Wind-driven Dead space Dead space Density currents Dead spa (a) Slug input Tracer response of tracer curves for slug input (d) Fig. 4-5 Definition sketch for short circuiting caused by(density currents caused by temperature diferences; (b)wind circulation patterns, (c)inadequate mixing; (d)fluidadvection (shi and dispersion
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
Factors leading to nonideal flow in reactors include 1. Temperature differences. In complete-mix and plug-flow reactors, nonideal flow(short circuiting) can be caused by density currents due to temperature differences. When the water entering the reactor is colder or warmer than the water in the tank, a portion of the water can travel to the outlet along the bottom of or across the top of the reactor without mixing completely(see Fig 4-5a) 2. Wind-driven circulation patterns. In shallow reactors wind-circulation patterns can be set up that will transport a portion of the incoming water to the outlet in a fraction of the actual detention time(see Fig. 4-5b)
Factors leading to nonideal flow in reactors include: 1. Temperature differences. In complete-mix and plug-flow reactors, nonideal flow (short circuiting) can be caused by density currents due to temperature differences. When the water entering the reactor is colder or warmer than the water in the tank, a portion of the water can travel to the outlet along the bottom of or across the top of the reactor without mixing completely (see Fig. 4-5a). 2. Wind-driven circulation patterns. In shallow reactors, wind-circulation patterns can be set up that will transport a portion of the incoming water to the outlet in a fraction of the actual detention time (see Fig. 4-5b)
3. Inadequate mixing. Without sufficient energy input portions of the reactor contents may not mix with the incoming water(see Fig 4-5c) 4. Poor design. Depending on the design of the inlet and outlet of the reactor relative to the reactor aspect ratio dead zones may develop within the reactor that will not mix with the incoming water(see Fig 4-5d) 5. Axial dispersion in plug-flow reactors. In plug-flow reactors the forward movement of the tracer is due to advection and dispersion. Advection is the term used to describe the movement of dissolved or colloidal material with the current velocity. Dispersion is the term used to describe the axial and longitudinal transport of material brought about by velocity differences, turbulent eddies and molecular diffusion
3. Inadequate mixing. Without sufficient energy input, portions of the reactor contents may not mix with the incoming water (see Fig. 4-5c). 4. Poor design. Depending on the design of the inlet and outlet of the reactor relative to the reactor aspect ratio, dead zones may develop within the reactor that will not mix with the incoming water (see Fig. 4-5d). 5. Axial dispersion in plug-flow reactors. In plug-flow reactors the forward movement of the tracer is due to advection and dispersion. Advection is the term used to describe the movement of dissolved or colloidal material with the current velocity. Dispersion is the term used to describe the axial and longitudinal transport of material brought about by velocity differences, turbulent eddies, and molecular diffusion
Need for Tracer Analysis 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
Need for Tracer Analysis 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
()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
(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