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 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 Static in-line mixers Used for chemicals instantaneous mixing such as alum (Al In-line mixers Used for chemicals instantaneous mixing such as alum (A High speed induction device Used for chemicals instantaneous mixing such as alum(AP) ferric chlorine(Fe) cationic polymer, 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 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 ofthe5-12 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