volume due to short circuiting resulting from temperature differences. the presence of dead zones resulting from poor design. inadequate mixing, and dispersion(see Fig 4-6) can result in reduced treatment rformance. Morrill examined the effects of short circuiting on the performance of sedimentation tanks Fig 4-5 Definition sketch for short circuiting caused by(a)density currents caused by temperature differences;(b)wind circulation patterns;(c)inadequate mixing;( fluid advection(via)and dispersion Need for Tracer Analysis One of the more important practical considerations involved in reactor design is how to achieve the ideal conditions postulated in the analysis of their performance. The use of dves and tracers for measuring the residence time distribution curves is one of the simplest and most successful methods now used to assess ce of full-scal tors.(2)the contact time in chlorine contact basins. 3) the assessment of the hydraulic approach co lIons Ir reactors and (4)the assessment of flow patterns in constructed wetlands and other natural treatment stems.Tracer studies are also of critical importance in assessing the degree of success that has been d with corrective measures Types of Tracers Over the vears. a number of tracers have been used to evaluate the hydraulic performance of reactors. Important characteristics for a tracer include: The tracer should not affect the flow (should have essentially the same densi when v The tracer must be conservative so that a mass balance can be performed. It must be possible to inject the tracer over a short ti The tracer should be able to be analyzed conveniently v The tracer should not be absorbed on or react with the exposed reactor surfaces. v The tracer should not be absorbed on or react with the particles in wastewater. Dves and chemicals that have been used successfully in tracer studies include congo-red fluorescein fluorosilicic acid(H2SiF6), hexafluoride gas( SF6). lithium chloride( LiCD. Pontacyl Brilliant Pink B. tassium, potassium p anate rhodam T, and sodium chloride(NaCD. Pontacyl Brilliant Pink B(the acid form of rhodamine WD) is especially useful in the not readily adsorbed onto surfaces. Because fluorescein. rhodamine WT. and Pontacyl Brilliant Pink B can be detected at very low concentrations using a fluorometer, they are the dye tracers used most commonl in the evaluation of wastewater-treatment facilities. Lithium chloride is commonly used for the study of dium chloride. used commonly in the past. has a tendency to form density currents less mixed Hexafluoride gas(SF6) is used most commonly for tracing the movement of groundwater Conduct of Tracer Tests In tracer studies, typically a tracer (i.e, a dye, most commonly) is introduced into the influent end of the Tracer response reactor or basin to be studied The time of its arrival at the effluent end is determined by collecting a series of grab samples for a given Mixer to period of time or by measuring the arrival of a Uy banks ELuent weir The method used to introduce the tracer willo tracer using instrumental methods(see Fig. 4-6) control the type of response observed at the downstream end. Two types of dye input are used, the choice depending on the influent and effluent configurat Stacked plan view of Fig 4-6 Schematic of setup used to control of tracer using positive curve for continuous nput tracer studies of plug-flow reactor (a)slug of tracers added to flow; (b)continuous input of tracer added to flow. Tracer response curve is measured continuousl The first method involves the injection of a quantity of dye(sometimes referred to as a pulse or slug of dye)over a short period of time. Initial mixing is usually accomplished with a static mixer or an auxiliary mixer. With the slug injection method it is important to keep the initial mixing time short relative to the 4-74-7 volume due to short circuiting resulting from temperature differences, the presence of dead zones resulting from poor design, inadequate mixing, and dispersion (see Fig. 4-6) can result in reduced treatment performance. Morrill examined the effects of short circuiting on the performance of sedimentation tanks. Fig. 4-5 Definition sketch for short circuiting caused by (a)density currents caused by temperature differences; (b)wind circulation patterns; (c)inadequate mixing; (d)fluid advection(平流) and dispersion Need for Tracer Analysis One of the more important practical considerations involved in reactor design is how to achieve the ideal conditions postulated in the analysis of their performance. The use of dyes and tracers for measuring the residence time distribution curves is one of the simplest and most successful methods now used to assess the hydraulic performance of full-scale reactors. Important applications of tracer studies include (1) the assessment of short circuiting in sedimentation tanks and biological reactors, (2) the assessment of the contact time in chlorine contact basins, (3) the assessment of the hydraulic approach conditions in UV reactors, and (4) the assessment of flow patterns in constructed wetlands and other natural treatment systems. Tracer studies are also of critical importance in assessing the degree of success that has been achieved with corrective measures. Types of Tracers Over the years, a number of tracers have been used to evaluate the hydraulic performance of reactors. Important characteristics for a tracer include : ✓ The tracer should not affect the flow (should have essentially the same density as water when diluted). ✓ The tracer must be conservative so that a mass balance can be performed. ✓ It must be possible to inject the tracer over a short time period. ✓ The tracer should be able to be analyzed conveniently. ✓ The molecular diffusivity of the tracer should be low. ✓ The tracer should not be absorbed on or react with the exposed reactor surfaces. ✓ The tracer should not be absorbed on or react with the particles in wastewater. Dyes and chemicals that have been used successfully in tracer studies include congo-red, fluorescein, fluorosilicic acid (H2SiF6), hexafluoride gas (SF6), lithium chloride (LiCl), Pontacyl Brilliant Pink B, potassium, potassium permanganate, rhodamine WT, and sodium chloride (NaCl). Pontacyl Brilliant Pink B (the acid form of rhodamine WT) is especially useful in the conduct of dispersion studies because it is not readily adsorbed onto surfaces. Because fluorescein, rhodamine WT, and Pontacyl Brilliant Pink B can be detected at very low concentrations using a fluorometer , they are the dye tracers used most commonly in the evaluation of wastewater-treatment facilities. Lithium chloride is commonly used for the study of natural systems. Sodium chloride, used commonly in the past, has a tendency to form density currents unless mixed. Hexafluoride gas (SF6) is used most commonly for tracing the movement of groundwater. Conduct of Tracer Tests In tracer studies, typically a tracer (i.e., a dye, most commonly) is introduced into the influent end of the reactor or basin to be studied. The time of its arrival at the effluent end is determined by collecting a series of grab samples for a given period of time or by measuring the arrival of a tracer using instrumental methods (see Fig. 4-6). The method used to introduce the tracer will control the type of response observed at the downstream end. Two types of dye input are used, the choice depending on the influent and effluent configurations. Fig 4-6 Schematic of setup used to control tracer studies of plug-flow reactors (a)slug of tracers added to flow; (b)continuous input of tracer added to flow. Tracer response curve is measured continuously. The first method involves the injection of a quantity of dye (sometimes referred to as a pulse or slug of dye) over a short period of time. Initial mixing is usually accomplished with a static mixer or an auxiliary mixer. With the slug injection method it is important to keep the initial mixing time short relative to the