United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-85-002 Mar. 1985
Project Summary
Recommended Practices for
On-Line Measurement of
Residual Chlorine in
Wastewaters
Gershon Kulin
A manual was developed to provide
improved guidelines for selecting, in-
stalling, calibrating, and maintaining
instruments used to monitor and control
chlorine in wastewater treatment plants.
Data are combined from several sources
and placed in an easily referenced
format. The document briefly addresses
the most common concerns of treat-
ment plant personnel and refers to the
specific section of the document that
covers the topic in detail.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Chlorine concentration is a commonly
measured para meter i n wastewater treat-
ment plants. The measurement is re-
quired for permit reports of the National
Pollutant Discharge Elimination System
and for in-plant process control. The
devices used to measure chlorine con-
centration are diverse and have different
principles of operation, installation re-
quirements, effective measuring ranges,
and uncertainties of measurement (ac-
curacy).
The purpose of this document is to
publish in one concise manual the key
information required for:
• Selecting the proper device for an
application
• Designing the installation
• Operating and maintaining the device
• Calibrating the device
Also included in the manual are the
principles of operation for generically
different devices and several means of
verifying the calibration of an installed
chlorine analyzer.
Recommended Practices
Meters
The manual covers amperometric,
colorimetric, and polarographic sensors.
The amperometric and colorimetric sen-
sors are used as flow-through devices. A
sample is continuously withdrawn from
the process, and after conditioning with
reagents as necessary, it is made to flow
through the sensor. Polarographic sen-
sors are separated from the sample by a
membrane and, in principle, could be
directly immersed in the wastewater.
Installation
The care used in the installation of a
chlorine analyzer and its associated sam-
pling system is critical to the accuracy
and utility of the measurement, and to the
ability of operating personnel to maintain
the analyzer in optimum condition. The
importance of proper installation is illus-
trated by the following excerpt from the
installation requirements section of the
manual.
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"Flow-ThroughSampling: An ana-
lyzer using flow-through sampling
should have either a sampler that is
an integral part of the instrument or
an independent sampling system
with its own pump or pressure time
to provide adequate flow rate and ap-
propriate velocity of sample flow to
the analyzer as described in the
following:
"The contact time is equal to the
time required for a volume element
of liquid to go from the beginning of
the contact tank to the point at which
the sample is withdrawn, plus the
time required to move through the
sample line to the sensor. Thus, the
velocity of sample flow and the length
of the sampling line are to be chosen
such that the arrival at the sensor of a
volume element of sample corre-
sponds to the desired contact time.
This means, for example, that if the
analyzer is located far from the
contact tank, the sample may have to
be withdrawn close to the injection
point if it is to be representative of the
contact-chamber effluent by the time
it reaches the analyzer. See Figure 1.
"The velocities selected are subject
to the following additional con-
straints:
— The velocity must be maintained
high enough to prevent deposition
of suspended matter and to dis-
courage slime formation in the
line;
— The velocity must not be so high
as to break up suspended material
and expose new surfaces to chlo-
rine action; and
— Velocities of about 1.5 m/sec (5
ft/sec) but no higher than 3
m/sec(10ft/sec)will meetthese
constraints.
"The foregoing requirements can
be satisfied by selecting appropriate
combinations of pump capacity and
sample-line size."
Flow Rate
The system must include means for
determining that the sample flow rate is
within proper operating limits. If a flow-
meter is necessary to satisfy this require-
ment, the manufacturer must furnish it
as an integral part of the equipment.
Samp/ing Pump
If the sampling pump is not furnished
as an integral part of the analyzer system,
the manufacturer must provide the spec-
ification necessary for its selection and
installation. Pumps that subject the sam-
ple to minimal shears (and suspended
material breakup) are preferred. Positive
displacement pumps are generally supe-
rior to centrifugal pumps in this regard.
Check Samples
Provision must be made for withdraw-
ing a discrete check sample where the
sample enters the analyzer. The accuracy
of the analyzer can thereby be calibrated
or checked by an independent method.
Keeping Sample Lines Clean
Filtering
Since effluents of wastewater treat-
ment plants often contain suspended
solids, steps must be taken to prevent
constriction of the sampling system.
Filtering at the inlet to the sampler is one
possibility. Where appropriate, the ana-
lyzer should also incorporate a fine filter
or its equivalent ahead of the sensor cell.
Users should be furnished with guidelines
for determining visually or otherwise
whether analyzer filters need cleaning or
replacement.
Contact Chamber Flow
Sample Withdrawn
Sensor
Sample Line
Waste
Ls
T1 = Contact Time Until Sample Withdrawal
Effective Contact Time = T1 + Ls/Vs
Figure 1. Allowing for length of sample line.
2
Flushing
Some means must be available for
cleaning the sample lines of deposits and
coatings. These may include backflushing
capability, convenient disconnection of
sample line from the analyzer, or other
means. As a related requirement, provi-
sion should be made to keep liquid
flowing in the line at all times, even when
the sensor is out of service.
Materials
The sampling system should be con-
structed of materials that are corrosion
resistant and will not affect readings by
reaction with the residual chlorine.
Direct Immersion Sensors
If the analyzer is the type that uses a
sensor immersed directly in the flow, the
manufacturer must inform the user of
any requirements concerning the velocity
of the flow relative to the sensor. If an
agitator or other type of motion inducer is
required, the manufacturer must either
make it available for purchase as part of
the system or provide all information and
specifications necessary for its procure-
ment and installation.
Other Considerations
The sampling procedure should ensure
that the residual chlorine in the sample
that reaches the analyzer is represent-
ative of the average value in the chlorine
contact tank after a specific time. Thus at
the point of withdrawal, the sample must
be representative of conditions in the
corresponding sectional plane of the
contact tank. Furthermore, an accounting
must be given of possible changes in the
sample as it proceeds through the sample
line. Users should refer also to ASTM
D-3370.
Representative Samples
This section applies to both flow-
through and direct-immersion sensors. If
the injected chlorine has been well mixed
with the effluent stream, and if what is
essentially a plug flow prevails in the
contact tank, the chlorine concentration
will be uniform at a cross-section and a
sample will be representative of the
sectional plane at which it is withdrawn.
However, the existence of these condi-
tions should be verified by checking the
constancy of numerous residual chlorine
measurements made across the sampling
plane of the contact tank using a portable
analyzer. (Note that for this application
the repeatability of the analyzer is more
important than its accuracy.)
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As an alternative to the use of a
portable analyzer, samples may be with-
drawn from these points for measure-
ment with an amperometric titrator,
provided that the flow (wastewater and
chlorine) is constant while the samples
are being drawn and provided that the
period of time between the taking of each
sample and its measurement is the same
for all samples.
These measurements should be made
for both a high and a low plant flow. They
may be waived if hydrodynamic principles
or other means can establish to the
satisfaction of the involved parties that
complete lateral mixing has been
achieved.
In addition to mixing the residual
chlorine, decay must be characterized as
described in the following excerpt from
the sampling section of the manual.
"Characterizing the residual chlorine
decay curve. When measured resid-
ual chlorine concentration is plotted
against contact time, the resulting
curves usually display a relatively
rapid drop for the first few minutes
with a gradual exponential decay
thereafter. See Figure 2. It is impor-
tant that the approximate shape of
these curves be known so that the
effects of contact time on a sample
can be taken into account if neces-
sary. After plant startup, but prior to
activation of the residual chlorine
analyzers, make residual chlorine
measurements with a portable ana-
lyzer at points along the contact
basin corresponding to a range of
post-injection times; for example,
from five to sixty minutes.
— Enough repetitions of this survey
should be made to yield the
residual chlorine decay curve
(Figure 2) or curves that are
characteristic of the basin.
— This survey can also be made by
performing measurements on
grab samples. In that event, the
time elapsed between sample
withdrawal and titration should
be taken into account in plotting
Figure 2.
"Use the residual chlorine decay
curves (Figure 2) as a basis for
checking the condition of the sample
line. If the residual chlorine loss in
the sample line is greater than the
reduction during the corresponding
contact time, it is likely that the
sampling line requires cleaning.
"Subsequent to the initial determi-
nation of the decay curves (Figure 2),
periodic check measurements should
be made in the contact tank to
confirm their continued applicability.
The frequency of these measure-
01
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•c
o
I
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I
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Figure 2.
10 20 30 40
Contact Time, Minutes
Typical variation of residual chlorine with contact time.
50
60
ments must be established by exper-
ience."
Flow-Through Sensors
The following recommendations apply
only to flow-through sensors. The prev-
ious recommendations for sample lines
and pumps should be followed to enhance
the resistance of samples to mechanical
degradation. Another factor that affects
mechanical degradation of the sample is
organic growth on the inside walls of the
sample line. Such growth can result in
systematically low residual chlorine read-
ings. This effect can be particularly impor-
tant in long sample lines or in lines
following dechlorination.
As soon as the analyzer is placed in
service, the sample should be checked for
mechanical degradation by comparing
the residual chlorine concentrations of
samples withdrawn from the contact tank
(at points ranging from the intake to the
sample line) with those of samples taken
from the sample line at the entrance to
the analyzer. Equal chlorine concentra-
tions indicate the absence of degradation.
In long sample lines, however, the effect
of additional contact time must be taken
into account. Periodic measurements
thereafter will reveal the onset of organic
growth and will help establish a flushing
frequency for sample-line maintenance.
How often the chlorine residuals should
be measured depends on the operating
condition of the plant. A weekly check is
recommended initially, but the frequency
can be reduced as experience warrants.
The full report was submitted in fulfill-
ment of Interagency Agreement No. 78-
D-X0024-1 by the National Bureau of
Standards under the sponsorship of the
U.S. Environmental Protection Agency.
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Gershon Kulin is with the National Bureau of Standards, Washington, DC 20234.
Walter Schuk is the EPA Project Officer (see below).
The complete report, entitled "RecommendedPractices for On-Line Measurement
of Residual Chlorine in Wastewaters." (Order No. PB 85-147 478/AS; Cost:
$8.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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