The F.B. Leopold Company Inc.

The Mechanisms of Solids Removal in a Granular Filter

There are two basic mechanisms of solids removal in a granular filter: straining and non-straining.

Straining
The pores between media are commonly 7 to 10 percent as large as the grains. Particles larger than the pores will be deposited on the media and removed or "strained" out of the flow of water. Some particles smaller than the pores will be deposited by interception and sedimentation. However, most smaller particles such as giardia cyst, cryptosporidium, silt, color, and other colloidal matter will pass through a filter.

Non-Straining
Colloidal dispersions are discrete particles separated by the dispersion medium. They are too small and light to settle out with time. They are larger than individual atoms or molecules but are small enough to possess properties greatly different from coarse dispersions. Colloidal particles usually range in size from about 1 to 100 millimicrons.

Colloidal particles, such as color and silt, and micro-organisms, such as cryptosporidium and giardia cyst, are too small to filter out by normal straining mechanisms. These smaller particles must be coagulated and conditioned properly in order to filter them out.

The removal mechanisms for very small particles are interception, sedimentation, and brownian diffusion. Interception and sedimentation usually predominate with particles larger than 1 to 10 microns. Brownian diffusion usually controls for smaller particles.

Interception
Interception occurs when the stream of water brings the particles in contact with a grain of media. The particle then either sticks to or becomes caught between two grains of media.

Sedimentation
Some particles may be heavy enough that its settling velocity is greater than the velocity of water between the media grains. In that case, the particle will settle out between the media grains and be removed from the stream of water.

Brownian Diffusion
The energy of thc molecules in a dispersion medium is high enough for them to bombard the colloids and make them move about in a random manner. The colloids will eventually contact a media grain. The removal of colloids and very small particles in this manner depend on the proper chemical treatment. The colloids must be destabilized to allow them to attach themselves or "stick to" the media grains. Coagulants and filter aid polymers are used for that purpose.