Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual
particles that are generally invisible to the naked eye, similar to smoke in air.
The measurement of turbidity is 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, 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 is also applied to transparent 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.
Causes Turbidity in open water may be caused by growth
of phytoplankton.
Human activities that disturb land, such as construction, mining and agriculture, can
lead to high sediment levels entering water bodies during rain storms due to storm water
runoff.
Areas prone to high bank erosion rates as well as urbanized areas also contribute large
amounts of turbidity to nearby waters, through stormwater pollution from paved surfaces such
as roads, bridges and parking lots.
Certain industries such as quarrying, mining and coal recovery can generate very high levels
of turbidity from colloidal rock particles.
In drinking water, the higher the turbidity level, the higher the risk that people may
develop gastrointestinal diseases.
This is especially problematic for immunocompromised people, because contaminants like viruses
or bacteria can become attached to the suspended solids.
The suspended solids interfere with water disinfection with chlorine because the particles
act as shields for the virus and bacteria.
Similarly, suspended solids can protect bacteria from ultraviolet sterilization of water.
In water bodies such as lakes, rivers and reservoirs, high turbidity levels can reduce
the amount of light reaching lower depths, which can inhibit growth of submerged aquatic
plants and consequently affect species which are dependent on them, such as fish and shellfish.
High turbidity levels can also affect the ability of fish gills to absorb dissolved
oxygen.
This phenomenon has been regularly observed throughout the Chesapeake Bay in the eastern
United States.
For many mangrove areas, high turbidity is needed to support certain species, such as
to protect juvenile fish from predators.
For most mangroves along the eastern coast of Australia, in particular Moreton Bay, turbidity
levels as high as 600 Nephelometric Turbidity Units are needed for proper ecosystem health.
Measurement
The most widely used measurement unit for turbidity is the FTU.
ISO refers to its units as FNU.
There are several practical ways of checking water quality, the most direct being some
measure of attenuation of light as it passes through a sample column of water.
The alternatively used Jackson Candle method 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 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 in JTUs.
The propensity of particles to scatter a light beam focused on them is now considered a more
meaningful measure of turbidity in water.
Turbidity measured this way uses an instrument called a nephelometer with the detector set
up 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.
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, a correlation between turbidity and total suspended solids 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 is then recorded as a measure of the transparency of the water.
The Secchi disk has the advantages of integrating turbidity over depth, 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 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 allows determination of turbidity
based on a contrast disk in its bottom, being analogous 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 Linke turbidity factor.
Standards and test methods
Drinking water standards Governments have set standards on the allowable
turbidity in drinking water.
In the United States, systems that use conventional or direct filtration methods turbidity cannot
be higher than 1.0 nephelometric turbidity units at the plant outlet and all samples
for turbidity must be less than or equal to 0.3 NTU for at least 95 percent of the samples
in any month.
Systems that use filtration other than the conventional or direct filtration must follow
state limits, which must include turbidity at no time exceeding 5 NTU.
Many drinking water utilities strive to achieve levels as low as 0.1 NTU.
Ambient water standards The U.S. Environmental Protection Agency has
published water quality criteria for turbidity.
These criteria are scientific assessments of the effects of turbidity, which are used
by states to develop water quality standards for water bodies.
Some states have promulgated water quality standards for turbidity, including:
Louisiana.
25, 50 or 150 NTU, or background plus 10 percent, depending on the water body.
Vermont.
10 NTU or 25 NTU, depending on water body classification.
Washington.
5 NTU over background, or 10 percent increase when background is over 50 NTU.
Analytical methods Published analytical test methods for turbidity
include: ISO 7027 "Water Quality: Determination of
Turbidity" US EPA Method No.
180.1, "Turbidity" "Standard Methods," No. 2130B.
Treatment Turbidity is commonly treated using either
a settling or filtration process.
Depending on the application, chemical reagents, will be dosed into the wastewater stream to
increase the effectiveness of the settling or filtration process.
Potable water treatment and municipal wastewater plants often remove turbidity with a combination
of sand filtration, settling tanks and clarifiers.
In-situ water treatment or direct dosing for the treatment of turbidity is common when
the affected water bodies are dispersed, when the problem is not consistent or when a low
cost solution is required.
In-situ treatment of turbidity involves the addition of a reagent, generally a flocculant,
evenly dispensed over the surface of the body of water.
The flocs then settle at the bottom of the water body where they remain or are removed
when the water body is drained.
This method is commonly used at coal mines and coal loading facilities where stormwater
collection ponds have seasonal issues with turbidity.
A number of companies offer portable treatment systems for in-situ water treatment or direct
dosing of reagents.
Reagents There are a number of chemical reagents that
are available for treating turbidity.
Reagents available for treating turbidity include aluminium sulfate or alum3.xH2O),
ferric chloride, gypsum, poly-aluminium chloride, long chain acrylamide-based polymers and numerous
proprietary reagents.
The water chemistry must be carefully considered when chemical dosing as some reagents, such
as alum, will alter the pH of the water.
The dosing process must also be considered when using reagents as the flocs may be broken
apart by excessive mixing.
See also
Transparency and translucency Bottom trawling
McFarland standard Stormwater
Total suspended solids Water purification
In-situ water treatment References
External links Water on the Web: "Turbidity"
"Understanding Turbidity."
United States Geological Survey USGS field manual on measuring turbidity
Water Quality - Stormwater, Washington State Water Quality - Stormwater, Construction,
Washington State