Settling Basin Theoretical Design

For any clay-sized or larger particle in suspension in a fluid, the settling rate is a function of the gravitational force (downward) and the frictional resistance (opposite). Because the mass of a particle increases with the cube of the radius, but drag surface area only increases with the square of the radius, larger particles settle more quickly than small particles. Very small particles (such as the colloidal particles in milk), can be kept in solution indefinately by static charges and brownian motion, so settling basins are ineffective at removing these particles. The essence of the design process is to determine a specific residence time, dependent on a particle size removal goal. The sediment removal will include all particles with a velocity > Vc, plus that fraction of the slower (smaller) particles that enter low enough in the column to also settle to the sludge layer before passing out of the basin.


 

In order to theoretically calculate the critical velocity of the smallest consistently removed particle using Stoke's law (for small Reynolds numbers), we must know the particle density, fluid viscosity, and the drag coefficient. In practice, a settling column experiment is often used to determine the settling velocity of the different fractions of a suspension, along with the mass of sediment in each fraction. Details on this theory, additional readings and problems related to sedimentation basin design are available.

The settling basin is sized for smallest particle to be removed, using the following equation:

Q=AVc

    where Q = flowrate
    A = surface area of basin, and
    V    c is the terminal settling velocity of the smallest particle for which settling is desired.

Note that the size of the basin required is a function of area but not depth, so shallow systems are most efficient (and are therefore sometime stacked in municipal and industrial applications).

In practice, we must adjust the resulting Q/A (=Vc ) for the effects of inlet and outlet turbulence, non-uniform fluid flow, and sludge storage.

Most settling basins are designed to achieve a 5 to 10 minute residence time, which should settle 50 to 75% of the solids from open feedlot runoff. At 6 minutes residence time, the nominal size of the smallest consistently separated particule is 35 microns, with a calculated terminal velocity Vc of 0.84 cm/sec (Lorimor, J., 1993, Iowa State Univ.).

An empirical approach to settling basin design is outlined in Iowa State University's Livestock/Environment Home Study Series - Open Feedlot Runoff publication. Practical Notes on Settling Basin Design were developed to complement that approach, and discuss conversion to metric units among other issues.

 

 


isu107Department of Agricultural and Biosystems  Engineering
100 Davidson Hall, Ames, Iowa 50011-3080 USA