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