ASTM-D7725 › Standard Test Method for the Continuous Measurement of Turbidity Above 1 Turbidity Unit (TU)
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Scope
1.1 This test method covers the online and inline 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 Figure 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 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 turbidity unit (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 method can cover very different ranges of turbidity.
1.5 Many of the turbidity units and instrument designs covered in this 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 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 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 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 FNU, a 1 FAU, a 1 BU, and so forth.
1.6 This standard does not purport to cover all available technologies for high-level turbidity measurement.
1.7 This test method was tested on different waters, and with standards that will serve as surrogates to samples. It is the user's responsibility to ensure the validity of this test method for waters of untested matrices.
1.8 Those samples with the highest particle densities typically prove to be the most difficult to measure. In these cases, the process monitoring method can be considered with adequate measurement protocols installed.
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Refer to the MSDSs for all chemicals used in this procedure.
TABLE 1 Technologies for Measuring Turbidity Greater Than 1 TU that can be used for In-Line or On-Line Applications
| Design and Reporting Unit | Prominent Application | Key Design Features | Typical Instrument Range | Suggested Application |
|---|---|---|---|---|
| Nephelometric non- ratio (NTU) | White light turbidimeters Comply with EPA 180.1 for low level turbidity monitoring. | Detector centered at 90 degrees relative to the incident light beam. Uses a white light spectral source. | 0.012 to 40 NTU | Regulatory reporting of clean water |
| Ratio White Light tur bidimeters (NTRU) | Complies with ISWTR regulations and Standard Method 2130B. Can be used for both low and high level measurement. | Used a white light spectral source. Primary detector centered at 90°. Other detectors located at other angles. An instrument algorithm uses a combination of detector readings to generate the turbidity reading. | 0.012 - 10,000 NTRU | Regulatory Reporting of clean water |
| Formazin Nephelometric, near- IR turbidimeters, non- ratiometric (FNU) | Complies with ISO 7027. The wavelengthis less susceptible to color interferences. Applicable for samples with color and good for low level monitoring. | Detector centered at 90 degrees relative to the incident light beam. Uses a near-IR (780-900 nm) monochromatic light source. | 0.012 - 1,000 FNU | 0 - 40 FNU ISO 7027 Regulatory reporting |
| Formazin Nephelometric near-IR turbidimeters, ratio metric(FNRU) | Complies with ISO 7027. Applicable for samples with high levels of color and for monitoring to high turbidity levels. | Uses a near-IR monochromatic light source (780-900 nm). Primary detector centered at 90°. Other detectors located at other angles. An instrument algorithm uses a combination of detector readings to generate the turbidity reading. | 0.012 - 1,000 FNU | 0- 40 FNRU ISO 7027 Regulatory reporting |
| Surface Scatter Turbi dimeters (SSU) | Turbidity is determined through light scatter from a defined volume beneath the surface of a sample. Applicable for reporting for USEPA compliance monitoring. | Detector centered at 90 degrees relative to the incident light beam. Uses a “white light” spectral source. | 0.012-10,000 FNRU | 10 - 10,000 SSU |
| Formazin Nephelometric Turbidity Multibeam Unit (FNMU) | Is applicable to EPA regulatory method GLI Method 2. Applicable to drinking water and wastewater monitoring applications. | Detectors are geometrically centered at 0° and 90°. An instrument algorithm uses a combination of detector readings, which may differ for turbidities varying magnitude. | 0.012 to 4000 NTMU | 0 to 40 NTMU Reporting for EPA and ISO compliane |
| Formazin Attenuation Unit (FAU) | Compliance Reporting for ISO 7027 for samples that exceed 40 units | Uses a near-IR light source at 860±30 nm and the detector is 0 degrees relative to the centerline of the incident light beam. The measurement is an attenuation measurement. | 10 - 10,000+ FAU | 100 - 10,000+ FAU Reporting for ISO 7027 for levels in excess of 40 units |
| Attenuation Unit (AU) | Not applicable for regulatory purposes. Best applied for samples with high level turbidity. | Uses a white light spectral source (400-680 nm range). Detector geometry is 0° relative to the incident light beam. | 10 - 10,000+ AU | 100 - 10,000+ AU |
| Formazin Back Scatter (FBU) | Not applicable for regulatory purposes. Best applied to high turbidity samples. Backscatter is common probe technology and is best applied in higher turbidity samples. | Uses a near-IR monochromatic light source in the 780-900 nm range. Detector geometry is between 90 and 180° relative to the incident light beam. | 10,000+ FBU | 10,000 FBU |
| Forward Scatter Ratio Unit (FSRU) | The technology encompasses a single, light source and two detectors. Light sources can vary from single wavelength to polychromatic sources. The detection angle for the forward scatter detector is between 0 and 90- degrees relative to the centerline of the incident light beam. | The technology is sensitive to turbidities as low as 1 TU. The ratio technology helps to compensate for color interference and fouling. | The measurement of ambient waters such as streams, lakes, and rivers. The range is typically from about 1 – 800 FSRU, depending on the manufacturer. | Forward Scatter Ratio Unit (FSRU) |
Significance and Use
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.
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.
The instrumentation used must allow for the continuous on-line monitoring of a sample stream.
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.
Table 1 describing technologies and reporting results. Those technologies listed are appropriate for the range of measurement prescribed in this method are mentioned, though others may come available. Figure X3–1 from Appendix 3 contains a flowchart to assist in technology selection.
For a specific design that falls outside of these reporting ranges, the turbidity should be reported in turbidity units (TU) with a subscripted wavelength value to characterize the light source that was used.
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 method.
Keywords
calibration; calibration verification; continuous; formazin; measurement; monitoring; nephelometer; nephelometric; on-line; standard; styrenedivinylbenzene; turbidimeter; turbidity; turbidity standards; ICS Number Code 13.060.60 (Examination of water for physical properties)
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13.060.60 (Examination of physical properties of water)
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Document Number
ASTM-D7725-17R23
Revision Level
2017 R23 EDITION
Status
Current
Modification Type
New
Publication Date
Dec. 4, 2023
Document Type
Test Method
Page Count
23 pages
Committee Number
D19.03