Solid-Liquid Separation

Dissolved solids are not visible to the naked eye and cannot be removed from the water by filtration. The standard definition of dissolved solids is material generally smaller than 0.45 microns in size. There are two types of dissolved solids:

• Normally-Soluble Substances: Materials that become more soluble with increasing temperature, such as table salt or sugar. The hotter the water, the more of these materials will dissolve.
• Inversely-Soluble Substances: Normally referred to as hardness ions and generally limited to salts of calcium, magnesium, strontium, and barium in water treatment, these materials become less soluble as temperatures increase, hence their tendency to form scale on the hot surfaces of boiler or heat exchanger tubes. Water treatment would be a simple matter were it not for the inversely-soluble hardness salts.

These are solids not quite small enough to be considered dissolved but not quite large enough to be considered suspended. Generally, colloidal materials will appear as a haze in the water, and distinct particles will not be visible with the naked eye. Colloidal materials are typically within the size range of approximately 0.45–2.0 microns. Colloidal solids do not settle out from the water because they are so small they are greatly affected by their ionic surface charges. Thus, a colloidal suspension in water is said to be a stable suspension.

Color is a type of colloidal suspension. Organic molecules that contribute color to raw surface water are simply macromolecules that fall into the smaller colloidal size range. In water, these macromolecules take on an ionic surface charge that stabilizes them so they cannot settle out.