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Turbidity: units, methods, examples

Many of these particles originate from the activity carried out by man in construction, mining, as well as in agriculture. This generates particles that are transported to rivers, lakes and reservoirs that are the main sources of water consumed by populations.

Glasses of water with different degrees of turbidity

Turbidity has serious ecological consequences. For example, suspended particles absorb heat from solar radiation, which brings about an increase in temperature, and also a reduction in the oxygen concentration of the water.

On the other hand, suspended particles scatter sunlight, limiting its access to the waterbed. This affects the photosynthetic activity of aquatic plants , and thus reduces the presence and development of aquatic fauna.

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Turbidity units

The turbidity of this channel prevents its earthy bottom from being seen. Source: Sebastian Ballard / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0)

There are different units to measure turbidity:

FTU

The turbidity measurement units, with the exception of the Jackson method unit, have been elaborated on the basis of formazin.

The turbidity pattern of formazin is formed by the combination of hexamethylene-tetramine (C 6 H 12 N 4 ) with hydrazine sulfate (N 2 H 6 SO 4 ).

The FTU (Formazin Turbidity Units) unit was the most widely used turbidity measurement unit. Subsequently, ISO (International Standardization Organization) suggested using the FNU (Formazin Nephelometric Units) as the turbidity unit, when applying the ISO 7027 (European) turbidity method.

NTU

The NTU (Nephelometric Turbidity Units) unit is most frequently used when the US-EPA method 180.1 or the standard method for the examination of tap water and wastewater is used in the study of turbidity. Formazin is also used for making the NTU unit to express turbidity.

For this purpose, a formazin suspension was created by mixing aqueous solutions of hexamethylenetetramine and hydrazine sulfate in known proportions to form a turbidity pattern of 400 NTU.

Formerly, the Jackson method of the candle was used to measure turbidity, using the JTU unit (Jackson Turbidity Units). Although they use different methods to measure turbidity, the value of the units JTU and NTU has been equated.

So 40 JTU is roughly equal to 40 NTU. Therefore, the equivalences between the values ​​of the units are not necessary. Also, the NTU, FNU, FTU and FAU units are based on the same formazin standards, so the value of each of these units is the same.

A relationship has been established between the NTU units and the mass of the suspended particles: a mass of solutes in suspension of 1 mg / L or ppm is equivalent to 3 NTU. Meanwhile, a mass of 300 mg / L or ppm is equivalent to 900 NTU.

The Jackson method

It is the oldest method for determining the turbidity of water. It consists of the use of a support medium, equipped with a glass material platform, under which a lit candle is placed.

The water tested for turbidity is continuously poured into a cylinder, placed on the glass platform, observing from the top of the cylinder the moment when the candlelight ceases to be seen. The height of the water column in the cylinder is then measured and the result analyzed.

The higher the height of the water column, the lower the turbidity of the water. Turbidity is expressed in JTU units (Jackson Turbidity Units).

Formazin attenuation method

In this method, the amount of light that is transmitted through the suspension to which the turbidity is being determined is measured. In this case, the light emitting lamp, the examined sample and the transmitted light detector form an angle of 180 °.

The method has the difficulty that the turbidity is not always caused by the same type of suspended particles, which is why there is a significant variation in the scattering of light between samples examined for turbidity.

This affects the transmission of light through the particle suspensions examined for their degree of turbidity. Therefore, this method has little use.

In this method, the FAU (Formazin Attenuation Units) unit is used to express the degree of turbidity of a water sample.

Nephelometric method

In the nephelometric method, the light from a lamp or diode falls on a suspension of particles, causing their dispersion. In this method the light detector is positioned in such a way that it forms an angle of 90º in relation to the sample.

Therefore, only the light scattered in that direction is measured. This method has two versions: ISO 7027, followed in Europe, and US EPA 180.1.

ISO 7027 standard

This standard recommends as a light source a diode supplying a wavelength of 860 nM, which corresponds to infrared light. The use of this wavelength has the advantage that it eliminates the interference produced by the coloration of the particle suspension, only its turbidity being measured.

But the sensitivity to detect small particles is low at the 860 nM wavelength. This is a limitation on the use of ISO 7027.

The unit to express the degree of turbidity of the ISO Standard is the FNU (Formazin Nephelometric Units).

US EPA 180.1 standard

Among the differences with the ISO 7027 Standard, is the use of a tungsten lamp as a light source. And also, the light used is white, not infrared. The standard expresses the degree of turbidity using the NTU unit (Nephelometric Turbidity Units).

Secchi disk

Secchi disk. Source: Wikimedia Commons.

It is a method used to estimate turbidity in lakes and water reservoirs. The artifact consists of a disk with a diameter of 20 to 30 cm, divided into four portions: two are painted white, and two are black, the portions of the same color being placed in front of each other (upper image).

The disc has in its central part a ring that serves to tie a string to it, through which its sinking and recovery of the water is guided. The disk sinks into the water until it can no longer be seen, the length of the disk string being used to estimate the turbidity of the lake.

Water turbidity

There is a set of standards in different countries that establish tolerance limits for water turbidity. For example, in the United States at the outlet of water processing plants, which use conventional filtration methods, the turbidity should not exceed 1 NTU unit.

But in any case, the turbidity should not exceed 5 NTU units. Meanwhile, in the European Union the turbidity tolerance limit has been established at 4 NTU units.

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