LaMotte 1964-EPA, Manual

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INTRODUCTION
TURBIDITY
What is Turbidity?
Turbidity is an aggregate property of the solution, water in most cases. Turbidity is not
specific to the type of particle in the water. They could be suspended or colloidal
matter and they can be inorganic, organic or biological. At high concentrations
turbidity is perceived as cloudiness or 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 passing through a
window is a scattered by dust particles in the air, allowing them 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 filtration. In 1998, the EPA
published the IESWTR (interim enhanced surface water treatment rule) mandating
turbidities in combined filter effluent 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 field units. The optical design and low detection
limit of the TC-3000 allow 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.
INTRODUCTION 19