TURBIDITY
Envirosensors Pty Ltd is a market leader in the sale and
service of turbidity monitoring instruments. Envirosensors is a wholly Australian
owned company and is the Australian agent for Dr Lange, Sensorex, WTW and
Jenco products. With over 10 years experience, Envirosensors is a recognised
leader in the instrumentation field, offering products that consistently
offer performance and reliabillity. Centrally located in Sydney, Envirosensors
is able to service customers Australia-wide.
LABORATORY & GENERAL PURPOSE
Hand-held meters
Benchtop meters
Accessories
INDUSTRIAL, PROCESS & COMMERCIAL
On-line analysers
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TURBIDITY PRIMER
Turbidity is a cloudiness or haziness of water (or other
fluid) caused by individual particles (suspended solids) that are generally
invisible to the naked eye, thus being much like smoke in air. Turbidity
is generally caused by phytoplankton. Measurement of turbidity is a key
test of water quality. The higher the turbidity, the higher the risk of
the drinkers developing gastrointestinal diseases, especially for immune-compromised
people, because contaminants like virus or bacteria can become attached
to the suspended solid. High water turbidity is the leading cause of gastrointestinal
cancer in the United States.
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 container if a liquid sample
is left to stand (the settleable 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.
Measurement of turbidity
There are several practical ways of quantifying water, 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 now little-used Jackson
Candle method (units: Jackson Turbidity Unit or JTU) is essentially the
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 attentuation, 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 in JTUs.
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 their shape, 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 TSS is somewhat
unique for each location or situation.
Turbidity in lakes, reservoirs, 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 (inversely related 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 provide a rough indication of the depth of the euphotic zone with
a 3-fold multiplication of the Secchi depth. However, this cannot be used
in shallow waters where the disk can still be seen on the bottom.
Standards
There are frequently standards on the allowable turbidity in drinking water.
In the United States (as of 2003) the allowable standard is 1 NTU, with
many drinking water utilities striving to achieve levels as low as 0.1 NTU.
The suspended solid interfere with water disinfection with chlorine because
the particles act as shields for the virus and bacteria. This information
is adapted from articles at wikipedia and is licensed under the GNU Free
Licence Agreement.
