**A[ppendix]{.smallcaps} C. T[urbidity]{.smallcaps}
S[tandard]{.smallcaps}**

**M[ethod]{.smallcaps}**

2130 TURBIDITY[^1]

2130 A. Introduction

1\. Sources and Significance

Clarity of water is important in producing products destined for human
consumption and in many manufacturing operations. Beverage producers,
food processors, and potable water treatment plants drawing from a
surface water source commonly rely on fluid-particle separation
processes such as sedimentation and filtration to increase clarity and
insure an acceptable product. The clarity of a natural body of water is
an important determinant of its condition and productivity.

Turbidity in water is caused by suspended and colloidal matter such as
clay, silt, finely divided organic and inorganic matter, and plankton
and other microscopic organisms. Turbidity is an expression of the
optical property that causes light to be scattered and absorbed rather
than transmitted with no change in direction or flux level through the
sample. Correlation of turbidity with the weight or particle number
concentration of suspended matter is difficult because the size, shape,
and refractive index of the particles affect the light-scattering
properties of the suspension. When present insignificant concentrations,
particles consisting of light-absorbing materials such as activated
carbon cause a negative interference. In low concentrations these
particles tend to have a positive influence because they contribute to
turbidity. The presence of dissolved, color-causing substances that
absorb light may cause a negative interference. Some commercial
instruments may have the capability of either correcting for a slight
color interference or optically blanking out the color effect.

2\. Selection of Method

Historically, the standard method for determination of turbidity has
been based on the Jackson candle turbidimeter; however, the lowest
turbidity value that can be measured directly on this device is 25
Jackson Turbidity Units (JTU). Because turbidities of water treated by
conventional fluid-particle separation processes usually fall within the
range of 1 to 1 unit, indirect secondary methods were developed to
estimate turbidity. Electronic nephelometers are the preferred
instruments for turbidity measurement.

Most commercial turbidimeters designed for measuring low turbidities
give comparatively good indications of the intensity of light scattered
in one particular direction, predominantly at right angles to the
incident light. Turbidimeters with scattered-light detectors located at
90 to the incident beam are called nephelometers. Nephelometers are
relatively unaffected by small differences in design parameters and
therefore are specified as the standard instrument for measurement of
low turbidities. Instruments of different make and model may vary in
response.^^ However, interinstrument variation may be effectively
negligible if good measurement techniques are used and the
characteristics of the particles in the measured suspensions are
similar. Poor measurement technique can have a greater effect on
measurement error than small differences in instrument design.
Turbidimeters of nonstandard design, such as forward-scattering devices,
may be more sensitive than nephelometers to the presence of larger
particles. While it may not be appropriate to compare their output with
that of instruments of standard design, they still may be useful for
process monitoring.

An additional cause of discrepancies in turbidity analysis is the use of
suspensions of different types of particulate matter for instrument
calibration. Like water samples, prepared suspensions have different
optical properties depending on the particle size distributions, shapes,
and refractive indices. A standard reference suspension having
reproducible light-scattering properties is specified for nephelometer
calibration.

Its precision, sensitivity, and applicability over a wide turbidity
range make the nephelometric method preferable to visual methods. Report
nephelometric measurement results as nephelometric turbidity units
(NTU).

3\. Storage of Sample

Determine turbidity as soon as possible after the sample is taken.
Gently agitate all samples before examination to ensure a representative
measurement. Sample preservation is not practical; begin analysis
promptly. Refrigerate or cool to 4C, to Minimize microbiological
decomposition of solids, if storage is required. For best results,
measure turbidity immediately without altering the original sample
conditions such as temperature or pH.

2130 B. Nephelometric Method

1\. General Discussion

*a. Principle:* this method is based on a comparison of the intensity of
light scattered by the sample under defined conditions with the
intensity of light scattered by a standard reference suspension under
the same conditions. The higher the intensity of scattered light, the
higher the turbidity. Formazin polymer is used as the primary standard
reference suspension. The turbidity of a specified concentration of
formazin suspension is defined as 4000 NTU.

*b. Interference:* Turbidity can be determined for any water sample that
is free of debris and rapidly settling coarse sediment. Dirty glassware
and the presence of air bubbles give false results. "True color," i.e.,
water color due to dissolved substances that absorb light, causes
measured turbidities to be low. This effect usually is not significant
in treated water.

1.  Apparatus

*a. Laboratory or process nephelometer* consisting of a light source for
illuminating the sample and one or more photoelectric detectors with a
readout device to indicate intensity of light scattered at 90 to the
path of incident light. Use an instrument designed to minimize stray
light reaching the detector in the absence of turbidity and to be free
from significant drift after a short warmup period. the sensitivity of
the instrument should permit detecting turbidity differences of 0.02 NTU
or less in the lowest range in waters having a turbidity of less than 1
NTU. Several ranges may be necessary to obtain both adequate coverage
and sufficient sensitivity for low turbidities. Differences in
instrument design will cause differences in measured values for
turbidity even though the same suspension is used for calibration. To
minimize such differences, observe the following design criteria:

1\) Light source--- Tungsten-filament lamp operated at a color
temperature between 2200 and 3000K.

2\) Distance traversed by incident light and scattered light within the
sample tube--- Total not to exceed 10 cm.

3\) Angle of light acceptance by detector--- Centered at 90 to the
incident light path and not to exceed 30 from 90. The detector and
filter system, if used, shall have a spectral peak response between 400
and 600 nm.

*b. Sample cells:* Use sample cells or tubes of clear, colorless glass
or plastic. Keep cells scrupulously clean, both inside and out, and
discard if scratched or etched. Never handle them where the instrument's
light beam will strike them. Use tubes with sufficient extra length, or
with a protective case, so that they may be handled properly. fill cells
with samples and standards that have been agitated thoroughly and allow
sufficient time for bubbles to escape.

Clean sample cells by thorough washing with laboratory soap inside and
out followed by multiple rinses with distilled or deionized water; let
cells air-dry. Handle sample cells only by the top to avoid dirt and
fingerprints within the light path.

Cells may be coated on the outside with a thin layer of silicone oil to
mask minor imperfections and scratches that may contribute to stray
light. Use silicone oil with the same refractive index as glass. Avoid
excess oil because it may attract dirt and contaminate the sample
compartment of the instrument. Using a soft, lint-free cloth, spread the
oil uniformly and wipe off excess. The cell should appear to be nearly
dry with little or no visible oil.

Because small differences between sample cells significantly impact
measurement, use either matched pairs of cells or the same cell for both
standardization and sample measurement.

3\. Reagents

*a. Dilution water:* High-purity water will cause some light scattering,
which is detected by nephelometers as turbidity. To obtain low-turbidity
water for dilutions, nominal value 0.02 NTU, pass laboratory
reagent-grade water through a filter with pore size sufficiently small
to remove essentially all particles larger than 0.1 m;[^2] the usual
membrane filter used for bacteriological examination is not
satisfactory. rinse collecting flask at least twice with filtered water
and discard the next 200 mL.

Some commercial bottled demineralized waters have a low turbidity. These
may be used when filtration is impractical or a good grade of water is
not available to filter in the laboratory. Check turbidity of bottled
water to make sure it is lower than the level that can be achieved in
the laboratory.

*b. Stock primary standard formazin suspension:*

1\) Solution I--- Dissolve 1.000 g hydrazine sulfate,
(NH~2~)~2~H~2~SO~4~, in distilled water and dilute to 100 mL in a
volumetric flask. C[aution]{.smallcaps}: *Hydrazine sulfate is a
carcinogen; avoid inhalation, ingestion, and skin contact. Formazin
suspensions can contain residual hydrazine sulfate.*

2\) Solution II--- Dissolve 10.00 g hexamethylenetetramine,
(CH~2~)~6~N~4~, in distilled water and dilute to 100 mL in a volumetric
flask.

3\) In a flask, mix 5.0 mL Solution I and 5.0 mL Solution II. Let stand
for 24 h at 25  3C. This results in a 4000-NTU suspension. Transfer
stock suspension to an amber glass or other UV-light-blocking bottle for
storage. Make dilutions from this stock suspension. The stock suspension
is stable for up to 1 year when properly stored.

*c. Dilute turbidity suspensions:* Dilute 4000 NTU primary standard
suspension with high-quality dilution water. Prepare immediately before
use and discard after use.

*d. Secondary standards:* Secondary standards are standards that the
manufacturer (or an independent testing organization) has certified will
give instrument calibration results equivalent (within certain limits)
to the results obtained when the instrument is calibrated with the
primary standard, i.e., user-prepared formazin. Various secondary
standards are available including: commercial stock suspensions of 4000
NTU formazin, commercial suspensions of microspheres of
styrene-divinylbenzene co-polymer, [^3] and items supplied by instrument
manufacturers, such as sealed sample cells filled with latex suspension
or with metal oxide particles in a polymer gel. The U.S. Environmental
Protection Agency^1^ designates user-prepared formazin, commercial stock
formazin suspensions, and commercial styrene-divinylbenzene suspensions
as "primary standards," and reserves the term "secondary standard" for
the sealed standards mentioned above.

Secondary standards made with suspensions of microspheres of
styrene-divinylbenzene coploymer typically are as stable as concentrated
formazin and are much more stable than diluted formazin. Thes
suspensions can be instrument-specific; therefore, use only suspensions
formulated for the type of nephelometer being used. Secondary standards
provided by the instrument manufacturer (sometimes called "permanent"
standards) many be necessary to standardize some instruments before each
reading and in other instruments only as a calibration check to
determine when calibration with the primary standard is necessary.

All secondary standards, even so-called "permanent" standards, change
with time. replace them when their age exceeds the shelf life.
Deterioration can be detected by measuring the turbidity of the standard
after calibrating the instrument with a fresh formazin or microsphere
suspension. If there is any doubt about the integrity or turbidity value
of any secondary standard, check instrument calibration first with
another secondary standard and then, if necessary, with user-prepared
formazin. Most secondary standards have been carefully prepared by their
manufacturer and should, if properly used, give good agreement with
formazin. Prepare formazin primary standard only as a last resort.
Proper application of secondary standards is specific for each make and
model of nephelometer. Not all secondary standards have to be discarded
when comparison with a primary standard shows that their turbidity value
has changed. In some cases, the secondary standard should be simply
relabeled with the new turbidity value. Always follow the manufacturer's
directions.

4\. Procedure

*a. General measurement techniques:* Proper techniques are important in
minimizing the effects of instrument variables as well as stray light
and air bubbles. Regardless of the instrument used, the measurement will
be more accurate, precise, and repeatable if close attention is paid to
proper measurement techniques.

Measure tubidity immediately to prevent temperature changes and particle
flocculation and sedimentation from changing sample characteristics. if
flocculation is apparent, break up aggregates by agitation. Avoid
dilution whenever possible. Particles suspended in the original sample
may dissolve or otherwise change characteristics when the temperature
changes or when the sample is diluted.

Remove air or other entrained gases in the sample before measurement.
Preferably degas even if no bubbles are visible. Degas by applying a
partial vacuum, adding a nonfoaming-type surfactant, using an ultrasonic
bath, or applying heat. In some cases, two or more of these techniques
may be combined for more effective bubble removal. For example, it may
be necessary to combine addition of a surfactant with use of an
ultrasonic bath for some severe conditions. Any of these techniques, if
misapplied, can alter sample turbidity; *use with care*. If degassing
cannot be applied, bubble formation will be minimized if the samples are
maintained at the temperature and pressure of the water before sampling.

Do not remove air bubbles by letting sample stand for a period of time
because during standing, turbidity-causing particulates may settle and
sample temperature may change. Both of these conditions alter sample
turbidity, resulting in a nonrepresentative measurement.

Condensation may occur on the outside of surface of a sample cell when a
cold sample is being measured in a warm, humid environment. This
interferes with turbidity measurement. Remove all moisture form the
outside of the sample cell before placing the cell in the instrument. If
fogging recurs, let sample warm slightly by letting it stand at room
temperature or by partially immersing it in a warm water bath for a
short time. Make sure samples are again well mixed.

*b. Nephelometer calibration:* Follow the manufacturer\'s operating
instructions. Run at least one standard I each instrument range to be
used. Make certain the nephelometer gives stable reading in all
sensitivity ranges used. Follow techniques outlined in ¶s 2*b* and 4*a*
for care and handling of sample cells, degassing, and dealing with
condensation.

*c. Measurement of turbidity:* Gently agitate sample. Wait until air
bubbles disappear and pour sample into cell. When possible, pour well
mixed sample into cell and immerse it in an ultrasonic bath for 1 to 2 s
or apply vacuum degassing, causing complete bubble release. Read
turbidity directly form instrument display.

*d. Calibration of continuous turbidity monitors:* Calibrate continuous
turbidity monitors for low turbidities by determining turbidity of the
water flowing out of them, using a laboratory-model nephelometer, or
calibrate the instruments according to manufacturer\'s instructions with
formazin primary standard or appropriate secondary standard.

5\. Interpretation of Results

Report turbidity readings as follows:

+-----------------------------------+-----------------------------------+
| Turbidity Range                   | Report to the                     |
|                                   |                                   |
| *NTU*                             | Nearest                           |
|                                   |                                   |
|                                   | *NTU*                             |
+-----------------------------------+-----------------------------------+
| 0-1.0                             | 0.05                              |
+-----------------------------------+-----------------------------------+
| 1-10                              | 0.1                               |
+-----------------------------------+-----------------------------------+
| 10-40                             | 1                                 |
+-----------------------------------+-----------------------------------+
| 40-100                            | 5                                 |
+-----------------------------------+-----------------------------------+
| 100-400                           | 10                                |
+-----------------------------------+-----------------------------------+
| 400-1000                          | 50                                |
+-----------------------------------+-----------------------------------+
| \>1000                            | 100                               |
+-----------------------------------+-----------------------------------+

When comparing water treatment efficiencies, do not estimate turbidity
more closely than specified above. Uncertainties and discrepancies in
turbidity measurements make it unlikely that results can be duplicated
to greater precision than specified.

6\. Reference

1.  U.S. E[nvironmental]{.smallcaps} P[rotection]{.smallcaps}
    A[gency]{.smallcaps}, 1993. Methods for Determination of Inorganic
    Substances in Environmental Samples. EPA-600/R/93/100 - Draft.
    Environmental Monitoring Systems Lab., Cincinnati, Ohio.

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7.  Bibliography

Hach, C.C., R.D. Vanous & J.M. Heer, 1985. Understanding Turbidity
Measurement, Hach Co., Technical Information Ser., Booklet 11, Loveland,
Colo.

Katz, E.L. 1986. The stability of turbidity in raw water and its
relation ship to chlorine demand. J. Amer. Water Works Assoc. 78:72.

McCoy, W.F. & B.H. Olson, 1986. Relation ship among turbidity particle
counts and bacteriological quality within water distribution lines.
Water Res. 20:1023.

Bucklin, K.E., G.A. McFeters & A. Amirtharajah, 1991. Penetration of
coliform through municiopal drinking water filters. Water Res. 25:1013.

Hernandez, E., R.A. Baker & P.C. Crandall, 1991. Model for evaluation
turbidity in cloudy beerages. J. Food Sci. 56:747.

Hart, V.S., C.E. Johnson & R.D. Letterman, 1992. An analysis of
low-level turbidity measurements. J. Amer. Water Works Assoc.,
84(12);40.

LeChevallier, M.W. & W.D. Norton, 1992. Examining relationship between
particle counts and Giardia, Cryptosporidium, and turbidity. J. Amer.
Water Works Assoc. 84(12):54.

2150 ODOR[^4]

2150A. Introduction

1\. Discussion

Odor, like taste, depends on contact of a stimulating substance with the
appropriate human receptor cell. The stimuli are chemical in nature and
the term \"chemical senses\" often is applied to odor and taste. Water
is a neutral medium, always present on or at the receptors that perceive
sensory response. In its pure form, water cannot produce odor or taste
sensations. Man and other animals can avoid many potentially toxic foods
and waters because of adverse sensory response. These senses often
provide the first warning of potential hazards in the environment.

Odor is recognized^1^ as a quality factor affecting acceptability of
drinking water (and foods prepared with it), tainting of fish and other
aquatic organisms, and esthetics of recreational waters. Most organic
and some inorganic chemicals contribute taste or odor. These chemicals
may originate form municipal and industrial waste discharges, form
natural sources such as decomposition of vegetable matter, or from
associated microbial activity, and from disinfectants or their products.

The potential for impairment of the sensory quality of water has
increased as a result of expansion in the variety and quantity of waste
materials, demand for water disposal of captured air pollutants, and
increased reuse of available water supplies by a growing population.
Domestic consumers and process industries such as food, beverage, and
pharmaceutical manufacturers require water essentially free of tastes an
odors.

Some substances, such as certain inorganic salts, produce taste without
odor and are evaluated by taste testing (Section 2160). Many other
sensations ascribed to the sense of taste actually are odors, even
though the sensation is not noticed until the material is taken into the
mouth. Because some odorous materials are detectable when present in
only a few nanograms per liter, it is usually impractical and often
impossible to isolate and identify the odor-producing chemical. The
ultimate odor-testing device is the human nose. Odor tests are performed
to provide qualitative descriptions and approximate quantitative
measurements of odor intensity. The method for intensity measurement
presented here is the *threshold odor* test, based on a method of
limits.^2^ This procedure, while not universally preferred,^3^ has
definite strengths.^4^

Sensory tests are useful as a check on the quality of raw and finished
water and for control of odor through the treatment process. They can
assess the effectiveness of different treatments and provide a means of
tracing the source of contamination.

Section 6040B provides an analytical procedure for quantifying several
organic odor-producing compounds including geosmin and methylisoborneol.

2\. References

1.  [U.S. Environmental Protection Agency]{.smallcaps}, 1973. Proposed
    Criteria for Water Quality. Vol. 1. Washington, D.C.

2.  [American Society for Testing and Materials Committee
    E-18]{.smallcaps}, 1968. STP 433, Basic principles of sensory
    evaluation: STP 434, Manual on sensory testing methods; STP 440,
    Correlation of subjective-

[^1]: ^^ Approved by Standard Methods Committee. 1994.

    ^^ Nephelometers that instrument manufacturers claim meet the
    design specifications of this method may not give the same reading
    for a given suspension, even when each instrument has been
    calibrated using the manufacturer's manual. This differential
    performance is especially important when measurements are made for
    regulatory purposes. Consult regulatory authorities when selecting a
    nephelometer to be used for making measuremts that will be reported
    for regulatory purposes.

[^2]: ^\*^ Nuclepore Corp., 7035 Commerce Circle, Pleasanton, Calif., or
    equivalent.

[^3]: ^^ AMCO-AEPA-1 Standard, Advanced Polymer Systems, 6396 Haven
    Ave., Redwood City, Calif., or equivalent.

[^4]: ^^ Approved by Standard Methods Committee, 1991.