# Importance of Turbidity

## 7.1 Overview {#overview .unnumbered}

Section 2 of this guidance manual is included to present an overview on
the definition and sources of turbidity. Understanding turbidity, its
causes and sources, and the significance to human health will provide
the background on which the new turbidity standards are based.

## 7.2 Turbidity: Definition, Causes, and History as a Water Quality Parameter {#turbidity-definition-causes-and-history-as-a-water-quality-parameter .unnumbered}

Turbidity is a principal physical characteristic of water and is an
expression of the optical property that causes light to be scattered and
absorbed by particles and molecules rather than transmitted in straight
lines through a water sample. It is caused by suspended matter or
impurities that interfere with the clarity of the water. These
impurities may include clay, silt, finely divided inorganic and organic
matter, soluble colored organic compounds, and plankton and other
microscopic organisms. Typical sources of turbidity in drinking water
include the following (see Figure 7-1):

-   Waste discharges;

-   Runoff from watersheds, especially those that are disturbed or
    eroding;

-   Algae or aquatic weeds and products of their breakdown in water
    reservoirs, rivers, or lakes;

-   Humic acids and other organic compounds resulting from decay of
    plants, leaves, etc. in water sources; and

-   High iron concentrations which give waters a rust-red coloration
    (mainly in ground water and ground water under the direct influence
    of surface water).

-   Air bubbles and particles from the treatment process (e.g.,
    hydroxides, lime softening)

Simply stated, turbidity is the measure of relative clarity of a liquid.
Clarity is important when producing drinking water for human consumption
and in many manufacturing uses. Once considered as a mostly aesthetic
characteristic of drinking water, significant evidence exists that
controlling turbidity is a competent safeguard against pathogens in
drinking water.

![](media/image1.png){width="5.995833333333334in"
height="3.9652777777777777in"}

[]{#__RefHeading___Toc448537940 .anchor}Figure 7-1. Typical Sources of
Turbidity in Drinking Water

The first practical attempts to quantify turbidity date to 1900 when
Whipple and Jackson developed a standard suspension fluid using 1,000
parts per million (ppm) of diatomaceous earth in distilled water (Sadar,
1996). Dilution of this reference suspension resulted in a series of
standard suspensions, which were then used to derive a ppm-silica scale
for calibrating turbidimeters.

The standard method for determination of turbidity is based on the
Jackson candle turbidimeter, an application of Whipple and Jackson\'s
ppm-silica scale (Sadar, 1996). The Jackson candle turbidimeter consists
of a special candle and a flat-bottomed glass tube (Figure 7-2), and was
calibrated by Jackson in graduations equivalent to ppm of suspended
silica turbidity. A water sample is poured into the tube until the
visual image of the candle flame, as viewed from the top of the tube, is
diffused to a uniform glow. When the intensity of the scattered light
equals that of the transmitted light, the image disappears; the depth of
the sample in the tube is read against the ppm-silica scale, and
turbidity was measured in Jackson turbidity units (JTU). Standards were
prepared from materials found in nature, such as Fuller\'s earth,
kaolin, and bed sediment, making consistency in formulation difficult to
achieve.

![](media/image2.wmf){width="2.3055555555555554in"
height="7.583333333333333in"}

Source: Sadar, 1996.

[]{#__RefHeading___Toc448537941 .anchor}Figure 7-2. Jackson Candle
Turbidimeter

In 1926, Kingsbury and Clark discovered formazin, which is formulated
completely of traceable raw materials and drastically improved the
consistency in standards formulation. Formazin is a suitable suspension
for turbidity standards when prepared accurately by weighing and
dissolving 5.00 grams of hydrazine sulfate and 50.0 grams of
hexamethylenetetramine in one liter of distilled water. The solution
develops a white hue after standing at 25C for 48 hours. A new unit of
turbidity measurement was adopted called formazin turbidity units (FTU).

Even though the consistency of formazin improved the accuracy of the
Jackson Candle Turbidimeter, it was still limited in its ability to
measure extremely high or low turbidity. More precise measurements of
very low turbidity were needed to define turbidity in samples containing
fine solids. The Jackson Candle Turbidimeter is impractical for this
because the lowest turbidity value on this instrument is 25 JTU. The
method is also cumbersome and too dependent on human judgement to
determine the exact extinction point.

Indirect secondary methods were developed to estimate turbidity. Several
visual extinction turbidimeters were developed with improved light
sources and comparison techniques, but all were still dependent of human
judgement. Photoelectric detectors became popular since they are
sensitive to very small changes in light intensity. These methods
provided much better precision under certain conditions, but were still
limited in ability to measure extremely high or low turbidities.

Finally, turbidity measurement standards changed in the 1970\'s when the
nephelometric turbidimeter, or nephelometer, was developed which
determines turbidity by the light scattered at an angle of 90 from the
incident beam (Figure 7-3). A 90 detection angle is considered to be
the least sensitive to variations in particle size. Nephelometry has
been adopted by *Standard Methods* as the preferred means for measuring
turbidity because of the method\'s sensitivity, precision, and
applicability over a wide range of particle size and concentration. The
nephelometric method is calibrated using suspensions of formazin polymer
such that a value of 40 nephelometric units (NTU) is approximately equal
to 40 JTU (AWWARF, 1998). The preferred expression of turbidity is NTU.

## Turbidity\'s Significance to Human Health

Excessive turbidity, or cloudiness, in drinking water is aesthetically
unappealing, and may also represent a health concern. Turbidity can
provide food and shelter for pathogens. If not removed, turbidity can
promote regrowth of pathogens in the distribution system, leading to
waterborne disease outbreaks, which have caused significant cases of
gastroenteritis throughout the United States and the world. Although
turbidity is not a direct indicator of health risk, numerous studies
show a strong relationship between removal of turbidity and removal of
protozoa.

![](media/image3.wmf){width="6.063194444444444in"
height="3.0555555555555554in"}

Source: Sadar, 1996; photo revised by SAIC, 1998.

[]{#__RefHeading___Toc448537942 .anchor}Figure 7-3. Nephelometric
Turbidimeter

The particles of turbidity provide "shelter" for microbes by reducing
their exposure to attack by disinfectants (Figure 7-4). Microbial
attachment to particulate material or inert substances in water systems
has been documented by several investigators (Marshall, 1976; Olson et
al., 1981; Herson et al., 1984) and has been considered to aid in
microbe survival (NAS, 1980). Fortunately, traditional water treatment
processes have the ability to effectively remove turbidity when operated
properly.

### Waterborne Disease Outbreaks

Notwithstanding the advances made in water treatment technology,
waterborne pathogens have caused significant disease outbreaks in the
United States and continue to pose a significant problem. Even in
developed countries, protozoa have been identified as the cause of half
of the recognized waterborne outbreaks (Rose et al., 1991). The most
frequently reported waterborne disease in the United States is acute
gastrointestinal illness, or gastroenteritis (Huben, 1991). The symptoms
for this disease include fever, headache, gastrointestinal discomfort,
vomiting, and diarrhea. Gastroenteritis is usually self-limiting, with
symptoms lasting one to two weeks in most cases. However, if the immune
system is suppressed, as with the young, elderly and those suffering
from HIV or AIDS, the condition can be very serious and even life
threatening. The causes are usually difficult to identify but can be
traced to various viruses, bacteria, or protozoa.

![](media/image4.wmf){width="5.416666666666667in"
height="4.666666666666667in"}

Source: LeChevallier and Norton, 1991.[]{#__RefHeading___Toc448537943
.anchor}

Figure 7-4. Particles of Turbidity May Provide Protection for
Microorganisms

*Giardia* and *Cryptosporidium* are the two most studied organisms known
to cause waterborne illnesses. These two protozoa are believed to be
ubiquitous in source water, are known to occur in drinking water
systems, have been responsible for the majority of waterborne outbreaks,
and treatments to remove and/or inactivate them are known to be
effective for a wide range of waterborne parasites (LeChevallier and
Norton, in Craun, 1993). *Giardia* and *Cryptosporidium* have caused
over 400,000 persons in the United States to become ill since 1991,
mostly due to a 1993 outbreak in Milwaukee, Wisconsin.

*Giardia* and viruses are addressed under the 1989 SWTR. Systems using
surface water must provide adequate treatment to remove and/or
inactivate at least 3-log (99.9%) of the *Giardia lamblia* cysts and at
least 4-log (99.99%) of the enteric viruses. However, *Cryptosporidium*
was not addressed in the SWTR due to lack of occurrence and health
effects data. In the mid-1980\'s, the United States experienced its
first recognized waterborne disease outbreak of cryptosporidiosis
(D\'Antonio et al., 1985). It was soon discovered that the presence of
*Cryptosporidium* in drinking water, even in very low concentrations,
could be a significant health hazard (Gregory, 1994). In 1993, a major
outbreak of cryptosporidiosis occurred even though the system was in
full compliance with the SWTR. Several outbreaks caused by this pathogen
have been reported (Smith et al., 1988; Hayes at al., 1989; Levine and
Craun, 1990; Moore et al., 1993; Craun, 1993). The ESWTR\'s primary
focus is to establish treatment requirements to further address public
health risks from pathogen occurrence, and in particular,
*Cryptosporidium*.

Table 7-1 displays several instances of past outbreaks of
cryptosporidiosis in systems using surface water as a source, along with
general information about the plant and turbidity monitoring. In three
out of four of the cases displayed in the table (Milwaukee, Jackson
County, and Carrollton), turbidity over 1.0 NTU was occurring in
finished water during the outbreaks.

[]{#__RefHeading___Toc448537951 .anchor}Table 7-1. Cryptosporidium
Outbreaks vs. Finished Water Turbidity

+----------------+----+-------------------------+---------------------+
| Location of    | Ye | General Plant           | Turbidity           |
| Outbreak       | ar | Information             | Information         |
+================+====+=========================+=====================+
| Las Vegas,     | 1  | No apparent             | The raw water       |
| Nevada         | 99 | deficiencies or         | averaged 0.14 NTU   |
|                | 3- | problems with this      | between January     |
| (CDC, 1996)    | 19 | community system; SWTR  | 1993 and June 1995, |
|                | 94 | compliant; system       | with a high of 0.3  |
|                |    | performed               | NTU; the maximum    |
|                |    | pre-chlorination,       | turbidity of        |
|                |    | filtration (sand and    | finished water      |
|                |    | carbon), and filtration | during this time    |
|                |    | of lake water; outbreak | was 0.17 NTU.       |
|                |    | affected mostly persons |                     |
|                |    | infected with the human |                     |
|                |    | immunodeficiency virus  |                     |
|                |    | (HIV)                   |                     |
+----------------+----+-------------------------+---------------------+
| Milwaukee,     | 19 | Community system; SWTR  | Dramatic temporary  |
| Wisconsin      | 93 | compliant; however,     | increase in         |
|                |    | deterioration in source | finished water      |
| (CDC, 1996,    |    | (lake) raw-water        | turbidity levels;   |
|                |    | quality and decreased   | reported values     |
| Logsdon, 1996) |    | effectiveness of the    | were as high as 2.7 |
|                |    | coagulation-filtration  | NTU. (Turbidity had |
|                |    | process                 | never exceeded 0.4  |
|                |    |                         | NTU in the previous |
|                |    |                         | 10 years.)          |
+----------------+----+-------------------------+---------------------+
| Jackson        | 19 | Poor plant performance  | Earlier in the year |
| County, Oregon | 92 | (excessive levels of    | when outbreak       |
|                |    | algae and debris); no   | occurred, filtered  |
| (USEPA, 1997)  |    | pre-chlorination before | water had averaged  |
|                |    | filtration              | 1 NTU or greater.   |
+----------------+----+-------------------------+---------------------+
| Carrollton,    | 19 | Conventional filtration | Filtered water      |
| Georgia        | 87 | plant; sewage           | turbidity from one  |
|                |    | overflowed into water   | filter reached 3    |
| (USEPA, 1997,  |    | treatment intake,       | NTU about three     |
|                |    | followed by operational | hours after it was  |
| Logsdon, 1996) |    | irregularities in       | returned to service |
|                |    | treatment; filters were | without being       |
|                |    | placed back into        | washed.             |
|                |    | service without being   |                     |
|                |    | backwashed.             |                     |
+----------------+----+-------------------------+---------------------+

### The Relationship Between Turbidity Removal and Pathogen Removal

Low filtered water turbidity can be correlated with low bacterial counts
and low incidences of viral disease. Positive correlations between
removal (the difference between raw and plant effluent water samples) of
pathogens and turbidity have also been observed in several studies. In
fact, in every study to date where pathogens and turbidity occur in the
source water, pathogen removal coincides with turbidity/particle removal
(Fox, 1995).

As an example, data gathered by LeChevallier and Norton (in Craun, 1993)
from three drinking water treatment plants using different watersheds
indicated that for every log removal of turbidity, 0.89 log removal was
achieved for the parasites *Cryptosporidium* and *Giardia* (Figures 7-5
and 7-6). Of course, this exact relationship does not hold for all
treatment plants. Table 7-2 lists several other studies in addition to
LeChevallier and Norton\'s, and their conclusions on the relationship of
turbidity to protozoan removal.

All studies in Table 7-2 show turbidity as a useful predictor of
parasite removal efficiency. This evidence suggests that although a very
low turbidity value does not completely ensure that particles are
absent, it is an excellent measure of plant optimization to ensure
maximum public health protection.

![](media/image5.wmf){width="4.305555555555555in"
height="3.0694444444444446in"}

Source: LeChevallier and Norton, 1991.

[]{#__RefHeading___Toc448537944 .anchor}Figure 7-5. Relationship Between
Removal of *Giardia* and Turbidity

![](media/image6.wmf){width="4.114583333333333in"
height="2.7041666666666666in"}

Source: LeChevallier and Norton, 1991.[]{#__RefHeading___Toc448537945
.anchor}

Figure 7-6. Relationship Between Removal of *Cryptosporidium* and
Turbidity

[]{#__RefHeading___Toc448537952 .anchor}Table 7-2. Studies on the
Relationship between Turbidity Removal\
and Protozoa Removal

+---------------+------------------------------------------------------+
| Re            | Discovery/Conclusion on Turbidity                    |
| ference/Study |                                                      |
+===============+======================================================+
| Patania et    | Four systems using rapid granular filtration, when   |
| al., 1995\*   | treatment conditions were optimized for turbidity    |
|               | and particle removal, achieved a median turbidity    |
|               | removal of 1.4 log and median particle removal of 2  |
|               | log. The median cyst and oocyst removal was 4.2 log. |
|               | A filter effluent turbidity of less than 0.1 NTU or  |
|               | less resulted in the most effective cyst removal, by |
|               | up to 1.0 log greater than when filter effluent      |
|               | turbidities were greater than 0.1 NTU (within the    |
|               | 0.1 to 0.3 NTU range).                               |
+---------------+------------------------------------------------------+
| Nieminski and | [Pilot plant study]{.underline}: Source water        |
| Ongerth,      | turbidity averaged 4 NTU (maximum = 23 NTU),         |
| 1995\*        | achieving filtered water turbidities of 0.1-0.2 NTU. |
|               | *Cryptosporidium* removals averaged 3.0 log for      |
|               | conventional treatment and 3.0 log for direct        |
|               | filtration, while *Giardia* removals averaged 3.4    |
|               | log for conventional treatment and 3.3 log for       |
|               | direct filtration.                                   |
|               |                                                      |
|               | [Full scale plant study]{.underline}: Source water   |
|               | had turbidities typically between 2.5 and 11 NTU     |
|               | (with a peak level of 28 NTU), achieving filtered    |
|               | water turbidities of 0.1-0.2 NTU. *Cryptosporidium*  |
|               | removals averaged 2.25 log for conventional          |
|               | treatment and 2.8 log for direct filtration, while   |
|               | *Giardia* removals averaged 3.3 log for conventional |
|               | treatment and 3.9 log for direct filtration.         |
+---------------+------------------------------------------------------+
| Ongerth and   | Using very low-turbidity source waters (0.35 to 0.58 |
| Pecoraro,     | NTU), 3 log removal for both cysts were obtained,    |
| 1995\*        | with optimal coagulation. (With intentionally        |
|               | suboptimal coagulation, the removals were only 1.5   |
|               | log for *Cryptosporidium* and 1.3 log for            |
|               | *Giardia*.)                                          |
+---------------+------------------------------------------------------+
| LeChavallier  | Data gathered from three drinking water treatment    |
| and Norton    | plants using different watersheds indicated that for |
| (in Craun,    | every log removal of turbidity, 0.89 log removal was |
| 1993)         | achieved for *Cryptosporidium* and *Giardia*.        |
+---------------+------------------------------------------------------+
| Nieminski,    | A high correlation (r^2^=0.91) exists between        |
| 1992          | overall turbidity removal and both *Giardia* and     |
|               | *Cryptosporidium* removal through conventional water |
|               | treatment.                                           |
+---------------+------------------------------------------------------+
| Ongerth, 1990 | *Giardia* cyst removal by filtration of              |
|               | well-conditioned water results in 90% or better      |
|               | turbidity reduction, which produces effective cyst   |
|               | removal of 2-log (99%) or more.                      |
+---------------+------------------------------------------------------+
| LeChavallier  | In a study of 66 surface water treatment plants      |
| et al.,       | using conventional treatment, most of the utilities  |
| 1991\*        | achieved between 2 and 2.5 log removals for both     |
|               | *Cryptosporidium* and *Giardia*, and a significant   |
|               | correlation (p=0.01) between removal of turbidity    |
|               | and *Cryptosporidium* existed.                       |
+---------------+------------------------------------------------------+
| LeChavallier  | In source water turbidities ranging from 1 to 120    |
| and Norton,   | NTU, removal achieved a median of 2.5 log for        |
| 1992\*        | *Cryptosporidium* and *Giardia* at varying stages of |
|               | treatment optimization. The probability of detecting |
|               | cysts and oocysts in finished water supplies         |
|               | depended on the number of organisms in the raw       |
|               | water; turbidity was a useful predictor of *Giardia* |
|               | and *Cryptosporidium* removal.                       |
+---------------+------------------------------------------------------+
| Foundation    | Raw water turbidity ranged from 1 to 30 NTU, and     |
| for Water     | *Cryptosporidium* removal was between 2 and 3 log.   |
| Research,     | Investigators concluded that any measure which       |
| 1994\*        | reduces filter effluent turbidity should reduce risk |
|               | from *Cryptosporidium*.                              |
+---------------+------------------------------------------------------+
| Hall et al.,  | Any measure which reduces filtrate turbidity will    |
| 1994          | reduce the risk from *Cryptosporidium*; a sudden     |
|               | increase in the clarified water turbidity may        |
|               | indicate the onset of operational problems with a    |
|               | consequent risk from cryptosporidiosis.              |
+---------------+------------------------------------------------------+
| Gregory, 1994 | Maintaining the overall level of particulate         |
|               | impurities (turbidity) in a treated water as low as  |
|               | possible may be an effective safeguard against the   |
|               | presence of oocysts and pathogens.                   |
+---------------+------------------------------------------------------+
| Anderson et   | In a pilot plant study, the removal of particles \>  |
| al., 1996     | 2m was significantly related to turbidity reduction |
|               | r=0.97 (p\<0.0001); the removal of *Cryptosporidium* |
|               | oocysts may be related to the removal of *Giardia*,  |
|               | r=0.79 (p\<0.14); the reduction of turbidity may be  |
|               | related to the removal of *Giardia* cysts, r=0.67    |
|               | (p\<0.13) and *Cryptosporidium* oocysts (p\<0.08)    |
+---------------+------------------------------------------------------+

-   as discussed in EPA\'s Notice of Data Availability (USEPA, 1997)

## References

1\. Anderson, W.L., et al. 1996. "Biological Particle Surrogates for
Filtration Performance Evaluation."

2.  CDC (Centers for Disease Control). 1996. "Surveillance for
    Waterborne-Disease Outbreaks - United States, 1993-1994." *Morbidity
    and Mortality Weekly Report*, 45(SS-1).

3\. D\'Antonio, R.G., R.E. Winn, J.P. Taylor, et al. 1985. "A Waterborne
Outbreak of Cryptosporidiosis in Normal Hosts." *Annals of Internal
Medicine*. 103:886-888.

4.  Fox, K.R. 1995. "Turbidity as it relates to Waterborne Disease
    Outbreaks." Presentation at M/DBP Information Exchange, Cincinnati,
    Ohio. AWWA white paper.

5.  Gregory, J. 1994. "*Cryptosporidium* in Water: Treatment and
    Monitoring Methods." *Filtration & Separation*. 31:283-289.

6.  Hall, T., J. Presdee, and E. Carrington. 1994. "Removal of
    *Cryptosporidium* oocysts by water treatment processes." Foundation
    for Water Research.

7.  Herson, D.S., D.R. Marshall, and H.T. Victoreen. 1984. "Bacterial
    persistence in the distribution system." *J. AWWA.* 76:309-22.

8.  LeChevallier, M.W., W.D. Norton, and R.G. Lee. 1991. "*Giardia* and
    *Cryptosporidium* in Filtered Drinking Water Supplies." *Applied and
    Environmental Microbiology*. 2617-2621.

9.  LeChevallier, M.W. and W.D. Norton. 1992. "Examining Relationships
    Between Particle Counts and *Giardia*, *Cryptosporidium*, and
    Turbidity." *J. AWWA*.

10\. LeChevallier, M.W. and W.D. Norton. "Treatments to Address Source
Water Concerns: Protozoa." *Safety of Water Disinfection: Balancing
Chemical and Microbial Risks*. G.F. Craun, editor. ILSI Press,
Washington, D.C.

11\. Marshall, K.C. 1976. *Interfaces in microbial ecology*. Harvard
University Press, Cambridge, MA.

12\. NAS (National Academy of Sciences). 1980. *National Research
Council: drinking water and health*, Volume 2. National Academy Press,
Washington, D.C.

13\. Nieminski, E.C. 1992. "*Giardia* and *Cryptosporidium* - Where do
the cysts go." Conference proceedings, AWWA Water Quality Technology
Conference.

14\. Olson, B.H., H.F. Ridgway, and E.G. Means. 1981. "Bacterial
colonization of mortar-lined and galvanized iron water distribution
mains." Conference proceedings, AWWA National Conference. Denver, CO.

15\. Ongerth, J.E. 1990. "Evaluation of Treatment for Removing *Giardia*
Cysts." *J. AWWA*. 82(6):85-96.

16. Sadar, M.J. 1996. *Understanding Turbidity Science*. Hach Company
    Technical Information Series - Booklet No. 11.

17. USEPA. 1997. *Occurrence Assessment for the Interim Enhanced Surface
    Water Treatment Rule, Final Draft*. Office of Ground Water and
    Drinking Water, Washington, D.C.

18. USEPA. 1983. *Turbidity Removal for Small Public Water Systems*.
    Office of Ground Water and Drinking Water, Washington, D.C.

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7\. Importance of Turbidity [7-1](#importance-of-turbidity)

7.1 Overview [7-1](#overview)

7.2 Turbidity: Definition, Causes, and History as a Water Quality
Parameter
[7-1](#turbidity-definition-causes-and-history-as-a-water-quality-parameter)

7.3 Turbidity\'s Significance to Human Health
[7-4](#turbiditys-significance-to-human-health)

7.3.1 Waterborne Disease Outbreaks [7-5](#waterborne-disease-outbreaks)

7.3.2 The Relationship Between Turbidity Removal and Pathogen Removal
[7-7](#the-relationship-between-turbidity-removal-and-pathogen-removal)

7.4 References [7-10](#references)

Figure 7-1. Typical Sources of Turbidity in Drinking Water
[7-2](#__RefHeading___Toc448537940)

Figure 7-2. Jackson Candle Turbidimeter
[7-3](#__RefHeading___Toc448537941)

Figure 7-3. Nephelometric Turbidimeter
[7-5](#__RefHeading___Toc448537942)

Figure 7-4. Particles of Turbidity May Provide Protection for
Microorganisms [7-6](#__RefHeading___Toc448537943)

Figure 7-5. Relationship Between Removal of *Giardia* and Turbidity
[7-8](#__RefHeading___Toc448537944)

Figure 7-6. Relationship Between Removal of *Cryptosporidium* and
Turbidity [7-8](#__RefHeading___Toc448537945)

Table 7-1. Cryptosporidium Outbreaks vs. Finished Water Turbidity
[7-7](#__RefHeading___Toc448537951)

Table 7-2. Studies on the Relationship between Turbidity Removal and
Protozoa Removal [7-9](#__RefHeading___Toc448537952)