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Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the
naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to norma ...
, similar to smoke in air. The measurement of turbidity is a key test of
water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
. 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 (the settable 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. Turbidity (or haze) 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 and effects

Turbidity in open water may be caused by growth of
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
. Human activities that disturb land, such as
construction Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and ...
,
mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the econom ...
and
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
, can lead to high
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
levels entering water bodies during rain storms due to storm water
runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock marke ...
. Areas prone to high bank erosion rates as well as urbanized areas also contribute large amounts of turbidity to nearby waters, through stormwater
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, th ...
from
paved Pavement may refer to: * Pavement (architecture), an outdoor floor or superficial surface covering * Road surface, the durable surfacing of roads and walkways ** Asphalt concrete, a common form of road surface * Sidewalk or pavement, a walkway alo ...
surfaces such as roads, bridges, parking lots and airports. Some industries such as quarrying,
mining Mining is the extraction of valuable minerals or other geological materials from the Earth, usually from an ore body, lode, vein, seam, reef, or placer deposit. The exploitation of these deposits for raw material is based on the econom ...
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
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
es or
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
can become attached to the suspended solids. The suspended solids interfere with water disinfection with
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...
because the particles act as shields for the virus and bacteria. Similarly, suspended solids can protect bacteria from ultraviolet (UV) sterilization of water. In water bodies such as
lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
s, rivers and reservoirs, high turbidity levels can reduce the amount of light reaching lower depths, which can inhibit growth of submerged
aquatic plants Aquatic plants are plants that have adapted to living in aquatic environments (saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that g ...
and consequently affect species which are dependent on them, such as
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
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 The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / ...
in the eastern United States. For many mangrove areas, high turbidity is needed in order to support certain species, such as to protect juvenile fish from predators. For most mangroves along the eastern coast of
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
, in particular Moreton Bay, turbidity levels as high as 600 Nephelometric Turbidity Units (NTU) are needed for proper
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
health.


Measurement

There are two standard units for reporting turbidity: Formazin Nephelometric Units (FNU) from ISO 7027 and Nephelometric Turbidity Units (NTU) from USEPA Method 180.1. ISO 7027 and FNU is mostly widely used in Europe, whereas NTU is mostly widely used in the U.S. The ISO 7027 provides the method in water quality for the determination of turbidity. It is used to determine the
concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', ''number concentration'', ...
of suspended particles in a sample of water by measuring the incident light scattered at right angles from the sample. The scattered light is captured by a photodiode, which produces an electronic signal that is converted to a turbidity.
Open source hardware Open-source hardware (OSH) consists of physical artifacts of technology designed and offered by the open-design movement. Both free and open-source software (FOSS) and open-source hardware are created by this open-source culture movement and ...
has been developed following the ISO 7027 method to measure turbidity reliably using an Arduino microcontroller and inexpensive LEDs. There are several practical ways of checking water quality, the most direct being some measure of
attenuation In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at var ...
(that is, reduction in strength) of light as it passes through a sample column of water. The alternatively 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 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 many small particles scattering the source beam than if there are few. The units of turbidity from a calibrated nephelometer can be either NTU or FTU, depending on the standard method used. 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 total suspended solids (TSS) is somewhat unusual for each location or situation. Turbidity in lakes, reservoirs, channels, and the ocean can be measured using a
Secchi disk The Secchi disk (or Secchi disc), as created in 1865 by Angelo Secchi, is a plain white, circular disk in diameter used to measure water transparency or turbidity in bodies of water. The disc is mounted on a pole or line, and lowered slowly down ...
. 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 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.Myre, E, Shaw, R. The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field
"The Turbidity Tube"
!-- https://www2.virginia.edu/blandy/blandy_web/education/Bay/TurbidityTubeConstruction&Use_Myre_Shaw.pdf may be the same -->
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 (TL).


Standards and test methods


Drinking water standards

Governments have set standards on the allowable turbidity in drinking water. In the United States, public water systems that use conventional or direct filtration methods must not have a turbidity higher than 1.0 NTU 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. The European turbidity standard is 4 NTU.


Ambient water standards


United States

The US
Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale ...
(EPA) 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. (States may also publish their own criteria.) Some states have promulgated water quality standards for turbidity, including: *
Louisiana Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is bord ...
. 25, 50 or 150 NTU, or background plus 10 percent, depending on the water body. *
Vermont Vermont () is a U.S. state, state in the northeast New England region of the United States. Vermont is bordered by the states of Massachusetts to the south, New Hampshire to the east, and New York (state), New York to the west, and the Provin ...
. 10 NTU or 25 NTU, depending on water body classification. *
Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...
. 5 NTU over background (when background is 50 NTU or less), 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 Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, ...
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 (i.e. there are numerous water bodies spread out over a geographical area, such as small drinking water reservoirs), when the problem is not consistent (i.e. when there is turbidity in a water body only during and after the wet season) 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 that are available for treating turbidity include aluminium sulfate or alum (Al2(SO4)3·nH2O),
ferric chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
(FeCl3),
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywa ...
(CaSO4·2H2O), poly- aluminium chloride, long chain
acrylamide Acrylamide (or acrylic amide) is an organic compound with the chemical formula CH2=CHC(O)NH2. It is a white odorless solid, soluble in water and several organic solvents. From the chemistry perspective, acrylamide is a vinyl-substituted primar ...
-based
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s and numerous proprietary reagents.Earth Systems, Clear Solutions newsletter - Focus on Turbidit

2003.
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 Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from pr ...
may be broken apart by excessive mixing.


See also

* - reference for microbiological analysis * Ocean optics * *
Water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...


References


External links


Water on the Web: "Turbidity"
(University of Minnesota, Duluth and Lake Superior College)
"Understanding Turbidity."
United States Geological Survey (USGS)




Water Quality - Stormwater, Construction, Washington State
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