Значение и использование

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.

Область применения

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
(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 NTURegulatory 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 NTRURegulatory 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 FNU0 - 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 FNU0- 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 FNRU10 - 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 NTMU0 to 40 NTMU Reporting for
EPA and ISO compliane Formazin Attenuation Unit
(FAU)Compliance Reporting for ISO
7027 for samples that exceed 40
unitsUses 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+ FAU100 - 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+ 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
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+ FBU10,000 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)