Published in Water & Waste Water Asia Magazine (www.
waterwastewaterasia.com) May/June 2016 Issue
32
astewater is produced
everywhere from homes,
schools, hospitals,
businesses, factories, refineries, power
plants, and more. Regardless of the
source, whether municipal or industrial,
all of that wastewater eventually has to
be treated and
disinfected before it
can be released back
into the environment
safely. Wastewater is
over 99% water and
about 0.3% dissolved
and suspended solid
material. Wastewater
treatment involves
several steps to make
sure that the
wastewater streams
meet local and
national regulations
that safeguard the
e nv i ro n m e nt a n d
water quality.
At a minimum,
treatment removes suspended solids
and reduces the concentration of
organic matter. Other steps may
remove nutrients, such as nitrogen
and phosphorus. Treatment must also
kill disease-causing microorganisms.
Chlorine gas and sodium hypochlorite
are common disinfectants.
The accurate measurement of
total chlorine is the wastewater
treatment plant’s first line of defense
in assuring that the water has been
sufficiently treated and the chlorine is
at appropriate levels to meet regulatory
requirements for safe discharge into the
environment. Yet, selecting the right
chlorine analyzer in water treatment
applications can be a challenge for
instrument technicians. Sometimes
it is difficult for technicians to select
the right analyzer. For example, in an
application that requires measurement
of free residual chlorine, a technician
may incorrectly select a free chlorine
analyzer, but a total chlorine analyzer
is actually more suitable in this case.
This article explores and discusses
the typical chlorination process in
wastewater. The selection of a chlorine
analyzer in seawater chlorination is also
examined with the use of an actual case
study performed at a global chemical
customer site.
microorganisms that were used in the
previous treatment stages. Disinfection
inactivates or destroys pathogenic
organisms and prevents the spread of
waterborne diseases to downstream
users and the environment. Chlorine
gas is often used, but some plants
use their own chlorine
solution, such as
sodium hypochlorite.
Effective Chlorine
Measurement in Wastewater
to Meet Environmental
Requirements
May June 2016
By Ryo Hashimoto,
Senior Manager, Emerson Process Management,
Rosemount Asia Pacific
Wastewater Treatment
Typically, wastewater treatment
includes four basic treatment stages:
primary, secondary, sludge, and final
treatment. In the primary treatment
stage, larger solids are removed
from wastewater by screening and
settling. Secondary treatment is a large
biological process for further removal
of the remaining suspended and
dissolved solids. Sludge is generated
during the first two stages and is
treated to convert it into a stable
organic solid. In the final treatment,
wastewater is disinfected to kill any
remaining harmful microorganisms
prior to being released. The various
stages of wastewater treatment include
physical, chemical, and biological
treatment processes.
Chlorination and dechlorination
are involved in the final stage of
wastewater treatment. In this stage,
disinfectants are added to the water
to kill disease-causing organisms and
Chlorination
Process in
Wastewater
Treatment
When chlorine is
added to wastewater,
a portion of it
is consumed by
reactions with
chemicals in the water,
making it unavailable
for disinfection.
Moreover, the chlorine
that remains has often
been converted to
forms that are significantly poorer
Rosemount TCL system from Emerson
Process Management offers continuous
total chlorine measurement with low
reagent requirements.
Water & Wastewater Asia
33
d i si nfectants th an th e so d i u m
hypochlorite or chlorine gas originally
added.
Only two forms of chlorine have
disinfecting properties: free chlorine
and combined chlorine. Free chlorine,
which is by far the better disinfectant,
is produced when sodium hypochlorite
or chlorine gas is added to water.
Combined chlorine is monochloramine
and dichloramine, which are
compounds formed by the reaction
between free chlorine and ammonia.
Because wastewater treatment seldom
completely removes ammonia and
organic amines, chlorinated wastewater
typically contains combined chlorine,
primarily monochloramine. Free
chlorine will appear only if enough
chlorine has been added to completely
oxidise the chloramines, so-called
Water & Wastewater Asia
breakpoint chlorination. Breakpoint
chlorination is rarely practiced in
municipal treatment plants.
The sum of free and combined
chlorine is often called total chlorine.
The term is misleading because standard
analytical methods define total chlorine
by the way it is measured, not as
the aggregate of certain chemicals.
Accordingly, total chlorine is all the
oxidants, some of which may not
contain chlorine, in the sample capable
of oxidising potassium iodide to iodine
when the pH is between 3.5 and 4.5. The
amount of iodine produced under these
conditions is defined as total chlorine.
For chlorine, either free or combined,
to be effective, the concentration
and contact time must be controlled.
Most treatment plants have a contact
chamber where chlorine is injected,
mixed, and allowed to remain in contact
with the water for the required time.
Flow rate into the chamber and the
chlorine concentration at the outlet are
used to control the chlorine injection
rate.
Dechlorination Process
In many areas, local and national
government agencies regulate the
amount of chlorine allowed in the
final plant effluent before being
discharged into lakes, rivers, or the
ocean. This requires dechlorination,
which removes the free and
combined chlorine residuals to
reduce the toxicity after chlorination
and before discharge. The chlorine
concentration in reused wastewater
can be as high as several ppm. In
discharged wastewater, the permitted
May June 2016
34
Figure 1
concentration is low, typically
between 0.01 to 0.30 ppm of chlorine.
Chlorine is closely regulated because
even small amounts are harmful to
aquatic organisms. Typically, plants are
required to monitor their waste streams
and report chlorine levels to a regulatory
agency. Agencies can require either
continuous or grab sample testing.
Chlorine is added to the effluent
from the final clarifier as it enters the
chlorine contact chamber. Typically,
the chemical injection rate is controlled
using the flow rate and the chlorine
concentration at the outlet of the
contact chamber (see Figure 1). A second
contact chamber for dechlorination is
not needed because dechlorination
reactions are almost instantaneous.
Most wastewater applications require
measuring total chlorine, although
free chlorine might also be required.
Excess chlorine is removed in a
dechlorination stage by adding sulfur
dioxide, sodium bisulfite, sodium sulfite,
or sodium metabisulfite. The chlorine
concentration is measured in both the
May June 2016
chlorination and dechlorination stages.
Direct measurement of trace
chlorine can be difficult. If the discharge
limit is close to the detection limit, the
relative measurement error will be
high. If the discharge limit is below the
detection limit, the analyzer cannot
be used at all. Many chlorine sensors,
particularly amperometric ones, are
calibrated against the results of a grab
sample test. Because chlorine sensors
cannot be accurately calibrated if the
sample contains only a trace of chlorine,
there must be a provision to interrupt
the normal sample flow and replace
it with chlorinated water. Chlorinated
water is also needed for periodically
checking the response of the analyzer
to confirm that low chlorine readings
are valid and not the result of a failed
sensor.
Seawater Chlorination
In a reaction of chlorination in normal
water, sodium hypochlorite or chlorine
gas forms hypochlorous acid (HOCl)
and hypochlorite ions (OCl-). A free
chlorine amperometic sensor reacts
with HOCl and converts it into a free
residual chlorine measurement. When
it comes to seawater chlorination, free
chlorine (HOCl and OCl-) is not an issue
as it does not exist in chlorinated
seawater because of its bromide (Br-)
content. Seawater contains as much as
65 ppm bromide. When chlorine is added
to seawater, it oxidises with bromide to
create a mixture of hypobromous acid
(HOBr) and hypobromite ion (OBr-). In the
process, free chlorine becomes chloride
(Cl-), so free chlorine no longer
exists. Free chlorine will also oxidise
trace iodide (I-) in seawater to
hypoiodous acid (HOI) and hypoiodite
ions (OI-). Finally, if ammonia and
organic amines are present,
inorganic and organic chloramines
and bromamines might form. For this
reason, free chlorine won’t be present in
chlorinated seawater, so when
measuring it, it is best to monitor total
chlorine. Typical chlorine applications
are listed in Table 1 on the following
page.
Water & Wastewater Asia
35
A Real-World Example
A major global chemical processing
company uses seawater for process
cooling. Because that cooling water
is later returned to the nearby
estuary, the plant must use only the
minimum concentration of chlorine
to be sure it meets environmental
discharge regulations. The company
had previously used an analyzer
based on diethyl-p-phenylenediamine
(DPD) to monitor chlorine, but this
technology had become outdated
and was expensive to run. The
DPD analyzer was a colorimetric
method and wasn’t well-suited for
seawater. The water is salty, contains
a l ga e , s e a we e d , m u s s e l s , a n d
sand, and these contents impact
the accuracy and reliability of the
measurement. As a result, the sensor
had to be cleaned nearly every day.
The plant shifted to amperometric
chlorine analyzers for total chlorine
measurement. As chlorine diffuses
into the sensor through a membrane,
an electrochemical reaction generates
current proportional to the diffusion
rate, which itself is proportional to the
chlorine concentration. The company
found this method to be effective
in dirty seawater and to provide
a consistent and accurate measurement
as the water content and pollution
and temperature changes do not
affect its reliability. It also provides
a faster measurement, so problems
with chlorine levels can be identified
a n d a d d r e s s e d m o r e q u i c k l y,
improving environmental compliance
and avoiding regulatory penalties
and fines. Faster, more accurate
measurement of total chlorine allows
the plant to optimise both the chlorine
content and the microbiological
activity.
When considering the chlorine
analyzers and sensors the plant chose,
one important selection factor was
the reagent used in the systems. The
analyzer they opted for had very low
reagent requirements so the reagent
lasted for more than three months,
which reduced costs and maintenance.
Because air is dosed in the analyzer
the plant selected, it offers a selfcleaning aspect, so the measuring cell
rarely if ever needs cleaning, a huge
maintenance improvement over the
previous system.
Responsible for Regulatory
Compliance
Regardless of whether treated water
was used in industrial processes,
such as for cooling water, or used
by municipalities in homes, schools,
businesses, and more, the resulting
wastewater must be treated so it can be
returned to the environment. Chlorine
is critical to disinfecting water to prevent
disease, but it also poses its own risk
as a dangerous poison to sea life and
the environment, so water treatment
plants, both industrial and municipal,
must ensure its careful removal before
water is discharged. Accurate, reliable
chlorine monitoring is vital to ensure
safe discharge water, compliance with
environmental regulations, and an
optimised treatment process. WWA
About the Author
Ryo Hashimoto is senior manager, Emerson
Process Management, Rosemount liquid
analytical in Asia-Pacific. Further information
can be found at www.EmersonProcess.com/
Rosemount-Water Quality Panels.
Table 1. Typical Chlorine Applications
FRC: Free Residual Chlorines • CAC: Combined Available Chlorines •TRC: Total Residual Chlorines
Water & Wastewater Asia
May June 2016