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TURBIDITY
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WHAT IS TURBIDITY?
Turbidity is an aggregate property of the solution, which is water in most
cases. Turbidity is not specifi c to the types of particles in the water.
The particles could be suspended or colloidal matter, and they can
be inorganic, organic or biological. At high concentrations, turbidity
is perceived as cloudiness, haze or an absence of clarity in the water.
Turbidity is an optical property that results when light passing through
a liquid sample is scattered. The scattering of light results in a change
in the direction of the light passing through the liquid. This is most
often caused when the light strikes particles in solution and is scattered
backward, sideways and forward. If the turbidity is low, much of the light
will continue in the original direction. Light scattered by the particles
allows the particle to be ”seen” or detected in solution just as sunlight
allows dust particles in the air to be seen.
In the past 10 years, turbidity has become more than just a measure
of water clarity. Because of the emergence of pathogens such as
Cryptosporidium and Giardia, turbidity now holds the key to assuring
proper water fi ltration. In 1998, the EPA published the IESWTR (interim
enhanced surface water treatment rule) mandating turbidities in
combined fi lter effl uent to read at or below 0.3 NTU. By doing so, the
EPA hoped to achieve a 2 log (99%) removal of Cryptosporidium. There
is presently consideration to lower this to 0.1 NTU. The trend has been
to check the calibration of on-line turbidimeters with hand-held fi eld
units. The optical design and low detection limit of the 2020we/wi allows
very accurate readings for such calibrations.
The meter also allows the user to choose the units of measure for
expressing turbidity. While nephelometric turbidity unit (NTU) has
been the standard for years, FNU (formazin nephelometric unit) and
FAU (formazin attenuation unit) are now being used in ISO 7027 units.
American Society of Brewing Chemists (ASBC) units and European
Brewery Convention (EBC) units allow the brewing industry to check
process waters.
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HOW IS TURBIDITY MEASURED?
Turbidity is measured by detecting and quantifying the scattering of light
in water (solution). Turbidity can be measured in many ways. There are
visual methods and instrumental methods. Visual methods are more
suitable for samples with high turbidity. Instrumental methods can be
used on samples with both high and low levels of turbidity.
Two visual methods are the Secchi Disk method and the Jackson
Candle method. The Secchi Disk method is often used in natural waters.
A black and white Secchi Disk is lowered into the water until it can no
longer be seen. It is then raised until it can be seen again. The average
of these two distances is known as the “Secchi Depth”. The Jackson
Candle method uses a long glass tube over a standard candle. Water
is added or removed from the tube until the candle fl ame becomes
indistinct. The depth of the water measured with a calibrated scale is
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