Navigating the Turbidity Units Standards Around Water Quality Monitoring A paper edited by: Ludovic Grosjean, OCEANX GROUP Dana Galbraith, OBSERVATOR INSTRUMENTS August 2017 Abstract: This paper presents a discussion on current standards and units used in the Water Quality Monitoring industry. The contents of this Paper are presented to create discussion in the turbidity unit standards on this topic; the contents of this paper are not to be considered an adopted standard of any kind. ABN 56 007 283 963 8-10 Keith Campbell Court, PO Box 9039 Scoresby, VIC 3179 AUSTRALIA Tel: +61 3 8706 5000, Fax: +61 3 8706 5049 Email: info.au@observator.com Web: www.observator.com
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Table of Contents Introduction 4 Turbidity measurement 5 Turbidity Measurement issues... 5 Description of Turbidity Standards 6 Determination of Units 8 Safety concerns & Alternatives 9 Choice of Units 10 Analite series range of sensors... 10 Summary 11 References 12 Papers and Briefings... 12 Formal Specifications & Standards... 12 www.observator.com A Paper Published by Observator Instruments 3
Introduction This document is written to provide guidance to readers of various Turbidity Units standards and specifications published by the Environmental Protection Agency (EPA), and the International Organization for Standardization (ISO), on how these standards and specifications relate to each other. It is also intended to help clarify which units are relevant to the measurement of turbidity to help better understand the standards landscape. This document does not detail all of the relevant Turbidity Unit standards, but rather focuses on the distinguishing differences between NTU and FNU. As stated earlier, it is intended to serve as a guide to the reader to understand probe specification. We recognize that many other units also play an important role in water quality monitoring; but those two units are the most universally Internationally recognized and other measurement methods are beyond the scope of this document. This document covers all audiences, although a particular document referenced may be aimed at a narrower audience, such as water quality experts seeking to leverage unit reference as part of their work. www.observator.com A Paper Published by Observator Instruments 4
Turbidity measurement Turbidity is an indication of the amount of light that pass through a column of water. It is measured for multiple reason including coloration indication, photosynthetic analysis, biological activity indication, or contamination analysis. In natural water, turbidity is most often measured by an optical sensor according to the ISO 7027 standard by suspended particles at 90 degrees to a beam of infrared light. Turbidity Measurement issues As there is no analytical definition for the measurement of Turbidity, there is no way to establish if a turbidity measurement is correct. The measured turbidity, depends on the wavelength of light and the angle at which the detector is positioned. For various reasons, turbidity probes do not all use the same light sources, angles of measurement to detect the scattered light, and signal processing strategies. As a result, measurements from different makes/models of turbidity probes may not be comparable to one other. [3] There is also a significant latitude in the ISO 7027 which explains why different sensor are giving different readings. There are several ways to linearize the sensor output and several calibration solution including formazin and polymer beads are also available. There are dozens of turbidity measurement units, including Nephelometric Turbidity Unit (NTU), Formazin Nephelometric Unit (FNU), Formazin Attenuation Unit (FAU), Formazin Turbidity Units (FTU), etc, which complicates comparisons of turbidity measurements. When a formazin standard is measured, the value for each of these units will be the same, however the value on samples may differ significantly. [6] Another technique, called optical backscatter, which can measure turbidities corresponding to up to 7,000NTU or more, where ISO 7027 sensor is limited to about 3,000NTU. Turbidity uses a two-point calibration; one point is zero (turbidity-free water) and the other point should be a standard approximating the turbidity of the water you intend to monitor. [4] www.observator.com A Paper Published by Observator Instruments 5
Description of Turbidity Standards While the Turbidity definitions differ slightly, the open standards organizations EPA and ISO referenced two main Units based on the Wavelength of Light Source and the method of detection: The most widely used measurement unit for turbidity is the Formazin Turbidity Unit (FTU). ISO refers to its units as Formazin Nephelometric Units (FNU). 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. [7] The propensity of particles to scatter a light beam focused on them is 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). [7] The United States and European standards use conventional NTU units which is Internationally recognized as a reference. The U.S. Environmental Protection Agency (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. ISO standards also exist 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 (ISO 7027:1999). [8] More generally, turbidity sensors deployed in situ for continuous monitoring shall meet the ISO 7027:1999 Water Quality Turbidity Standard. The key features of ISO 7027 are: light source: monochromatic infrared beam with a maximum 1.5 convergence angle measurement wavelength: 860 nm ± 30 nm measurement angle: 90 ± 2.5 calibration standard: formazin reporting units: Formazin Nephelometric Units (FNU), and operational range: 0 4000 FNU. [5] www.observator.com A Paper Published by Observator Instruments 6
ISO 7027-1:2016 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); Turbidities measured according to this method are presented as Nephelometric Turbidity Units (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). 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). Turbidity measured by the second method is expressed in formazin attenuation units (FAU), results typically range between 40 FAU and 4 000 FAU. [9] Data from instruments meeting neither the ISO 7027 nor EPA 180.1 Standard but using a back-scatter-type detection system and using formazin as the calibration standard are reported in Formazin Backscatter Units (FBU). [5] www.observator.com A Paper Published by Observator Instruments 7
Determination of Units To help categorize the differences in turbidity probe design and distinguish among turbidity measurements that may not be comparable, new units of measurement were established and assigned based on the probe design. The following table summarizes the units and the characteristics of the probes assigned to those units. The differences are based mainly on the wavelength of the light used to make the measurement, the angle at which the scattered light is detected, the number of detectors, and any special signal processing. Units of Measurement for Turbidity Sensors Detector Geometry White or broadband: peak spectral output of 400-680nm Wavelength of Light Source Infrared monochromatic: typical output in 780-900nm range Single Illumination Beam Light Source Standard Nephelometric Turbidity Unit (NTU) Limited to instruments that comply with EPA Method 180.1 Formazin Nephelometric Unit (FNU) Pertains to instruments that comply with ISO 7027 90 degrees Nephelometric Turbidity Ratio Unit (NTRU) Formazin Nephelometric Ratio Unit (FNU) 180 degrees to incident beam (attenuation) Attenuation Unit (AU) Formazin Attenuation Unit (FAU) [3], [10] Formazin Nephelometric Unit (FNU) is similar to a Nephelometric Turbidity Unit (NTU) in that both measure scattered light at 90 degrees from the incident light beam, but the FNU is measured with an infrared light source according to the ISO 7027 method whereas the NTU is measured with a white light according to EPA method 180.1. Because suspended particles scatter light of different wavelengths with varying efficiency, turbidity data collected with infrared light sources are not directly comparable to turbidity data collected with white light sources. Always be careful when using turbidity data that have different units of measurement; data of different units are not necessarily comparable. www.observator.com A Paper Published by Observator Instruments 8
Safety concerns & Alternatives International Standards and Governance frameworks are defining Formazin as the reference solution to be used for calibration reference. While those standards are providing a certain latitude, the use of Formazin can be discussed. It has been proven that Formazin is only stable for a few days and it is also a suspected carcinogen. It may cause allergic skin reaction (H317), cancer (H350), or can be harmful to aquatic life with long lasting effects (H350). [11] Alternatives such as AMCO Clear Certified Reference Standards are now used by the US EPA, as an approved alternative to formazin products for the calibration of turbidity monitors. There are many advantages in using this solution as a calibration reference replacement: Non-hazardous, safe, non-toxic and disposable Accurate: Within 1% lot to lot to the master formazin dilution requires no shaking, mixing or inversion will not settle Easy to use: No dilution or preparation necessary Stable: Guaranteed 1-year shelf life -- even after opening N.I.S.T. Traceable Custom Values and suspensions available Requires no mixing or inverting, easy-to-use / No dilutions or preparations Stable - does not settle out of suspension Accurate to 1% lot-to-lot As a non-hazardous aqueous polymer suspension that is extremely stable, it remains suspended without shaking or inverting. On a quality instrument the obtained value remains unchanged since there is no particle settling taking place. AMCO Clear is NIST traceable: Particle shape, size and distribution are consistent from lot to lot and traceable to NIST particle standards. Optical Density of each value is consistent and determined on a Certified Visible Spectrophotometer. The solution is available not as a concentrate such as formazin, but in the ready to use value of your choice. Specific calibration points for particular instrument are available from 0.02 to 10,000 Turbidity Units. Finally, AMCO Clear is preferred by many governmental agencies for calibration of their turbidity sensors used in Water Quality and Suspended Sediment Concentration and Transport. [1], [2] www.observator.com A Paper Published by Observator Instruments 9
Choice of Units Which products are relevant to you depends on what standard your project and organization is referring to. The unit will vary on the detection method and can sometimes be misleading. Analite series range of sensors Analite Turbidity series range of sensors are using an Infrared Monochromatic detector measures scattered light at a 90-degree angle from the incident light. While Formazyn is recognized as a primary standard, Observator Instruments performs the probe calibration using AMCO Clear solution instead for safety and environmental concerns. [6] Consequently, all Analite probes readings should be expressed using Formazin Nephelometic Units (FNU). However, for International commodity, units are still expressed in NTU in our documentation. www.observator.com A Paper Published by Observator Instruments 10
Summary An abundance of units has emerged from the standards organizations of ISO and EPA. This document was written to help the Turbidity Measurement User community at large to navigate the myriad of overlapping units and understand the key differences between NTU and FNU. Users of Analite Turbidity series produced by Observator Instruments are being provided Turbidity measurements expressed in NTU, the current most commonly recognized unit used on the market. However, the infrared light used in the sensor implies that Observator Instruments complies with ISO 7027, which implies that measurement should be expressed in FNU instead. We anticipate continuing the collaborative efforts to communicate in our specifications the International standards and their meaning to ensure that they continue to evolve in as consistent and complete a manner as possible. www.observator.com A Paper Published by Observator Instruments 11
References Papers and Briefings [1] AMCO Clear Certified Reference Standards for Turbidity Meters, WATER ONLINE: www.wateronline.com/doc/amco-clear-certified-reference-standards-for-0001 [2] Providing the Highest Quality Standards and Instruments Available, AMCO Clear : www.amcoclear.com/ [3] Turbidity -- Units of Measurement, USGS: https://or.water.usgs.gov/grapher/fnu.html [4] Turbidity probes and monitoring used stand alone or a sensor for sonde, Eureka Water Probes: www.waterprobes.com/turbidity [5] National Environmental Monitoring Standard, NEMS Turbidity Recording, Measurement, Processing and Archiving of Turbidity Data, Version 1.1, LAWA, October 2016: www.lawa.org.nz [6] Flint River Green Notebook, Flint River, April 2011: www.flintriver.org [7] Turbidity, Wikipedia: https://en.wikipedia.org/wiki/turbidity Formal Specifications & Standards [8] ISO 7027:1999 Preview Water quality -- Determination of turbidity: www.iso.org/standard/30123.html [9] ISO 7027-1:2016 Preview Water quality -- Determination of turbidity -- Part 1: Quantitative methods: www.iso.org/standard/62801.html [10] Method 180.1: Determination of Turbidity by Nephelometry, James W. O'Dell, Revision 2.0 August 1993: www.epa.gov/sites/production/files/2015-08/documents/method_180-1_1993.pdf [11] Turbidity Standard (Formazin), 4000NTU, Safety Data Sheet, LabChem, Version 1.0, July 2014: www.labchem.com/tools/msds/msds/lc26290.pdf About Observator Instruments Observator Group provides instruments for meteorological and hydrological solutions, process monitoring, air and climate technology. Observator Group is also specialized in window wipers, sunscreens and high precision mechanical production. A company with 90 years experience in these markets. Observator products and systems are sold worldwide through the several offices (Netherlands, Germany, Greece, Australia, United Kingdom and Singapore) and through her distributors network. Further information on Observator Group can be found at www.observator.com/ www.observator.com A Paper Published by Observator Instruments 12