Results of tracer tests in rock-plant filters

Most commonly used design procedures for rock-plant filters assume the hydraulic regime to be plug flow. However, an increasing number of investigators now believe this assumption to be an oversimplification. The purpose of this research was to examine the flow pattern through two sets of filters; one set with an L:W ratio equal to 5:1 and the other with a L:W ratio of 1:5. Tracer tests were used to determine mean detention time and dispersion number (D/ul), a measure of departure from plug flow, for each cell. Results showed that the "long" cells (large L:W ratio) exhibited an intermediate amount of dispersion whereas the wide cells (low L:W ratio) exhibited a large amount of dispersion. The flow in the wide cells appeared substantially two-dimensional in nature. The effects of two-dimensional flow are manifested as dispersion when analyzed using one-dimensional equations. Thus, the value of a dispersion number computed from tracer data assuming one-dimensional how is dependent, in part, on the L:W ratio of the cell. In this study the wide cells produced higher dispersion numbers than the long cells even when the detention times were approximately equal.