Turbidity: measurement, sensors, sediment, suspended solids, algae, PAR, pollution, NTU, calibration and more!

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What is turbidity?

Turbidity (or 'cloudiness')is the inverse of transparency. To understand what turbidity is, it is illustrative to look at an early 'sensor' for measuring turbidity: the Secchi disk. This black-and-white segmented disk is lowered into the water until it is no longer visible. The depth at which this occurs (the 'secchi depth') is a measure of the transparency of the water. The greater the secchi depth, the higher the transparency and the lower the turbidity.

Although the concept of turbidity applies to fluids in general, this website focuses on the turbidity of water only. More specifically, turbidity is considered in the context of environmental monitoring for rivers, channels, streams, lakes, oceans, harbors and surface waters in general.

Causes of turbidity

In extremely clear (transparent) water, the depth at which the Secchi disk becomes invisible can be over 40 meter. Transparency is reduced by particles in the water. The list of possible particles is practically endless. To name a few major sources of turbidity:

Apart from suspended solids (particles), the turbidity of water may also depend on dissolved substances. For example, a dye may color the water and thereby increase its turbidity.

Importance of turbidity

Turbidity is relevant for the following:

These aspects are further described below. However, first it is important to note that it is not always the absolute turbidity value that is important, but it is the change in turbidity that is often relevant. For example, the same turbidity value that is dangerously high for a coral reef may be totally acceptable (or even exceptionally low) for a fishing pond. In either case it is important to have a longer term reference to establish what is a 'normal' turbidity level for a particular location.

Another reason why 'changes' in turbidity are more relevant than 'absolute' values, is that turbidity is not a very well defined parameter (refer to turbidity measurement methods) and that different measurement methods (instruments) may yield different (absolute) values.

Turbidity and sediment

In some cases, the single most important cause of turbidity is the suspension of sediment. For example, during dredging operations a typical plume is created. Other human activities, or erosion in combination with rain fall, can also cause an increase in suspended sediment. The suspended sediment can be detected and traced by turbidity measurements. These measurements can be used to support the modeling of sediment transport and plume behavior.

Turbidity and PAR

Turbidity reduces the penetration of sunlight in the water. The amount of sunlight (or more specifically the Photosynthetically Active Radiation, or PAR) is relevant for photosynthesis. Thus an increase in turbidity, for a given amount of incident sunlight, decreases PAR.

Turbidity and pollution

Turbidity itself is just an optical property. But combined with knowledge of the local situation, turbidity becomes much more. For example, suppose a pond is known to suffer from occasional algae blooms. In that case, an increase in turbidity indicates the likely onset of an algae bloom. Another example: Suppose silt is known to be polluted with heavy metals or nutrients. And suppose the silt has to be dredged. Then, if at some distance from the dredging site an increase in turbidity is measured, this clearly points to a likely increase in heavy metals or nutrients.

In all cases, the relatively simple and cheap turbidity measurement can be used as an indicator for substances that themselves are more difficult or expensive to measure.

Turbidity measurement methods

Modern turbidity sensors use a LED or other light source to shine a small beam of light through the water and measure the amount of reflected light. Water itself does not reflect light (the turbidity is zero), but particles in the water do reflect light and increase turbidity.

Nephelometer

The most common way to measure turbidity is by using a Nephelometer. This means that the reflected light is measured at a 90 degree angle from the emitted light. This method has been standardized by ISO under the name ISO 7027:1999. The ISO 7027 method prescribes an infra red LED with a wavelength of 860nm. The principle is shown in fig x. The filter passes only the relevant wavelength (860 nm) and blocks most of the ambient light. IR light is highly attenuated in water, hence IR light from the sun penetrates only a few centimeters. Thus sun light will not affect the measurements, if the sensor is submerged for more than, say, 10 cm. Also, the sample volume extends just a few centimeters from the sensor. Obviously, there should be no obstacles within this volume. Nephelometers may also use wide bandwidth light sources like a tungsten lamp. (Not ISO 7027 compliant)

Optical Backscatter

Measuring the reflected light at a 90 degree angle is not the only optical method of measuring turbidity; it is also possible to measure the reflection at 180 degree (light coming the same way back) or at other angles. It is also possible to measure the attenuation (rather that the reflection) at the other side of a sample volume.

Acoustic turbidity measurement

Instead of optical measurements, turbidity can also be measured acoustically. An acoustic instrument emits an ultrasonic sound pulse and measures the reflections. The bigger the reflection, the higher the turbidity. The advantage of this method is that it can cover a large sample volume (a cone of more than 100 meters of length and more than 10 meters wide at the farthest point ). This in contrast to a sample volume of a few centimeters in case of a Nephelometer with an infra-red LED. A disadvantage is that reflection does not only occur on particles, but also on air bubbles, the water surface and even on fish.

Comparing turbidity measurements

It is important to realize that different meters will measure different turbidity values in the same water. This is not only true when comparing meters using different principles (like reflection at 90 or 180 degree, or different light sources, or acoustic versus optical), but also for meters of different make using the same principle. The precise dimensioning of the optics affects the results. This is one of the reasons why the absolute value of turbidity is of limited use, and why changes (detected by the same instrument) are more relevant.

##To be done:More details on non-linearity with increasing turbidity, effect of particle size and reflective properties, color, effect of ambient light, propagation length of IR and on Calibration.

Turbidity versus suspended solids

Suspended solids are usually the main cause of turbidity; hence the measure of 'turbidity' is related to the measure of 'Total Suspended Solids' (TSS). However, turbidity is a purely optical property of the water, whereas TSS is the weight of the suspended solids per liter of water (mg/liter). Obviously there is no way to calculate one from the other in general, because the turbidity does not only depend on the concentration (mg/l), but also on the size and the refractive index of the particles.

Despite the absence of a general relation between turbidity and TSS, a specific relation can be established for a specific situation. The value and accuracy of this method is somewhat disputable. The basic idea is to calculate TSS from turbidity by establishing the relation from grab samples. The turbidity is measured frequently (little effort), and occasionally a grab sample is taken. The grab sample is used to measure TSS (much effort). This allows a relation between turbidity and TSS to be established for that particular location under those particular circumstances.

Units of turbidity

The most common unit of turbidity is the NTU (Nephelometric Turbidity Units) NTU is the turbidity as measured by the Nephelometer as described in Turbidity measurement methods.

To get a feeling of what a certain NTU value actually means, it is illustrative to compare it to the secchi depth, as described in What is turbidity? Remember that the secchi depth is the depth at which the black-white segmented Secchi disk becomes invisible. The conversion between turbidity and water clarity varies, however a rough rule of thumb for South Australian estuaries (developed by the SA EPA) is included below:

NTU Secchi depth (m)
2 10
5 4
10 2
25 0.9
100 0.2

Calibration

##To be done