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Turbidity Measurement Methods & Regulations

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Turbidity Measurement Methods & Regulations

[Photo Credit: USGS Water Science School]

Measuring the "turbidity" (e.g. suspended sediment, cloudiness or haziness in a liquid) is one method that is used to discern the suitability of water for drinking, swimming or fishing. High rates of turbidity can be associated with contamination and/or pathogens. Turbidity may be caused by coastal zone erosion, storm surge, industrial effluent, agricultural runoff, or the runoff from urban impervious surfaces.

The American Society for Testing and Materials (ASTM) defines turbidity as "an expression of the optical properties of a liquid that causes light rays to be scattered and absorbed rather than transmitted in straight lines through a sample." The U.S. EPA describes turbidity as "...a measure of the cloudiness of water. It is used to indicate water quality and filtration effectiveness (such as whether disease-causing organisms are present). Higher turbidity levels are often associated with higher levels of disease-causing microorganisms such as viruses, parasites and some bacteria. These organisms can cause symptoms such as nausea, cramps, diarrhea, and associated headaches."

Several early methods for testing turbidity were introduced primarily in order to monitor water-borne diseases. According to the Illinois State Water Survey, "The normal procedure in 1912 used the turbidity standard adopted by the U. S. Geological Survey: "water which contains 100 parts of silica per million in such a state of fineness that a bright platinum wire one millimeter in diameter can just be seen when the center of the wire is 100 millimeters below the surface of the water and the eye of the observer is 1.2 meters above the wire." A rod with a platinum wire on the end was calibrated by placing graduation marks on the rod, at various distances from the end, and this was lowered into the water as far as the wire could be seen. The vanishing depth was compared to a table of known values to get the measured turbidity."

George C. Whipple and Daniel D. Jackson in 1899, as reported by the American Water Works Association (AWWA) in Operational Control of Coagulation and Filtration Processes (2011), "developed a standard suspension of fluid using 1,000 ppm of diatomaceous earth in distilled water. Dilution of this reference suspension resulted in a series of standard suspensions used to derive a ppm-silica scale for calibrating contemporary turbidimeters." Jackson is credited for applying the scale to a turbidimeter, resulting in the "Jackson Candle Turbidimeter."

As early as 1911, Jackson's invention was deployed for a variety of applications, as evidenced by:

Muer, Henry. "The Determination of Sulphur in Coal by means of the Jackson Candle Turbidimiter," The Journal of Industrial and Engineering Chemistry, Volume 3, August, 1911, pp. 553-557. candle turbidimeter&f=false ]

The Units of Measurement for Turbidity, sbsequent to the precursor Jackson Turbidity Unit (JTU), now include a variety of alternative methodologies:

Nephelometric Turbidity Unit (NTU) - Standard in the USA

Formazin Nephelometric Unit (FNU) - International Standard

Formazin Attenuation Unit (FAU)

The following is a link to a spreadsheet listing of manufacturers, model numbers, prices and description of features for a selected array of Turbidimeters and Nephelometers.

Regulatory Standards:

Dec. 13, 2017 marked the end of the 21 day public comment period allowed consequent to the U.S. Environmental Protection Agency's proposed revision/rescinding of the Clean Water Rule, initially jointly adopted in May, 2015 by the EPA and the U.S. Army's Corps of Engineers. The Rule offered clarification with respect to which waterways were subject to protection under the ****Clean Water Act, as amended in 1972.

US EPA: "For systems that use conventional or direct filtration, at no time can turbidity (cloudiness of water) go higher than 1 Nephelometric Turbidity Unit (NTU), and samples for turbidity must be less than or equal to 0.3 NTUs in 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 NTUs."

"International Office for Standardization (ISO) 7027-1:2016 (European) turbidity method: specifies two quantitative methods using optical turbidimeters or nephelometers for the determination of turbidity of water:

a) nephelometry, procedure for measurement of diffuse radiation, applicable to water of low turbidity (for example drinking water);

b) turbidimetry, procedure for measurement of the attenuation of a radiant flux, more applicable to highly turbid waters (for example waste waters or other cloudy waters).

Turbidities measured according to the first method are presented as nephelometric turbidity units (NTU). The results typically range between <0,05 NTU and 400 NTU. Depending on the instrument design, it can also be applicable to waters of higher turbidity. There is numerical equivalence of the units NTU and formazin nephelometric unit (FNU).

Turbidity measured by the second method is expressed in formazin attenuation units (FAU), results typically range between 40 FAU and 4 000 FAU."

Requirements: Drinking Water

National Pollutant Discharge Elimination System (NPDES) Permits

Turbidity Limit at Outfall

State-Specific Water Quality Standards Effective under the Clean Water Act (CWA) US EPA


Effective February 6, 2017:

Chapter 283. Pollution Discharge Elimination (PDF)(22 pp, 67 K)

"(Effective February 6, 2017) The document grants the department of natural resources all authority necessary to establish, administer, and maintain a state pollutant discharge elimination system. Chapter 283.15 describes the process for establishing water quality standard variances. Chapter 283.16 establishes the statewide variance for phosphorus."

References [Including Selected Abstracts]:

John R. Gray, G. Douglas Glysson, Federal Interagency Subcommittee on Sedimentation (U.S.) Proceedings of the Federal Interagency Workshop on Turbidity and Other Sediment Surrogates, April 30-May 2, 2002, Reno, Nevada. U.S. Department of the Interior, U.S. Geological Survey, 2003 - 56 pages

Béla G. Lipták, Kriszta Venczel. Analysis and Analyzers, Volume 2. CRC Press, Nov 25, 2016, 1332 pages

American Water Works Association (AWWA) Staff. Operational Control of Coagulation and Filtration Processes, 3rd Ed. (M37). Jan 12, 2011

Randy D. Down, Jay H. Lehr. Environmental instrumentation and analysis handbook. Wiley-Interscience, 2005 - Science - 1068 pages

A.I. Dogliotti, K.G. Ruddick, B. Nechad, D. Doxaranc & E. Knaeps. "A single algorithm to retrieve turbidity from remotely-sensed data in all coastal and estuarine waters." Remote Sensing of Environment. Volume 156, January 2015, Pages 157-168

Chauncey W. Anderson. TURBIDITY 6.7, Turbidity, Version 2.1 (9/2005).

"Although turbidity is not an inherent property of water, as is temperature or pH (Davies-Colley and Smith, 2001), the recognition of turbidity as an indicator of the environmental health of water bodies has increased over the past decade, resulting in a growing demand for high-quality and objective turbidity measurements. To meet this demand, relatively inexpensive, yet sophisticated instruments have been developed that allow for nearly continuous monitoring and data logging of turbidity in natural waters. Gray and Glysson (2003) note the following examples of disparate uses for turbidity data:

Regulating and maintaining drinking water clarity. Determining water clarity for aquatic organisms. Indicating visual impairment in water. Real-time monitoring that indicates watershed conditions. Developing surrogates for concentration of suspended sediment (SSC) and other constituents. Monitoring the effects of land development and related human activities and subsequent management of natural resources. Determining transport of contaminants associated with suspended materials." "Turbidity, Version 2.1" (9/2005) U.S. Geological Survey TWRI Book 9


"Although technological advances in turbidity measurement have produced a variety of instrument types to meet one or more of these differing objectives, turbidity instruments of different designs commonly do not yield identical or equivalent results. Moreover, the mixing of different source waters or dilutions of environmental samples may not produce linear results when measuring for turbidity because of the variety of factors that contribute to and can have an effect on turbidity. Selection of the appropriate turbidity instrument requires, therefore, consideration of project objectives, data requirements, and the physical and chemical properties of the water body."

"This section on turbidity provides protocols and guidelines for selecting appropriate field and laboratory instruments and procedures for instrument calibration and maintenance, turbidity measurement, data storage, and quality assurance that meet stated objectives for U.S. Geological Survey (USGS) data-collection efforts.1"

Octavian A. Postolache ; P. M. B. Silva Girao ; J. M. Dias Pereira ; Helena Maria G. Ramos. "Multibeam Optical System and Neural Processing for Turbidity Measurement," IEEE Sensors Journal ( Volume: 7, Issue: 5, May 2007 ), 677 - 684.


"This paper presents a turbidity measuring system based on a modulated four infrared (IR) light beam architecture with advanced data processing. The turbidity sensing component consists of a pair of IR light-emitting diodes (LEDs) connected to a current drive controlled through the pulsewidth modulated (PWM) outputs of a multifunction input/output board. The scattered and transmitted IR light in the media under test is detected by a two-channel IR photodiode module that includes a set of transimpedance and programmable gain amplifier. The voltages proportional to the detectors' output currents, are acquired using a 12-bit ADC included in a microcontroller and RS232 transmitted to a laptop personal computer (PC) that works as an advanced control and processing unit. Using optimal neural network processing architectures, an accurate extraction of the turbidity information is performed. A practical approach concerning the neural network architectures [multilayer perceptron single-input-single-output (SISO), multiple-input-single-output (MISO)] including neural network training and testing is discussed in the paper. The multi-input architectures prove to be a robust and general solution for the proposed application. Results from a turbidity measuring system that was designed for automated standalone remote operation with sensing channel autocalibration capabilities are presented."

Telesnicki, Guy J.; Goldberg, Walter M.N"Comparison of Turbidity Measurement by Nephelometry and Transmissometry and its Relevance to Water Quality Standards,"

Bulletin of Marine Science, Volume 57, Number 2, September 1995, pp. 540-547(8)

Publisher: University of Miami - Rosenstiel School of Marine and Atmospheric Science.

ABSTRACT: "The standard method for measuring turbidity in United States coastal waters is by nephelometric analysis with Formazin calibration. This study examined relationships between field measurements and various standards, and compared the performance of nephelometry with transmissometry. Turbidity generated during a beach restoration project in Florida was compared with Formazin and marl standards. For each datum, paired readings were taken by nephelometry and transmissometry, and compared using regression analysis. Both instruments measured individual standards in proportion to their concentration over a broad range of turbidity. Turbidity in the field was optically heterogeneous, i.e., more variable than standards, and did not correspond to either instrument using Formazin calibration. Marl predicted turbidity in the field within 95% confidence limits only below 11 nephelometric turbidity units (NTU). The State of Florida's statutory limit on turbidity of 29 NTU corresponds to 4.4 (±1.2) % transmission (%T) using field turbidity data. The use of Formazin to calibrate these instruments at this level underestimated turbidity in the field by about 50% at 29 NTU; marl underestimated field turbidity by about 24%. Weight of silt/clay in the field was linear as a function of percent transmission (Fig. 2b), but not using nephelometric analysis. Marl weight as a nephelometric standard produced a linear response at all concentrations, in contrast to Formazin which failed to produce a consistent nephelometric response at concentrations below 25 mg˙liter--1. Turbidity produced by known weight of Formazin and marl did not correspond using either instrument, and considerably underestimated silt/clay concentrations in the field. Water quality standards are discussed with respect to these findings."

Illinois State Water Survey. Prairie Research Institute

Richard M. Duchrow & W. Harry Everhart. "Turbidity Measurement". Transactions of the American Fisheries Society Vol. 100 , Iss. 4,1971 Pages 682-690 |

Published online: 09 Jan 2011

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"A quick and reliable method of measurement is necessary to set standard limits on the amount of suspended sediment to be tolerated in streams near land-use operations. Turbidity measurements may be useful if a major portion of the total turbidity is contributed by settleable solids, if a relationship exists between turbidity readings and weight per unit of volume of suspended sediment, and if a reliable meter is available. Water with turbidity readings greater than one JTU (Jackson Turbidity Unit) is generally composed mostly of settleable solids unless distorted by color. Non-filterable and total dissolved solids contribute variable amounts of light penetration reduction. Percentage contribution to turbidity of settleable solids is highly variable from sample to sample and from station to station."

"A high correlation exists between turbidity readings and weight for individual sediment types of suspension, but a poor relationship exists when sediment type is varied. Experiments conducted on the Hach model 2100, the Hellige, and the Jackson Candle turbidimeters resulted in a highly significant difference (α = 0.01) between readings on the same sample of in suspended sediment. Turbidity is a questionable measure of suspended solids in water. A more accurate index would be suspended solids measured gravimetrically."