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 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 Lining the basin and local wind conditions If a floating aerator is used prevent septicity and odor formation. a minimum operating level is needed aerator Typically, the minimum Fig. 5-10 Typical open tpe flow fre equalization basins: ( opical section 1.5 to 2 m. with floating through a lined earthen basin; (shallow concrete basin; (cldeepconcrete basin should be provided below ie aerators to minimize erosion. To prey wind-induced erosion in the upper portions of the to protect the slopes with riprap. soil cement or a partial concrete laver. 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/mof 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 015 m/m,. 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 Operational Appurtenances. Among the appurtenances that should be included in the design of equalization basins are(1)facilities for flushing any solids and grease b and foam cumulate on the may tend o ad on of foam on the sides of the basin and to ai odor control facilities where covered equalization basins must be used. Solids removed from equalization 5-11
5-11 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 Fig. 5-10 Typical open type flow equalization basins: (a)typical section through a lined earthen basin;(b)shallow concrete basin; (c)deep concrete basin Minimum allowable operating level to protect aerator freeboard
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(D) mixing of one tewater liquid suspensions, and (5) heat transfer. Most mixing operations in wastewater can be classified as continuous-rapid (less than 30 s) or continuous (ie, ongoing Continuous Rapid Mixing in Wastewater Treatment Continuous rapid mixing is used. most often. where one substance is to be mixed with another. The ns of continuous ra (e.g. the addition of alum or iron salts prior to flocculation and settling or for dispersing chlorine and ypochlorite 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 Mixing device Typical mixing Application/remarks Static in-line mixers Used for chemicals instantaneous mixing such as alum (Al ferric chlorine(Fe),cat In-line mixers Used for chemicals instantaneous mixing such as alum(AP), ferric chlorine(Fe)cationic polvmer, chlorine High speed induction device Used for chemicals instantaneous mixing such as alum(AP) ferric chlorine(Fe f ). cationi mer. chlorine Pressurized water jets Used in water treatment practice and for reclaimed water Turbine and propeller mixers sed 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 Chemicals to be mixed are introduced in the suction intake of the Other hydraulic mixers I 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 fom aggregates or flocs from finely divided particles and from chemically destabilized particles. Flocculation is a icles that can be removed read ettling or filtration. Although not used routinely, flocculation of stewater 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 Differential gradient setting 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 ofthe
5-12 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
wwo tpes offlocculation:(a)micro-flocculation(due to Brownian motion, perikinetic flocculation); (b)mcaro-flocculation (orthokinetic flocculation)due to fluid shear andor 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 as orthokinetic flocculation) is the term used to refer to the aggregation of particles greater than 1 or 2 Macroflocculation can be brought about by(1) induced velocity gradients and(2) differential settling. Particles can be brought together (ie, 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 lar rticle 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 th colloidal particles reach a size of l or 2 um through contacts produced by Brownian motion. For examp 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 suspension. In biological treatment systems the mixing device is also used to provide the oxvgen needed for the process. Thus, the aeration equipment must be able to provide the oxygen needed for the process mechanical aerators and dissolved aeration devices are used mixing is used to homogenize the contents of the digester to accelerate the biological conversion process. nd to distribute uniformly the heat generated from biological conversion reaction Timescale in Mix 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 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 de hydraulic lumps, 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 Dilute Turbulence created Angled vanes orifice plate and promote turbulence nozzle discharge hemical fee Chemical is injected ( through manifold Concentrated at four points chemical feed Turbulence created by orifice turbine mixers and nozzle discharge Guide vanes nternal mixer
5-13 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
feed induced by pumping action of Chemical feed Flow direction can be perpendicular or parallel to mixer shaft Flow nozzle Fig. 5-12 Typical mixers used in wastewntertreatmentforrapid mixing : (a in ine static mixerwith internal vanes, (b)inLine static mixer with orifice formixing dilute chemicals, (canine mixer, in-line mixerwith intemal mixer,(ejhgh-speed induction mixer Pressurized water jet mixerwith reactor tu 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 g 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 varving from about 12 mm to 3 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 (ie, 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 r for mixing is supplied by the energy dissipation caused by the orifice plate and by the power input to the propeller mixer. ligh-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 ind ical 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
5-14 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