Effects of Tracer Density on the Hydrodynamics of Two Disinfection Reactors

Tracer studies and fluid dynamic analyses were employed to illustrate the impact of tracer density on reactor hydrodynamics. The studies were performed using sodium chloride solution as the tracer in two typical disinfection reactors – a bench-scale reactor in which the flow of the tracer is in the direction of gravity (vertical column reactor) and a pilot scale reactor in which flow is perpendicular to gravity (horizontal serpentine reactor). In the vertical reactor, the high-density tracer influences the flow field by projecting downward into the background fluid and promoting dispersion. For the vertical reactor, the measured residence time distribution is insensitive to flow rate in the reactor. Above a tracer concentration of 125 mg/L the tracer has a marked effect on the residence time distribution. At and below 125 mg/L there is no apparent effect. In the horizontal serpentine reactor, stratification is pronounced, despite a relatively low (1%) difference in density between the tracer and background fluid. Data analyses indicate that for the horizontal reactor, during washout (when the influent water has lower density than the water in the reactor) at a flow rate of 8 m3/s there are two flow streams – a main flow in which roughly 65% of the fluid flows and a secondary flow in which the remaining 35% flows. Computational fluid dynamics (CFD) analyses identified pronounced stratification in the horizontal reactor and indicate that the tracer density reduces the volume through which the main fluid flow occurs, reduces mixing in the reactor and significantly advances breakthrough. For both the horizontal and vertical reactors, density effects on hydrodynamics must be accounted for when tracer data are used in scale-up or design. In addition, the success of CFD in modeling tracer flow indicate that validated CFD models provide more dependable characterization of reactor hydrodynamics than tracer studies conducted with non-ideal tracers.