Turbidity

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Turbidity standards of 5, 50, and 500 NTU TurbidityStandards.jpg
Turbidity standards of 5, 50, and 500 NTU

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.

Haze atmospheric phenomenon in which dust, smoke, and other dry particulates obscure the clarity of the sky

Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates obscure the clarity of the sky. The World Meteorological Organization manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic ash, dust, sand, and snow. Sources for haze particles include farming, traffic, industry, and wildfires.

In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress, or external force. Fluids are a phase of matter and include liquids, gases and plasmas. They are substances with zero shear modulus, or, in simpler terms, substances which cannot resist any shear force applied to them.

In marine and freshwater ecology, a particle is a small object. Particles can remain in suspension in the ocean or freshwater. However, they eventually settle and accumulate as sediment. Some can enter the atmosphere through wave action where they can act as cloud condensation nuclei (CCN). Many organisms filter particles out of the water with unique filtration mechanisms. Particles are often associated with high loads of toxins which attach to the surface. As these toxins are passed up the food chain they accumulate in fatty tissue and become increasingly concentrated in predators. Very little is known about the dynamics of particles, especially when they are re-suspended by dredging. They can remain floating in the water and drift over long distances. The decomposition of some particles by bacteria consumes a lot of oxygen and can cause the water to become hypoxic.

Contents

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.

Settling The process by which particulates settle to the bottom of a liquid and form a sediment

Settling is the process by which particulates settle to the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction exerted by that force. For gravity settling, this means that the particles will tend to fall to the bottom of the vessel, forming a slurry at the vessel base.

Colloid A mixture of an insoluble or solube substance microscopically dispersed throughout another substance

In chemistry, a colloid is a mixture in which one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance. Sometimes the dispersed substance alone is called the colloid; the term colloidal suspension refers unambiguously to the overall mixture. Unlike a solution, whose solute and solvent constitute only one phase, a colloid has a dispersed phase and a continuous phase that arise by phase separation. To qualify as a colloid, the mixture must be one that does not settle or would take a very long time to settle appreciably.

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. [1]

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, parking lots and airports. [2] Some industries such as quarrying, mining and coal recovery can generate very high levels of turbidity from colloidal rock particles.

Phytoplankton Autotrophic members of the plankton ecosystem

Phytoplankton are the autotrophic (self-feeding) components of the plankton community and a key part of oceans, seas and freshwater basin ecosystems. The name comes from the Greek words φυτόν (phyton), meaning "plant", and πλανκτός (planktos), meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high enough numbers, some varieties may be noticeable as colored patches on the water surface due to the presence of chlorophyll within their cells and accessory pigments in some species. About 1% of the global biomass is due to phytoplankton.

Construction Process of the building or assembling of a building or infrastructure

Construction is the process of constructing a building or infrastructure. Construction differs from manufacturing in that manufacturing typically involves mass production of similar items without a designated purchaser, while construction typically takes place on location for a known client. Construction as an industry comprises six to nine percent of the gross domestic product of developed countries. Construction starts with planning, design, and financing; it continues until the project is built and ready for use.

Mining The extraction of valuable minerals or other geological materials from the earth

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. These deposits form a mineralized package that is of economic interest to the miner.

In drinking water, the higher the turbidity level, the higher the risk that people may develop gastrointestinal diseases. [3] 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 (UV) sterilization of water. [4]

Virus Type of non-cellular infectious agent

A virus is a small infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

Bacteria A domain of prokaryotes – single celled organisms without a nucleus

Bacteria are a type of biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of the earth's crust. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the bacterial phyla have species that can be grown in the laboratory . The study of bacteria is known as bacteriology, a branch of microbiology.

Chlorine Chemical element with atomic number 17

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 is a yellow-green gas at room temperature. It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity on the Pauling scale, behind only oxygen and fluorine.

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. [5] [6]

Lake A body of relatively still water, in a basin surrounded by land

A lake is an area filled with water, localized in a basin, surrounded by land, apart 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, and therefore are distinct from lagoons, and are also larger and deeper than ponds, though there are no official or scientific definitions. Lakes can be contrasted with rivers or streams, which are usually flowing. Most lakes are fed and drained by rivers and streams.

Fish vertebrate animal that lives in water and (typically) has gills

Fish are gill-bearing aquatic craniate animals that lack limbs with digits. They form a sister group to the tunicates, together forming the olfactores. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered paraphyletic by excluding the tetrapods. Because in this manner the term "fish" is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology, unless it is used in the cladistic sense, including tetrapods. The traditional term pisces is considered a typological, but not a phylogenetic classification.

Shellfish culinary and fisheries term for exoskeleton-bearing aquatic invertebrates

Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. In addition, a few species of land crabs are eaten, for example Cardisoma guanhumi in the Caribbean. Shellfish are among the most common food allergens.

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, in particular Moreton Bay, turbidity levels as high as 600 Nephelometric Turbidity Units (NTU) are needed for proper ecosystem health.

Measurement

Turbid creek water caused by heavy rains. Runoff torbidity.jpg
Turbid creek water caused by heavy rains.

The most widely used measurement unit for turbidity is the Formazin Turbidity Unit (FTU). ISO refers to its units as FNU (Formazin Nephelometric Units). ISO 7027 provides the method in water quality for the determination of turbidity. It is used to determine the 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 has been developed following the ISO 7027 method to measure turbidity reliably using an Arduino microcontroller and inexpensive LEDs. [7]


There are several practical ways of checking water quality, the most direct being some measure of attenuation (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 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 (NTU). 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. 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. [8] [9] 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). [10] [11]

Standards and test methods

Turbidimeters used at a water purification plant to measure turbidity (in NTU) of raw water and clear water after filtration. Turbidimeters.JPG
Turbidimeters used at a water purification plant to measure turbidity (in NTU) of raw water and clear water after filtration.

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 must not have a turbidity higher than 1.0 nephelometric turbidity units (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. [12] The European standards for turbidity state that it must be no more than 4 NTU. [13] The World Health Organization, establishes that the turbidity of drinking water should not be more than 5 NTU, and should ideally be below 1 NTU. [14]

Ambient water standards

US

The US Environmental Protection Agency (EPA) has published water quality criteria for turbidity. [15] 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. 25, 50 or 150 NTU, or background plus 10 percent, depending on the water body. [16] .
  • Vermont. 10 NTU or 25 NTU, depending on water body classification. [17]
  • Washington. 5 NTU over background (when background is 50 NTU or less), or 10 percent increase when background is over 50 NTU. [18]

Analytical methods

Published analytical test methods for turbidity include:

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 (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 available for treating turbidity include aluminium sulfate or alum (Al2(SO4)3·nH2O), ferric chloride (FeCl3), gypsum (CaSO4·2H2O), poly-aluminium chloride, long chain acrylamide-based polymers and numerous proprietary reagents. [22] 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

Related Research Articles

Water treatment Process that improves the quality of water

Water treatment is any process that improves the quality of water to make it more acceptable for a specific end-use. The end use may be drinking, industrial water supply, irrigation, river flow maintenance, water recreation or many other uses, including being safely returned to the environment. Water treatment removes contaminants and undesirable components, or reduces their concentration so that the water becomes fit for its desired end-use. This treatment is crucial to human health and allows humans to benefit from both drinking and irrigation use.

Nephelometer Instrument for measuring the concentration of suspended particulates

A nephelometer is an instrument for measuring the concentration of suspended particulates in a liquid or gas colloid. A nephelometer measures suspended particulates by employing a light beam and a light detector set to one side of the source beam. Particle density is then a function of the light reflected into the detector from the particles. To some extent, how much light reflects for a given density of particles is dependent upon properties of the particles such as their shape, color, and reflectivity. Nephelometers are calibrated to a known particulate, then use environmental factors (k-factors) to compensate lighter or darker colored dusts accordingly. K-factor is determined by the user by running the nephelometer next to an air sampling pump and comparing results. There are a wide variety of research-grade nephelometers on the market as well as open source varieties.

Secchi disk A circular disk used to measure water transparency or turbidity

The Secchi disk, as created in 1865 by Angelo Secchi, is a plain white, circular disk 30 cm (12 in) 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 in the water. The depth at which the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity. Since its invention, the disk has also been used in a modified, smaller 20 cm (8 in) diameter, black and white design to measure freshwater transparency.

Water quality chemical, physical, biological, and radiological characteristics of water

Water quality refers to the chemical, physical, biological, and radiological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which compliance, generally achieved through treatment of the water, can be assessed. The most common standards used to assess water quality relate to health of ecosystems, safety of human contact, and drinking water.

Wastewater treatment Converting wastewater into an effluent for return to the water cycle

Wastewater treatment is a process used to remove contaminants from wastewater or sewage and convert it into an effluent that can be returned to the water cycle with minimum impact on the environment, or directly reused. The latter is called water reclamation because treated wastewater can then be used for other purposes. The treatment process takes place in a wastewater treatment plant (WWTP), often referred to as a Water Resource Recovery Facility (WRRF) or a Sewage Treatment Plant (STP). Pollutants in municipal wastewater are removed or broken down.

Settling basin structure using sedimentation to remove matter from wastewater

A settling basin, settling pond or decant pond is an earthen or concrete structure using sedimentation to remove settleable matter and turbidity from wastewater. The basins are used to control water pollution in diverse industries such as agriculture, aquaculture, and mining. Turbidity is an optical property of water caused by scattering of light by material suspended in that water. Although turbidity often varies directly with weight or volumetric measurements of settleable matter, correlation is complicated by variations in size, shape, refractive index, and specific gravity of suspended matter. Settling ponds may be ineffective at reducing turbidity caused by small particles with specific gravity low enough to be suspended by Brownian motion.

Total suspended solids (TSS) is the dry-weight of suspended particles, that are not dissolved, in a sample of water that can be trapped by a filter that is analyzed using a filtration apparatus. It is a water quality parameter used to assess the quality of a specimen of any type of water or water body, ocean water for example, or wastewater after treatment in a wastewater treatment plant. It is listed as a conventional pollutant in the U.S. Clean Water Act. Total dissolved solids is another parameter acquired through a separate analysis which is also used to determine water quality based on the total substances that are fully dissolved within the water, rather than undissolved suspended particles.

Sand filter sand

Sand filters are used as a step in the water treatment process of water purification.

A particulate air filter is a device composed of fibrous or porous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. Filters containing an adsorbent or catalyst such as charcoal (carbon) may also remove odors and gaseous pollutants such as volatile organic compounds or ozone. Air filters are used in applications where air quality is important, notably in building ventilation systems and in engines.

A particle counter is an instrument that detects and counts physical particles.

Total dissolved solids

Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular suspended form. Generally, the operational definition is that the solids must be small enough to survive filtration through a filter with 2-micrometer pores. Total dissolved solids are normally discussed only for freshwater systems, as salinity includes some of the ions constituting the definition of TDS. The principal application of TDS is in the study of water quality for streams, rivers, and lakes. Although TDS is not generally considered a primary pollutant, it is used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants.

Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to the motion of the water,suspended solids can be removed by the sedimentation because of their comparatively large size. It is used as one indicator of water quality.

Trophic state index A measure of how much nutrient are in a body of water.

Trophic State Index (TSI) is a classification system designed to rate bodies of water based on the amount of biological activity they sustain. Although the term "trophic index" is commonly applied to lakes, any surface body of water may be indexed.

Clarifier Settling tanks for continuous removal of solids being deposited by sedimentation

Clarifiers are settling tanks built with mechanical means for continuous removal of solids being deposited by sedimentation. A clarifier is generally used to remove solid particulates or suspended solids from liquid for clarification and (or) thickening. Concentrated impurities, discharged from the bottom of the tank are known as sludge, while the particles that float to the surface of the liquid are called scum.

Sediment control

A sediment control is a practice or device designed to keep eroded soil on a construction site, so that it does not wash off and cause water pollution to a nearby stream, river, lake, or sea. Sediment controls are usually employed together with erosion controls, which are designed to prevent or minimize erosion and thus reduce the need for sediment controls. Sediment controls are generally designed to be temporary measures, however, some can be used for storm water management purposes.

Ocean turbidity A measure of the amount of cloudiness or haziness in sea water caused by individual particles that are too small to be seen without magnification

Ocean turbidity is a measure of the amount of cloudiness or haziness in sea water caused by individual particles that are too small to be seen without magnification. Highly turbid ocean waters are those with many scattering particulates in them. In both highly absorbing and highly scattering waters, visibility into the water is reduced. The highly scattering (turbid) water still reflects a lot of light while the highly absorbing water, such as a blackwater river or lake, is very dark. The scattering particles that cause the water to be turbid can be composed of many things, including sediments and phytoplankton.

Formazine polymer

Formazine (formazin) is a heterocyclic polymer produced by reaction of hexamethylenetetramine with hydrazine sulfate.

Raw water is water found in the environment that has not been treated and does not have any of its minerals, ions, particles, bacteria, or parasites removed. Raw water includes rainwater, ground water, water from infiltration wells, and water from bodies like lakes and rivers.

Microtox bioassay

Microtox is an in vitro testing system which uses bioluminescent bacteria to detect toxic substances in different substrates such as water, air, soils and sediments. Allivibrio fischeri are non-pathogenic, marine, bacteria that luminesce as a natural part of their metabolism. When exposed to a toxic substance, the respiratory process of the bacteria is disrupted, reducing light output. Allivibrio fischeri have demonstrated high sensitivity across a wide variety of toxic substances. Response to toxicity is observed as a change in luminescence, which is a by-product of cellular respiration. This change can be used to calculate a percent inhibition of Allivibrio fischeri that directly correlates to toxicity.

ISO 7027:1999 is an ISO standard for water quality that enables the determination of turbidity. The ISO 7027 technique is used to determine the 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.

References

  1. Haze technical definition Archived August 22, 2015, at the Wayback Machine
  2. U.S. Environmental Protection Agency (EPA). Washington, D.C. "National Management Measures to Control Nonpoint Source Pollution from Urban Areas." Chapters 7 and 8. Document No. EPA 841-B-05-004. November 2005.
  3. A.G. Mann, C.C. Tam, C.D. Higgins, & L.C. Lodrigues. (2007). The association between drinking water turbidity and gastrointestinal illness: a systematic review. BMC Public Health. 7(256): 1 - 7
  4. "Turbidity of water and its measurment [sic] – disinfection not possible without removing suspended particles".
  5. U.S. Fish and Wildlife Service. Annapolis, Maryland. "Decline of Submerged Plants in Chesapeake Bay."
  6. EPA. Chesapeake Bay Program. Annapolis, Maryland. "Sediments." Archived 2011-09-27 at the Wayback Machine
  7. Bas Wijnen, G. C. Anzalone and Joshua M. Pearce, Open-source mobile water quality testing platform. Journal of Water, Sanitation and Hygiene for Development, 4(3) pp. 532–537 (2014). doi : 10.2166/washdev.2014.137, open access preprint
  8. [ dead link ]Waterwatch Australia, Module 4 - physical and chemical parameters "Methods Turbidity"
  9. [ dead link ]Myre, E, Shaw, R. The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field. "The Turbidity Tube"
  10. HelioClim (Center for Energy and Processes). Paris, France. "Linke Turbidity Factor." Archived 2011-07-23 at the Wayback Machine
  11. Kasten, F. The linke turbidity factor based on improved values of the integral Rayleigh optical thickness. Solar Energy 56:3, 269 (1996) doi : 10.1016/0038-092X(95)00114-7 .
  12. EPA. Washington, District of Columbia. "Drinking Water Contaminants." 2009-09-11.
  13. http://dwi.defra.gov.uk/consumers/advice-leaflets/standards.pdf
  14. http://www.lenntech.com/turbidity.htm#ixzz3R3yPreK7
  15. EPA. Washington, District of Columbia. "Quality Criteria for Water." (Commonly known as the "Gold Book.") 1986. Document No. EPA-440/5-86-001.
  16. Louisiana Department of Environmental Quality. Baton Rouge, Louisiana."Surface Water Quality Standards." Louisiana Administrative Code (LAC). Title 33, Part IX, Chapter 11. August 6, 2007.
  17. Vermont Water Resources Board. Montpelier, Vermont. "Vermont Water Quality Standards." Archived 2013-12-31 at the Wayback Machine January 25, 2006.
  18. Washington Department of Ecology. Olympia, Washington."Water Quality Standards for Surface Waters of the State of Washington." Washington Administrative Code (WAC). Chapter 173-201A. November 18, 1997.
  19. International Organization for Standardization. Geneva, Switzerland. "ISO 7027-1:2016 Water quality – Determination of turbidity – Part 1: Quantitative Methods." 2016 and "ISO 7027-2:2019 Water quality – Determination of turbidity – Part 2: Semi-quantitative methods for the assessment of transparency of waters." 2019.
  20. EPA. Environmental Monitoring Systems Laboratory. Cincinnati, Ohio. "Method 180.1: Determination of Turbidity by Nephelometry; Revision 2.0." August 1993.
  21. Clescearl, Leonore S(Editor), Greenberg, Arnold E.(Editor), Eaton, Andrew D. (Editor). Standard Methods for the Examination of Water and Wastewater (20th ed.) American Public Health Association, Washington, District of Columbia. ISBN   0-87553-235-7. This is also available on CD-ROM and online by subscription.
  22. Earth Systems, Clear Solutions newsletter - Focus on Turbidity 2003.