ASTM D7725 - 12

    Standard Test Method for the Continuous Measurement of Turbidity Above 1 Turbidity Unit (TU)

    Active Standard ASTM D7725 | Developed by Subcommittee: D19.03

    Book of Standards Volume: 11.01

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    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 X31 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.

    1. 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 UnitProminent ApplicationKey Design FeaturesTypical Instrument RangeSuggested Application
    Nephelometric non- ratio
    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
    0.012 to 40 NTURegulatory reporting of clean
    Ratio White Light tur bidimeters
    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 NTRURegulatory Reporting of clean
    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 FNU0 - 40 FNU ISO 7027
    Regulatory reporting
    Formazin Nephelometric near-IR
    turbidimeters, ratio
    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
    0.012 - 1,000 FNU0- 40 FNRU ISO 7027
    Regulatory reporting
    Surface Scatter Turbi dimeters
    Turbidity is determined through
    light scatter from a defined
    volume beneath the surface of a
    sample. Applicable for reporting
    for USEPA compliance
    Detector centered at 90 degrees
    relative to the incident light
    beam. Uses a white light
    spectral source.
    0.012-10,000 FNRU10 - 10,000 SSU
    Formazin Nephelometric
    Turbidity Multibeam Unit
    Is applicable to EPA regulatory
    method GLI Method 2.
    Applicable to drinking water and
    wastewater monitoring
    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 NTMU0 to 40 NTMU Reporting for
    EPA and ISO compliane
    Formazin Attenuation Unit
    Compliance Reporting for ISO
    7027 for samples that exceed 40
    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
    10 - 10,000+ FAU100 - 10,000+ FAU Reporting for ISO
    7027 for levels in excess of 40
    Attenuation Unit (AU)
    Not applicable for regulatory
    purposes. Best applied for
    samples with high level
    Uses a white light spectral
    source (400-680 nm range).
    Detector geometry is 0° relative
    to the incident light beam.
    10 - 10,000+ AU100 - 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
    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+ FBU10,000 FBU
    Forward Scatter Ratio Unit
    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
    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

    2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

    ASTM Standards

    D1129 Terminology Relating to Water

    D2777 Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water

    D3370 Practices for Sampling Water from Closed Conduits

    D3864 Guide for On-Line Monitoring Systems for Water Analysis

    D6698 Test Method for On-Line Measurement of Turbidity Below 5 NTU in Water

    D7315 Test Method for Determination of Turbidity Above 1 Turbidity Unit (TU) in Static Mode

    ICS Code

    ICS Number Code 13.060.60 (Examination of water for physical properties)

    UNSPSC Code

    UNSPSC Code 47101516(Turbidimeters); 41113026(Nephelometers)

    Referencing This Standard
    Link Here
    Link to Active (This link will always route to the current Active version of the standard.)

    DOI: 10.1520/D7725-12

    Citation Format

    ASTM D7725-12, Standard Test Method for the Continuous Measurement of Turbidity Above 1 Turbidity Unit (TU), ASTM International, West Conshohocken, PA, 2012,

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