Standard Historical Last Updated: Nov 22, 2017
ASTM D6855-12

Standard Test Method for Determination of Turbidity Below 5 NTU in Static Mode

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 is often accomplished by coagulation, settling, and filtration. Measurement of turbidity provides a rapid means of process control for when, how, and to what extent the water must be treated to meet specifications.

This test method is suitable to turbidity such as that found in drinking water, process water, and high purity industrial water.

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 describes technologies and reporting results (see also Refs (1),(2),(3)). Those technologies listed are appropriate for the range of measurement prescribed in this method. Others may come available in the future. Fig. X5.1 provides a flow chart to aid in selection of the appropriate technology for low-level static turbidity applications.

If a design that falls outside of the criteria listed in Table 1 is used, the turbidity should be reported in turbidity units (TU) with a subscripted wavelength value to characterize the light source that was used.

TABLE 1 Applicable Technologies Available for Performing Static Turbidity Measurements Below 5 NTU

Design and
Reporting Unit
Prominent ApplicationKey Design FeaturesTypical Instrument RangeSuggested Application
non-ratio (NTU)
White light turbidimeters. Comply
with USEPA Method 180.1 (1)
for low level turbidity monitoring.
Detector centered at 90° relative
to the incident light beam. Uses
a white light spectral source.
0.020 to 40Regulatory reporting
of clean water
Ratio White Light
turbidimeters (NTRU)
Complies with ISWTR regulations and
Standard Method 2130B. (2)
Can be used for both
low and high level
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.
0.020 to10 000Regulatory Reporting of
clean water
Nephelometric, near-IR
non-ratiometric (FNU)
Complies with ISO 7027.
The wavelength is less susceptible
to color interferences.
Applicable for samples with color
and good for low level monitoring.
Detector centered at 90° relative to
the incident light beam. Uses a near-IR
(780-900 nm) monochromatic light source.
0.012 to 10000 - 40 ISO 7027
Regulatory reporting
Nephelometric near-IR
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 to 10 0000 - 40 ISO 7027
Regulatory reporting
Nephelometric Turbidity
Multibeam Unit (NTMU)
Is applicable to EPA regulatory
method GLI Method 2. (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 40000 to 40 Reporting for
EPA and ISO compliance
mNTUIs applicable to reporting of clean
waters and filter performance
monitoring. Very sensitive to
turbidity changes in low
turbidity samples. (3)
Nephelometric method involving a
laser-based light source at 660-nm and
a high sensitivity photo-multplier tube
(PMT) detector for light scattered at 90°.
1000 mNTU = 1 NTU
5 to 5000 mNTU or
0.005 to 5.000 NTU
0-5000 mNTU, for EPA
compliance reporting on
drinking water systems


1.1 This test method covers the static determination of turbidity in water (see 4.1).

1.2 This test method is applicable to the measurement of turbidities under 5.0 nephelometric turbidity units (NTU).

1.3 This test method was tested on municipal drinking water, ultra-pure water and low turbidity samples. It is the user's responsibility to ensure the validity of this test method for waters of untested matrices.

1.4 This test method uses calibration standards are defined in NTU values, but other assigned turbidity units are assumed to be equivalent.

1.5 This test method assigns traceable reporting units to the type of respective technology that was used to perform the measurement. Units are numerically equivalent with respect to the calibration standard. For example, a 1.0 NTU formazin standard is also equal to a 1.0 FNU standard, a 1.0 FNRU standard and so forth.

1.6 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.

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Book of Standards Volume: 11.01
Developed by Subcommittee: D19.07
Pages: 15
DOI: 10.1520/D6855-12
ICS Code: 19.100