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 Application | Key Design Features | Typical Instrument Range | Suggested Application |
| Nephelometric 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 40 | Regulatory 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 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. | 0.020 to10 000 | Regulatory Reporting of clean water |
| Nephelometric, near-IR turbidimeters, 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 1000 | 0 - 40 ISO 7027 Regulatory reporting |
| 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 to 10 000 | 0 - 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 4000 | 0 to 40 Reporting for EPA and ISO compliance |
| mNTU | Is 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 |
Scope
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.
