Breakpoint Chlorination Plays Important Role in RO - Crown Solutions

Pretreatment: Breakpoint
Chlorination Plays
Important Role in RO
Pretreatment
By James McDonald, PE, CWT
Originally Published: Ultrapure Water, January 2003, Volume 20, Number 1
When chlorination is used in reverse osmosis (RO) pretreatment,
breakpoint chlorination can make or break the system. This can be especially
critical when treating surface waters, wastewaters, or recycle streams. Too
low a chlorination level can lead to microbiological fouling of the RO
membranes resulting in reduced RO performance and increased operational
costs.
Typically, the DPD free chlorine test method is used to monitor free
available chlorine levels. Free available chlorine is defined as the amount of
chlorine which exists in the treated system as hypochlorous acid and
hypochlorite ions after the chlorine demand has been satisfied. The DPD
free chlorine test method has several interfering compounds that can affect
the test results. One important interference to consider is monochloramine,
which is why breakpoint chlorination can be such an important issue.
When chlorine gas (Cl2) or bleach (NaOCl) are added to water, they rapidly
hydrolyze and dissociate to form hypochlorous acid (HOCl) and hypochlorite
ions (OCl - ). Hypochlorous acid is the much stronger of the two biocides and
can react very quickly with inorganics such as ammonia. Some dissolved
organic materials also react rapidly, but the completion of many organochlorine
reactions can take hours. (1)
Chloramines
If ammonia exists in the water being pretreated for RO use, the reaction
between hypochlorous acid and ammonia is a very important reaction that
must be taken into account. Hypchlorous acid and ammonia combine to
form inorganic chloramines: monochloramine (NH2Cl), dichloramine
(NHCl2) and trichloramines or nitrigen trichloride (NCl3). The relative
amounts of the chloramines formed are a function of chlorine fed, the
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chlorine/ammonia ratio, temperature, and pH. In general, monochloramine is
formed above pH 7 and dominates at pH 8.3.
Monochloramine is a much weaker biocide than hypochlorous acid. The
killing power of free residual chlorine (i.e., hypochlorous acid and
hypochlorite ion) is as much as 25 times higher than the killing power of
combined available chlorines (i.e., monochloramines). (2)
Why is all this important? As mentioned, monochloramine is an interference
to the DPD free chlorine test. As Table A shows, the interference in the DPD
free chlorine test can be rather high considering many control ranges are in
the 0.25 to 0.5 parts per million (ppm) free chlorine range. Your free
chlorine tests may be showing a free chlorine residual of 0.4 ppm, but if you
have ammonia in the source water, this reading may be affected by
monochloramine interference. You think you have free chlorine residual as a
biocide, but you really only have monochloramine. How do you ensure that
monochloramine is not interferring with your free chlorine test? You achieve
and exceed breakpoint chlorination.
Table A - DPD Free Chlorine Interference (ppm) (3)
Monochloramine Sample Temperature °F (°C)
(NH2Cl) Level (ppm) 40 (5) 50 (10) 68 (20) 83 (30)
1.2 +0.15 0.19 0.30 0.29
2.5 +0.35 0.38 0.55 0.61
3.5 +0.38 0.56 0.69 0.73
5.0 +0.68 0.75 0.93 1.05
Breakpoint Chlorination
Breakpoint chlorination is the application of sufficient chlorine to maintain a
free available chlorine residual. The principle purpose of breakpoint
chlorination is to ensure effective disinfection by satisfying the chlorine
demand of the water. In waters that contain ammonia such as wastewater,
breakpoint chlorination is a means of eliminating ammonia to achieve a true
free chlorine residual.
Figure 1 shows the theoretical breakpoint chlorination curve. Adding
chlorine to water that contains ammonia or nitrogen-containing organic
matter produces an increased combined chlorine residual. Between points A
and B on the curve, mono- and dichloramines are formed. Point B represents
the point where all ammonia has been oxided to monochloramine and
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dichloramine. Complete monochloramine oxidation to dichloramine,
occurring between points B and C, results in a decline in the combined
available residuals initially formed. Point C is the breakpoint: the point at
which chlorine demand has been satisfied and additional chlorine appears as
free residuals. The free available residual chlorine increases in direct
proportion to the amount of chlorine applied between points C and D.
Many factors affect breakpoint chlorination including the initial ammonia
nitrogen concentration, pH,
temperature, and demand
exerted by other inorganic and
organic species. A weight ratio
of 8:1 or greater of chlorine
applied to initial ammonia
nitrogen is required for
breakpoint chlorination to be
reached. If the weight ratio is
less, there is insufficient
chlorine present to oxidize the
chlorinated nitrogen
compounds initially formed.
For instantaneous chlorine
residual, the weight ratio
required may be 20:1 or
more. Reaction rates are
Available Total Cl2 Residual
(ppm)
fastest at high temperatures and pH 7-8. (1)
Theortical Breakpoint
Chlorination Curve
Figure 1 - Theoretical Breakpoint
Chlorination Curve
Determining Breakpoint
A field test can lead you in the right direction to finding the chlorination
breakpoint. Although the test cannot replicate the exact conditions of the
system, it is a starting point. The following procedure has been used with
success at several locations.
Test Calculation Data:
• Household Bleach ≈ 5.25% NaOCl
• NaOCl MW = 74.5
• Cl2 MW = 71
• 71 / 74.5 * 5.25% = 5% as equivalent free Cl2 or 50,000
ppm in household bleach
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7
6
5
4
3
2
1
0
A
B
C
Zero
Chlorine
Demand
0 5 10
Cl2 Dosage (ppm)
D

Test Procedure:
1. Test the ammonia level in the unchlorinated water to evaluated.
Record the result.
2. Add 1 milliliter (mL) of original bleach solution to 99 mL
distilled water. This makes approximately a 500 ppm solution.
3. Check strength of 500 ppm solution by adding 0.2 mL with a
syringe to 100 mL distilled water. This should give 1 ppm free
chlorine. Test using the DPD free chlorine test. Multiply the
test result by 5. Record the result. This is the amount of Cl2 per
mL that will be add to a 100 mL sample in step 5.
4. In five beakers, add 100 mL each of the water to be evaluated for
breakpoint chlorination. Do not filter.
5. Add chlorine solution to each beaker. The amount of chlorine
solution added per beaker would be dictated by the dosage (ppm)
of Cl2 desired. The amount of chlorine added per mL of
prepared solution is: mL added * (ppm Cl2/mL calculated in
step 3).
To achieve breakpoint chlorination, a minimum ratio of 8:1 of
chlorine to ammonia must be achieved. It is recommend that
beakers 1 and 2 be at dosages less than the 8:1 ratio, beaker 3 at
the 8:1 ratio, and beakers 4 and 5 be greater than the 8:1 ratio.
This should give a good breakpoint chlorination curve. If you
have to add more than 10 mL of chlorine solution, make a
stronger chlorine solution and start at step 2.
6. Wait 30 minutes.
7. Test for free chlorine residual.
8. Graph your results.
Another test that can be run on the same five beakers in the above procedure
is monochloramine. Hach offers a Monochlor-F test procedure for
monochloramine. Monochloramine concentrations will be zero when
breakpoint chlorination is achieved (test accuracy is +/-0.1 ppm as Cl2).
Analytical Test Options
As mentioned before, monochloramines interfere with the results of the DPD
free chlorine test. This also holds true for the Nessler and Salicylate test
methods for ammonia testing. Table A summarizes monochloramines effects
on analytical test options available.
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Table B - Analytical Test Options with Monochloramine
Analysis Test Method Monochloramine
Interference
Free Chlorine DPD Yes
Total Chlorine DPD No
Monochloramine Monochlor-F No
Ammonia
Salicylate Yes
Nessler Yes
Ion Selective Electrode No
Testing for ammonia alone using an ion selective electrode (ISE) will not
determine when breakpoint chlorinate has been reached since the ammonia
concentration will go to zero ppm prior to breakpoint chlorination. Point B
in Figure 1 represents the point when ammonia concentrations are zero ppm.
Case Study #1
A large industrial plant recovered wastewater for cooling tower makeup by
using RO units. Chlorine was added upstream of the RO with a
dechlorination step immediately before the RO. Membrane fouling was
becoming a real problem. RO capacity was being reduced and membrane
cleaning frequency was increasing. The plant was under pressure to recover
more wastewater via the RO system. Membrane biopsies revealed
microbiological fouling.
One of the first steps to take when approaching a problem is to first
determine if the steps currently being taken are being done properly. The
plant was feeding chlorine at the proper point. Chlorine was being feed into
the Clear Well which was the point of lowest chlorine demand prior to the
RO system. The test records showed a consistent free chlorine residual being
maintained. So far so good, but was the free chlorine residual they were
testing using the DPD free chlorine test method really showing free chlorine
or was there monochloramine interference?
Water tested prior to chlorine addition showed ammonia levels that ranged
from 2.5 ppm to 19 ppm. With water temperatures of 80°F and a free
chlorine residual control range of 0.25 to 0.5 ppm, you can easily see in
Table A that free chlorine residual results could be entirely due to
monochloramine interference! The plant thought they were getting proper
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chlorination prior to the RO, but were getting a much weaker biocide
(monochloramine) instead.
The breakpoint chlorination test procedure described earlier was conducted.
Figure 2 shows the results from one round of tests. As you can see, the free
chlorine residual curve closely resembles that in Figure 1. Ammonia was
also tested using the Salicylate method. Even though monochloramine is an
interference for this method, Figure 2 shows that the ammonia level as zero
at the breakpoint where all monochloroamine had been oxidized. At the
breakpoint and beyond, monochloramine does not exist and is not an
interference to chlorine or ammonia testing.
The two solutions available to the plant were to increase chlorine feed or
supplement with another biocide. Because of variation in ammonia levels
and the large chlorine demand required to reach breakpoint chlorination, the
plant decided to use DBNPA as a supplemental biocide. With a
comparatively minimal DBNPA usage rate, the plant was able to
significantly increase membrane life and the time between cleanings.
Test Results (ppm)
18
16
14
12
10
8
6
4
2
0
Monochloramine
Interferrence
Breakpoint Chlorination of Clear Well Study
7/24/01
Free Chlorine Residual Test Ammonia
Monochloramine
Interferrence
0 20 40 60 80 100 120 140 160 180
Chlorine Added (ppm)
Breakpoint
True
Free Chlorine
Residual
Figure 2 - Breakpoint Chlorination Example
Case Study #2
A large industrial plant used river water as makeup to a 2,000 gpm RO
system for boiler feedwater makeup. Due to fouling problems, each bank of
membranes was cleaned twice a week. Cleaning at this frequency is not only
bad for the membranes, but requires a lot of manpower commitment. RO
pressure differences, permeate quality, and RO feed pressure were
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significantly affected by the fouling. The plant knew this was a problem, but
had already had many "experts" review their system over the years with no
solution. Nine separate companies had already tried. They were resistant to
trying anything else and did not want the RO touched.
The advantages of solving this problem were obvious: longer service runs,
less damage to membranes, minimized membrane replacement costs, reduced
manpower costs, lower water production costs, decreased pumping costs, etc.
First, the concepts of breakpoint chlorination were applied. The ammonia
concentration in the makeup water was determined. The breakpoint
chlorination test procedure previously described was conducted to find the
proper chlorine dosage to reach breakpoint chlorination. The plant's current
chlorine dosage was no where near that required for breakpoint chlorination.
A higher dosage was required for proper disinfection of the raw water prior
to being introduced to the RO.
Next, to prove the findings, a pilot RO unit was set up parallel with the
current RO system. Breakpoint chlorination dosages of chlorine were added
to the pilot RO pretreatment train with the water being dechlorinated prior to
entering the RO. Once breakpoint chlorination was achieved and a true free
chlorine residual was maintained throughout the pretreatment train, the pilot
RO performance was greatly improved over that of the current RO system.
Much longer service runs between cleanings were observed. Pressure drops
across the membranes, feed water pressure, and permeate quality were each
greatly improved.
With the result of the pilot study as proof, the plant implemented breakpoint
chlorination dosages on the current RO and experienced a similar success.
Conclusion
The application of breakpoint chlorination with RO pretreatment has
successfully been used to solve baffling microbiological fouling problems of
RO membranes. Although the customers thought they were applying
sufficient chlorine for disinfection, what they were actually measuring was
monochloramine interference to the DPD free chlorine test. Determining the
breakpoint chlorination allowed the customers to better disinfect their RO
feedwater and resulted in longer service runs between membrane cleanings.
RO's are complicated systems with many factors to consider. The
application of breakpoint chlorination is just one of those factors that must be
considered. When approaching any problem, one of the first steps should be
to ensure that the current technology and treatments are being properly
applied. Taking into account breakpoint chlorination, as described in this
article, is a good method to determine if chlorine chemistry is being properly
applied.
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References
1. "Betz Handbook of Industrial Water Conditioning", 9 th Edition, Betz
Laboratories, Inc., Trevose, PA, pp. 196-199, 1991.
2. George Glifford White, "The Handbook of Chlorination", 2 nd Edition,
Van Nostrand Reinhold Company, New York, New York, pp. 162-167,
1986.
3. "Water Analysis Handbook", 3 rd Edition, Hach Company, Loveland, CO,
p. 351, 1992.
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