Turbidity

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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, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and water quality.

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.

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 and effects

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.

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 viruses and bacteria. Similarly, suspended solids can protect bacteria from ultraviolet (UV) sterilization of water.[ citation needed ]

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. [4] [5]

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.[ citation needed ]

Measurement

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

The measurement of turbidity is a key test of both water clarity and water quality.[ citation needed ] 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 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. [6]

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. [7] 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. [7]

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. [7]

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. [8]

An additional device, which may help measuring turbidity in shallow waters is the turbidity tube. [9] [8] 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, 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. [12] The European turbidity standard is 4 NTU. [13]

Ambient water standards

United States

The US Environmental Protection Agency (EPA) has published water quality criteria for turbidity. [14] 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. [15]
  • Vermont. 10 NTU or 25 NTU, depending on water body classification. [16]
  • Washington. 5 NTU over background (when background is 50 NTU or less), or 10 percent increase when background is over 50 NTU. [17]

Analytical methods

Published analytical test methods for turbidity include:

Treatment

Turbidity is commonly treated using a settling or filtration process, or both settling and filtration. Depending on the application, flocculants may be dosed into the water stream to increase the effectiveness of the settling or filtration process. [21] [22] Potable water treatment and municipal wastewater plants often remove turbidity with a combination of settling tanks, granular media filtration, 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 (FeCl3), gypsum (CaSO4·2H2O), poly-aluminium chloride, long chain acrylamide-based polymers and numerous proprietary reagents. [23] 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

<span class="mw-page-title-main">Water purification</span> Process of removing impurities from water

Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water that is fit for specific purposes. Most water is purified and disinfected for human consumption, but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The history of water purification includes a wide variety of methods. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.

<span class="mw-page-title-main">Nephelometer</span> Instrument for measuring the concentration of suspended particulates

A nephelometer or aerosol photometer 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.

<span class="mw-page-title-main">Secchi disk</span> 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.

<span class="mw-page-title-main">Water quality</span> Assessment against standards for use

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 treatment of the water, can be assessed. The most common standards used to monitor and assess water quality convey the health of ecosystems, safety of human contact, extent of water pollution and condition of drinking water. Water quality has a significant impact on water supply and oftentimes determines supply options.

<span class="mw-page-title-main">Settling basin</span>

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 known as sintered glass crucible. TSS 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.

<span class="mw-page-title-main">Sand filter</span> Water filtration device

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

<span class="mw-page-title-main">Total dissolved solids</span> Measurement in environmental chemistry

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. TDS are often measured in parts per million (ppm). TDS in water can be measured using a digital meter.

Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to motion of the water. Suspended solids can be removed by sedimentation if their size or density is comparatively large, or by filtration. It is used as one indicator of water quality and of the strength of sewage, or wastewater in general. It is an important design parameter for sewage treatment processes.

<span class="mw-page-title-main">Water testing</span> Procedures used to analyze water quality

Water testing is a broad description for various procedures used to analyze water quality. Millions of water quality tests are carried out daily to fulfill regulatory requirements and to maintain safety.

<span class="mw-page-title-main">Clarifier</span> 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. Inside the clarifier, solid contaminants will settle down to the bottom of the tank where it is collected by a scraper mechanism. 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.

<span class="mw-page-title-main">Sediment control</span>

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.

<span class="mw-page-title-main">Ocean turbidity</span> Measure of cloudiness of sea water

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. Highly scattering (turbid) water still reflects much light, while 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.

Water chemistry analyses are carried out to identify and quantify the chemical components and properties of water samples. The type and sensitivity of the analysis depends on the purpose of the analysis and the anticipated use of the water. Chemical water analysis is carried out on water used in industrial processes, on waste-water stream, on rivers and stream, on rainfall and on the sea. In all cases the results of the analysis provides information that can be used to make decisions or to provide re-assurance that conditions are as expected. The analytical parameters selected are chosen to be appropriate for the decision making process or to establish acceptable normality. Water chemistry analysis is often the groundwork of studies of water quality, pollution, hydrology and geothermal waters. Analytical methods routinely used can detect and measure all the natural elements and their inorganic compounds and a very wide range of organic chemical species using methods such as gas chromatography and mass spectrometry. In water treatment plants producing drinking water and in some industrial processes using products with distinctive taste and odours, specialised organoleptic methods may be used to detect smells at very low concentrations.

<span class="mw-page-title-main">Formazine</span> Heterocyclic polymer produced by reacting methenamine and hydrazine sulfate

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.

Turbidimetry is the process of measuring the loss of intensity of transmitted light due to the scattering effect of particles suspended in it. Light is passed through a filter creating a light of known wavelength which is then passed through a cuvette containing a solution. A photoelectric cell collects the light which passes through the cuvette. A measurement is then given for the amount of absorbed light.

<span class="mw-page-title-main">Microtox bioassay</span>

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.

<span class="mw-page-title-main">Water clarity</span> How deeply visible light penetrates through water

Water clarity is a descriptive term for how deeply visible light penetrates through water. In addition to light penetration, the term water clarity is also often used to describe underwater visibility. Water clarity is one way that humans measure water quality, along with oxygen concentration and the presence or absence of pollutants and algal blooms.

References

  1. ↑ Thermallaminatingfilms.com. Haze technical definition Archived August 22, 2015, at the Wayback Machine
  2. ↑ National Management Measures to Control Nonpoint Source Pollution from Urban Areas (Report). Washington, D.C.: U.S. Environmental Protection Agency (EPA). November 2005. EPA 841-B-05-004. Chapters 7-8.
  3. ↑ Mann, A.G.; Tam, C.C.; Higgins, C.D.; Lodrigues, L.L. (2007-09-21). "The association between drinking water turbidity and gastrointestinal illness: a systematic review". BMC Public Health. Springer Nature. 7 (256): 256. doi: 10.1186/1471-2458-7-256 . PMC   2174477 . PMID   17888154.
  4. ↑ Stevenson, J. Court; Piper, Catherine B.; Confer, Nedra (1979). "Decline of Submerged Plants in Chesapeake Bay". Annapolis, MD: U.S. Fish and Wildlife Service. Archived from the original on 2019-04-28.
  5. ↑ "Sediments". Annapolis, MD: Chesapeake Bay Program. 2009-08-06. Archived from the original on 2011-09-27.
  6. ↑ Wijnen, Bas; Anzalone, G.C.; Pearce, Joshua M. (2014-05-09). "Open-source mobile water quality testing platform". Journal of Water, Sanitation and Hygiene for Development. London: IWA Publishing. 4 (3): 532–537. doi:10.2166/washdev.2014.137. S2CID   111183462.
  7. 1 2 3 "Measuring Turbidity, TSS, and Water Clarity". Fairborn, OH: Fondriest Environmental, Inc. Retrieved 1 February 2021.
  8. 1 2 Myre, E, Shaw, R. The Turbidity Tube: Simple and Accurate Measurement of Turbidity in the Field. "The Turbidity Tube" [ dead link ]
  9. ↑ Waterwatch Australia, Module 4 - physical and chemical parameters "Methods Turbidity" [ dead link ]
  10. ↑ HelioClim (Center for Energy and Processes). Paris, France. "Linke Turbidity Factor." Archived 2011-07-23 at the Wayback Machine
  11. ↑ Kasten, F. (March 1996). "The linke turbidity factor based on improved values of the integral Rayleigh optical thickness". Solar Energy. Elsevier. 56 (3): 239–244. Bibcode:1996SoEn...56..239K. doi:10.1016/0038-092X(95)00114-7..
  12. ↑ "National Primary Drinking Water Regulations". EPA. 2022-01-26.
  13. ↑ "What are the drinking water standards?" (PDF). London: Drinking Water Inspectorate (UK). June 2017. Archived from the original (PDF) on 2020-11-11.
  14. ↑ Quality Criteria for Water (PDF) (Report). EPA. 1986. EPA 440/5-86-001. (Commonly known as the "Gold Book.").
  15. ↑ 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.
  16. ↑ Vermont Water Resources Board. Montpelier, Vermont. "Vermont Water Quality Standards." Archived 2013-12-31 at the Wayback Machine January 25, 2006.
  17. ↑ 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.
  18. ↑ 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.
  19. ↑ EPA. Environmental Monitoring Systems Laboratory. Cincinnati, Ohio. "Method 180.1: Determination of Turbidity by Nephelometry; Revision 2.0." August 1993.
  20. ↑ Eaton, Andrew D.; Greenberg, Arnold E.; Rice, Eugene W.; Clesceri, Lenore S.; Franson, Mary Ann H., eds. (2005). Standard Methods For the Examination of Water and Wastewater (21 ed.). American Public Health Association. ISBN   978-0-87553-047-5. Also available on CD-ROM and online by subscription.
  21. ↑ Water Treatment, Grade 1. Denver, CO, USA: American Water Works Association. 2016.
  22. ↑ Darshan Singh Sarai (2006). Water Treatment Made Simple For Operators. John Wiley.
  23. ↑ Earth Systems, Clear Solutions newsletter - Focus on Turbidity 2003.
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