Turbidity, or cloudiness, in water is caused by very small particles that remain suspended and tend to “float” because of their very low density. The standard analysis measurement for turbidity is reported in Nephelometric Turbidity Units (NTU), which have superseded the Jackson Turbidity Units (JTU) of measurement formerly used in water analysis. Turbidity in potable water cannot exceed 0.5 NTU, according to current U.S. Environmental Protection Agency (EPA) Primary Drinking Water Standards. A reading of 5.0 NTU triggers a mandatory “boil water” alert in public water systems.
Temporary cloudiness in water, such as may be noticed in a freshly drawn glass, is often caused by excess air. This cloudiness disappears rapidly upon standing. Another cause of cloudiness in a glass of drawn hot water can be extremely fine precipitants created during the heating; this condition generally clears itself quickly. Still another form of cloudiness in water may be the rare case of methane gas (CH4) common in marsh water sources.6
Some turbidity (both organic or inorganic in nature) in surface water will settle out when the water is allowed to stand. One the other hand, a portion of this material may be present as finely divided, colloidal matter that cannot be removed by settling. In general, most turbidity in residential water treatment can be removed by passing the water through a bed (tank) of granular-type media in a sediment filter.
The finer the particle size of a given filter medium, the greater the filter’s ability to remove the particulate. Some turbidity and color in water are composed of such small particles that they slip right through the conventional filter medium.
In commercial applications, removal of these extra-fine species usually requires the help of a chemical feed addition. Often a chemical such as alum (aluminum sulfate) is added in low dosages to the stream of water to neutralize the electrical charge or to destabilize the particles, thereby causing them to adhere to one another and, in turn, form larger particles. These particles are then removed first by settling, then filtration.7
In the home and on the farm where turbidity and sediment are encountered, a more easily maintained and automatic system is needed. The operation of chemical feed devices is generally more than the average homeowner or businessperson wants to be bothered with. Very often a small automatic filter can do the task. Most domestic filters are either porous media-type whole house units or disposable cartridge filters at one or two faucets for drinking water, depending on the amount and type of particulate to be removed. Where softening, demineralization, or reverse osmosis is involved, filtering would be the first (or pretreatment) step so a clear stream of water would be feeding these subsequent treatment modes.