometry;(2) basin construction including cleaning, access, and safety; (3) mixing and air requirements; (4)operational appurtenances; and(5) pump and pump control systems Basin Geometry. The importance of basin geometry varies somewhat, depending on whether in-line or off-line equalization is used. If in-line equalization is used to dampen both the flow and the mass loadings. it is important to use a geometry that allows the basin to function as a continuous-flow stirred-tank reactor 5-10
5-10 ✓ What are the features that should be incorporated into design? ✓ How can the deposition of solids and potential odors be controlled? Location of Equalization Facilities. The best location for equalization facilities must be determined for each system. Because the optimum location will vary with the characteristics of the collection system and the wastewater to be handled, land requirements and availability, and the type of treatment required, detailed studies should be performed for several locations throughout the system. Where equalization facilities are considered for location adjacent to the wastewater-treatment plant, it is necessary to evaluate how they could be integrated into the treatment process flowsheet. In some cases, equalization after primary treatment and before biological treatment may be appropriate. Equalization after primary treatment causes fewer problems with solids deposits and scum accumulation. If flow-equalization systems are to be located ahead of primary settling and biological systems, the design must provide for sufficient mixing to prevent solids deposition and concentration variations, and aeration to prevent odor problems. In-Line or Off-Line Equalization. As shown on Fig. 5-8, it is possible to achieve considerable damping of constituent mass loadings to the downstream processes with in-line equalization, but only slight damping is achieved with off-line equalization. Volume Requirements for the Equalization Basin. The volume required for flowrate equalization is determined by using an inflow cumulative volume diagram in which the cumulative inflow volume is plotted versus the time of day. The average daily flowrate, also plotted on the same diagram, is the straight line drawn from the origin to the endpoint of the diagram. Diagrams for two typical flowrate patterns are shown on Fig. 5-9. To determine the required volume, a line parallel to the coordinate axis, defined by the average daily flowrate, is drawn tangent to the mass inflow curve. The required volume is then equal to the vertical distance from the point of tangency to the straight line representing the average flowrate (see Fig. 5-9a). If the inflow mass curve goes above the line representing the average flowrate (see Fig. 5-9b), the inflow mass diagram must be bounded with two lines that are parallel to the average flowrate line and tangent to extremities of the inflow mass diagram. The required volume is then equal to the vertical distance between the two lines. The physical interpretation of the diagrams shown on Fig. 5-9 is as follows. At the low point of tangency (flowrate pattern A) the storage basin is empty. Beyond this point, the basin begins to fill because the slope of the inflow mass diagram is greater than that of the average daily flowrate. The basin continues to fill until it becomes full at midnight. For flowrate pattern B, the basin is filled at the upper point of tangency. Fig. 5-9 Schematic mass diagrams for the determination of the required equalization basin storage volume for two typical flowrate patterns In practice, the volume of the equalization basin will be larger than that theoretically determined to account for the following factors: ✓ Continuous operation of aeration and mixing equipment will not allow complete drawdown, although special structures can be built. ✓ Volume must be provided to accommodate the concentrated plant recycle streams that are expected, if such flows are returned to the equalization basin (a practice that is not recommended unless the basin is covered because of the potential to create odors). ✓ Some contingency should be provided for unforeseen changes in diurnal flow. ✓ Although no fixed value can be given, the additional volume will vary from 10 to 20 percent of the theoretical value, depending on the specific conditions. Basin Configuration and Construction. In equalization basin design, the principal factors that must be considered are (1) basin geometry; (2) basin construction including cleaning, access, and safety; (3) mixing and air requirements; (4) operational appurtenances; and (5) pump and pump control systems. Basin Geometry. The importance of basin geometry varies somewhat, depending on whether in-line or off-line equalization is used. If in-line equalization is used to dampen both the flow and the mass loadings, it is important to use a geometry that allows the basin to function as a continuous-flow stirred-tank reactor