ASTM D7725-17(2023)
(Test Method)Standard Test Method for the Continuous Measurement of Turbidity Above 1 Turbidity Unit (TU)
Standard Test Method for the Continuous Measurement of Turbidity Above 1 Turbidity Unit (TU)
SIGNIFICANCE AND USE
4.1 Turbidity is undesirable in drinking water, plant effluent waters, water for food and beverage processing, and for a large number of other water dependent manufacturing processes. Removal of suspended matter is accomplished by coagulation, settling, and filtration. Measurement of turbidity provides a rapid means of process control to determine when, how, and to what extent the water must be treated to meet specifications.
4.2 This test method is suitable for the on-line monitoring of turbidity such as that found in drinking water, process water, and high purity industrial waters.
4.3 The instrumentation used must allow for the continuous on-line monitoring of a sample stream.
4.4 When reporting the measured result, appropriate units should also be reported. The units are reflective of the technology used to generate the result, and if necessary, provide more adequate comparison to historical data sets.
4.4.1 Table 1 describing technologies and reporting results. Those technologies listed are appropriate for the range of measurement prescribed in this test method are mentioned, though others may come available. Fig. X3.1 from Appendix X3 contains a flowchart to assist in technology selection.
4.4.2 For a specific design that falls outside of these reporting ranges, the turbidity should be reported in TU with a subscripted wavelength value to characterize the light source that was used.
4.4.3 Ratio white light turbidimeters are common as bench top instruments but not as a typical process instrument. However, if fitted with a flow-cell they meet the criteria of this test method.
SCOPE
1.1 This test method covers the on-line and in-line determination of high-level turbidity in water that is greater than 1.0 turbidity units (TU) in municipal, industrial and environmental usage.
1.2 In principle, there are three basic applications for on-line measurement set ups. This first is the slipstream (bypass) sample technique. For the slipstream sample technique a portion of sample is transported out of the process and through the measurement apparatus. It is then either transported back to the process or to waste. The second is the in-line measurement where the sensor is brought directly into the process (see Fig. 8). The third basic method is for in-situ monitoring of sample waters. This principle is based on the insertion of a sensor into the sample itself as the sample is being processed. The in-situ use in this test method is intended for the monitoring of water during any step within a processing train, including immediately before or after the process itself.
1.3 This test method is applicable to the measurement of turbidities greater than 1.0 TU. The absolute range is dictated by the technology that is employed.
1.4 The upper end of the measurement range is left undefined because different technologies described in this test method can cover very different ranges of turbidity.
1.5 Many of the turbidity units and instrument designs covered in this test method are numerically equivalent in calibration when a common calibration standard is applied across those designs listed in Table 1. Measurement of a common calibration standard of a defined value will also produce equivalent results across these technologies. This test method prescribes the assignment of a determined turbidity values to the technology used to determine those values. Numerical equivalence to turbidity standards is observed between different technologies but is not expected across a common sample. Improved traceability beyond the scope of this test method may be practiced and would include the listing of the make and model number of the instrument used to determine the turbidity values.
1.5.1 In this test method, calibration standards are often defined in NTU values, but the other assigned turbidity units, such as those in Table 1 are equivalent. For example, a 1 NTU formazin standard is also a 1...
General Information
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Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7725 − 17 (Reapproved 2023)
Standard Test Method for the
Continuous Measurement of Turbidity Above 1 Turbidity
Unit (TU)
This standard is issued under the fixed designation D7725; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope values to the technology used to determine those values.
Numerical equivalence to turbidity standards is observed
1.1 This test method covers the on-line and in-line determi-
between different technologies but is not expected across a
nation of high-level turbidity in water that is greater than 1.0
common sample. Improved traceability beyond the scope of
turbidity units (TU) in municipal, industrial and environmental
this test method may be practiced and would include the listing
usage.
of the make and model number of the instrument used to
1.2 In principle, there are three basic applications for on-line
determine the turbidity values.
measurement set ups. This first is the slipstream (bypass)
1.5.1 In this test method, calibration standards are often
sample technique. For the slipstream sample technique a
defined in NTU values, but the other assigned turbidity units,
portion of sample is transported out of the process and through
such as those in Table 1 are equivalent. For example, a 1 NTU
the measurement apparatus. It is then either transported back to
formazin standard is also a 1 FNU, a 1 FAU, a 1 BU, and so
the process or to waste. The second is the in-line measurement
forth.
where the sensor is brought directly into the process (see Fig.
1.6 This test method does not purport to cover all available
8). The third basic method is for in-situ monitoring of sample
waters. This principle is based on the insertion of a sensor into technologies for high-level turbidity measurement.
the sample itself as the sample is being processed. The in-situ
1.7 This test method was tested on different waters, and with
use in this test method is intended for the monitoring of water
standards that will serve as surrogates to samples. It is the
during any step within a processing train, including immedi-
user’s responsibility to ensure the validity of this test method
ately before or after the process itself.
for waters of untested matrices.
1.3 This test method is applicable to the measurement of
1.8 Those samples with the highest particle densities typi-
turbidities greater than 1.0 TU. The absolute range is dictated
cally prove to be the most difficult to measure. In these cases,
by the technology that is employed.
the process monitoring method can be considered with ad-
1.4 The upper end of the measurement range is left unde-
equate measurement protocols installed.
fined because different technologies described in this test
1.9 The values stated in SI units are to be regarded as
method can cover very different ranges of turbidity.
standard. No other units of measurement are included in this
1.5 Many of the turbidity units and instrument designs
standard.
covered in this test method are numerically equivalent in
1.10 This standard does not purport to address all of the
calibration when a common calibration standard is applied
safety concerns, if any, associated with its use. It is the
across those designs listed in Table 1. Measurement of a
responsibility of the user of this standard to establish appro-
common calibration standard of a defined value will also
priate safety, health, and environmental practices and deter-
produce equivalent results across these technologies. This test
mine the applicability of regulatory limitations prior to use.
method prescribes the assignment of a determined turbidity
Refer to the MSDSs for all chemicals used in this procedure.
1 1.11 This international standard was developed in accor-
This test method is under the jurisdiction of ASTM Committee D19 on Water
dance with internationally recognized principles on standard-
and is the direct responsibility of Subcommittee D19.03 on Sampling Water and
Water-Formed Deposits, Analysis of Water for Power Generation and Process Use,
ization established in the Decision on Principles for the
On-Line Water Analysis, and Surveillance of Water.
Development of International Standards, Guides and Recom-
Current edition approved Nov. 1, 2023. Published December 2023. Originally
mendations issued by the World Trade Organization Technical
approved in 2012. Last previous edition approved in 2017 as D7725 – 17. DOI:
10.1520/D7725-17R23. Barriers to Trade (TBT) Committee.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7725 − 17 (2023)
TABLE 1 Technologies for Measuring Turbidity Greater Than 1 TU That Can be Used for In-Line or On-Line Applications
Design and Reporting
Prominent Application Key Design Features Typical Instrument Range Suggested Application
Unit
Nephelometric Non-Tatio White light turbidimeters comply with Detector centered at 90 degrees 0.012 to 40 NTU Regulatory reporting of
(NTU) EPA 180.1 for low-level turbidity relative to the incident light beam. clean water
monitoring. Uses a white light spectral source.
Ratio White Light Turbidi- Complies with Interim Enhanced Used a white light spectral source. 0.012–10 000 NTRU Regulatory reporting of
meters (NTRU) Surface Water Treatment Rule Primary detector centered at 90°. clean water
(ISWTR) regulations and Standard Other detectors located at other
Method 2130B. Can be used for angles. An instrument algorithm
both low- and high-level measure- uses a combination of detector read-
ment. ings to generate the turbidity read-
ing.
Formazin Nephelometric, Complies with ISO 7027. The wave- Detector centered at 90 degrees 0.012–1 000 FNU 0–40 FNU ISO 7027 regu-
Near-IR Turbidimeters, lengthis less susceptible to color in- relative to the incident light beam. latory reporting
Non-Ratiometric (FNU) terferences. Applicable for samples Uses a near-IR (780–900 nm)
with color and good for low-level monochromatic light source.
monitoring.
Formazin Nephelometric Complies with ISO 7027. Applicable Uses a near-IR monochromatic light 0.012–1 000 FNU 0–40 FNRU ISO 7027 regu-
Near-IR Turbidimeters, for samples with high levels of color source (780–900 nm). Primary de- latory reporting
Ratio Metric (FNRU) and for monitoring to high turbidity tector centered at 90°. Other detec-
levels. tors located at other angles. An in-
strument algorithm uses a
combination of detector readings to
generate the turbidity reading.
Surface Scatter Turbidi- Turbidity is determined through light Detector centered at 90 degrees 0.012–10 000 FNRU 10–10 000 SSU
meters (SSU) scatter from a defined volume be- relative to the incident light beam.
neath the surface of a sample. Appli- Uses a “white light” spectral source.
cable for reporting for EPA compli-
ance monitoring.
Formazin Nephelometric Is applicable to EPA regulatory Detectors are geometrically centered 0.012 to 4000 NTMU 0 to 40 NTMU Reporting for
Turbidity Multibeam unit method GLI Method 2. Applicable to at 0° and 90°. An instrument algo- EPA and ISO compliane
(FNMU) drinking water and wastewater moni- rithm uses a combination of detector
toring applications. readings, which may differ for tur-
bidities varying magnitude.
Formazin Attenuation Unit Compliance Reporting for ISO 7027 Uses a near-IR light source at 860 ± 10–10 000+ FAU 100–10 000+ FAU Report-
(FAU) for samples that exceed 40 units. 30 nm and the detector is 0 degrees ing for ISO 7027 for levels
relative to the centerline of the inci- in excess of 40 units
dent light beam. The measurement
is an attenuation measurement.
Attenuation Unit (AU) Not applicable for regulatory pur- Uses a white light spectral source 10–10 000+ AU 100–10 000+ AU
poses. Best applied for samples with (400–680 nm range). Detector ge-
high-level turbidity. ometry is 0° relative to the incident
light beam.
Formazin Back Scatter Not applicable for regulatory pur- Uses a near-IR monochromatic light 10 000+ FBU 10 000 FBU
(FBU) poses. Best applied to high turbidity source in the 780–900 nm range.
samples. Backscatter is common Detector geometry is between 90
probe technology and is best applied and 180° relative to the incident light
in higher turbidity samples. beam.
Forward Scatter Ratio The technology encompasses a The technology is sensitive to tur- The measurement of ambient Forward Scatter Ratio Unit
Unit (FSRU) single, light source and two detec- bidities as low as 1 TU. The ratio waters such as streams, (FSRU)
tors. Light sources can vary from technology helps to compensate for lakes, and rivers. The range is
single wavelength to polychromatic color interference and fouling. typically from about 1–800
sources. The detection angle for the FSRU, depending on the
forward scatter detector is between manufacturer.
0 and 90–degrees relative to the
centerline of the incident light beam.
D7725 − 17 (2023)
2. Referenced Documents 3.2.4 calibration-verification standards, n—defined stan-
2 dards used to verify the accuracy of a calibration in the
2.1 ASTM Standards:
measurement range of interest.
D1129 Terminology Relating to Water
3.2.4.1 Discussion—These standards may not be used to
D2777 Practice for Determination of Precision and Bias of
perform calibrations, only calibration verifications. Included
Applicable Test Methods of Committee D19 on Water
verification standards are opto-mechanical lightscatter devices,
D3370 Practices for Sampling Water from Flowing Process
gel-like standards, or any other type of stable-liquid standard.
Streams
Calibration verification standards may be instrument specific.
D3864 Guide for On-Line Monitoring Systems for Water
Analysis
3.2.5 detection angle, n—the angle formed with its apex at
D6698 Test Method for On-Line Measurement of Turbidity
the center of the analysis volume of the sample, and such that
Below 5 NTU in Water (Withdrawn 2023)
one vector coincides with the centerline of the incident light
D7315 Test Method for Determination of Turbidity Above 1
source’s emitted radiation and the second vector projects to the
Turbidity Unit (TU) in Static Mode
center of the primary detector’s view.
2.2 Other Publications:
3.2.5.1 Discussion—This angle is used for the differentia-
EPA 180.1 Methods for Chemical Analysis of Water and
tion of turbidity-measurement technologies that are used in this
Wastes, Turbidity
test method.
GLI Method 2 Great Lakes Instruments (GLI) — Turbidity
3.2.6 forward-scatter-detection angle, n—the angle that is
ISO 7027 Water Quality — Determination of Turbidity
formed between the incident light source and the primary
Standard Method 2130B Standard Methods for the Exami-
detector, and that is between 0 and 90-degrees.
nation of Water and Wastewater
3.2.6.1 Discussion—Most designs will have an angle be-
3. Terminology
tween 10 and 45 degrees.
3.1 Definitions:
3.2.7 nephelometric-detection angle, n—the angle that is
3.1.1 For definitions of terms used in this standard, refer to
formed between the incident light source and the detector, and
Terminology D1129.
that is at 90-degrees
3.2 Definitions of Terms Specific to This Standard:
3.2.8 nephelometric-turbidity measurement, n—the mea-
3.2.1 attenuation-detection angle, n—the angle that is
surement of light scatter from a sample in a direction that is at
formed between the incident light source and the primary
90° with respect to the centerline of the incident-light path.
detector, and that is at exactly 0-degrees.
3.2.8.1 Discussion—Units are NTU (nephelometric turbid-
3.2.1.1 Discussion—This is typically a transmission mea-
ity units). When ISO 7027 technology is employed units are
surement.
FNU (formazin nephelometric units).
3.2.2 backscatter-detection angle, n—the angle that is
3.2.9 ratio-turbidity measurement, n—the measurement de-
formed between the incident light source and the primary
rived through the use of a nephelometric detector that serves as
detector, and that is greater than 90-degrees and up to 180-
the primary detector, and one or more other detectors used to
degrees.
compensate for variation in incident-light fluctuation, stray
3.2.3 calibration turbidity standard, n—a turbidity standard
light, instrument noise, or sample color.
that is traceable and equivalent to the reference turbidity
3.2.10 reference-turbidity standard, n—a standard that is
standard to within statistical errors; calibration turbidity stan-
synthesized reproducibly from traceable raw materials by the
dards include commercially prepared 4000 NTU formazin,
user.
stabilized formazin, and styrenedivinylbenzene (SDVB).
3.2.10.1 Discussion—All other standards are traced back to
3.2.3.1 Discussion—These standards may be used to cali-
this standard. The reference standard for turbidity is formazin.
brate the instrument. Calibration turbidity standards may be
instrument specific.
3.2.11 seasoning, n—the process of conditioning labware
with the standard that will be diluted to a lower value.
3.2.11.1 Discussion—The process reduces contamination
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM and dilution errors. See Appendix X2 for suggested procedure.
Standards volume information, refer to the standard’s Document Summary page on
3.2.12 slipstream, n—an on-line technique for analysis of a
the ASTM website.
The last approved version of this historical standard is referenced on
sample as it flows through a measurement chamber of an
www.astm.org.
instrument.
Available from United States Environmental Protection Agency (EPA), William
3.2.12.1 Discussion—The sample is transported from the
Jefferson Clinton Bldg., 1200 Pennsylvania Ave., NW, Washington, DC 20460,
http://www.epa.gov.
source into the instrument (for example a turbidimeter),
Available from Hach Company, P.O. Box 389, Loveland, Colorado 80539,
analyzed, and then transported to drain or back to the process
https://www.hach.com.
6 stream. The term is synonymous with the terms “on-line
Available from International Organization for Standardization (ISO), ISO
instrument” or “continuous-monitoring instrument.”
Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
Geneva, Switzerland, http://www.iso.org.
3.2.13 stray light, n—all light reaching the detector other
Available from American Public Health Ass
...
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