10
TURBIDITY
WHAT IS TURBIDITY?
Turbidity is an optical property that results when light passes through a liquid sample
and is scattered by particulate matter. The scattering of light results in a change in the
direction of the light passing through the liquid. If the turbidity is low, the particles may
be invisible to the naked eye and much of the light will continue in the original direction.
As the quantity of particles increases in samples with greater turbidity, the light strikes
particles in solution and is scattered backward, sideways and forward. Light scattered by
the particles allows the particles in the solution to be ”seen” or detected just as sunlight
allows dust particles in the air to be seen. At high concentrations, turbidity is perceived as
cloudiness, haze or an absence of clarity. Turbidity is not specifi c to the types of particles in
the sample. The particles may be suspended or colloidal and can have inorganic, organic or
biological origins.
In drinking water, turbidity may indicate a treatment problem or signal conditions with an
increased risk of gastrointestinal diseases. Because pathogens such as
Cryptosporidium
and
Giardia
cause measurable amounts of turbidity, turbidity monitoring can hold the
key to assuring adequate water fi ltration. In 1998, the EPA published the IESWTR (interim
enhanced surface water treatment rule) mandating turbidities in combined fi lter effl
uent
should 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 used to monitor drinking
water with hand-held fi eld units. The optical design and low detection limit of the 2020t/i
allows very accurate readings for such calibrations. Drinking water that is turbid is not
always harmful to human health but does impart an unpalatable appearance.
Turbidity in environmental waters reduces the amount of benefi cial sunlight that reaches
submerged aquatic vegetation, raises surface water temperature, buries eggs and bottom
dwelling creatures, and can carry sediment and pesticides through the water system.
HOW IS TURBIDITY MEASURED?
Turbidity is measured by detecting and quantifying the scattering of light in a solution. The
amount of light that is scattered is infl uenced by particulate properties of color, shape, size
and refl ectivity. Turbidity can be measured by various methods including visual methods
and instrumental methods. Visual methods are more suitable for samples with high
turbidity. Instrumental methods can be used for samples with turbidity at all levels.
Examples of visual methods are the Secchi Disk method and the Jackson Candle method.
The Secchi Disk method is often used in natural waters. A Secchi Disk with black and white
quadrants 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 measurements is known as the “Secchi
Depth”. The Jackson Candle method uses a long glass calibrated tube placed over a
standardized candle. Water is added or removed from the tube until the candle fl ame
becomes indistinct. The height of the water in the tube is measured with a calibrated scale
and is reported as Jackson Turbidity Units (JTU). The lowest level of turbidity that can
accurately be determined with this method is about 25 JTU.
Instrumental methods for measuring turbidity involve a combination of detection angles
and light sources to optimize accuracy in various samples and to meet regulatory
requirements. The 2020t and 2020i turbidimeters off er the option of three calibration
curves for measuring turbidity that is based on the characteristics of the sample.
In the nephelometric mode, which is the default mode, the detector that is located
90-degrees from the light source measures the scattered light from a light beam passing
through a sample. In the 2020t, this confi guration and the tungsten lamp, with a color
temperature of 2,200–3,000 °K, meet the requirements of EPA method 180.1. The 2020i,
