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 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 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 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:
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 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 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 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.
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 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?
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 insofar as possible. Therefore, elongated designs should be avoided, and the inlet and outlet configurationally should be arranged to minimize short circuiting. Discharging the influent near the mixing equipment usually minimizes short circuiting. If the geometry of the basins is controlled by the available land area and an elongated geometry must be used, it may be necessary to use multiple inlets and outlets. Provisions should be included in the basin design for access by cleaning equipment such as front-end loaders. Multiple compartments are also desirable to reduce cleaning costs and for odor control.
Basin Construction. New basins may be of earthen, concrete, or steel construction; earthen basins are generally the least expensive. Depending on local conditions, the interior side slopes may vary between 3:1 and 2:1. A section through a typical earthen basin is shown on Fig. 5-10. In most installations, a liner is required to prevent ground-water contamination. Basin depths will vary depending on land availability, ground-water level, and topography, if a liner is used in areas of high groundwater, the effects of hydraulic uplift on the liner must be considered. The freeboard required depends on the surface area of the basin and local wind conditions. If a floating aerator is used to provide mixing and prevent septicity and odor formation, a minimum operating level is needed to protect the aerator. Typically, the minimum water depth can vary from 1.5 to 2 m. With floating aerators, a concrete pad should be provided below the aerators to minimize erosion. To prevent wind-induced erosion in the upper portions of the basin, it may be necessary to protect the slopes with riprap, soil cement, or a partial concrete layer. Fencing should also be provided to prevent public access to the basins.
In areas of high groundwater, drainage facilities should be provided to prevent embankment failure. To further ensure a stable embankment, the tops of the dikes should be of adequate width. The use of an adequate dike width will facilitate the use of mechanical equipment for maintenance and will also reduce construction costs, especially where mechanical compaction equipment is used.
Mixing and Air Requirements. The proper operation of both in-line and off-line equalization basins generally requires proper mixing and aeration. Mixing equipment should be sized to blend the contents of the tank and to prevent deposition of solids in the basin. To minimize mixing requirements, grit-removal facilities should precede equalization basins where possible. Mixing requirements for blending a medium-strength municipal wastewater, having a suspended solids concentration of approximately 210 mg/L, range from 0.004 to 0.008 kW/m3 of storage. Aeration is required to prevent the wastewater from becoming septic and odorous. To maintain aerobic conditions, air should be supplied at a rate of 0.01 to 0.015 m3/m3.min. In equalization basins that follow primary sedimentation and have short detention times (less than 2 h), aeration may not be required.
Where mechanical aerators are used, baffling may be necessary to ensure proper mixing, particularly with a circular tank configuration. To protect the aerators in the event of excessive level drawdown, low-level shutoff controls should be provided. Because it may be necessary to dewater the equalization basins periodically, the aerators should be equipped with legs or draft tubes that allow them to come to rest on the bottom of the basin without damage. Various types of diffused air systems may also be used for mixing and aeration including static tube, jet, and aspirating aerators.
Operational Appurtenances. Among the appurtenances that should be included in the design of equalization basins are (1) facilities for flushing any solids and grease that may tend o accumulate on the basin walls; (2) a high water takeoff for the removal of floating material and foam; (3) water sprays to prevent the accumulation of foam on the sides of the basin and to aid in scum removal; and (4) separate odor control facilities where covered equalization basins must be used. Solids removed from equalization basins should be returned to the head of the plant for processing.
Pumps and Pump Control. Because flow equalization imposes all additional head requirement within the treatment plant, pumping facilities are frequently required. Pumping may precede or follow equalization, but pumping into the basin is generally preferred for reliability of treatment operation. In some cases, pumping of both basin influent and equalized flows will be required.
An automatically controlled flow-regulating device will be required where gravity discharge from the basin is used. Where basin effluent pumps are used, instrumentation should be provided to control the preselected equalization rate. Regardless of the discharge method used, a flow-measuring device should be provided on the outlet of the basin to monitor the equalized flow.
5-4 Mixing and Flocculation
Mixing is an important unit operation in many phases of wastewater treatment including (1) mixing of one substance completely with another, (2) blending of miscible liquids, (3) flocculation of wastewater particles, (4) continuous mixing of liquid suspensions, and (5) heat transfer. Most mixing operations in wastewater can be classified as continuous-rapid (less than 30 s) or continuous (i.e., ongoing). Continuous Rapid Mixing in Wastewater Treatment
Continuous rapid mixing is used, most often, where one substance is to be mixed with another. The principal applications of continuous rapid mixing are in (1) the blending of chemicals with wastewater (e.g., the addition of alum or iron salts prior to flocculation and settling or for dispersing chlorine and hypochlorite into wastewater for disinfection),(2) the blending of miscible liquids, and (3) the addition of chemicals to sludge and biosolids to improve their dewatering characteristics. Typical examples of the types of mixers used in wastewater-treatment facilities for rapid mixing are reported in Table 5-5.
Tab. 5-5 Typical mixing times and applications for different mixing and flocculation devices in wastewater
Mixing device
Typical mixing times, s
Application/remarks
Static in-line mixers
<1
Used for chemicals instantaneous mixing such as alum (Al3+), ferric chlorine (Fe3+),cationic polymer, chlorine
In-line mixers
<1
Used for chemicals instantaneous mixing such as alum (Al3+), ferric chlorine (Fe3+),cationic polymer, chlorine
High speed induction device
<1
Used for chemicals instantaneous mixing such as alum (Al3+), ferric chlorine (Fe3+),cationic polymer, chlorine
Pressurized water jets
<1
Used in water treatment practice and for reclaimed water applications
Turbine and propeller mixers
2-20
Used in back mix reactors for the mixing of alum in sweep floc applications. Actual time depends on the configuration of the vessel in which mixing is take place. Mixing of chemicals in solution feed tanks.
Pumps
<1
Chemicals to be mixed are introduced in the suction intake of the pump.
Other hydraulic mixers
1-10
Hydraulic jumps, weirs, Parshall flumes,etc.
Continuous Mixing in Wastewater Treatment
Continuous mixing is used where the contents of a reactor or holding tank or basin must be kept in suspension such as in equalization basins, flocculation basins, suspended-growth biological treatment processes, aerated lagoons, and aerobic digesters.
Flocculation in Wastewater Treatment. The purpose of wastewater flocculation is to form aggregates or flocs from finely divided particles and from chemically destabilized particles. Flocculation is a transport step that brings about the collisions between the destabilized particles needed to form larger particles that can be removed readily by settling or filtration. Although not used routinely, flocculation of wastewater by mechanical or air agitation may be considered for (1) increasing removal of suspended solids and BOD in primary settling facilities, (2) conditioning wastewater containing certain industrial wastes. (3) improving performance of secondary settling tanks following the activated-sludge process, and (4) as a pretreatment step for the filtration of secondary effluent. When used, flocculation can be accomplished in separate tanks or basins specifically designed for the purpose, in in-line facilities such as in the conduits and pipes connecting the treatment units, or in combination with flocculator-clarifiers.
Fig. 5-11 Schematic illustrations of the two types of flocculation: (a)micro-flocculation(due to Brownian motion, perikinetic flocculation);(b)mcaro-flocculation (orthokinetic flocculation)due to fluid shear and/or differential settling
Flocculation typically follows rapid mixing where chemicals have been added to destabilize the particles. There are two types of flocculation: (1) microflocculation and (2) macroflocculation. The distinction between these two types of flocculation is based on the particle sizes involved.
Microflocculation (also known as perikinetic flocculation) is the term used to refer to the aggregation of particles brought about by the random thermal motion of fluid molecules. The random thermal motion of fluid molecules is also known as Brownian motion or movement (see Fig. 5-11a). Microflocculation is significant for particles that are in the size range from 0.001 to about 1μm..Macroflocculation (also known as orthokinetic flocculation) is the term used to refer to the aggregation of particles greater than 1 or 2μ m. Macroflocculation can be brought about by (1) induced velocity gradients and (2) differential settling. Particles can be brought together (i.e., flocculated) by inducing velocity gradients in a fluid containing the particles to be flocculated. As illustrated on Fig. 5-11b, faster-moving particles will overtake slower-moving particles in a velocity field. If the particles that collide stick together, a larger particle will be formed that will be easier to remove by gravity separation.
In macroflocculation by differential settling (see Fig. 5-11b), large particles over-take smaller particles during gravity settling. When the two particles collide and stick together, a larger particle is formed that settles at a rate that is greater than that of the larger particle before the two particles collided.
It should be noted that flocculation brought about by induced velocity gradients is ineffectual until the colloidal particles reach a size of 1 or 2 μm through contacts produced by Brownian motion. For example, macroflocculation cannot be used to aggregate viruses, which are 0.1 m in size or smaller, until they are microflocculated or adsorbed or enmeshed in larger flocs or particles.
Maintaining Material in Suspension. Continuous mixing operations are used in biological treatment processes such as the activated-sludge process to maintain the mixed liquor suspended solids in suspension. In biological treatment systems the mixing device is also used to provide the oxygen needed for the process. Thus, the aeration equipment must be able to provide the oxygen needed for the process and must be able to deliver the energy needed to maintain mixed conditions within the reactor. Both mechanical aerators and dissolved aeration devices are used. In both aerobic and anaerobic digestion, mixing is used to homogenize the contents of the digester to accelerate the biological conversion process, and to distribute uniformly the heat generated from biological conversion reactions.
Timescale in Mixing
The timescale for mixing is an important consideration in the design of mixing facilities and operations. For example, if the reaction rate between the substance being mixed into a liquid and the liquid is rapid, the time of mixing is extremely important. For slowly reacting substances, the time of mixing is not as critical.It should be noted that achieving extremely short mixing times becomes increasingly difficult as the flowrate increases. In some applications, it may be preferable to use multiple mixing devices to achieve optimal mixing times.
Types of Mixers Used for Rapid Mixing in Wastewater Treatment
Many types of mixing devices are available, depending on the application and the time scale required for mixing (see Table 5-5). The principal devices used for rapid mixing in wastewater-treatment applications include static in-line mixers, high-speed induction mixers, pressurized water jets, and propeller and turbine mixers. Mixing can also be accomplished in pumps and with the aid of hydraulic devices such as hydraulic jumps, Parshall flumes, or weirs. Although hydraulic mixing can sometimes be highly efficient, the principal problem is that the energy input varies with the flowrate, and incomplete and ineffective mixing can occur at low flowrates.
Fig. 5-12 Typical mixers used in wastewater treatment for rapid mixing : (a)in-line static mixer with internal vanes, (b)in-line static mixer with orifice for mixing dilute chemicals , (c)in-line mixer, (d)in-line mixer with internal mixer, (e)high-speed induction mixer, (f)pressurized water jet mixer with reactor tube
Static Mixers. Static in-line mixers contain internal vanes or orifice plates that bring about sudden changes in the velocity patterns as wall as momentum reversals. Static mixers are principally identified by their lack of moving parts. Typical examples include in-line static mixers that contain elements that bring about sudden changes in the velocity patterns as well as momentum reversals (see Fig. 5-12a) and mixers that contain orifice plates and nozzles (see Fig. 5-12b). Static in-line mixers are used most commonly for mixing of chemicals with wastewater. In-line mixers are available in sizes varying from about 12 mm to 3 m×3 m open channels. Low-pressure-drop round, square, and rectangular in-line static mixers have been developed for chlorine mixing in open channels and tunnels for flowrates varying from 0.22 to over 8.76 m3/s.
For static in-line mixers with vanes, the longer the mixing elements, the better the mixing; however, the pressure loss increases. It should also be noted that the shear rate and the scale (i.e., size) of the turbulent eddies formed in static mixers with vanes are more limited in range as compared to the wide range of values obtained with mechanical mixers. Mixing also occurs in a plug-flow regime in static in-line mixers. Mixing times in static mixers are quite short, typically less than 1 s. The actual mixing time will vary with the length of the mixer, which depends on the number of mixing elements used, and the internal volume occupied by the mixing element. Thus, because the nature of the mixing that occurs in static mixers is quite different from that of mechanical mixers, use of the velocity gradient concept is inappropriate for static mixers.
In-line Mixers. In-line mixers are similar to static mixers but contain a rotating mixing element to enhance the mixing process. Typical examples of in-line mixers are illustrated on Fig. 5-12c and d. In the in-line mixer shown on Fig. 5-12c, the power required for mixing is supplied by an external source. For the mixer shown on Fig. 5-12d, the power for mixing is supplied by the energy dissipation caused by the orifice plate and by the power input to the propeller mixer.
High-Speed Induction Mixer. The high-speed induction mixer is an efficient mixing device for a variety of chemicals. A proprietary device, shown on Fig. 5-12e for chlorine mixing, consists of a motor-driven open propeller that creams a vacuum in the chamber directly above the propeller. The vacuum created by the impeller induces the chemical to be mixed directly from the storage container without the need for dilution water. The high operating speed of the impeller (3450 r/min) provides a thorough mixing of the chemical that is being added to the water by the high velocity of the fluid leaving the impeller of the mixing device.
Pressurized Water Jets. Pressurized water jet mixers, such as illustrated on 5-12f, can also be used to mix chemicals. An important design feature of pressurized water jet mixers is that the velocity of the jet containing the chemical to be mixed must be sufficient to achieve mixing in all parts of the pipeline. As shown on Fig. 5-12f, a reactor tube has been added to achieve effective mixing. With pressurized water jet mixers, the power for mixing is provided by an external source (i.e., the solution feed pump).
Turbine and Propeller Mixers. Turbine and propeller mixers are used commonly in wastewater-treatment processes for mixing and blending of chemicals, for keeping material in suspension, and for aeration. Turbine or propeller mixers are usually constructed with a vertical shaft driven by a speed reducer and electric motor. Two types of impellers are used for mixing: (1) radial-flow impellers and (2) axial-flow impellers. Radial-flow impellers generally have flat or curved blades located parallel to the axis of the shaft. The vertical flat-blade turbine impeller is a typical example of a radial-flow impeller. Axial-flow impellers make an angle of less than 90o with the drive shaft. Axial-flow impellers are further classified as variable pitch-constant angle of attack and constant pitch-variable angle of attack. Propellers and hydrofoils are typical examples. Propeller mixers may be provided with more than one set of propeller blades on a shaft. Typical impellers used for mixing are shown on Fig. 5-13, and information on the different types of impellers is presented in Table 5-6.
Rapid mixing in wastewater-treatment processes usually occurs in the regime of turbulent flow in which inertial forces predominate. As a general rule, the higher the velocity and the greater the turbulence, the more efficient the mixing.
Types of Mixers Used for Flocculation in Wastewater Treatment
The principal types of mixers used for flocculation can be classified as (1) static mixers. (2) paddle mixers, and (3) turbine and propeller mixers.
Static Mixers. In the most common type of static mixer, the liquid to be treated is subjected to a series of flow reversals in which the direction of flow is changed. Static mixers can be comprised of over and under narrow flow channels, such as shown on Fig. 5-12a, or the narrow flow channels can be laid out horizontally. The headloss caused by frictional resistance offered by the flow channels and the flow reversals prorides the energy for flocculation, in some designs, the channel spacing is varied to provide a decreasing energy gradient so that the large floc particles formed toward the end of the flocculation basin will not be broken apart.
Fig. 5-13 Typical impellers used for mixing in wastewater treatment facilities: (a)disk-type radial-flow impeller, (b)axial-flow pitched blade impeller, (c)axial-flow hydro-foil-type impeller, (d)propeller mixer
Tab. 5-6 Typical types of mixing impellers used in wastewater treatment
Type of impeller
Flow
Shear
Pumping capacity
Applications
Vertical flat blade turbine
Radial
High
Low
Vertical-flow mixing. Suspension of solids, gas dispersion
Disk turbine
Radial
High
Low
Mixing ,gas dispersion
Surface impeller
Radial
High
Moderate
Gas transfer
Pitch-blade turbine
Axial
Moderate
Moderate
Horizontal flash mixing, suspensions of solids
Low-shear hydrofoil
Axial
Low
High
Horizontal flow flash mixing, suspension of solids, blending, flocculation
Propeller
Axial
Very low
high
Horizontal flow flash mixing, suspension of solids, blending, flocculation
Paddle Mixers. Paddle mixers are used as flocculation devices when coagulants, such as aluminum or ferric sulfate, and coagulant aids, such as polyelectrolytes and lime, are added to wastewater or solids (sludge). Paddle flocculators consist of a series of appropriately spaced paddles mounted on either a horizontal or vertical shaft. Flocculation is promoted by gentle mixing brought about by the slow-moving paddles, which, as shown on Fig, 5-14b, rotate the liquid and promote mixing. Increased particle contact promotes floc growth, but, if the mixing is too vigorous, the increased shear forces will break up the floc into smaller particles. Agitation should be controlled carefully so that the floc particles will be of suitable size and will settle readily. Variable-speed drives are often used to regulate the paddle speed. There bas been a movement away from paddle flocculators to the use of turbine flocculators because of the maintenance problems associated with paddle flocculators.
Turbine-and Propeller-Type Flocculators. The rotating element of turbine-and propeller-type flocculators consists of three or four blades attached to a vertical shaft (see Fig. 5-14c). The flocculator is driven with an external gear reduction system powered by a variable-speed drive. The blades of the propeller may be rectangular in shape or have the shape of a hydrofoil. Blades shaped as hydrofoils are used to limit the amount of floc shearing while at the same time providing the velocity gradients and pumping capacity needed for mixing. In sizing turbine- or propeller-type flocculators, both the power and the pumping requirements must be considered. In addition, the tip speed and the superficial velocity must also be considered.
Types of Mixers Used for Continuous Mixing in Wastewater Treatment
Continuous mixing operations are used ink biological treatment processes such as the activated-sludge process to maintain the mixed liquor suspended solids uniformly mixed state. In biological treatment systems the mixing device is also used to provide the oxygen needed for the process. Thus, the aeration equipment must be able to provide the oxygen needed for the process and the energy needed to maintain mixed conditions within the reactor. Both mechanical aerators and dissolved aeration devices are used. Diffused air is often used to fulfill both the mixing and oxygen requirements. Alternatively, mechanical turbine-aerator mixers may be used. Horizontal, submersible propeller mixers are often used to maintain channel velocities in oxidation ditches, mix the contents of anoxic reactors (see Fig. 5-15), and aid in the destratification of reclaimed water storage reservoirs.
Fig. 5-14 Typical mixer used for flocculation in wastewater treatment: (a)over and under baffled reactor, (b)paddle mixed in baffled tank, (c)turbine mixer in a baffled tank
Pneumatic Mixing. In pneumatic mixing, a gas (usually air or oxygen) is injected into the bottom of mixing or activated-sludge tanks, and the turbulence caused by the rising gas bubbles serves to mix the fluid contents of the tank. In aeration, soft bubbles are formed with an average diameter of 5 mm while the air flow is about 10 percent of the liquid flow. The velocity gradients due to bubble formation range from a Gavg < 200 s-1 to Gmax = 8200 s-1.
Mechanical Aerators and Mixers. The principal types of mechanical aerators used for continuous mixing are high-speed surface aerators and slow-speed surface aerators. Typical power requirements for mixing with mechanical aerators range from 20 to 40 kW/103 m3, depending on the type of mixer and the geometry of the tank, lagoon, or basin.
Fig. 5-15 Submerged propeller mixers used
to mix the contents of an anoxic reactor
.
5-5 Gravity Separation Theory
The removal of suspended and colloidal materials from wastewater by gravity separation is one of the most widely used unit operations in wastewater treatment. A summary of gravitational phenomena is presented in Table 5-7. Sedimentation is the term applied to the separation of suspended particles that are heavier than water, by gravitational settling. The terms sedimentation and settling are used interchangeably. A sedimentation basin may also be referred to as a sedimentation tank, clarifier, settling basin or settling tank. Accelerated gravity settling involves the removal of particles in suspension by gravity settling in an accelerated flow field.
Tab.5-7 Types of gravitational phenomena utilized in wastewater treatment
Description
Sedimentation is used for the removal of grit, TSS in primary settling basins, biological floc removal in the activated-sludge settling basin, and chemical floc removal when the chemical coagulation process is used. Sedimentation is also used for solids concentration in sludge thickeners. In most cases, the primary purpose is to produce a clarified effluent, but it is also necessary to produce sludge with a solids concentration that can be handled and treated easily.
On the basis of the concentration and the tendency of particles to interact, four types of gravitational settling can occur: (1) discrete particle, (2) flocculent, (3) hindered (also called zone), and (4) compression. Because of the fundamental importance of the separation processes in the treatment of wastewater, the analysis of each type of separation process is discussed separately.
Particle Settling Theory
The settling of discrete, nonflocculating particles can be analyzed by means of the classic laws of sedimentation formed by Newton and Stokes. Newton's law yields the terminal particle velocity by equating the gravitational force of the particle to the frictional resistance, or drag. The gravitational force is given by
FG=(ρp-ρw)gVp
WhereFG =gravitational force, MLT-2(kg·m/s2)
ρp =density of particles, MLT-3(kg /m3)
ρw =density of water, MLT-3(kg /m3)
g=acceleration due to gravity, LT-2(9.18m/s2)
Vp=volume of particles, L3(m3)
Settling in the Laminar Region. For Reynolds numbers less than about 1.0, viscosity is the predominant force governing the settling process, and the first term in the above equation predominates. Assuming spherical particles, then yields Stokes' law:
Wherevp =terminal velocity of particles, LT-1(m/s)
dp =diameter of particles, L(m)
sgp =specific gravity of the particle
μ=dynamic viscosity, MTL-2(N·s/m2)
v=kinematic , viscosity, L-2T-1 (m2/s)
Flocculent Particle Settling
Particles in relatively dilute solutions will not act as discrete particles but will coalesce during sedimentation. As coalescence or flocculation occurs, the mass of the particle increases, and it settles faster. The extent to which flocculation occurs is dependent on the opportunity for contact, which varies with overflow rate, depth of the basin, velocity gradients in the system, concentration of particles, and range of particle sizes. The effects of these variables can be determined only by sedimentation tests.
The settling characteristics of a suspension of flocculent particles can be obtained by using a settling column test. Such a column can be of any diameter but should be equal in height to the depth of the proposed tank. The solution containing the suspended matter should be introduced into the column in such a way that a uniform distribution of particle sizes occurs from top to bottom, Care should be taken to ensure that a uniform temperature is maintained throughout the test to eliminate convection currents. Settling should take place under quiescent conditions. The duration of the test should be equivalent to the settling time in the proposed tank.
At the conclusion of the settling time, the settled matter that has accumulated at the bottom of the column is drawn off, the remaining liquid is mixed, and the TSS of the liquid is measured. The TSS of the liquid is then compared to the sample TSS before settling to obtain the percent removal.
The more traditional method of determining settling characteristics of a suspension is to use a column similar to the one described above but with sampling ports inserted at approximately 0.5 m intervals. At various time intervals, samples are withdrawn from the ports and analyzed for suspended solids. The percent removal is computed for each sample analyzed and is plotted as a number against time and depth.
Fig. 5-16 Definition sketch for the idealized settling of discrete particles in three different types of settling basins: (a)rectangle, (b)circular, (c)upflow
Inclined Plate and Tube Settling
Inclined plate and tube settlers are shallow settling devices consisting of stacked offset trays or bundles of small plastic tubes of various geometries (see Fig. 5-17) that are used to enhance the settling characteristics of sedimentation basins. They are based on the theory that settling depends on the settling area rather than detention time. Although they are used predominantly in water-treatment applications, plate and tube settlers are used in wastewater-treatment for primary, secondary, and tertiary sedimentation. In primary sedimentation applications, however, fine screening should be provided ahead of the settling operation to prevent plugging of the plates or tubes.
To be self-cleaning, plate ox tube settlers are usually set at an angle between 45 and 60o above the horizontal. When the angle is increased above 60o, the efficiency decreases. If the plates and tubes are inclined at angles less than 45% solids will tend to accumulate within the plates or tubes. Nominal spacing between plates is 50 mm, with an inclined length of 1 to 2 m. To control biological growths and the production of odors (the principal problems encountered with their use), the accumulated solids must be flushed out periodically (usually with a high-pressure water). The need for flushing poses a problem with the use of plate and tube settlers when the characteristics of the solids to be removed vary from day to day.
The main objective in inclined settler development has been to obtain settling efficiencies close to theoretical limits. Attention must be given to providing equal flow distribution to each settler, producing good flow distribution within each settler, and collecting settled solids while preventing re-suspension.
Fig. 5-16 Plate and tube settlers: (a)module of inclined tubes, (b)tubes installed in a rectangular sedimentation tank, (c)operation, (d)definition sketch for the analysis of settling in a tube settler
Inclined settling systems are generally constructed for use in one of three ways with respect to the direction of liquid flow relative to the direction of particle settlement: (1) countercurrent, (2) co-current, and (3) cross-flow. The flow patterns are shown schematically on Fig. 5-17.
Countercurrent Settling. With countercurrent flow, wastewater suspension in the basin passes upward through the plate or tube modules and exits from the basin above the modules (see Fig. 5-17a). The solids that settle out within the plates or tubes move by gravity countercurrently downward and out of the modules to the basin bottom (see Fig. 5-17c). Tube settlers are mostly used in the countercurrent mode.
A proprietary settler, the Lamella Gravity Settler, manufactured by the Parkson Corporation, is based on countercurrent settling with modifications (see Fig. 5-18). The feed stream is introduced into the settler by means of a feed duct to the feed box, which is a bottomless channel between plate sections. The flow is directed downward toward individual side-entry plate slots. The feed is distributed across the width of the plates and flows upward under laminar flow conditions. The plates are inclined 55o from the horizontal. The solids settle on the plates and the clean supernatant exits the plates through orifice holes. The orifice holes are placed immediately above each plate and are sized to induce a calculated pressure drop to ensure the feed is hydraulically distributed equally among the plates.
The solids slide down the plates into a collection hopper. Further thickening of the solids occurs in the hopper due to compression in the quiescent zone made possible by feeding the plates from the side rather than from the bottom.. Plate packs can also be retrofitted into existing clarifiers to improve performance.
Co-current Settling. In co-current settling, the solids suspension is introduced above the inclined surfaces and the flow is down through the tubes or plates (see Fig. 5-17b). The time for a particle to settle the vertical distance between two surfaces is the same as for countercurrent settling.
Cross-Flow Settling. In cross-flow settling, the liquid flow is horizontal and does not interact with the vertical settling velocity.
5-6 Grit Removal
Removal of grit from wastewater may be accomplished in grit chambers or by the centrifugal separation of solids. Grit chambers are designed to remove grit, consisting of sand, gravel, cinders, or other heavy solid materials that have subsiding velocities or specific gravities substantially greater than those of the organic putrescible solids in wastewater Grit chambers are most commonly located after the bar screens and before the primary sedimentation tanks. Primary sedimentation tanks function for the removal of the heavy organic solids. In some installations, grit chambers precede the screening facilities. Generally, the installation of screening facilities ahead of the grit chambers makes the operation and maintenance of the grit removal facilities easier.
Locating grit chambers ahead of wastewater pumps, when it is desirable to do so, would normally involve placing them at considerable depth at added expense. It is therefore usually deemed more economical to pump the wastewater, including the grit, to grit chambers located at a convenient position ahead of the treatment plant units, recognizing that the pumps may require greater maintenance.
Types of Grit Chambers
Grit chambers are provided to (1) protect moving mechanical equipment from abrasion and accompanying abnormal wear; (2) reduce formation of heavy deposits in pipelines, channels, and conduits; and (3) reduce the frequency of digester cleaning caused by excessive accumulations of grit. The removal of grit is essential ahead of centrifuges, heat exchangers, and high-pressure diaphragm pumps.
There are three general types of grit chambers: horizontal flow, of either a rectangular or a square configuration; aerated; or vortex type. In the horizontal-flow type, the flow passes through the chamber in a horizontal direction and the straight-line velocity of flow is controlled by the dimensions of the unit, an influent distribution gate, and a weir at the effluent end. The aerated type consists of a spiral-flow aeration tank where the spiral velocity is induced and controlled by the tank dimensions and quantity of air supplied to the unit. The vortex type consists of a cylindrical tank in which the flow enters tangentially creating a vortex flow pattern; centrifugal and gravitational forces cause the grit to separate. Design of grit chambers is commonly based on the removal of grit particles having a specific gravity of 2.65 and a wastewater temperature of 15.5oC. However, analysis of grit-removal data indicates the specific gravity
ranges from 1.3 to 2.7.
Horizontal Flow Grit Chambers
Rectangular and square horizontal-flow grit chambers have been used for many years. Their use, however, in new installations has been limited in favor of aerated and vortex-type grit chambers. Representative design data for horizontal-flow grit chambers are presented in Table 5-7.
Tab. 5-7 Typical design information for horizontal-flow chambers
Unit
Range
Typical
Detention time
s
45-90
60
Horizontal velocity
m/s
0.25-0.4
0.3
Settling velocity for removal of :
0.21mm(65-mesh)material
0.15mm(65-mesh)material
m/min
m/min
1.0-1.3
0.6-0.9
1.15
0.75
Headloss in a control section as percent of depth in channel
%
30-40
36
Added length allowance for inlet and outlet turbulence
%
25-50
30
Rectangular Horizontal-Flow Grit Chambers. The oldest type of grit chamber used is the rectangular horizontal-flow, velocity-controlled type. These units were designed to maintain a velocity as close to 0.3 m/s as practical and to provide sufficient time for grit particles to settle to the bottom of the channel. The design velocity will carry most organic particles through the chamber and will tend to resuspend any organic particles that settle but will permit the heavier grit to settle out.
The basis of design of rectangular horizontal-flow grit chambers is that, under the most adverse conditions, the lightest particle of grit will reach the bed of the channel prior to its outlet end. Normally, grit chambers are designed to remove all grit particles that will be retained on a 0.21-mm-diameter (65-mesh) screen, although many chambers have been designed to remove grit particles retained on a 0.15-mm-diameter (100-mesh) screen. The length of channel will be governed by the depth required by the settling velocity and the control section, and the cross-sectional area will be governed by the rate of flow and by the number of channels. Allowance should be made for inlet and outlet turbulence.
Grit removal from horizontal- flow grit chambers is accomplished usually by a conveyor with scrapers, buckets, or plows. Screw conveyors or bucket elevators are used to elevate the removed grit for washing or disposal. In small plants, grit chambers are sometimes cleaned manually.
Square Horizontal-Flow Grit Chambers. Square horizontal-flow grit chambers, such as those shown on Fig. 5-19, have been in use for over 60 years. Influent to the units is distributed over the cross section of the tank by a series of vanes or gates, and the distributed wastewater flows in straight lines across the tank and overflows a weir in a free discharge. Where square grit chambers are used, it is generally advisable to use at least two units. These types of grit chambers are designed on the basis of over-flow rates that are dependent on particle size and the temperature of the wastewater. They are nominally designed to remove 95 percent of the 0.15-mm-diameter (100- mesh) particles at peak flow.
In square grit chambers, the solids are removed by a rotating raking mechanism to a sump at the side of the tank. Settled grit may be moved up an incline by a reciprocating rake mechanism (see Fig. 5-19) or grit may be pumped from the tank through a cyclone degritter to separate the remaining organic material and concentrate grit. The concentrated grit then may be washed again in a classifier using a submerged reciprocating rake or an inclined-screw conveyor. By either method, organic solids are separated from the grit and flow back into the basin, resulting in a cleaner, dryer grit.
Aerated Grit Chambers
In aerated grit chambers, air is introduced along one side of a rectangular tank to create a spiral flow pattern perpendicular to the flow through the tank. The heavier grit particles that have higher settling velocities settle to the bottom of the tank. Lighter, principally organic, particles remain in suspension and pass through the tank. The velocity of roll or agitation governs the size of particles of a given specific gravity that will be removed. If the velocity is too great, grit will be carried out of the chamber; if it is too small, organic material will be removed with the grit. Fortunately, the quantity of air is easily adjusted. With proper adjustment, almost 100 percent removal will be obtained, and the grit will be well washed. Grit that is not well washed and contains organic matter is an odor nuisance and attracts insects.
Fig. 5-20 Typical section through an aerated grit chamber
Aerated grit chambers are nominally designed to remove 0.21-mm-diameter (65-mesh) or larger, with 2- to 5-minute detention periods at the peak hourly rate of flow. The cross section of the tank is similar to that provided for spiral circulation in activated-sludge aeration tanks, except that a grit hopper about 0.9 m deep with steeply sloping sides is located along one side of the tank under the air diffusers (see Fig. 5-20). The air diffusers are located about 0.45 to 0.6 m above the normal plane of the bottom. Influent and effluent baffles are used frequently for hydraulic control and improved gilt-removal effectiveness.
Wastewater will move through the tank in a spiral path (see Fig. 5-21) and will make two to three passes across the bottom of the tank at maximum flow and more passes at lesser flows. Wastewater should be introduced in the direction of the roll. To determine the required headloss through the chamber, the expansion in volume caused by the air must be considered.
For grit removal, aerated grit chambers are often provided with grab buckets traveling on monorails and centered over the grit collection and storage trough (see Fig. 5-22). An added advantage of a grab bucket grit-removal system is that dropping the grit from the bucket through the tank contents can further wash grit.
Fig. 5-21 Helical flow pattern in an aerated grit chamber
Other installation are equipped with chain-and-bucket conveyors, running the full length of the storage troughs, which move the grit to one end of the trough and elevate it above the wastewater level in a continuous operation. Screw conveyors, tubular conveyors, jet pump and airlifts have also been used. Grit-removal equipment for aerated grit chambers is subject to the same wear as experienced in the horizontal-flow units. For large installations, traveling-bridge grit collectors, as shown on Fig. 5-23, can be used. Grit pumps are immersed in the grit chambers and travel the entire length, pumping grit into a stationary grit trough collection system. The pumps can operate continuously or they can be programmed to run on cycles based on time or flow.
Fig. 5-22 Grab bucket used to remove grit from aerated grit chamber
In areas where industrial wastewater is discharged to the collection system, the release of volatile organic compounds (VOCs) by the air agitation in aerated grit chambers needs to be considered. The release of significant amounts of VOCs can be a health risk to the treatment plant operators. Where release of VOCs is an important consideration, covers may be required or nonaerated-type grit chambers used.
Vortex-Type Grit Chambers
Grit is also removed in devices that use a vortex flow pattern. Two types of devices are shown on Fig. 5-24. In one type, illustrated on Fig. 5-24a, wastewater enters and exits tangentially. The rotating turbine maintains constant flow velocity, and its adjustable-pitch blades promote separation of organics from the grit. The action of the propeller produces a toroidal flow path for grit particles. The grit settles by gravity into the hopper in one revolution of the basin's contents. Solids are removed from the hopper by a grit pump or an airlift pump.
Fig. 5-23 Aerated grit chamber with traveling bridge-type grit removal system
Grit removed by a grit pump can be discharged to a hydroclone for removal of the remaining organic material. Grit removed by an airlift may be dawatered on a wedgewire screen.
In the second type, illustrated on Fig. 5-24b, a vortex is generated by the flow entering tangentially at the top of the unit. Effluent exits the center of the top of the unit from a rotating cylinder, or "eye" of the fluid. Centrifugal and gravitational forces within this cylinder minimize the release of particles with densities greater than water. Grit settles by gravity to the bottom of the unit, while organics, including those separated from grit particles by centrifugal forces, exit principally with the effluent. Organics remaining with the settled grit are separated as the grit particles move along the unit floor. Headloss in the unit is a function of the size particle to be removed and increases significantly for very fine particles. Vortex grit-removal units are sized to handle peak flowrates up to 0.3 m3/s per unit. Grit is removed from the unit by a cleated belt conveyor. Because of its overall height, this type of grit system requires a deep basement, or a lift station if it is installed above grade.
Fig. 5-24 Vortex-type grit chambers: (a)Pista; (b)Teacup
Solids (Sludge) Degritting
Where grit chambers are not used and the grit is allowed to settle in the primary settling tanks, grit removal is accomplished by pumping dilute quantities of primary sludge to a cyclone degritter. The cyclone degritter acts as a centrifugal separator in which the heavy particles of grit and solids are separated by the action of a vortex and discharged separately from the lighter particles and the bulk of the liquid. The principal advantage of cyclone degritting is the elimination of the cost of building, operating, and maintaining grit chambers. The disadvantages are (1) pumping of dilute quantities of solids usually requires solids thickeners, and (2) pumping of grit with the liquid primary solids increases the cost of operating and maintaining solids collectors and the primary sludge pumps.
Fig. 5-25 Schematic of grit separation and washing unit
Grit Characteristics, Quantities, Processing, and Disposal
Grit consists of sand, gravel, cinders, or other heavy materials that have specific gravities or settling velocities considerably greater than those of organic particles. In addition to these materials, grit includes eggshells, bone chips, seeds, coffee grounds, and large organic particles.
Characteristics of Grit. Generally, what is removed as grit is predominantly inert and relatively dry material. However, grit composition can be highly variable, with moisture content ranging from 13 to 65 percent, and volatile content from 1 to 56 percent. The specific gravity of clean grit particles reaches 2.7 for inerts but can be as low as 1.3 when substantial organic material is agglomerated with inerts. A bulk dentity of 1600 kg/m3 is commonly used for grit. Often, enough organics are present in the grit so that it quickly putrefies if not properly handled after removal from the wastewater. Grit particles 0.2 mm and larger have been cited as the cause of most downstream problems.
The actual size distribution of retained grit exhibits variation due to differences in collection system characteristics, as well as variations in grit-removal efficiency. Generally, most grit particles are retained on a 0.15-mm (100-mesh) sieve, reaching nearly 100 percent retention in some distances; however, grit can be much finer. In the south eastern United States, where fine sand known as "sugar sand" constitutes a portion of the grit, less than 60 percent of the grit was retained on a 0.15-mm (100-mesh) screen in some cases.
The character of grit normally collected in grit chambers and from cyclone degritters varies widely from what might be normally considered as clean grit, to grit that includes a large proportion of putrescible organic material. Unwashed grit may contain 50 percent or more of organic material, has a distinctly disagreeable odor, and, unless promptly disposed of, may attract insects and rodents.
Quantities of Grit. The quantities of grit will vary greatly from one location to another, depending on the type of sewer system, the characteristics of the drainage area, the condition of the sewers, the frequency of street sanding to counteract icing conditions, the types of industrial wastes, the number of household garbage grinders served, and in areas with sandy soils.
It is difficult to interpret grit-removal data because grit itself is poorly characterized and almost no data exist on relative removal efficiencies. The information on grit characteristics derives from what has been removed as grit. Sieve analyses are not normally performed on grit chamber influents and effluents. For these reasons, the efficiencies of grit-removal systems cannot be compared.
Grit Separation and Washing. Grit separators and grit washers may accomplish removal of a major part of the organic material contained in grit. When some of the heavier organic matter remains with the grit, grit washers are commonly used to provide a second stage of volatile solids separation. Examples of grit separation and washing units are shown on Fig. 5-25.
Two principal types of grit washers are available. One type relies on an inclined submerged rake that provides the necessary agitation for separation of the grit from the organic materials and, at the same time, raises the washed grit to a point of discharge above water level. Another type of grit washer uses an inclined screw and moves the grit up the ramp. Both types can be equipped with water sprays to assist in the cleansing action. Hydroclone separators are often installed at the inlet to the grit washer to improve grit separation and organics removal.
Disposal of Grit. The most common method of grit disposal is transport to a landfill. In some large plants, grit is incinerated with solids. As with screenings, some states require grit to be lime stabilized before disposal in a landfill. Disposal in all cases should be done in conformance with the appropriate environmental regulations.
In larger plants where trucks are used to transfer grit, elevated grit storage facilities may be provided with bottom-loading gates. Difficulties experienced in getting the grit to flow freely from the storage hoppers have been minimized by using steep slopes on the storage hoppers, by applying air beneath the grit, and by the use of hopper vibrators. Drainage facilities for collection and disposal of drippings from the bottom-loading gates are desirable. Grab buckets operating on a monorail system may also be used to load trucks directly from the grit chambers.
Neumatic conveyors are sometimes used to convey grit short distances. Advantages of pneumatic conveying include (1) no elevated storage hoppers are required, and (2) attendant odor problems associated with storage are eliminated. The principal disadvantage is the considerable wear on piping, especially at bends.
5-7 Primary Sedimentation
The objective of treatment by sedimentation is to remove readily settleable solids and floating material and thus reduce the suspended solids content. Primary sedimentation is used as a preliminary step in the further processing of the wastewater. Efficiently designed and operated primary sedimentation tanks should remove from 50 to 70 percent of the suspended solids and from 25 to 40 percent of the BOD.
Sedimentation tanks have also been used as stormwater retention tanks, which are designed to provide a moderate detention period (10 to 30 min) for overflows from either combined sewers or storm sewers. The purpose of sedimentation is to remove substantial portion of the organic solids that otherwise would be discharged directly to the receiving waters. Sedimentation tanks have also been used to provide detention periods sufficient for effective disinfection of such overflows.
Description
Almost all treatment plants use mechanically cleaned sedimentation tanks of standardized circular or rectangular design. The selection of the type of sedimentation unit for a given application is governed by the size of the installation, by rules and regulations of local control authorities, by local site conditions, and by the experience and judgment of the engineer. Two or more tanks should be provided so that the process may remain in operation while one tank is out of service for maintenance and repair work. At large plants, the number of tanks is determined largely by size limitations.
Rectangular Tanks. Rectangular sedimentation tanks may use either chain-and-flight solids collectors or traveling-bridge-type collectors. Equipment for settled solids removal generally consists of a pair of endless conveyor chains, manufactured of alloy steel, cast iron, or thermoplastic. Attached to the chains at approximately 3-m intervals are scraper flights made of wood or fiberglass, extending the full width of the tank or bay. The solids settling in the tank are scraped to solids hoppers in small tanks and to transverse troughs in large tanks. The transverse troughs are equipped with collecting mechanisms (cross collectors), usually either chain-and flight or screw-type collectors, which convey solids to one or more collection hoppers. In very long units (over 50 m), two collection mechanisms can be used to scrape solids to collection points near the middle of the tank length. Where possible, it is desirable to locate solids pumping facilities close to the collection hoppers.
Rectangular tanks may also be cleaned by a bridge-type mechanism that travels up and down the tank on rubber wheels or on rails supported on the sidewalls. One or more scraper blades are suspended from the bridge. Some of the bridge mechanisms are designed so that the scraper blades can be lifted clear of the solids blanket on the return travel.
Where cross collectors are not provided, multiple solids hoppers must be installed. Solids hoppers have operating difficulties, notably solids accumulation on the slopes and in the corners and arching over the solids drawoff piping. Wastewater may also be drawn through the solids hopper, bypassing some of the accumulated solids, resulting in a "rathole" effect. A cross collector is more advisable, except possibly in small plants, because a more uniform and concentrated solids can be withdrawn and many of the problems associated with solids hoppers can be eliminated.
Because flow distribution in rectangular tanks is critical, one of the following inlet designs is used: (1) full-width inlet channels with inlet weirs, (2) inlet channels with submerged ports or orifices, (3) or inlet channels with wide gates and slotted baffles. Inlet weirs, while effective in spreading flow across the tank width, introduce a vertical velocity component into the solids hopper that may resuspend the solids particles. Inlet ports can provide good distribution across the tank width if the velocities are maintained in the 3 to 9 m/min range, inlet baffles are effective in reducing the high initial velocities and distribute flow over the widest possible cross-sectional area. Where full-width baffles are used, they should extend from 150 mm below the surface to 300 mm below the entrance opening.
For installations of multiple rectangular tanks, below-grade pipe and equipment galleries can be constructed integrally with the tank structure and along the influent end. The galleries are used to house the sludge pumps and sludge drawoff piping. The galleries also provide access to the equipment for operation and maintenance. Galleries can also be connected to service tunnels for access to other plant units.
Scum is usually collected at the effluent end of rectangular tanks with the flights returning at the liquid surface. The scum is moved by the flights to a point where it is trapped by baffles before removal. Water sprays can also move the scum. The scum can be scraped manually up an inclined apron, or it can be removed hydraulically or mechanically, and for scum removal a number of means have been developed. For small installations, the most common scum drawoff facility consists of a horizontal slotted pipe that can be rotated by a lever or a screw. Except when drawing scum, the open slot is above the normal tank water level. When drawing scum, the pipe is rotated so that the open slot is submerged just below the water level, permitting the scum accumulation to flow into the pipe. Use of this equipment results in a relatively large volume of scum liquor.
Another method for removing scum is by a transverse rotating helical wiper attached to a shaft. Scum is removed from the water surface and moved over a short inclined apron for discharge to a cross-collecting scum trough. The scum may then be flushed to a scum ejector or hopper ahead of a scum pump. Another method of scum removal consists of a chain-and-flight type of collector that collects the scum at one side of the tank and scrapes it up a short incline for deposit in scum hoppers, whence it can be pumped to disposal units. Scum is also collected by special scum rakes in rectangular tanks that are equipped with the carriage or bridge type of sedimentation tank equipment. In installations where appreciable mounts of scum are collected, the scum hoppers are usually equipped with mixers to provide a homogeneous mixture prior to pumping. Scum is usually disposed of with the solids and biosolids produced al the plant; however, separate scum disposal is used at many plants.
Multiple rectangular tanks require less land area than multiple circular tanks and find application where site space is at a premium. Rectangular tanks also lend themselves to nesting with preaeration tanks and aeration tanks in activated-sludge plants, thus permitting common wall construction and reducing construction costs. They are also used generally where tank roofs or covers are required.
Circular Tanks. In circular tanks the flow pattern is radial (as opposed to horizontal in rectangular tanks). To achieve a radial flow pattern, the wastewater to be settled can be introduced in the center or around the periphery of the tank, as shown on Fig. 5-26. Both flow configurations have proved to be satisfactory generally, although the center-feed type is more commonly used, especially for primary treatment. In the center-feed design (see Fig. 5-26a), the wastewater is transported to the center of the tank in a pipe suspended from the bridge, or encased in concrete beneath the tank floor.
Fig. 5-26 Typical circular primary sedimentation tanks: (a)center feed; (b)peripheral feed
At the center of the tank, the wastewater enters a circular well designed to distribute the flow equally in all directions. The center well has a diameter typically between 15 and 20 percent often total tank diameter and ranges from 1 to 2.5 m in depth and should have a tangential energy-dissipating inlet within the feedwell.
The energy-dissipating device functions to collect influent from the center column and discharge it tangentially into the upper 0.5 to 0.7 m of the feedwell. The discharge ports are sized to produce a velocity of ≤ 0.75 m/s at maximum flow and 0.30 to 0.45 m/s at average flow. The feedwell should be sized so that the maximum downward velocity does not exceed 0.75 m/s. The depth of the feedwell should extend about 1 meter below the energy-dissipating inlet ports.
In the peripheral-feed design (see Fig. 5-26b), a suspended circular baffle forms an annular space into which the inlet wastewater is discharged in a tangential direction. The wastewater flows spirally around the tank and underneath the baffle, and the clarified liquid is skimmed off over weirs on both sides of a centrally located weir trough. Grease and scum are confined to the surface of the annular space. Peripheral feed tanks are used generally for secondary clarification.
Circular tanks 3.6 to 9 m in diameter have the solids-removal equipment supported on beams spanning the tank. Tanks 10.5 m in diameter and larger have a central pier that supports the mechanism and is reached by a walkway or bridge. The bottom of the tank is sloped at about 1 in 12 (vertical: horizontal) to form an inverted cone, and the solids are scraped to a relatively small hopper located near the center of the tank.
Multiple tanks are customarily arranged in groups of two or four. The flow is divided among the tanks by a flow-split structure, commonly located between the tanks. Solids are usually withdrawn by sludge pumps for discharge to the solids processing and disposal units.
Combination Flocculator-Clarifier. Combination flocculator-clarifiers are often used in water treatment and sometimes used for wastewater treatment, especially in cases where enhanced settling, such as for industrial wastewater treatment or for biosolids concentration, is required. Inorganic chemicals or polymers can be added to improve flocculation. Circular clarifiers are ideally suited for incorporation of an inner, cylindrical flocculation compartment (see Fig. 5-27). Wastewater enters through a center shalf or well and flows into the flocculation compartment, which is generally equipped with a paddle type or low-speed mixer. The gentle stirring causes flocculent particles to form. From the flocculation compartment, flow then enters the clarification zone by passing down and radially outward. Settled solids and scum are collected in the same way as in a conventional clarifier.
Fig. 5-27 Typical flocculator clarifier: view of empty tank
Stacked (Multilevel) Clarifiers. Stacked clarifiers originated in Japan in the 1960s where limited land area is available for the construction of wastewater-treatment facilities. Since that time, stacked clarifiers have been used in the United States, the most notable installation of which is at the Deer Island Wastewater Treatment Plant constructed in Boston harbor. Design of these types of clarifiers recognizes the importance of settling area to settling efficiency. Operation of stacked rectangular clarifiers is similar to conventional rectangular clarifiers in terms of influent and effluent flow patterns and solids collection and removal. The stacked clarifiers are actually two (or more) tanks, one located above the other, operating on a common water surface (see Fig. 5-28). Each clarifier is fed independently, resulting in parallel flow through the lower and upper tanks. Settled solids are collected from each tank with chain and flight solids collectors, discharging to a common hopper. In addition to saving space, advantages claimed for stacked clarifiers include less piping and pumping requirements. Because the facilities are more compact and have less exposed surface area, better control of odors and volatile organic compound emissions is possible. Disadvantages include higher construction cost than conventional clarifiers and more complex structural design. Design criteria for stacked clarifiers, as regards overflow and weir rates, are similar to conventional primary and secondary clarifiers.
Sedimentation Tank Performance
The efficiency of sedimentation basins with respect to the removal of BOD and TSS is reduced by (1) eddy currents formed by the inertia of the incoming fluid, (2) wind-induced circulation cells formed in uncovered tanks, (3) thermal convection currents, (4) cold or warm water causing the formation of density currents that move along the bottom of the basin and warm water rising and flowing across the top of the tank, and (5) thermal stratification in hot arid climates.
Short Circuiting and Hydraulic Stability. In an ideal sedimentation basin, a given block of entering water should remain in the basin for the full detention time. Unfortunately, in practice sedimentation basins are seldom ideal and considerable short circuiting will be observed for one or more of the reasons cited above. To determine if short circuiting exists and to what extent, tracer studies should be performed. Time-concentration curves should be developed for analysis. If in the repeated tests the time-concentration curves are similar, then the basin is stable. If the time-concentration curves are not repeatable, the basin is unstable and the performance of the basin will be erratic. The method of influent flow distribution, as discussed above, will also affect short circuiting.
Fig 5-28 Typical section through a stacked clarifier: (a)series flow; (b)parallel flow type
Temperature Effects. Temperature effects can be significant in sedimentation basins. It has been shown that a 1。Celsius temperature differential between the incoming wastewater and the wastewater in the sedimentation tank will cause a density current to form (see Fig. 5-29b and c). The impact of the temperature effects on performance will depend on the material being removed and its characteristics.
Wind Effects. Wind blowing across the top of open sedimentation basins can cause circulation cells to form (see Fig. 5-29d). When circulation cells form, the effective volumetric capacity of the basin is reduced. As with temperature effects, the impact of the reduced volume on performance will depend on the material being removed and its characteristics.
Fig. 5-29 Typical flow patterns observed in rectangular sedimentation: (a)ideal flow; (b)effect of density flow or thermal stratification; (c)effect of thermal stratification; (d)formation of wind-driven circulation cell
Design Considerations
If all solids in wastewater were discrete particles of uniform size, uniform density, uniform specific gravity, and uniform shape, the removal efficiency of these solids would be dependent on the surface area of the tank and time of detention. The depth of the tank would have little influence, provided that horizontal velocities would be maintained below the scouring velocity. However, the solids in most wastewaters are not of such regular character but are heterogeneous in nature, and the conditions under which they are present range from total dispersion to complete flocculation.
Detention Time. The bulk of the finely divided solids reaching primary sedimentation tanks is incompletely flocculated but is susceptible to flocculation. Floccalation is aided by eddying motion of the fluid within the tanks and proceeds through the coalescence of fine particles, at a rate that is a function of their concentration and of the natural ability of the particles to coalesce upon collision. As a general role, coalescence of a suspension of solids becomes more complete as time elapses, thus, detention time is a consideration in the design of sedimentation tanks. The mechanics of flocculation are such, however, that as the time of sedimentation increases, less and less coalescence of remaining particles occurs.
Normally, primary sedimentation tanks are designed to provide 1.5 to 2.5 h of detention based on the average rate of wastewater flow. Tanks that provide shorter detention periods (0.5 to 1 h), with less removal of suspended solids, are sometimes used for preliminary treatment ahead of biological treatment units. In cold climates, increases in water viscosity at lower temperatures retard particle settling in clarifiers and reduce performance at wastewater temperatures below 20oC. Thus, in cold climates, safety factors should be considered in clarifier design to ensure adequate performance.
Surface Loading Rates. Sedimentation tanks are normally designed on the basis of a surface loading rate (commonly termed "overflow rate") expressed as cubic meters per square meter of surface area per day(m3/m2·d). The selection of a suitable loading rate depends on the type of suspension to be separated. Designs for municipal plants must also meet the approval of state regulatory agencies, many of which have adopted standards for surface loading rates that must be followed. When the area of the tank has been established, the detention period in the tank is governed by water depth. Overflow rates in current use result in nominal detention periods of 2.0 to 2.5 h, based on average design flow.
The effect of the surface loading rate and detention time on suspended solids removal varies widely depending on the character of the wastewater, proportion of settleable solids, concentration of solids, and other factors. It should be emphasized that overflow rates must be set low enough to ensure satisfactory performance at peak rates of flow, which may vary from over 3 times the average flow in small plants to 2 times the average flow in large plants.
Weir Loading Rates. In general, weir loading rates have little effect on the efficiency of primary sedimentation tanks and should not be considered when reviewing the appropriateness of clarifier design.
Scour Velocity. To avoid the resuspension (scouring) of settled particles, horizontal velocities through the tank should be kept sufficiently low.
Characteristics and Quantities of Solids (Sludge) and Scum
Scum consists of a variety of floatable materials, and solids concentrations vary widely.
In primary sedimentation tanks used in activated-sludge plants, provision may be required for handling the excess activated sludge that may be discharged into the influent of the primary tanks for settlement and consolidation with the primary sludge. For treatment plants where waste-activated sludge is returned to the primary sedimentation tanks, the primary sedimentation tanks should include provisions for light flocculent solids of 98 to 99.5 percent moisture and for concentrations ranging from 1500 to 10,000 mg/L in the influent mixed liquor.
The volume of solids produced in primary settling tanks must be known or estimated so that these tanks and subsequent solids pumping, processing, and disposal facilities can be properly designed. The solids volume will depend on (1) the characteristics of the untreated wastewater, including strength and freshness; (2) the period of sedimentation and the degree of purification to be effected in the tanks; (3) the condition of the deposited solids, including specific gravity, water content, and changes in volume under the influence of tank depth or mechanical solids-removal devices; and (4) the period between solids-removal operations.
5-8 High-rate Clarification
High-rate clarification employs physical/chemical treatment and utilizes special flocculation and sedimentation systems to achieve rapid settling. The essential elements of high-rate clarification are enhanced particle settling and the use of inclined plate or tube settlers. Advantages of high-rate clarification are (1) units are compact and thus reduce space requirements, (2) start-up times are rapid (usually less than 30 min) to achieve peak efficiency, and (3) a highly clarified effluent is produced.
Fig. 5-30 Schematic microsand ballasted floc particles. Polymer layer is used to absorb chemical flocs onto sand grains.
Enhanced Particle Flocculation
Enhanced particle flocculation has been used in Europe for more than 15 years but has only been introduced relatively recently in the United States. In its most basic form, enhanced particle flocculation involves the addition of an inert ballasting agent (usually silica sand or recycled chemically conditioned sludge) and a polymer to a coagulated and partially flocculated suspension. The polymer appears to coat the ballasting particles and forms the "glue' that binds the chemical floc to the ballasted particles (see Fig. 5-30). After contact with these particles to grow. The particles grow as the larger, faster-settling particles overtake and collide with slower-settling particles. The velocity gradient G for flocculation is important as a high gradient will cause a breakdown in the floc particles, and insufficient agitation will inhibit floc formation. Velocity gradients for enhanced particle settling of wastewater generally range from 200 to 400 s-1
Analysis of Ballasted Particle Flocculation and Settling
The settling velocity of the ballasted particle is increased when compared to an unballasted floc particle, by (1) increasing the density of the particle, (2) decreasing the coefficient of drag and increasing the Reynolds number, and (3) decreasing the shape factor through the formation of more dense spherical-shaped particles. The ballasted floc particles appear to be more spherical than the floc particles alone. In effect, ballasted flocculent particles settle with a velocity closer to that of a discrete particle than that of flocculent particles that have very high shape factors.
Process Application
Three basic types of process are used for high-rate clarification: (1) ballasted flocculation with Lamella plate clarification, (2) three-stage flocculation with Lamella plate clarification, and (3) dense-solids flocculation/clarification with Lamella plate clarification. Each of these processes can operate at high overflow rates that allow significant reduction in the physical size of the sedimentation units. A summary of the principal features of each process is presented in Table 5-8. Applications for high-rate clarification include (1) providing advanced primary treatment, (2) treating wet-weather flows and combined sewer overflows, (3) treating waste filter backwash water, and (4) treating return flows from solids-processing facilities. Ranges of overflow rates and BOD and TSS removals for treating wet-weather flows (domestic wastewater plus infiltration inflow) are reported in Table 5-9. The processes are illustrated on Fig. 5-31 and are discussed below.
Tab. 5-8 Summary of features of high-rate clarification processes
Process
Features
Microsand ballasted flocculation and clarification
Microsand provides nuclei for floc formation
Floc is dense and settles rapidly
Lamella clarification, when used, provides high-rate settling in a small tank volume
Chemical addition, multistage flocculation, and Lamella clarification
Three-stage flocculation enhances floc formation
Lamella clarification provides high-rate settling in a small tank volume
Two-stage flocculation with chemically conditioned recycled sludge followed by Lamella clarification
Settled sludge solids are recycled to accelerate floc formation
Dense floc is formed that settles rapidly
Lamella clarification provides high-rate settling in a small tank volume
Tab. 5-9 Ranges of overflow rates and BOD and TSS removals from high-rate clarification processes treating wet-weather flows
Fig. 5-31 High-rate clarification processes: (a)ballasted flocculation, (b)Lamella plate clarification, (c)dense-sludge
Ballasted Flocculation. Ballasted flocculation employs a proprietary process, shown on Fig. 5-31a, in which a flocculation aid and a ballasting agent (typically a silica microsand) are used to form dense microfloc particles. The resulting floc particles are thus "ballasted" and settle rapidly. The treatment system consists of three compartments ozones: a mixing zone, maturation zone, and setting zone. Depending on the manufacturer of the process equipment, separate, serial compartments can be used to perform the process functions, or the functions can be combined in a single vessel. Either Lamella plate settling or conventional gravity clarification can be used.
Typically, screened wastewater is introduced to the ballasted flocculation reactor where a chemical coagulant (typically an iron salt) is injected to destabilize the solids. The wastewater then enters a mixing zone where microsand and polymer are injected to maximize the efficiency of flocculation and enhance settling of suspended solids. In the mixing zone, the polymer acts as a bonding agent for adhering the destabilized solids to the microsand. The maturation zone follows and is used to keep the solids in suspension while floc particles continue to develop and grow. Once developed, the ballasted floc particles settle rapidly to the bottom of the clarifier. Sand and floc particles removed from the clarified water are pumped to a cyclone separator (hydroclone) for separation of the sand. The separated sand is returned to the injection tank, and solids from the hydroclone are sent to the biosolids-processing facilities. The microsand usually ranges in size from 100 to 150/μm for treating wastewater and combined wastewater flows and has a specific gravity greater than 2.6 to enhance settling.
Lamella Plate Clarification. Lamella plate clarification uses chemical addition followed by three-stage fluctuation and a Lamella plate clarifier. Coagulant and polymer are injected into the influent wastewater prior to entrance into the flocculation zone. When chemically conditioned wastewater passes through each of the three flocculation zones, the mixing energy gradient is decreased as the wastewater proceeds from one stage to the next. The chemically conditioned/flocculated wastewater then passes to the Lamella clarifier for solids separation. A portion of the clarifier underflow can be recycled to the influent of the process to enhance settling, or the entire underflow can be sent to a thickening tank and the solids-processing facilities.
Dense-Sludge process. The dense sludge system is a proprietary process and differs from ballasted flocculation in that recycled chemically conditioned solids are used to form microfloc particles with the incoming wastewater instead of microsand. As shown on Fig. 5-31c, the influent wastewater enters an air-mixing zone where grit separation occurs and coagulant (usually ferric sulfate) is injected. After mixing, the wastewater flows into the first stage of a two-stage flocculation tank where polymer is added together with chemically conditioned, recirculated solids. Recirculated solids accelerate the flocculation process and ensure the formation of dense, homogeneous floc particles. In the second stage of flocculation, grease and scum begin separating and are removed. Flow from the fluctuation tank enters a presettling zone and then passes into a Lamella plate settler. Most of the suspended flocculated solids are separated directly in the presettling zone; the residual flocculated particles are removed in the Lamella settler. A portion of the settled solids is recirculated, and the remainder is sent to the solids processing and disposal system.
5-9 Large-scale Swirl and Vortex Separators for Combined Wastewater and Stormwater
Solids-separation devices such as swirl concentrators and vortex separators have been used in Europe and, to a lesser extent, in the United States for the treatment of combined sewer overflows (CSOs) and stormwater. These devices are compact solids-separation units with no moving parts. A typical vortex-type CSO solids-separation unit is illustrated on Fig. 5-32. Operation of vortex separators is based on the movement of particles within the unit. Water velocity moves the particles in a swirling action around the separator, additional flow currents move the particles toward the vortex, gravity pulls particles down, and a sweeping action moves heavier particles across the sloping floor toward the central drain.
During wet weather, the outflow from the unit is throttled, causing the unit to fill and to self- induce a swirling vortexlike flow regime. In the device shown on Fig. 5-32, secondary flow currents rapidly separate settleable grit and floatable matter. Concentrated foul matter is intercepted for treatment while the cleaner, treated flow discharges to receiving waters. The device is intended to operate under extremely high flow regimes.
Fig. 5-32 Typical vortex separator used for solids removal from combined sewer overflow
A device more recently developed and termed the continuous deflection separator (CDS) differs from the more traditional vortex separator in that it utilizes a filtration mechanism for solids separation and does not rely on secondary flow currents induced by the vortex action.
Fig. 5-33 Schematic of dissolved-air flotation systems: (a)without recycle in which the entire flow is passed through the pressuring tank; (b)with recycle in which only the recycle flow is pressurized. The pressurized flow is mixed with the influent before being released into the flotation tank
5-10 Flotation
Flotation is a unit operation used to separate solid or liquid particles from a liquid phase. Separation is brought about by introducing fine gas (usually air) bubbles into the liquid phase. The bubbles attach to the particulate matter, and the buoyant force of the combined particle and gas bubbles is great enough to cause the particle to rise to the surface. Particles that have a higher density than the liquid can thus be made to rise. The rising of particles with lower density than the liquid can also be facilitated (e.g., oil suspension in water).
In wastewater treatment, flotation is used principally to remove suspended matter and to concentrate biosolids. The principal advantages of flotation over sedimentation are that very small or light particles that settle slowly can be removed more completely and in a shorter time.
Fig. 5-34 Dispersed-air flotation unit. Air is induced and dispersed into the liquid by pumping action of the inductors
Once the particles have been floated to the surface, they can be collected by a skimming operation.
Description
The present practice of flotation as applied to wastewater treatment is confined to the use of air as the flotation agent. Air bubbles are added or caused to form by (1) injection of air while the liquid is under pressure, followed by release of the pressure (dissolved-air flotation), and (2) aeration at atmospheric pressure (dispersed-air flotation). In these systems, the degree of removal can be enhanced through the use of various chemical additives. In municipal wastewater treatment dissolved-air flotation is frequently used, especially for thickening of waste biosolids.
Dissolved-Air Flotation. In dissolved-air flotation (DAF) systems, air is dissolved in the wastewater under a pressure of several atmospheres, followed by release of the pressure to the atmospheric level. In small pressure systems, the entire flow may be pressurized by means of a pump to 275 to 350 kPa with compressed air added at the pump suction. The entire flow is held in a retention tank under pressure for several minutes to allow time for the air to dissolve. It is then admitted through a pressure-reducing valve to the flotation tank where the air comes out of solution in very fine bubbles.
In the larger units, a portion of the DAF effluent (15 to 120 percent) is recycled, pressurized, and semi-saturated with air. The recycled flow is mixed with the unpressurized main stream just before admission to the flotation tank, with the result that the air comes out of solution in contact with particulate matter at the entrance to the tank. Pressure types of units have been used mainly for the treatment of industrial wastes and for the concentration of solids.
Dispersed-Air Flotation. Dispersed-air (sometimes referred to as induced-air) flotation is seldom used in municipal wastewater treatment, but it is used in industrial applications for the removal of emulsified oil and suspended solids from high-volume waste or process waters. In dispersed-air flotation systems, air bubbles are formed by introducing the gas phase directly into the liquid phase through a revolving impeller. The spinning impeller acts as a pump, forcing fluid through disperser openings and creating a vacuum in the standpipe (see Fig. 5-33). The vacuum pulls air (or gas) into the standpipe and thoroughly mixes it with the liquid. As the gas/liquid mixture travels through the disperser, a mixing force is created that causes the gas to form very fine bubbles, The liquid moves through a series of cells before leaving the unit. Oil particles and suspended solids attach to the bubbles as they rise to the surface. The oil and suspended solids gather in dense froth at the surface and are removed by skimming paddies. The advantages of a dispersed-air flotation system are (1) compact size, (2) lower capital cost, and (3) capacity to remove relatively free oil and suspended solids. The disadvantages of induced-air flotation include higher connected power requirements than the pressurized system, performance is dependent on strict hydraulic control, and less flocculation flexibility. The quantities of float skimmings are significantly higher than the pressurized unit: 3 to 7 percent of the incoming flow as compared to less that 1 percent for dissolved-air systems.
Chemical Additives. Chemicals are commonly used to aid the flotation process. These chemicals, for the most part, function to create a surface or a structure that can easily absorb or entrap air bubbles. Inorganic chemicals, such as the aluminum and ferric salts and activated silica, can be used to bind the particulate matter together and, in so doing, create a structure that can easily entrap air bubbles. Various organic polymers can be used to change the nature of either the air-liquid interface or the solid-liquid interface, or both. These compounds usually collect on the interface to bring about the desired changes.
Design Considerations for Dissolved-Air Flotation Systems
Because flotation is very dependent on the type of surface of the particulate matter, laboratory and pilot-plant tests should be performed to yield the necessary design criteria. Factors that must be considered in the design of flotation traits include the concentration of particulate matter, quantity of air used, the particle-rise velocity, and the solids loading rate.
The performance of a dissolved-air flotation system depends primarily on the ratio of the volume of air to the mass of solids (A/S) required to achieve a given degree of clarification. The ratio will vary with each type of suspension and must be determined experimentally using a laboratory flotation cell. Typical A/S ratios encountered in the thickening of solids and biosolids in wastewater-treatment plants vary from about 0.005 to 0.060.
The required area of the thickener is determined from a consideration of the rise velocity of the solids, 8 to 160 L/m2·min, depending on the solids concentration, degree of thickening to be achieved, and the solids loading rate.
5-11 Oxygen Transfer
Oxygen transfer, the process by which oxygen is transferred from the gaseous to the liquid phase, is a vital part of a number of wastewater-treatment processes. The functioning of aerobic processes, such as activated sludge, biological filtration, and aerobic digestion, depends on the availability of sufficient quantities of oxygen.
Description
The most common application of oxygen transfer is in the biological treatment of wastewater. Because of the low solubility of oxygen and the consequent low rate of oxygen transfer, sufficient oxygen to meet the requirements of aerobic waste treatment does not enter water through normal surface air-water interfaces. To transfer the large quantities of oxygen that are needed, additional interfaces must be formed. Either air or oxygen can be introduced into the liquid, or the liquid in the form of droplets can be exposed to the atmosphere.
Oxygen can be supplied by means of air or pure-oxygen bubbles introduced to the water to create additional gas-water interfaces. In wastewater-treatment plants, submerged-bubble aeration is most frequently accomplished by dispersing air bubbles in the liquid at depths up to 10 m; depths up to 30 m have been used in some European designs. Hydraulic shear devices may also be used to create small bubbles by impinging a flow of liquid at an orifice to break up the air bubbles into smaller sizes. Turbine mixers may be used to disperse air bubbles introduced below the center of the turbine; they are designed both to mix the liquid in the basin and to expose it to the atmosphere in the form of small liquid droplets.
Evaluation of Oxygen Transfer Coefficient
For a given volume of water being aerated, aeration devices are evaluated on the basis of the quantity of oxygen transferred per unit of air introduced to the water for equivalent conditions (temperature and chemical composition of the water, depth at which the air is introduced, etc.).
Oxygen Transfer in Clean Water. The accepted procedure for determining the overall oxygen transfer coefficient in clean water may be outlined as follows. The accepted test method involves the removal of dissolved oxygen (DO) from a known volume of water by the addition of sodium sulfite followed by re-oxygenation to near the saturation level. The DO of the water volume is monitored during the reaeration period by measuring DO concentrations at several different points selected to best represent the contents of the tank. The minimum number of points, their distribution, and range of DO measurements made at each determination point are specified in the procedure.
Oxygen Transfer in Wastewater. In an activated-sludge system, the KLa value can be determined by considering the uptake of oxygen by microorganisms. Typically, oxygen is maintained at a level of 1 to 3 mg/L and the microorganisms use the oxygen as rapidly as it is supplied. Prediction of oxygen transfer rates in aeration systems is nearly always based on an oxygen rate model. The overall oxygen mass transfer coefficient KLa is usually determined in test or full-scale facilities. If pilot-scale facilities are used to determine KLa values, scale-up must be considered. The mass transfer coefficient KLa is also a function of temperature, intensity of mixing and hence of the type of aeration device used and the geometry of the mixing chamber, and constituents in the water. The effects of temperature, mixing intensity, tank geometry, and wastewater characteristics and the application of correction factors are discussed below.
Effects of Mixing Intensity and Tank Geometry. Effects of mixing intensity and tank geometry are difficult to deal with on a theoretical basis but must be considered in the design process because aeration devices are often chosen on the basis of efficiency. Efficiency is strongly related to the KLa value associated with a given aeration unit. In most cases an aeration device is rated for a range of operating conditions using tap water having a low TDS concentration. A correction factor a is used to estimate the KLa value in the actual system. Values of a vary with the type of aeration device, the basin geometry, the degree of mixing, and the wastewater characteristics. Values of a vary from about 0.3 to 1.2. Typical values for diffused and mechanical aeration equipment, are in the range of 0.4 to 0.8 and 0.6 to 1.2, respectively. If the basin geometry in which the aeration device is to be used is significantly different from that used to test the device, great care must be exercised in selecting an appropriate, a value.
Effects of Wastewater Characteristics. The correction factor is used to correct the test system oxygen transfer rate for differences in oxygen solubility due to constituents in the water such as salts, particulates, and surface-active substances.
Values ofβvary from about 0.7 to 0.98. A β value of 0.95 is commonly used for wastewater. Because the determination of βis within the capability of most wastewater treatment plant laboratories, experimental verification of assumed values is recommended.
Application of Correction Factors. The actual amount of oxygen required must be obtained by applying factors to a standard oxygen requirement that reflect the effects of salinity-surface tension (beta factor), temperature, elevation, diffused depth (for diffused aeration systems), the desired oxygen operating level, and the effects of mixing intensity and basin configuration. The fouling factor F is used to account for both internal and external fouling of air diffusers. Internal fouling is caused by impurities in the compressed air, whereas external fouling is caused by the formation of biological slimes and inorganic precipitants. The oxygen necessary for the biological process can be supplied by using air or pure oxygen. Three methods of introducing oxygen to the contents of the aeration tank used commonly are (1) mechanical aeration, (2) injection of diffused air, and (3) injection of high-purity oxygen.
5-12 Aeration Systems
There are several types of aeration systems used for wastewater treatment. The systems used depend on the function to be performed, type and geometry of the reactor, and cost to install and operate the system.
Types of Aeration Systems
The various types of aeration systems used and their applications are described in Table 5-10.
Tab. 5-10 Description of commonly used devices for wastewater aeration
Tab. 5-11 Description of commonly used air diffusion devices
Diffused-Air Aeration
The two basic methods of aerating wastewater are (1) to introduce air or pure oxygen into the wastewater with submerged diffusers or other aeration devices or (2) to agitate the wastewater mechanically so as to promote solution of air from the atmosphere. A diffused-air system consists of diffusers that are submerged in the wastewater, header pipes, air mains, and the blowers and appurtenances through which the air passes.
Diffusers. In the past, the various diffusion devices have been classified as either fine bubble or coarse bubble, with the connotation that fine bubbles were more efficient in transferring oxygen. The definition of terms and the demarcation between fine and coarse bubbles, however, have not been clear, but they continue to be used. The current preference is to categorize the diffused aeration systems by the physical characteristics of the equipment. Three categories are defined: (1) porous or fine-pore diffusers, (2) nonporous diffusers, and (3) other diffusion devices such as jet aerators, aspirating aerators, and U-tube aerators. The various types of diffused-air devices are described in Table 5-11.
Porous Diffusers. Porous diffusers are made in many shapes, the most common being domes, disks, and membranes. Tubes are also used. Plates were once the most popular but are costly to install and difficult to maintain. Porous domes, disks, and membranes have largely supplanted plates in newer installations. Domes, disks, or tube diffusers are mounted on or screwed into air manifolds, which may run the length of the tank close to the bottom and along one or two sides, or short manifold headers may be mounted on movable drop pipes on one side of the tank. Dome and disk diffusers may also be installed in a grid pattern on the bottom of the aeration tank to provide uniform aeration throughout the tank.
Numerous materials have been used in the manufacture of porous diffusers. These materials generally fall into the categories of rigid ceramic and plastic materials and flexible plastic, rubber, or cloth sheaths. The ceramic materials consist of rounded or irregular-shaped mineral particles bonded together to produce a network of interconnecting passageways through which compressed air flows. As the air emerges from the surface pores, pore size, surface tension, and air flowrate interact to produce the bubble size. Porous plastic materials are newer developments. Similar to the ceramic materials, the plastics contain a number of interconnecting channels or pores through which the compressed air can pass. Thin, flexible sheaths made from soft plastic or synthetic rubber have also been developed and adapted to disks and tubes. Air passages are created by punching minute holes in the sheath material. When the air is turned on, the sheath expands and each slot acts as a variable aperture opening; the higher the air flowrate, the greater the opening.
Rectangular panels that use a flexible polyurethane sheet (see Fig. 5-35) are also used in activated-sludge aeration. The panels are constructed with a stainless-steel frame and are placed on or close to the bottom of the tank and anchored. Advantages cited for aeration panels are (1) ultra-fine bubbles are produced that significantly improve oxygen transfer and system energy efficiency, (2) large areas of the tank floor can be covered, which facilitates mixing and oxygen transfer, and (3) foulants can be dislodged by "bumping," i.e., increasing the airflow to flex the membrane. Disadvantages are (1) the panel is a proprietary design and thus lacks competitive bidding, (2) the membrane has a higher headloss, which may affect blower performance in retrofit applications, and (3) increased blower air filtration is required to prevent internal fouling.
With all porous diffusers, it is essential that the air supplied be clean and free of dust particles that might clog the diffusers. Air filters, often consisting of viscous-impingement and dry-barrier types, are commonly used. Precoated bag filters and electrostatic filters have also been used. The filters should be installed on the blower inlet.
Nonporous Diffusers Several types of nonporous diffusers are available (see Fig. 5-36a and b). Nonporous diffusers produce larger bubbles than porous diffusers and consequently have lower aeration efficiency; but the advantages of lower cost, less maintenance, and the absence of stringent air-purity requirements may offset the lower oxygen transfer efficiency and energy cost. Typical system layouts for orifice diffusers closely parallel the layouts for porous dome and disk diffusers; however, single- and dual-roll spiral patterns using narrow- or wide-band diffuser placement are the most common. Applications for orifice and tube diffusers include aerated grit chambers, channel aeration, flocculation basin mixing, aerobic digestion, and industrial waste treatment.
Fig. 5-36 Nonporous diffusers used for the transfer of oxygen: (a)orifice; (b)tube
In the static tube aerator (see Fig. 5-37a), air is introduced at the bottom of a circular tube that can vary in height from 0.5 to 1.25 m. Internally, the tubes are fitted with alternately placed deflection plates to increase the contact of the air with the wastewater. Mixing is accomplished because the tube aerator acts as an airlift pump. Static tubes are normally installed in a grid-type floor coverage pattern.Jet aeration (see Fig. 5-37b and c) combines liquid pumping with air diffusion. The pumping system recirculates liquid in the aeration basin, ejecting it with compressed air through a nozzle assembly. This system is particularly suited for deep (>8 m) tanks. Aspirating aeration (Fig. 5-37d) consists of a motor-driven aspirator pump. The pump draws air in through a hollow tube and injects it underwater where both high velocity and propeller action create turbulence and diffuse the air bubbles. The aspirating device can be mounted on a fixed structure or on pontoons.
Fig. 5-37 Other devices used for the transfer of oxygen : (a)static tube mixer where air is introduced at the base of the aerator that contains mixing elements ,(b)jet reactor in which pressurized air and liquid are combined in a mixing chamber, (c)jet aerator in a manifold arrangement, (d)aspirating aerator
Diffuser Performance. The efficiency of oxygen transfer depends on many factors, including the type, size, and shape of the diffuser; the air flowrate; the depth of submersion; tank geometry including the header and diffuser location; and wastewater characteristics. Aeration devices are conventionally evaluated in clean water and the results adjusted to process operating conditions through widely used conversion factors. Oxygen transfer efficiency (OTE) of porous diffusers may also decrease with use due to internal clogging or exterior fouling. Internal clogging may be due to impurities in the compressed air that have not been removed by the air filters. External fouling may be due to the formation of biological slimes or inorganic precipitants. The rate of fouling will depend on the operating conditions, changes in wastewater characteristics, and the time in service. The fouling rates are important in determining the loss of OTE and the expected frequency of diffuser cleaning. Fouling and the rate of fouling can be estimated by (1) conducting full-scale OTE tests over a period of time, (2) monitoring aeration system efficiency, and (3) conducting OTE tests of fouled and new diffusers.
Factors commonly used to convert the oxygen transfer required for clean water to wastewater are the alpha, beta, and theta factors. The alpha factor, the ratio of the KLa of wastewater to the KLa of clean water, is especially important because alpha factor varies with the physical features of the diffuser system, the geometry of the reactor, and the characteristics of the wastewater. Wastewater constituents may affect porous diffuser oxygen transfer efficiencies to a greater extent than other aeration devices, resulting in lower alpha factors. The presence of constituents such as detergents, dissolved solids, and suspended solids can affect the bubble shape and size and result in diminished oxygen transfer capability. Values of alpha varying from 0.4 to 0.9 have been reported for fine-bubble diffuser systems. Therefore, considerable care must be exercised in the selection of the appropriate alpha factors.
Another measure of the performance of porous diffusers is the combination of the alpha and fouling factors, designated by the term aF in a number of in-process studies, the values of aF have ranged widely, from 0.11 to 0.79 with a mean of < 0.5, and were significantly lower than anticipated. The variability of aF was found to be site-specific, and demonstrated the need for the designer to investigate and evaluate carefully the environmental factors that may affect porous diffuser performance in selecting an appropriate a or aF factor.
Because the amount of air used per kilogram (pound) of BOD removed varies greatly from one plant to another, and there is risk in comparing the air use at different plants, not only because of the factors mentioned above but also because of different loading rates, control criteria, and operating procedures. Extra-high air flowrates applied 'along one side of a tank reduce the efficiency of oxygen transfer and may even reduce the net oxygen transfer by increasing circulating velocities. The result is a shorter residence time of air bubbles as well as larger bubbles with less transfer surface.
Methods of cleaning porous diffusers may consist of retiring of ceramic plates, high-pressure water sprays, brushing, or chemical treatment with acid or caustic baths.
Blowers. There are three types of blowers commonly used for aeration: centrifugal, rotary lobe positive displacement, and inlet guide vane-variable diffuser. Centrifugal blowers are almost universally used where the unit capacity is greater than 425 m3/min of free air. Rated discharge pressures range normally from 48 to 62 kN/m2. Centrifugal blowers have operating characteristics similar to a low-specific-speed centrifugal pump. The discharge pressure rises from shutoff to a maximum at about 50 percent of capacity and then drops off. The operating point of the blower is determined, similar to a centrifugal pump, by the intersection of the head-capacity curve and the system curve.
In wastewater-treatment plants, the blowers must supply a wide range of airflows with a relatively narrow pressure range under varied environmental conditions. A blower usually can only meet one particular set of operating conditions efficiently. Because it is necessary to meet a wide range of airflows and pressures at a wastewater-treatment plant, provisions have to be included in the blower system design to regulate or turn down the blowers. Methods to achieve regulation or turndown are (1) flow blowoff or bypassing, (2) inlet throttling, (3) adjustable discharge diffuser, (4) variable-speed driver, and (5) parallel operation of multiple units. Inlet throttling and an adjustable discharge diffuser are applicable only to centrifugal blowers; variable-speed drivers are more commonly used on positive-displacement blowers. Flow blowoff and bypassing is also an effective method of controlling surging of a centrifugal blower, a phenomenon that occurs when the blower operates alternately at zero capacity and full capacity, resulting in vibration and overheating. Surging occurs when the blower operates in a low volumetric range.
For higher discharge pressure applications (> 55 kN/m2) and for capacities smaller than 425 m3/min of free air per unit, rotary-lobe positive-displacement blowers are commonly used. The positive-displacement blower is a machine of constant capacity with variable pressure. The units cannot be throttled, but capacity control can be obtained by the use of multiple units or a variable-speed drive. Rugged inlet and discharge silencers are essential.
A relatively new blower design, the inlet guide vane-variable diffuser that was developed in Europe, mitigates some of the problems and considerations associated with standard centrifugal and positive-displacement aeration blowers. The design is based on a single-stage centrifugal operation that incorporates actuators to position the inlet guide vane and variable diffusers to vary blower flowrate and optimize efficiency. The blowers are especially well suited to applications with medium to high fluctuations in inlet temperature, discharge pressure, and flowrate. Blower capacities range from 85 to 1700 m3/min at pressures up to 170 kN/m2. Turndown rates of up to 40 percent of maximum capacity are possible without significant reduction in operating efficiency over the range of operation. Principal disadvantages are high initial cost and a sophisticated computer control system to ensure efficient operation.
The performance curve for a centrifugal blower is a plot of pressure versus inlet air volume and resembles the performance curve for a centrifugal pump. The performance curve typically is a falling-head curve where the pressure decreases as the inlet volume increases. Blowers are rated at standard air conditions, defined as a temperature of 20oC, a pressure of 760 mm Hg, and a relative humidity of 36 percent. Standard air has a specific weight of 1.20 kg/m3. The air density affects the performance of a centrifugal blower; any change in the inlet air temperature and barometric pressure will change the density of the compressed air. The greater the gas density, the higher the pressure will rise. As a result, greater power is needed for the compression process
Air Piping. Air piping consists of mains, valves, meters, and other fittings that transport compressed air from the blowers to the air diffusers. Because the pressures are low(less than 70 kN/m2), lightweight piping can be used.
The piping should be sized so that losses in air headers and diffuser manifolds are small in comparison to the losses in the diffusers. Typically, if headlosses in the air piping between the last flow-split device and the farthest diffuser are less than 10 percent of the headloss across the diffusers, good air distribution through the aeration basin can be maintained. Valves and control orifices are an important consideration in piping design Meter losses can be estimated as a fraction of the differential velocity head across the meter, depending on the type of meter.
The discharge pressure at the blowers will be the sum of the above losses, the depth of water over the air diffusers, and the loss through the diffusers.
Because of the high temperature of the air discharged by blowers 60 to 80oC, condensation in the air piping is not a problem, except where piping is submerged in the wastewater. It is essential, however, that provisions be made for pipe expansion and contraction. Where porous diffusers are used, pipes must be made of nonscaling materials or must be lined with material that will not corrode. Pipe materials are often stainless steel, fiberglass, or plastics suitable for higher temperatures. Other materials used include mild steel or cast iron with external coatings (e.g., coaltar epoxy or vinyl). Interior surfaces include cement lining or coal tar or vinyl coatings.
Mechanical Aerators
Mechanical aerators are commonly divided into two groups based on major design and operating features: aerators with vertical axis and aerators with horizontal axis. Both groups are further subdivided into surface and submerged aerators. In surface aerators, oxygen is entrained from the atmosphere; in submerged aerators, oxygen is entrained from the atmosphere and, for some types, from air or pure oxygen introduced in the tank bottom. In either case, the pumping or agitating action of the aerators helps to keep the contents of the aeration tank or basin mixed.
Surface Mechanical Aerators with Vertical Axis. Surface mechanical aerators with a vertical axis are designed to induce either updraft or downdraft flows through a pumping action. Surface aerators consist of submerged or partially submerged impellers that are attached to motors mounted on floats or on fixed structures. The impellers are fabricated from steel, cast iron, noncorrosive alloys, and fiberglass-reinforced plastic and are used to agitate the wastewater vigorously, entraining air in the wastewater and causing a rapid change in the air-water interface to facilitate solution of the air. Surface aerators may be classified according to the type of impeller used: centrifugal, radial-axial, or axial; or the speed of rotation of the impeller: low and high speed. Centrifugal impellers belong to the low-speed category; the axial-flow impeller type aerators operate at high speed. In low-speed aerators, the impeller is driven through a reduction gear by an electric motor. The motor and gearbox are usually mounted on a platform that is supported either by piers extending to the bottom of the tank or by beams that span the tank. Low-speed aerators may also be mounted on floats. In high-speed aerators, the impeller is coupled directly to the rotating element of the electric motor. High-speed aerators are almost always mounted on floats. These units were originally developed for use in ponds or lagoons where the water surface elevation fluctuates, or where a rigid support would be impractical. Surface aerators may be obtained in sizes from 0.75 to 100 kW (1 to 150 hp).
Submerged Mechanical Aerators with Vertical Axis. Most surface mechanical aerators are upflow types that rely on violent agitation of the surface and air entrainment for their efficiency. With submerged mechanical aerators, however, air or pure oxygen may also be introduced by diffusion into the wastewater beneath the impeller or downflow of radial aerators. The impeller is used to disperse the air bubbles and mix the tank contents. A draft tube may be used with either upflow or downflow models to control the flow pattern. The draft tube is a cylinder, usually with flared ends, mounted concentrically with the impeller. The length of the draft tube depends upon the aerator manufacturer. Submerged mechanical aerators may be obtained in sizes from 0.75 to 100 kW (1 to 150 hp).
Mechanical Aerators with Horizontal Axis. Mechanical aerators with horizontal axis are divided into two groups: surface and submerged aerators. The surface aerator is patterned after the original Kessener brush aerator, a device used to provide both aeration and circulation in oxidation ditches. The brush-type aerator had a horizontal cylinder with bristles mounted just above the water surface. The bristles were submerged in the water and the cylinder was rotated rapidly by an electric motor drive, spraying wastewater across the tank, promoting circulation, and entraining air in the wastewater. Angle steel, steel of other shapes, or plastic bars or blades are now used instead of bristles. A typical horizontal-axis surface aerator is shown on Fig. 5-38.
Fig. 5-38
Horizontal-axis aerators: (a)rotary brush(Kessener brush), (b)disk aerator
Submerged horizontal-axis aerators are similar in principle to surface aerators except disks or paddles attached to rotating shafts are used to agitate the water. The disk aerator has been used in numerous applications for channel and oxidation ditch aeration. The disks are submerged in the wastewater for approximately one-eighth to three-eighths of the diameter and enter the water in a continuous, nonpulsating manner. Recesses in the disks introduce entrapped air beneath the surface as the disk turns. Spacing of the disks can vary depending on the oxygen and mixing requirements of the process. Typical power requirements are reported as 0.1 to 0.75 kW/disk.
Mechanical aerators are rated in terms of their oxygen transfer rate expressed as kilograms of oxygen per kilowatt-hour (pounds of oxygen per horsepower-hour) at standard conditions. Standard conditions exist when the temperature is 20oC, the dissolved oxygen is 0.0 mg/L, and the test liquid is tap water. Testing and rating are normally done under non-steady-state conditions using fresh water, deaerated with sodium sulfite. Commercial-size surface aerators range in efficiency from 1.20 to 2.4 kg O2/kW.h. Efficiency claims for aerator performance should be accepted by the design engineer only when they are supported by actual test data for the actual model and size of aerator under consideration.
Energy Requirement for Mixing in Aeration Systems
As with diffused-air systems, the size and shape of the aeration tank are very important if good mixing is to be achieved. Aeration tanks may be square or rectangular and may contain one or more aerators.
In diffused-air systems, the air requirement to ensure good mixing varies from 20 to 30 m3/103 m3.min of tank volume, for a spiral-roll aeration pattern. For a grid system of aeration in which the diffusers are installed uniformly along the aeration basin bottom, mixing rates of 10 to 15 m3/103 m3.min have been suggested). Typical power requirements for maintaining a completely mixed flow regime with mechanical aerators vary from 20 to 40 kW/103 m3, depending on the type and design of the aerator, the nature and concentration of the suspended solids, the temperature, and the geometry of the aeration tank, lagoon, or basin. In the design of aerated lagoons for the treatment of domestic wastewater, it is extremely important that the mixing power requirement be checked because, in most instances, it will be the controlling factor.
Generation and Dissolution of High-Purity Oxygen
After the quantity of oxygen required is determined, it is necessary, where high-purity oxygen is to be used, to specify the type of oxygen generator that will best serve the needs of the plant. There are two basic oxygen generator designs: (1) a pressure swing adsorption (PSA) system for smaller and more common plant sizes (less than 150,000 m3/d), and (2) the traditional cryogenic air-separation process for large applications. Liquid oxygen can 'also be tracked in and stored onsite in a liquid form. Pressure-Swing Adsorption. The pressure-swing adsorption system uses a multibed adsorption process to provide a continuous flow of oxygen gas. The operating principle of the pressure-swing adsorption generator is that the oxygen is separated from the feed air by adsorption at high pressure, and the adsorbent is regenerated by "blowdown" to low pressure. The process operates on a repeated cycle having two basic steps, adsorption and regeneration. During the adsorption step, feed air flows through one of the adsorber vessels until the adsorbent is partially loaded with impurity. At that time the feed-air flow is switched to another adsorber, and the first adsorber is regenerated. During regeneration, the impurities are cleaned from the adsorbent so that the bed will be available again for the adsorption step. Regeneration is carried out by depressurizing to atmospheric pressure, purging with some of the oxygen, and repressurizing back to the pressure of the feed air.
Cryogenic Air Separation. The cryogenic air separation process involves the liquefaction of air, followed by fractional distillation to separate it into its components (mainly nitrogen and oxygen). First, the entering air is filtered and compressed, and then it is fed to the reversing heat exchangers, which perform the dual function of cooling and removing the water vapor and carbon dioxide by freezing these mixtures out into the exchanger surfaces. Periodically switching or reversing the feed air and the waste nitrogen streams through identical passes of the exchangers to regenerate their water vapor and carbon dioxide removal capacity accomplishes this process.
Next, the air is processed through "cold and gel traps," which are adsorbent beds that remove the final traces of carbon dioxide as well as most hydrocarbons from the feed air. The processed air is then divided into two streams. The first stream is fed directly to the lower column of the distillation unit. The second stream is returned to the reversing heat exchangers and partially warmed to provide the required temperature difference across the exchanger. This stream is then passed through an expansion turbine and fed into the upper column of the distillation unit. An oxygen-rich liquid exits from the bottom of the lower column and the liquid nitrogen exits from the top. Both streams are then subcooled and transferred to the upper column. In this column, the descending liquid phase becomes progressively richer in oxygen, and the liquid that subsequently collects in the condenser reboiler is the oxygen product stream. This oxygen is recirculated continually through an adsorption trap to remove all possible residual traces of hydrocarbons. The waste nitrogen exits from the top portion of the upper column and is heat exchanged along with the oxygen product to recover all available refrigeration and to regenerate the reversing heat exchangers.
Dissolution of Commercial Oxygen. Oxygen is very insoluble in water--even pure oxygen--and requires special considerations to ensure high absorption efficiency. Oxygen dissolution equipment designed for air only optimizes energy consumption because the air is free and efficient oxygen absorption is not relevant. However, because of the cost of commercial oxygen, the facilities used for its dissolution must be designed to both efficiently absorb the commercial oxygen as well as minimize the unit energy consumption. These requirements rule out the more common aeration equipment alternatives.
Dissolution Time. A key feature that must be incorporated into a commercial oxygen dissolution system is oxygen retention time. To optimize the absorption of pure oxygen it has been found that a detention time of about 100 s is required. Further, two-phase flow must be maintained to avoid the coalescence of the oxygen bubbles to maintain absorption efficiency. Unfortunately, some pure oxygen dissolution systems consume as much energy to dissolve a ton of pure oxygen as standard surface aerators consume in dissolving a ton of oxygen from air.
| 课件名称： | 环境工程概论（一） |
| 课件分类： | 环境 |
| 课件类型： | 参考资料 |
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