Understanding Turbidity & Water Filtration

Turbidity, or cloudiness, in water is caused by very small particles that remain suspended and tend to “float” because of their very low density.

Turbidity Standards

The standard analysis measurement for turbidity is reported in Nephelometric Turbidity Units, or NTU, having superseded the Jackson Turbidity Units (JTU) of measurement formerly used in water analysis.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. The World Health Organization guidelines for drinking water suggest a turbidity less than 5 NTU and preferably less than 1 NTU. 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 case of cloudiness in water may be the rare case of methane gas, common in marsh water sources.

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Turbidity & Filtration

Some turbidity (both organic or inorganic in nature) in surface water will settle out when the water is allowed to stand. On 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 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 compesed of such small particles that they slip right through 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, for larger particles. These particles are often removed first be settling, then filtration.

Filtration alone can be easily maintained with an automatic system in most applications, however, in most commercial applications, when softening, demineralization, or reverse osmosis is involved, filtering should be the first (or pretreatment) step so a clear stream of water is able to feed these subsequent treatment modes.

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