# 3. Turbidity Methods & Measurement

## Introduction

The IESWTR requires systems to measure the turbidity of combined filter
effluent and individual filter effluent. Because these measurements are
used for reporting and compliance purposes (as described in Chapter 2),
accurate measurement and strict adherence to approved methods is of
paramount importance. The following chapter describes approved methods,
analytical issues associated with turbidimeters, quality assurance and
quality control issues, and data collection and management.

## Approved Turbidity Methods

Currently, the Agency has approved three methods for the measurement of
turbidity as described in §141.74. Systems must utilize turbidimeters
which conform to one of the following methods for compliance purposes.
If the instrument does not conform, then it may not be used for
monitoring under the requirements of the IESWTR. A brief description of
each of the methods is found below.

### 3.2.1 EPA Method 180.1

EPA method 180.1, "Determination of Turbidity by Nephelometry," is found
in the Agency's publication, *Methods for Chemical Analysis of Water and
Wastes*. The method is based upon 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. The higher the
intensity of scattered light, the higher the turbidity. Readings, in
NTUs, are made in a nephelometer designed according to specifications
laid out in the method. A primary standard suspension is used to
calibrate the instrument. A secondary standard suspension is used as a
daily calibration check and is monitored periodically for deterioration
using one of the primary standards. See Appendix B for EPA Method 180.1.

### 3.2.2 Standard Method 2130B

Standard Method 2130B, found in *Standard Methods* (1995), is similar to
EPA Method 180.1. The method is also 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. See Appendix C for Standard Method 2130B.

### 3.2.3 Great Lakes Instrument Method 2 (GLI 2)

Great Lakes Instruments Method 2 is an instrument specific, modulated
four beam method using a ratiometric algorithm to calculate the
turbidity value from the four readings that are produced. The comparison
is also based on a comparison of light scattered by the sample under
defined conditions with the intensity of the light scattered by the
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 specifications in the method. See Appendix D for
Great Lakes Instrument Method 2.

## Turbidimeters

As noted, turbidimeters must conform to one of the three approved
methods for measuring turbidity. For regulatory reporting purposes,
either an on-line or a benchtop turbidimeter may be used. A system may
find it appropriate to utilize on-line turbidimeters to monitor
individual filter effluent, while utilizing either a benchtop or on-line
turbidimeter for combined filter effluent. If a system chooses to
utilize on-line units for monitoring combined filter effluent, they must
validate the continuous measurements for accuracy on a regular basis
using a protocol approved by the State.

### Bench Top Turbidimeters

Bench top units are used exclusively for grab samples and include glass
cuvettes for holding the sample. Measurement with bench top units
requires strict adherence to the manufacturer's sampling procedure to
reduce errors from dirty glassware, air bubbles in the sample, and
particle settling. Plant operators should read and be fully familiar
with the operation manuals for all bench-top turbidimeters used in the
plant. Many maintenance and operational issues are specific to
turbidimeter make and model, and instruments are usually supplied with a
thorough user's manual.

#### Bench-top Basics 

Although durable, turbidimeters need to be stored and operated in a safe
and protected environment. Moisture and dust need to be prevented from
entering and accumulating inside turbidimeters. Humidity also needs to
be controlled to prevent condensation inside the instrument.
Turbidimeters should be located where they will not be exposed to
corrosive chemicals or fumes. Chemicals such as chlorine and acids can
ruin instrumentation. Finally, turbidimeters should be located in an
environment that is temperature controlled, at a consistent temperature
between 0and 50C.

Generally the instrument should be left on at all times (unless
otherwise specified in the user's manual). If any instrument is not left
on at all times, it may require a warm-up period before sample analysis.

The length of the sample piping or tubing from the sampling location to
the point where the sample is drawn off should be minimized. Long sample
lines can lead to problems with biological fouling and scaling which can
impact turbidity values. It is best to limit sample lines to ten feet or
less. Long sample lines can also cause confusion due to the lag time as
the sample travels through the piping. The longer the lag time, the more
difficult it is to correlate turbidity fluctuations to actual process
changes that might be occurring.

Sample taps in piping should be located on the sides of pipes. Samples
taken from the top or bottom will not accurately represent the turbidity
of the water. Samples taken from the bottom will often times contain
sediment while samples from the top may contain a greater number of air
bubbles. Ideally, sample taps should be angled into the water flow at an
angle of 0-45 degrees and extend into the center of the flow channel.
Sample taps should be located away from items which disturb flow such as
fittings, bends, meters, or pump discharges.

#### Operation and Maintenance

Preventative and routine maintenance should be carried out according to
manufacturers' instructions. Do not make repairs to the instrument
unless specified in the instruction manual. Even if a repair can be
made, consider sending the unit back to the manufacturer. Keep track of
maintenance and repair on a log sheet located next to the unit.

Maintain benchtop instruments in accordance with manufacturer
recommendations. Inspect the cleanliness of bulb and lenses daily. Clean
lenses, light sources, and other glassware with appropriate materials to
avoid scratches and dust accumulation. Avoid the use of chemicals or
other materials when cleaning unless instructed by the manufacturer. Do
not touch the optical components with bare hands (soft cotton gloves are
recommended). Recalibrate the instrument after any significant
maintenance or cleaning procedure.

Bench-top turbidimeters, just like most instruments, have an effective
service life. Various elements within the instrument can deteriorate
over time and with repeated use. Daily usage can result in wear on
electronics due to movement and temperature. Microprocessor based
electronics are also prone to memory loss during power supply
fluctuations. Service personnel can often provide insight on instrument
life and can make recommendations for specific maintenance items. Since
turbidimeters have become integral parts of a water treatment plant
operation and reporting, it is imperative to maintain instruments and
budget for replacements.

Replace incandescent turbidimeter lamps annually, or more frequently if
recommended by the manufacturer. Recalibrate the instrument whenever
optical components (e.g., lamp, lens, photodetector, etc.) of the
turbidimeter are replaced or cleaned.

#### Calibration

Calibration is an essential part of accurate turbidity measurement, and
as such, instrument calibration should be verified on a daily basis.
Calibration verification can be completed using primary or secondary
standards. If verification indicates significant deviation from the
standard (true) value (greater than ±10%), thoroughly clean and
recalibrate the instrument using a primary standard. If problems persist
the manufacturer should be contacted. At a minimum regardless of
calibration results, turbidimeters should be thoroughly cleaned and
calibrated with primary standards ***at least*** ***quarterly***.

After calibration, performance of the turbidimeter should be verified
with a secondary standard. If the instrument has internal electronic
diagnostics designed to assist in determining proper calibration, the
operator should use these tools to verify proper calibration and
operation. Calibration is discussed in significant detail in Section
3.4.5.

#### 3.3.2 On-Line Turbidimeters

On-line turbidimeters are process instruments which sample a side stream
split-off from the treatment process. The sample flows through the
on-line instrument for measurement and then wasted to a drain or
recycled through the treatment process.

Selection of the flow rate through on‑line turbidimeters should be in
accordance with manufacturer specifications. The sample flow should be
constant without variations due to pressure changes or surges.
Installation of a flow control device such as a rotameter on the sample
line can eliminate fluctuations in flow rate.

To the extent possible, turbidimeter samples should be obtained directly
from the process flow and not pumped to a remote instrument location.
Pumped samples can be non‑representative of the process flow due to
changes in the character of particles caused by the pump or the addition
of bubbles due to rapid pressure changes. If pumping is required, the
use of peristaltic pumps are desirable, as they have the least amount of
impact on particles in the sample.

Several of the on‑line turbidimeters available today have various sample
chamber sizes. It is important to note that the size of the sampling
chamber will affect the instrument response. The path length of the
light passing through the sample is inversely proportional to resolution
of the instrument. Therefore, the larger the sample size, the more
likely that the turbidity reading will be dampened.

#### Installation

On-line turbidimeters should be installed in accordance with
manufacturer instructions. The goal of proper installation is to ensure
proper operation, easy access for maintenance and calibration procedures
that should be performed, and obtain an accurate, representative and
timely sample.

Carefully consider the location of the sample tap. The tap should
provide a representative sample of the water being monitored. If an
individual filter is being monitored, locate the sample tap as close to
the filter as possible. The tap should provide a sample from the
centerline of the pipe, as opposed to the bottom or top of the pipe
where sediment or air bubbles may interfere with sample integrity.
Ideally, the sample will flow by gravity from the sample tap to the
turbidimeter without a sample pump. Sample pumps may have an effect on
turbidimeter measurements.

The length of conduit between the sample tap and the instrument should
be minimized, to the extent possible. Lengthy sample runs can delay
instrument response time and may cause changes in sample quality (i.e.,
settling of particulate matter, increased opportunity for biological
growth). In selecting sample tubing or pipe, the required sample flow
rate and pressure should be considered. Sample lines of insufficient
diameter may not provide adequate flow to the instrument and may be
prone to clogging. Excessively large diameter sample lines will delay
the instrument response and may permit settling of particulate matter.
Line flushing valves and ports may be necessary depending on the water
being sampled. Carefully consider these items when installing an on-line
turbidimeter.

A good sample tap location and plumbing arrangement will minimize the
potential for bubble formation. Most on-line turbidimeters have the
capability to eliminate minor bubble interference through baffles and/or
degassing chambers, but if the problem is severe, the turbidity
measurements may be affected.

The turbidimeter should be installed in a location that provides easy
access for routine maintenance and calibration procedures. It should be
protected from direct sunlight, extreme temperatures (\<32ºF/0ºC and
\>104ºF/40ºC), and rapid temperature fluctuations. It should also be
firmly mounted so as to avoid vibrations, which may interfere with the
accuracy of turbidity measurements.

The turbidimeter drain should provide easy access for flow verification
and collection of calibration verification samples. Flow rate and
calibration verification samples are important in establishing data
validity. Therefore, hard piping the turbidimeter drain without an
airgap is not recommended.

#### Operation and Maintenance

Preventative and routine maintenance should be carried out according to
manufacturer's instructions. A regular cleaning schedule is necessary to
ensure proper operation of on-line turbidimeters. A weekly inspection is
recommended, but this frequency may vary depending on the instrument
location and raw water quality. Warm or turbid samples may dictate more
frequent cleaning. An instrument mounted in a dusty environment may also
require more frequent cleaning. Items to inspect and clean include, but
are not limited to, lenses, light sources, sample reservoirs, air bubble
traps, and sample lines. Clean lenses, light sources, and other
glassware with appropriate materials to avoid scratches and dust
accumulation. During maintenance, care needs to be taken not to touch
the surface of any bulbs or detectors without proper covering on the
fingers. Soft cotton gloves should be worn when changing bulbs or
detectors. Recalibrate the instrument after any significant maintenance
or cleaning procedure.

On-line turbidimeters, just like most instruments, have an effective
service life. Various elements within the instrument can deteriorate
over time and with repeated use. Daily usage can result in wear on
electronics due to movement and temperature. Microprocessor based
electronics are also prone to memory loss during power supply
fluctuations. Many on‑line units with unsealed sensor electronics are
vulnerable to damage by outside contamination and splashing. Service
personnel can often provide insight on instrument life and can make
recommendations for specific maintenance items. Since turbidimeters have
become integral parts of a water treatment plant operation and
reporting, it is imperative to maintain instruments and budget for
replacements.

Incandescent turbidimeter lamps should be replaced annually or more
frequently if recommended by the manufacturer. The instrument should be
recalibrated whenever optical components (e.g., lamp, lens,
photodetectors, etc.) of the turbidimeter are replaced.

Systems should consider verifying sample flow rates on a weekly basis.
Flow rates should be within a range specified by the manufacturer.

#### Calibration

EPA recommends that on-line turbidimeters have calibration verified on a
weekly basis, if being utilized for combined filter effluent monitoring.
Less frequent verification may be more appropriate for turbidimeters
monitoring individual filter turbidity, but EPA recommends verification
be conducted with a frequency of **at least** once per month.

Calibration verification can be completed using primary standards,
secondary standards, or by comparison to a properly calibrated
turbidimeter. If verification indicates significant deviation from the
standard (true) value (greater than ±10%), the instrument should be
thoroughly cleaned and recalibrated using a primary standard. If
problems persist, the manufacturer should be contacted. Regardless of
calibration results, turbidimeters should be thoroughly cleaned and
calibrated with primary standards ***at least quarterly***.

***EPA does not recommend calibrating on-line instruments by comparison
with a bench-top turbidimeter***. It has been determined that this
procedure is likely to introduce unacceptable levels of error into the
calibration.

After calibration, verify instrument performance with a secondary
standard or by comparison with another properly calibrated instrument.
If the instrument has internal electronic diagnostics designed to assist
in determining proper calibration, the operator should use these tools
to verify proper calibration and operation. For additional information
on calibration see Section 3.4.5.

## Quality Assurance/Quality Control

Although using proper techniques and equipment is an important part of
conducting proper turbidity measurements, it is imperative that
operators are aware of factors in the processes which may lead to poor
quality data. Such factors include poor lab techniques, calculation
mistakes, malfunctioning or poorly functioning instrumentation, and
out-of-date and deteriorated chemicals. Development of a Quality
Assurance and Quality Control (QA/QC) Program ensures that lapses do not
occur which allow inaccurate measurements or erroneous reporting.
Systems may want to establish plans to provide assurance that
measurements are being made accurately and consistently.

### Quality Assurance Organization and Responsibilities 

A good QA/QC plan provides clear organization and defines who is
responsible for each of the aspects laid out in the plan and what their
responsibilities are. This section should include a list of the
positions (by title) that have responsibilities and what those
responsibilities are. The appropriate training or skills necessary for
each of the positions listed should also be included.

### Quality Assurance Objectives

The objectives of the Quality Assurance Program need to be laid out and
understood by the staff members. Objectives should be succinct, and
clear. SOPs should be developed with input from staff, enabling them to
effectively conduct work activities in compliance with applicable
requirements. Systems may wish to include one primary objective,
followed by a number of goals which all relate to the objective. An
example might look like the following:

**The primary objective of the Quality Assurance Program is to ensure
that turbidity measurements are accurate and consistent. Based on this,
the goals of the Quality Assurance Program at a generic water treatment
plant are the following:**

-   To adhere to proper sampling techniques as set forth in the Standard
    Operating Procedures.

-   To maintain and operate all turbidimeters at the plant properly in
    accordance with manufacturer instructions and Standard Operating
    Procedures.

-   To perform calibration of instruments on a routine and as-necessary
    basis.

-   To communicate and report all, malfunctions, abnormalities, or
    problems which may compromise the ability to accurately and
    consistently measure turbidity.

### Standard Operating Procedures 

Standard Operating Procedures (SOPs) are a way to ensure that activities
are accomplished in a consistent manner, and that each activity is
understood by all involved. SOPs should be kept as simple as possible in
order to ensure that each operator is consistent in undertaking the task
at hand. The title of the procedure should be clear, concise, and
descriptive of the equipment, process, or activity. Systems should
consider adopting SOPs for any of the following activities:

-   Cleaning turbidimeters

-   Creating Formazin Standards

-   Calibrating Turbidimeters

-   Referencing Index Samples

Instructional steps should be concise and precise, using the following
guidelines:

-   Steps should contain only one action.

-   Commands should be written with an action verb at the beginning.

-   Limits/and or tolerances for operating parameters should be specific
    values and consistent with the accuracy of the instrumentation.
    Procedures should not include mental arithmetic.

-   **"Cautions"** should be used to attract attention to information
    that is essential to safe performance.

-   **"Notes"** should be used to call attention to supplemental
    information. Notes present information that assists the user in
    making decisions or improving task performance.

-   Documentation methods should be incorporated as part of the
    procedure including what data needs to be recorded, if the
    individual needs to sign or date data, etc.

After developing an SOP, the author(s) should consider the following
questions:

-   Can the procedure be performed in the sequence it is written?

-   Can the user locate and identify all equipment referred to in the
    procedure?

-   Can the user perform the procedure without needing to obtain direct
    assistance or additional information from persons not specified by
    the procedure?

-   Are words, phrases, abbreviations, or acronyms that have special or
    unique meaning to the procedure adequately defined?

-   Is there a need for special controls on data collection and
    recordkeeping?

After completing the SOP it should be tested to the extent possible. It
is also a good idea to ask a technical reviewer to verify the accuracy
of the procedure. SOPs should be reviewed at least once every 2 years to
determine if the procedure and requirements are still accurate.

The following is a simplified example of an SOP written for the
development of Formazin.

### 3.4.4 Sampling Strategy and Procedures 

The procedure for conducting sampling should be laid out clearly and
concisely, preferably in SOPs (discussed in Section 3.4.3). It should
include information such as sampling location and frequency, collection
methods, sample handling, and any logistical considerations or safety
precautions which are necessary. Adherence to proper techniques is an
important step in minimizing the effects of instrument variables and
other interferences (Sadar, 1996). Measurements will be more accurate,
precise, and repeatable if operators follow and incorporate the
techniques listed in this section.

All turbidimeter manufacturers emphasize proper techniques and include
detailed instructions in their literature. Water treatment plant
operators responsible for conducting turbidity measurements are urged to
review these instructions and incorporate them into their SOPs. Specific
instruction for securing samples and measuring turbidity will differ for
the various instrument manufacturers and models, but there are certain
universally accepted techniques that should be utilized when conducting
measurements. The following paragraphs highlight some of these
techniques.

#### Handling of Cuvettes/Sample Tubes

Sample cells must be handled with absolute care to avoid contamination
or damage, such as marks and scratches, which might change the optical
characteristics of the glass. Scratches, fingerprints, and water
droplets on the sample cell or inside the light chamber can cause stray
light interference leading to inaccurate results. Cells can be acid
washed periodically and coated with a special silicone oil to fill small
scratches and mask the imperfections in the glass. Since the silicone
oil required for this application should have the same refractive
characteristics as glass, it is recommended that the oil be obtained
from the instrument manufacturer. Care should be taken to not apply
excessive oil that could attract dirt or contaminate the sample chamber
in the instrument. Once the oil has been applied to the cell, the excess
oil should be removed with a lint‑free cloth. The result should be a
sample cell surface with a dry appearance, but with all imperfections
filled with oil. Sample cells should always be handled at the top of the
cell or by the cap to avoid fingerprints or smudges. After a cell has
been filled with a sample and capped, the outside surface should be
wiped with a clean, lint‑free absorbent cloth until it is dry. Store
cells in an inverted position on clean surfaces to reduce contamination
by dirt or dust or store capped and filled with low turbidity water.

#### Orientation and Matching of Sample Cells

Since imperfections in the sample cell glass can influence light
scattering, the cell should be inserted in the turbidimeter with the
same orientation each time it is used. At the Philadelphia Water
Department, new cells are indexed and are not allowed to vary by more
than 0.01 NTUs. Philadelphia reports that as many as one quarter of the
cells are never used due to imperfections in sample cells (Burlingame,
1998).

Matched sample cells are required to minimize the effects of optical
variation among cells. If possible, it is better to use a single sample
cell for all measurements to minimize the variability due to
cell-to-cell imperfections. Once the orientation of a cell has been
established, the operator should always use the same orientation when
placing the sample cell into the instrument. Techniques for matching and
indexing are provided below.

######## Indexing Cells (Steps 1-2) Matching Cells (Steps 1-3)

Step 1. Pour ultra-pure dilution water into a sample cell (several cells
if performing matching) that has been cleaned according to the
techniques described previously in this section.

Step 2. Select sample cell, and place it into the turbidimeter. Rotate
the cell within the instrument until the display reads the lowest value.
Record the reading. Using a marker or pen, place a mark on the top of
the sample cells neck. **Do not put the mark on the cap.** Use this mark
to align sample cells each time a measurement is made.

Step 3. Select another sample cell, place it into the turbidimeter and
rotate the cell slightly until the reading matches that of the first
sample cell (within 0.01 NTUs). Using a marker or pen, place a mark on
the top of the sample cells neck. If unable to match the readings select
a different sample cell. Repeat the process until the appropriate number
of cells have been matched.

#### Degassing of the Sample

Water samples almost always contain substantial amounts of entrained
gases that can be released during turbidity measurement. Bubbles are
either generated during the filling of a sample container, occur due to
temperature fluctuations resulting in a reduced solubility of the gas in
a liquid, or are due to chemical and/or biological processes. Bubbles
within a sample act much like particles and can scatter light resulting
in an incorrect measurement. Many on‑line turbidimeters contain
apparatuses inside the instrument that serve to trap, collect, and vent
air bubbles. Usually these consist of baffled entries or membranous
chambers. Some vendors also manufacture add‑on units which can be placed
in the sample line before the on‑line turbidimeter. There are several
other options for removing bubbles from water (degassing) to reduce the
effect they have on measurements. The most commonly used methods
include, addition of a surfactant, application of a partial vacuum, and
use of an ultrasonic bath.

Addition of a surfactant compound to a water sample lowers the surface
tension of the water and allows entrained gases to readily escape. There
are a variety of surfactants used in turbidity measurements today.
Because of the variety in chemical composition, it is difficult to
provide guidance for their use. It is important to note that some
surfactants may have constituents which serve as a coagulant and cause
particles to aggregate and settle out. Other chemicals might contain
constituents with an ionic charge that cause particles to rise to the
surface. The use of surfactants is more appropriate for measurement of
highly turbid waters such as raw water. The most appropriate
instrument-specific advice regarding the use of surfactants can be
obtained by contacting the instrument manufacturer.

Application of a partial vacuum to a sample lowers the partial pressure
above the liquid surface and allows entrained gases to escape. Partial
vacuums can be created by a simple syringe or by use of a vacuum pump.
Some instrument manufacturers and suppliers provide pre‑made vacuum kits
that include syringes for degassing samples. The most common arrangement
is the use of a syringe and a stopper sized for the opening of the
sample cell or test tube.

The use of an ultrasonic bath creates vibrations in the sample to
facilitate the escape of gases. Ultrasonics is a specialty field/science
that utilizes an inaudible spectrum of sound frequencies ranging from
about 20,000 cycles per second to 100,000 cycles per second. Ultrasonic
baths are used for thoroughly cleaning supplies in the medical,
electronic, and metals industries. When high frequency sound waves are
passed through a cleaning fluid, such as water with suitable detergent
additive, many millions of microscopic bubbles form and then rapidly
collapse. The bubbles are the result of the stretch and compress phases
of the sound waves within the fluid, a process known as cavitation.
Ultrasonic devices may be most effective in severe turbidity conditions
or with viscous samples, however if used for degassing samples, samples
should be sonified **for no more than 1 to 2 seconds**. Sonification can
change particle size ranges, affecting a turbidimeters response if
improperly utilized (Burlingame, 1998).

#### Timeliness of Sample

Samples should be measured expeditiously after being secured to prevent
changes in particle characteristics due to temperature and settling.
Temperature can affect particles by changing their behavior or creating
new particles if precipitates are created. Dilution water may dissolve
particles or change their characteristics (Sadar, 1996). Operators are
encouraged to draw samples only when turbidimeters are ready to be
operated. Do not draw a sample and allow it to sit while the instrument
warms up or is being readied.

#### Other Important Sampling Techniques

-   Samples should not be violently agitated as particles can be broken
    apart or air may be entrained into the fluid. Gentle agitation such
    as swirling the sample cell is advisable to reduce particle
    settling.

-   Sample cells should be used only with the instruments for which they
    were intended. Do not mix and match.

-   Perform a visual observation of the sample cell every time a
    measurement is made. Verify that there are no visible bubbles in the
    sample and the cell is clean and free of scratches.

-   Samples entering the turbidimeters should be at the same temperature
    as the process flow samples. Changes in temperature can cause
    precipitation of soluble compounds and affect readings.

-   Sample cells should be evaluated with a low turbidity water (after
    cleaning) to determine if cells remain matched. If the evaluation
    determines that a cell is corrupted, discard the cell. Systems
    should consider conducting this evaluation weekly.

-   When in doubt, throw it out - If you have a question as to whether a
    sample cell is too scratched or stained get rid of it.

### Calibration

Turbidimeters, like all instrumentation, need to be calibrated
periodically to ensure that they are working properly and provide true
and accurate readings.

> ***Calibration should always be conducted according to manufacturer
> instructions.***

Determine the appropriate technical requirements for calibration based
on the following

-   Manufacturer

-   Model name and/or number

-   Parameters to be calibrated

-   Range to be calibrated

-   Acceptance criteria

-   Mandatory calibration procedures or standards

-   Required calibration program

After calibration, performance of the turbidimeter should be verified
with a secondary standard. If the instrument has internal electronic
diagnostics designed to assist in determining proper calibration, the
operator should use these tools to verify proper calibration and
operation.

#### Calibration Standards

A calibration standard must be used to conduct a calibration. Standards
are materials with a known value which, when placed in the instrument,
should be used to adjust the instrument to read the known value.

There are a variety of standards on the market today which are used to
calibrate turbidimeters. They are most often characterized as Primary,
Secondary, or Alternative standards. Standard Methods describes Primary
Standards as a standard which is prepared by the user from traceable raw
materials, using precise methodologies and under controlled
environmental conditions. (Standard Methods, 1995) Standard Methods
defines Secondary Standards as those standards a manufacturer (or an
independent testing organization) has certified to give instrument
calibration results equivalent (within certain limits) to results
obtained when an instrument is calibrated with a primary standard.

Standard Methods and EPA differ in their definitions of each of these
standards. EPA recognizes the following three Standards for approved use
in the calibration of turbidimeters.

-   FORMAZIN (user prepared and commercially produced)

-   AMCO-AEPA-1® MICROSPHERES

-   STABLCAL® (STABILIZED FORMAZIN)

Users need to realize that some instruments have been designed and
calibrated on specific primary standard(s) listed above. For optimal
results, users should contact the manufacturer of the instrument to
determine the recommended primary standard to be used for calibration.

Additionally, EPA recognizes Secondary Standards for use in monitoring
the day-to-day accuracy of turbidimeters by checking the calibration.
This check is used to determine if calibration with a Primary Standard
is necessary. Secondary Standards are used to check whether an
instrument produces measurements within acceptable limits around a
nominal value (typically 10%). Examples of *SECONDARY STANDARDS*
include:

-   GELEX®

-   GLASS/CERAMIC CUBES

-   MANUFACTURER PROVIDED INSTRUMENT SPECIFIC SECONDARY STANDARDS

The need to reconcile the definitions and differences among Primary and
Secondary Standards will be a continuing issue. It has been recognized
that the standards need to be unbiased, easy to use, safe, available for
a range of turbidities, and reproducible. Future efforts of the Agency,
in concert with other organizations and manufacturers, will focus on
ensuring the most appropriate, variation-free, and technologically
feasible standards are available and used for calibration of
turbidimeters.

#### Conducting the Calibration

All reputable turbidimeters have been factory-calibrated before leaving
the manufacturer. As described previously, turbidimeters, like most
instrumentation, tend to lose accuracy over time due to a variety of
factors, making periodic calibration very important to maintain accurate
measurements. The most important point to remember is:

***Calibration should [always]{.underline} be conducted according to
manufacturer instructions.***

Manufacturers differ in the steps to conduct a calibration, but the
following points are applicable to all calibrations.

-   Standards should be checked to ensure they have not expired. Never
    pour a standard back into its original container.

-   Care should be taken when preparing Formazin. If a spill occurs,
    clean up immediately according to the Material Safety Data Sheets
    (MSDSs) provided with your chemicals. Make sure to inspect the
    tube/cuvette for scratches and chips prior to pouring the solution
    in.

-   Check to make sure the tube/cuvette is lined up properly according
    to the indexing. Be sure not to scratch the tube when inserting, and
    ensure that the tube/cuvette is free of dust, smudges, and
    scratches.

-   When obtaining the reading, write the value legibly onto a form
    similar to the one found in Figure 3-1. Make sure to record the date
    of the calibration, the individual conducting the calibration, the
    value, and any peculiar situations or deviations from normal
    calibration procedures (e.g., switch to a new lot of Formazin,
    switch in standards, use of a new tube/cuvette, etc.) These
    measurements will allow for an understanding of whether the
    performance of a turbidimeter is in question. For example, if for 6
    months a turbidimeter reads approximately 20.152 when calibrated
    using polystyrene beads and one morning it reads 25.768, this could
    be an indication that the bulb in the turbidimeter has a problem.
    Conversely, if the standard in use was switched that morning, the
    resulting change might be due to change in standards.

-   Conduct the calibration the same way each time. Variations in how
    the calibration is conducted could yield inaccurate measurements.

-   It is extremely important that individuals who conduct the
    calibration have been trained to do so. Systems should consider
    creating Standard Operating Procedures to be read, learned, and
    followed by operators at the plant.

#### Frequency of Calibration

EPA recommends that the calibration of units be verified daily with
secondary standard and recalibration occur at least quarterly with
primary standards. Specific calibration procedures should be developed
for each individual instrument location. Listed below are several
guidelines for selecting calibration frequencies and procedures:

-   Select a frequency for checking instrument calibration with
    secondary standards and for full re-calibration of instrument with
    primary standards .

-   Establish the acceptable deviation from the primary standard during
    secondary verifications. Readings in excess of the deviation should
    trigger immediate re-calibration of the instrument. (10% is
    recommended by EPA)

-   Choose a time of day when full attention can be devoted to the
    calibration. Calibration at the end of a shift or right before a
    break can often lead to mistakes and sources of error. A calibration
    time should be established when operators are fully alert and
    focused on completing the task.

-   Identify and schedule in advance the dates for full turbidimeter
    calibration on the plant calendar or work scheduling chart.

-   Make preparations and maintain adequate supplies to prevent delays
    in the calibration schedule. It is important to keep an appropriate
    stock of standards. Due to the limited shelf-life of various
    standards, the age of the stored standards should be monitored so
    they can be replaced or reformulated as needed.

-   Assign calibration duties to a select group of individuals, and make
    it one of their standard activities. Train all appropriate
    individuals/operators in conducting a calibration in the event that
    one of the regular individuals is not available.

-   Create a Standard Operating Procedure for conducting a calibration
    and post next to the turbidimeter.

### Data Screening, Validation, and Reporting 

The methods for data screening, validation, and reporting should be
detailed to ensure that measurements are recorded calculated and
reported correctly. These methods should be designed to meet the Quality
Assurance Objectives. Again, the development and implementation of SOPs
will facilitate those goals.

  ------------------------------- -------------- ----------- -------------------- ----------------- -----
  **CALIBRATION CHECKLIST**                                                                         

  **Month\_\_\_\_\_\_\_\_\_\_**                                                                     

  **Year \_\_\_\_\_\_\_\_\_\_\_**                                                                   

  **Date**                        **Initials**   **Value**   **Standard**         **Comments**      

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    

                                                                                                    
  ------------------------------- -------------- ----------- -------------------- ----------------- -----

[]{#__RefHeading___Toc448537015 .anchor}Figure 3-1. Calibration
Checklist

### Performance and System Audits 

Performance and system audits should be conducted periodically to
determine the accuracy of the total measurement system(s) or component
parts thereof. Performance audits may include review of documentation
and log books for legibility and completeness. The systems audit
consists of evaluation of all components of the sampling and measurement
systems to determine their proper selection and use. This audit includes
a careful evaluation of both field and laboratory quality control
procedures and can include verification of written procedures and
analyst(s) understanding, verification and documentation of procedures,
as well as adherence to any SOPs.

### Preventative Maintenance

Preventive maintenance should be conducted on all instrumentation. The
maintenance program consists of scheduled (preventive) and non-scheduled
maintenance procedures. All maintenance performed and the results of
calibrations should be documented. Maintenance procedures and schedules
for equipment used should be available to the appropriate staff.
Adherence to maintenance schedules and procedures may be investigated
during a systems audit. A preventive maintenance schedule recommended by
the respective manufacturers should be followed for each instrument.
This preventive maintenance will include regular battery checks and
maintenance of a sufficient stock spare parts and supplies.
Manufacturers' procedures identify the schedule for servicing critical
items to minimize downtime of the measurement system.

## Data Collection and Management

The final steps in turbidity measurement deal with the collection of
data and management of collected data. The advent of the personal
computer revolution has provided much needed assistance to tasks that
were once time consuming, although automation still requires operators
who are skilled and trained in the use of sometimes sophisticated
equipment. This section describes the several methods available to
systems for the collection of data and provides a brief description of
the management of that data.

Data obtained from Supervisory Control and Data Acquisitions (SCADAs),
data recorders, or strip charts should be verified on a weekly basis by
comparing the turbidimeter reading with the data recording device
reading. If verification indicates greater than ±10% deviation, the
electronic signal should be recalibrated according to manufacturer
instructions.

### Data Collection Methods 

Acquisition of data from turbidimeters is an important step in the
turbidity measurement process. With the individual filter turbidity
requirements, systems will be required to continuously monitor each
filter. Each of the methods discussed below are typically used for
on-line turbidimeters. Readings using benchtop units are typically
recorded by hand or entered into a PC without the use of the data
collection equipment listed below. Systems may have experience using
these methods in monitoring other water quality parameters.

#### Strip Recorders and Circular Chart Recorders

Strip Chart and Circular Chart Recorders are a relatively established
technique for recording data. The units are set to obtain a reading at a
timed interval. A pen records the reading on paper at the interval. As
additional readings are taken, the pen moves back and forth (or up and
down in the case of a circular recorder) recording the values that are
being monitored.

Newer models include digital readouts as well as the capability to
transfer data to data loggers or other data acquisition systems. The
greatest disadvantages to using chart recorders is the difficulty in
incorporating data into electronic format and archiving such data.
Recorders also require the purchasing of replacement pens and charts.

#### Data Loggers

Data Loggers are "black boxes" which store data which is received from
input channels. The box records the data in memory which can then be
downloaded at a future time. Data loggers consist of two distinct
components: hardware and software.

######## Hardware

The units themselves typically consist of a device containing solid
state memory encased in a plastic weatherproof enclosure. Units have a
varying number of inputs that can be either analog (records actual
numbers) or digital (records a series of 0s and 1s), as well as an
output to download data. Systems most often are battery powered, but
some can be connected to existing power supplies. Nearly all systems
contain lithium or other batteries to keep memory active in the event of
a power failure.

######## Software

Two software components are important to data loggers/acquisition
devices. First, specialized software is necessary to configure the
logging unit. This configuration specifies the unit frequency at which
to obtain turbidity readings. The second part of the software is used to
retrieve the data from the logger and import it into a usable format on
a PC. Most companies offer integrated packages that allow users to
import the data and immediately plot and graph the data to depict trends
or produce reports. Data should be downloaded at regular intervals, as
data loggers cannot store data indefinitely.

Several methods exist to transfer data from the logger into the PC. Data
acquisition systems are often equipped to be compatible with telemetry
to upload data to PCs via telephone, cellular telephone, or radio.
Alternatively, either a laptop or palmtop can be connected to the unit
to download information, or the data logger can be brought into the
office where the PC is located and plugged into one of the input/output
ports on the PC. The better method could necessitate utilizing a second
data logger to take the place of the first logger when it is being
downloaded. Systems may wish to schedule downloads to occur at times
when a filter may not be in operation (when off-line or being
backwashed).

#### SCADA

SCADA systems are devices used for industrial measurement and control.
They consist of a central host (base unit), one or more field gathering
and control units (remotes), and a collection of standard and/or custom
software used to monitor and control remotely located field data
elements. The base unit and the remote units are linked via telemetry,
and the base unit receives data and provides instructions as specified
in the software. SCADA systems at treatment plants are also often times
referred to as Distributed Control Systems (DCS). DCSs function the same
as SCADA systems except that field gathering and control units are
located in a more confined area and communications may be via a local
area network (LAN) as opposed to remote telemetry.

SCADA systems can take inputs from a variety of sources and instruments.
These systems collect and display the data produced by a variety of
instruments so that the plant operator can monitor the entire treatment
process from one location. SCADA systems are typically used for a
variety of functions at a water treatment plant including flow control,
pH and temperature monitoring, automated disinfection dosing, and a host
of other functions. Control may be automatic or initiated by operator
commands. The inclusion of continuous turbidity monitoring could be
incorporated into the regime of items being measured and controlled by a
SCADA/DCS system at a treatment plant.

SCADA systems can also be used to log and store data for recording
purposes. Signals sent from remote instruments located on the plant site
are interpreted at the base unit. This unit provides the logic to
interpret all of the different signals and display real-time
measurements. The central unit can be programmed to automatically
transfer historical data to other storage media such as a tape drive or
Zip-drive.

### Data Management 

There are two distinct objectives to management of turbidity data: (1)
Regulatory Compliance, and (2) Checking Process Control and Treatment
Plant Optimization. The turbidity reporting and monitoring requirements
set forth in Chapter 2 establish the types of data which must be
collected and the analysis which must be done to meet the requirements
of the rule. In order to meet these requirements, operators must
understand three areas of data management:

-   Data Format;

-   Data Storage; and

-   Data Analysis.

#### Format

Storage of the data in a usable format is the first step to effective
data management. Operators should have the ability to download data from
their acquisition equipment into a usable and manageable format. Data is
typically placed in one of many different formats such as Excel, Access,
dBASE, and Lotus 123. Data should be converted into a format that can be
used by the facility. Many systems currently utilize software such as
those listed above. The key to selecting a format is the ease at which
the data can be viewed, manipulated, and or converted. Certain software
packages allow users to create reports, tables, or graphs based on the
data.

#### Storage

Storage of the data is the next step in effective data management.
Maintaining these data points for future analysis may pose a problem due
to the amount of disk space required. Systems should consider the use of
Zip-Drives or tape-drives for storage of data. Hard drives can be used
to store data while manipulating or evaluating. Tape and Zip-Drive
backups are also recommended due to the possibility of a PC crashing.

#### Interpreting and Analyzing Data

Data Analysis is the last step in effective data management. Systems are
encouraged to utilize the Data Collection Spreadsheets and Macros
developed for the Partnership for Safe Water. A description of the
Partnership for Safe Water is found in Chapter 4.

Spreadsheets were prepared for the Partnership for Safe Water to assist
utility partners in collecting performance data. The spreadsheets were
developed to capture turbidity data from raw water, sedimentation basin
effluent and filter effluent, but can be used to measure repetitive data
of any kind, from any point in the process for up to 365 days. Macros
have been written to generate frequency distributions on a monthly and
annual basis, to help evaluate trends and summarize large amounts of
data. Graphics capabilities of the spreadsheets are also built in to
automatically plot trend charts and frequency distributions. There are
also capabilities for generating summaries of the data to report as
background information. Other data summaries within the capabilities of
each spreadsheet software version could be generated as well. A disk
containing the software along with guidance for using the software is
found in the Composite Correction Program Handbook published by EPA.

The software provided with many of the data acquisition systems, which
can be custom designed for SCADA/DCS systems, also allow operators to
trend and analyze data. Easy-to-use software provides clear graphics for
operators to evaluate. Typically, data can be exported to various
spreadsheets or database programs for later analysis. Software is
typically interactive, with the ability to change colors, and graph
sizes.

Systems should analyze turbidity data to check process control and
treatment plant optimization. Systems may wish to evaluate backwash
turbidity spikes for individual filters, how storm events affect the
filtration capabilities, or the effect of various chemical dosages on
filtered effluent. Analysis could be undertaken to compare different
filters within a system or the effect of different flow rates. Chapter 5
provides information on conducting a Filter Self Assessment and analysis
which systems may wish to implement.

## References 

1.  AWWARF. 1998. *Treatment Process Selection for Particle Removal*,
    AWWARF International Water Supply Association.

2.  Burlingame, G.A., M.J. Pickel, and J.T. Roman. 1998. "Practical
    Applications of Turbidity Monitoring." *J. AWWA*. 90(8):57-69.

3.  CDHS (California Department of Health Services). 1998. *Turbidity
    Monitoring Guidelines*, June 18.

4.  Great Lakes Instruments, Inc. 1992. Turbidity. *GLI Method 2.*
    Milwaukee, WI.

5.  Great Lakes Instruments, Inc. "Turbidity Measurement." Technical
    Bulletin Number T1 Rev 2‑193. Milwaukee, WI.

6.  Hach Company. 1997. "Low Level Turbidity Measurement." Loveland
    Colorado, September.

7.  Hach Company. 1995. "Excellence in Turbidity Measurement."

8.  Hart, Johnson, and Letterman. 1992. "An Analysis of Low‑level
    Turbidity Measurements". *J. AWWA*. 84(12):40.

9.  International Organization for Standardization (ISO). 1990. *ISO
    7027 Water Quality‑Determination of Turbidity*.

10. King, K. 1991. "Four‑Beam Turbidimeter For Low NTU Waters." *J. of
    the Australian Water and Wastewater Association*, October.

11. Lex, D. 1994. "Turbidimeter Technology Turns on the High Beams."
    *Intech.* 41(6).

12. Meeting Notes. 1997. Turbidity Workshop, September 15‑16.

13. Sadar, M. 1996. *Understanding Turbidity Science,* Technical
    Information Series‑Booklet No. 11, Hach Company.

14. Sadar, M. *Turbidity Standards*, Technical Information
    Series‑Booklet 12, Hach Company, Loveland, CO.

15. Sethi, Patanaik, Biswas, Clark, and Rice. 1997. "Evaluation of
    Optical Detection Methods for Waterborne Suspensions." *J. AWWA*.
    89(2):98‑112.

16. Standard Methods. 1995. *Standard Methods for the Examination of
    Water and Wastewater*. Nineteenth Edition. Franson, M.H., Eaton,
    A.D., Clesceri, L.S., and Greenberg, A.E., (editors). American
    Public Health Association, AWWA, and Water Environment Federation.
    Port City Press, Baltimore, MD.

17. USEPA. 1998. *Handbook: Optimizing Water Treatment Plant Performance
    Using the Composite Correction Program*. EPA/625/6-91/027.

18. USEPA. 1997. "National Primary Drinking Water Regulations: Interim
    Enhanced Surface Water Treatment Rule Notice of Data Availability;
    Proposed Rule." 62 *FR* 59486. November 3.

19. USEPA. 1993. *Methods for the Determination of Inorganic Substances
    in Environmental Samples*, EPA‑600/R‑93‑100.

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### 

######### THIS PAGE INTENTIONALLY LEFT BLANK

[3. Turbidity Methods & Measurement
[1](#turbidity-methods-measurement)]{.smallcaps}

3.1 Introduction [1](#introduction)

3.2 Approved Turbidity Methods [1](#approved-turbidity-methods)

3.2.1 EPA Method 180.1 [1](#epa-method-180.1)

3.2.2 Standard Method 2130B [1](#standard-method-2130b)

3.2.3 Great Lakes Instrument Method 2 (GLI 2)
[2](#great-lakes-instrument-method-2-gli-2)

3.3 Turbidimeters [2](#turbidimeters)

3.3.1 Bench Top Turbidimeters [2](#bench-top-turbidimeters)

3.4 Quality Assurance/Quality Control
[6](#quality-assurancequality-control)

3.4.1 Quality Assurance Organization and Responsibilities
[7](#quality-assurance-organization-and-responsibilities)

3.4.2 Quality Assurance Objectives [7](#quality-assurance-objectives)

3.4.3 Standard Operating Procedures [7](#standard-operating-procedures)

3.4.4 Sampling Strategy and Procedures
[9](#sampling-strategy-and-procedures)

3.4.5 Calibration [12](#calibration-2)

3.4.6 Data Screening, Validation, and Reporting
[15](#data-screening-validation-and-reporting)

3.4.7 Performance and System Audits [16](#performance-and-system-audits)

3.4.8 Preventative Maintenance [16](#preventative-maintenance)

3.5 Data Collection and Management [17](#data-collection-and-management)

3.5.1 Data Collection Methods [17](#data-collection-methods)

3.5.2 Data Management [19](#data-management)

3.6 References [20](#references)

Figure 3-1. Calibration Checklist [16](#__RefHeading___Toc448537015)