# Appendix D. Turbidity GLI Method 2

## Revision Date: November 2, 1992

**1. SCOPE AND APPLICATION**

1.1 This method is applicable to drinking water samples in the range of
turbidity from 0 to 40 nephelometric units (NTU). Higher values may be
obtained with dilution of the sample. A method detection limit of .100
NTUs is recommended for this procedure.

> NOTE 1: NTUS are considered comparable to the previously reported
> Formazin Turbidity Units (FTU).

**2. SUMMARY OF METHOD**

> 2.1 The method is based upon a comparison of the intensity of light
> scatters by the sample under defined conditions with the intensity of
> the light scattered by a standard reference suspension. The higher the
> intensity of the scattered light, the higher the turbidity. Readings,
> in NTUs, are made in a nephelometer designed according to the
> specifications outlined in Apparatus. A standard suspension of
> Formazin, prepared under closely defined conditions, is used to
> calibrate the instrument.
>
> 2.1.1 Formazin polymer is used as the turbidity reference suspension
> for water because it is more reproducible than other types of
> standards previously used for turbidity standards.

**3. SAMPLE HANDLING AND PRESERVATION**

> 3.1 Collect each sample in a soft or hard plastic, or soft or hard
> glass container. Immediately refrigerate or ice the sample to 4C and
> analyze within 48 hours.

**4. Physical Characteristics**

4.1 The presence of floating debris and coarse sediments which settle
out rapidly will give low readings. Finely divided air bubbles will
affect the results in a positive manner.

> 4.2 The presence of color, that is the color of water which is due to
> dissolved substances which absorb light, will cause turbidities to be
> low, although this effect is generally not significant.

**5. APPARATUS**

5.1 The turbidimeter shall consist of a nephelometer with two light
sources for illuminating the sample and two detectors with a readout
device to indicate the intensity of light scattered at right angles to
the path of the incident light. The turbidimeter shall accomplish two
measurement phases every 0.5 seconds.

> In the first phase, light source shall pulse a beam of light directly
> into photodetector two. Simultaneously, photodetector one shall
> measure the light scattered at a 90 degree angle. Diffused light
> measured by photodetector two shall be called the reference signal
> while scattered light measured by photodetector one shall be called an
> active signal.
>
> In the second phase, light source two shall pulse a beam of light
> directly into photodetector one. Simultaneously, photodetector two
> shall measure the light scattered at a 90 degree angle. This time, the
> diffused light measured by photodetector one shall be called a
> reference signal and scattered light measured by photodetector two
> shall be called an active signal. The two phase measurement shall
> provide four independent measurements from the two light sources; two
> reference signals and two active signals. A ratiometric algorithm will
> then be used to calculate the turbidity value from these four
> readings.
>
> The turbidimeter should be designed that little stray light reaches
> the detector in the absence of turbidity and should be free from
> significant drift after a short warm-up period.
>
> 5.2 The sensitivity of the instrument should permit detection of a
> turbidity difference of 0.02 unit or less in waters with turbidities
> less than 1 unit. The instrument should measure from 0 to 40 units
> turbidity. Several ranges will be necessary to obtain both adequate
> coverage and sufficient sensitivity for low turbidities.
>
> 5.3 The sample tubes to be used with the available instrument must be
> of clear, colorless optical glass. They should be kept scrupulously
> clean, both inside and out, and discarded when the become scratched or
> etched. They must not be handled at all where the light strikes them,
> but should be provided with sufficient extra length, or with a
> protective case, so that they may be handled.
>
> 5.4 Any apparatus may be used provided that it complies with the
> following requirements:
>
> 5.4.1 the wavelength of the incident radiation shall be 860 nm;
>
> 5.4.2 the spectral bandwidth of the incident radiation shall be less
> than or equal to 60 nm;
>
> 5.4.3 there shall be no divergence from parallelism of the incident
> radiation and any convergence shall not exceed 1.5 degrees;
>
> 5.4.4 There shall be two light sources and two detectors
>
> 5.4.5 the measuring angle between the optical axis of the incident
> radiation and that of the diffused radiation for light pulsed through
> the sample by light source one shall be 90  2.5 degrees;
>
> 5.4.6 the measuring angle between the optical axis of the incident
> radiation and that of the diffused radiation for light pulsed through
> the sample by light source two shall be 90  2.5 degrees;
>
> The narrow definition of the light source makes it unnecessary to
> specify sensitivity of the photodetector.

**6. REAGENTS**

> 6.1 Turbidity-free water: Pass distilled water through a membrane
> filter having precision-sized holes of 0.2 m; the usual membrane
> filter used for bacterial examinations is not satisfactory (Sec. 3.1,
> EPA-approved Standard Method 214A, 16th edition).
>
> 6.2 Stock formazin turbidity suspension:
>
> Solution 1: Distilled 1.00 g hydrazine sulfate (NH~2~)H~2~SO~4,~, in
> distilled water and dilute to 100 ml in a volumetric flask.
>
> Solution2: Dissolve 10.00 g hexamethylenetetramine in distilled water
> and dilute to 100 ml in a volumetric flask. In a 100 ml volumetric
> flask, mix 5.0 ml Solution 1 with 5.0 ml Solution 2. Allow to stand 24
> hours at 24  degrees C, then dilute to the mark and mix.
>
> 6.3 Standard formazin turbidity suspension: Dilute 10.00 ml stock
> turbidity suspension to 100 ml with turbidity-free water. The
> turbidity of this suspension is defined as 40 units. Dilute portions
> of the standard turbidity suspension with turbidity-free water as
> required.

**7. PROCEDURE**

> 7.1 Turbidimeter calibration: The manufacturer=s operating
> instructions should be followed. Measure standards on the turbidimeter
> covering the range of interest. If the instrument is already
> calibrated in standard turbidity units, this procedure will check the
> accuracy of the calibration scales. At least one standard should be
> run in each instrument range to be used. Some instruments permit
> adjustments of sensitivity so that scale is not supplied, then
> calibration curves should be prepared for each range of the
> instrument.. If a pre-calibrated scale is not supplied, then
> calibration curves should be prepared for each range of the
> instrument.
>
> 7.2 Turbidities less than 40 units: Shake the sample thoroughly to
> disperse the solids. Wait until air bubbles disappear then pour the
> sample into the turbidimeter tube . Read the turbidity directly from
> the instrument scale or from the appropriate calibration curve.
>
> 7.3 Turbidities exceeding 40 units: Dilute the sample with one or more
> volumes of turbidity-free water until the turbidity falls below 40
> units. The turbidity of the original sample is then computed from the
> turbidity of the diluted sample and the dilution factor. For example
> if five volumes of turbidity-free water were added to 1 volume of
> sample, and the diluted sample showed a turbidity of 30 units, then
> the turbidity of the original sample was 180 units.

**8. CALCULATIONS**

> 8.1 Nephelometric turbidity units (NTU)
>
> = [A(B+C)]{.underline}
>
> C
>
> where: A = NTU found in the diluted sample
>
> B = volume of the dilution water, mL and
>
> C = sample volume taken for dilution, mL
>
> 8.2 Report results as follows:

+---------------------------------+------------------------------------+
| NTU                             | Record to Nearest                  |
+=================================+====================================+
| 0.0--1.0                        | 0.05                               |
|                                 |                                    |
| 1--10                           | 0.1                                |
|                                 |                                    |
| 10--40                          | 1                                  |
|                                 |                                    |
| 40--100                         | 5                                  |
|                                 |                                    |
| 100--400                        | 10                                 |
|                                 |                                    |
| 400--1,000                      | 50                                 |
|                                 |                                    |
| \>1,000                         | 100                                |
+---------------------------------+------------------------------------+

**9. PRECISION AND ACCURACY**

> 9.1 In a single laboratory, using surface waters filtered to minimum
> turbidity with 0.04 filters and dosed with formazin to levels of
> 0.47, 0.91, 5.6, 9.8, 39.3, 82.7 and 99.4 NTU, the  standard
> deviations were 0.007, 0.014, 0.1, 0.22, 0.45, and 0.83 units
> respectively.
>
> 9.2 Accuracy of the great Lakes Turbidity Method
>
> The range of the mean percent recoveries of turbidity from 10
> fortified drinking water samples, each analyzed in triplicate by the
> Great Lakes Instruments Turbidity Method, was as follows:

+-----------------------------------+-----------------------------------+
| Turbidity Added to Sample         | Range of Percent Recovery         |
+===================================+===================================+
| 4.5                               | 98.1--112.2                       |
|                                   |                                   |
| 9.5                               | 96.8--111.1                       |
|                                   |                                   |
| 34.5                              | 93.7--114.3                       |
+-----------------------------------+-----------------------------------+

PRECISION OF THE GREAT LAKES TURBIDITY METHOD

The range of the standard deviations and percent relative standard
deviations (or percent coefficient of variations) associated with the
triplicate observations of the total theoretical concentration of 10
fortified drinking water samples by the Great Lakes Instruments
turbidity Method were as follows:

+-----------------------+-----------------------+-----------------------+
| Total Theoretical     | Standard Deviation    | Relative Standard     |
| Turbidity (NTU)       | (NTU)                 | Deviation (%)         |
+-----------------------+-----------------------+-----------------------+
| 0.5                   | 0.01--0.06            | 1.1--10.7             |
|                       |                       |                       |
| 5.0                   | 0.00--0.10            | 0.0--2.0              |
|                       |                       |                       |
| 10.0                  | 0.00                  | 0.0--5.6              |
|                       |                       |                       |
| 35.0                  | 0.00--0.58            | 0.0--1.8              |
+-----------------------+-----------------------+-----------------------+

> 9.3 Accuracy and precision should be checked on a routine basis to
> monitor the overall performance of the instrument. A series of reagent
> blanks and check standards should be run to validate the quality of
> sample data. These checks should occur at frequency that is required
> for regulatory compliance.

**10. SAFETY**

> 10.1 Operators handling reagents should wear safety glasses, rubber
> gloves and appropriate protective clothing. Consult the Material
> Safety Data Sheets for additional safety information before working
> with reagents.

**11. QUALITY ASSURANCE**

> 11.1 Each laboratory using this method in regulated environmental
> monitoring is required to operate a formal quality assurance/control
> program. The minimum initial requirements of this program consist of
> the demonstration of the laboratory=s capability with this method. On
> a continuing basis, the laboratory should check its performance
> (accuracy and precision) by analyzing reagent blanks and check
> standards, fortified blanks, and/or fortified samples, preferably at a
> minimum frequency of 10% of the total samples analyzed by the method.
> The laboratory should maintain the performance records that define the
> quality of the data generated with the method.

**12. POLLUTION PREVENTION**

> 12.1 Solution samples should be used, collected and disposed of in
> accordance with all Federal, state and local regulations.

**13. WASTE MANAGEMENT**

> 13.1 In case of spill or release: Dilute with water. Dispose of in
> accordance with all Federal, state, and local regulations.

**14. References**

1\. ASTM. 1976. *Book of ASTM Standards.* Part 31. "Water." Standard
D1889-71.

2\. Standard Methods. 1975. *Standard Methods for the Examination of
Water and Wastewater*. Fourteenth Edition. Method 214A. APHA, USEPA, and
WEF. Port City Press, Baltimore, MD.

3\. Standard, Ref. No. ISO 7027-1984 (E).

4\. Standard Methods. 1985. *Standard Methods for the Examination of
Water and Wastewater*. Sixteenth Edition. Method 214A. APHA, USEPA, and
WEF. Port City Press, Baltimore, MD.

5\. USEPA. 1990. "Standard Methods for the Certification of Laboratories
Analyzing Drinking Water: Criteria and Procedures, Quality Assurance."
EPA/570/9-90/008. Office of Ground Water and Drinking Water, Cincinnati,
OH.