Current Team Research Focus - Spring '11
Effect of a Floc-Rollup Phenomenon on Flocs
Introduction
Traditionally, inclined plate and tube settlers are used to create compact sedimentation tanks. Conventional design guidelines are based on obtaining a desired sedimentation design capture velocity. Theoretically, this capture velocity can still be achieved while greatly reducing conventional plate spacing or tube diameter. Yet, the greatest concern with small plate spacing is the danger of settling sludge being swept out with the finished water - the phenomenon known as the floc-rollup. It is the purpose here to estimate the effect of the floc-rollup inside the plate settlers.
Particle Capture by a Lamella
The experimental testing was performed on tube settlers and the design capture velocity of tube settlers in which the ends of the tube are perpendicular to the axis of the tube is given by (Eq 1):
where
L is the length of the tube settlers,
D is the tube diameter,
Vup is the average vertical component of the fluid velocity in the tubes,
and VC is the terminal velocity of the slowest settling particle that is reliably captured by the tube settler.
Eq 1 illustrates that tube settler performance (as manifested by Vc) is maintained as long as the ratio of L/D is constant for a fixed . Thus, it is theoretically possible to reduce L by decreasing the diameter of the tube settlers, D. After a floc settles on the lower surface of a plate or a tube it continues to experience an upward drag caused by the fluid flow. The velocity at the centerline of the floc increases if the spacing between plates or the diameter of the tube is decreased while maintaining a constant average fluid velocity. Gravitational force will cause flocs to roll or slide down the incline while the fluid drag will tend to cause the floc to roll or slide up the incline. When the fluid drag and the gravitational forces balance, the floc remains stationary. This balance point is approximated by determining the point at which the velocity caused by fluid drag at the centerline of the floc is the same as the opposing component of its terminal velocity along the slope.
