Turbidity

Turbidity is the cloudiness or haziness of a fluid caused by
individual particles ( suspended solids ) that are generally invisible
to the naked eye , similar to smoke in air . The measurement of
turbidityis a key test of water quality .
Fluids can contain suspended solid matter consisting of particles of
many different sizes. While some suspended material will be large
enough and heavy enough to settle rapidly to the bottom of the
container if a liquid sample is left to stand (the settable solids ),
very small particles will settle only very slowly or not at all if the
sample is regularly agitated or the particles are colloidal . These
small solid particles cause the liquid to appear turbid.
Turbidity (or haze) is also applied totransparent solids such as glass
or plastic. In plastic production haze is defined as the percentage of
light that is deflected more than 2.5° from the incoming light
direction.
The most widely used measurement unit for turbidity is the FTU (
Formazin Turbidity Unit). ISO refers to its units as FNU (Formazin
Nephelometric Units).
There are several practical ways of checking water quality, the most
direct being some measure of attenuation (that is, reduction in
strength) of light as it passes through a sample column of water. The
alternatively used Jackson Candle method (units: Jackson Turbidity
Unit or JTU ) is essentiallythe inverse measure of the length of a
column of water needed to completely obscure a candle flame viewed
through it. The more water needed (the longer the water column), the
clearer the water. Of course water alone produces some attenuation,
and any substances dissolved in the water that produce color can
attenuate some wavelengths. Modern instruments do not use candles, but
this approach of attenuation of a light beam through a column of water
should be calibrated and reported inJTUs.
A property of the particles — that they will scatter a light beam
focused on them — is considered a more meaningful measure of turbidity
in water. Turbidity measured this way uses an instrument called a
nephelometer with the detector setup to the side of the light beam.
More light reaches the detector if there are lots of small particles
scattering the source beam than if there are few. The units of
turbidity from a calibrated nephelometer are called Nephelometric
Turbidity Units ( NTU ). To some extent, how much light reflects for a
given amount of particulates is dependent upon properties of the
particles like theirshape, color, and reflectivity. For this reason
(and the reason that heavier particles settle quickly and do not
contribute to a turbidity reading), a correlation between turbidity
and total suspended solids (TSS) is somewhat unique for each location
or situation.
Turbidity in lakes, reservoirs, channels, and the ocean can be
measured using a Secchi disk . This black and white disk is lowered
into the water until it can no longer be seen; the depth (Secchi
depth) is then recorded as a measure of the transparency of the water
(inverselyrelated to turbidity). The Secchi disk has the advantages of
integrating turbidity over depth (where variable turbidity layers are
present), being quick and easy to use, and inexpensive. It can
providea rough indication of the depth of the euphotic zone with a
3-fold division of the Secchi depth , however this cannot be used in
shallow waters where the disk can still be seen on the bottom.
An additional device, which may help measuring turbidity in shallow
waters is the Turbidity tube.
The Turbidity tube condenses water in a graded tube which
allowsdetermination of turbidity based on a contrast disk in its
bottom, being analog to the Secchi disk .
Turbidity in air, which causes solar attenuation, is used as a measure
of pollution. To model the attenuation of beam irradiance, several
turbidity parameters have been introduced, including the
Linketurbidity factor (T L ).

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