ULTRAPURE WATER® JANUARY 2003--UP20013636

WPRETREATMENTBREAKPOINT CHLORINATION PLAYS IMPORTANT ROLE IN ROPRETREATMENT

osmosis (RO) pretreatment, breakpointchlorination can make or break the sys-tem. This can be especially criticalwhen treating surface waters, wastewa-ters, or recycle streams. Too low achlorination level can lead to microbio-logical fouling of the RO membranes,resulting in reduced RO performanceand increased operational costs.

Typically, the N, N-diethyl-p-phe-nylenediamine (DPD)-free chlorine testmethod is used to monitor free availablechlorine levels. Free available chlorineis defined as the amount of chlorine thatexists in the treated system as hypo-chlorous acid and hypochlorite ions af-ter the chlorine demand has been satis-fied. The DPD-free chlorine test methodhas several interfering compounds thatcan affect the test results. One impor-tant interference to consider is mono-chloramine, which is why breakpointchlorination can be such an importantissue.

When chlorine gas (Cl2) or bleach(NaOCl) are added to water, they rapid-ly hydrolyze and dissociate to form hy-pochlorous acid (HOCl) and hypochlo-rite ions (OCl-). Hypochlorous acid isthe much stronger of the two biocidesand can react very quickly with inorgan-ics such as ammonia. Some dissolvedorganic materials also react rapidly, butthe completion of many organo-chlo-rine reactions can take hours (1).

ChloraminesIf ammonia exists in the water beingpretreated for RO use, the reaction be-tween hypochlorous acid and ammoniais a very important reaction that must betaken into account. Hypchlorous acidand ammonia combine to form inorgan-ic chloramines: monochloramine(NH2Cl), dichloramine (NHCl2), and tri-chloramines or nitrogen trichloride(NCl3). The relative amounts of the chlo-ramines formed are a function of chlo-rine fed, the chlorine/ammonia ratio, tem-perature, and pH. In general, mono-chloramine is formed above pH 7 anddominates at pH 8.3.

Monochloramine is a much weakerbiocide than hypochlorous acid. Thekilling power of free residual chlorine(i.e., hypochlorous acid and hypochlo-rite ion) is as much as 25 times higherthan the killing power of combined avail-able chlorines (i.e., monochloramines)(2).

Why is all this important? As men-tioned, monochloramine is interferenceto the DPD-free chlorine test. As TableA (3) shows, the interference in the DPD-free chlorine test can be rather high,considering many control ranges are inthe 0.25 to 0.5 parts per million (ppm)free chlorine range. Your free chlorine

hen chlorina-tion is usedin reverse

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By James McDonaldCROWN Solutions Inc.

tests may be showing a free chlorineresidual of 0.4 ppm, but if you haveammonia in the source water, this read-ing may be affected by monochlora-mine interference. You think you havefree chlorine residual as a biocide, butyou really only have monochloramine.How do you ensure that monochlora-mine is not interfering with your freechlorine test? You achieve and exceedbreakpoint chlorination.

Breakpoint ChlorinationBreakpoint chlorination is the applica-tion of sufficient chlorine to maintain afree available chlorine residual. Theprinciple purpose of breakpoint chlori-nation is to ensure effective disinfectionby satisfying the chlorine demand of thewater. In waters that contain ammoniasuch as wastewater, breakpoint chlori-nation is a means of eliminating ammo-nia to achieve a true free chlorine resid-ual.

Figure 1 shows the theoretical break-point chlorination curve. Adding chlo-rine to water that contains ammonia ornitrogen-containing organic matter pro-duces an increased combined chlorineresidual. Between points A and B on thecurve, mono- and dichloramines areformed. Point B represents the point

Figure 1. Theoretical breakpoint chlorination curve.

ULTRAPURE WATER® JANUARY 2003--UP200136 37

where all ammonia has been oxidized tomonochloramine and dichloramine.Complete monochloramine oxidation todichloramine, occurring between pointsB and C, results in a decline in thecombined available residuals initiallyformed. Point C is the breakpoint: thepoint at which chlorine demand hasbeen satisfied and additional chlorineappears as free residuals. The freeavailable residual chlorine increases indirect proportion to the amount of chlo-rine applied between Points C and D.

Many factors affect breakpoint chlori-nation including the initial ammonia ni-trogen concentration, pH, temperature,and demand exerted by other inorganicand organic species. A weight ratio of8:1 or greater of chlorine applied toinitial ammonia nitrogen is required forbreakpoint chlorination to be reached.If the weight ratio is less, there is insuf-ficient chlorine present to oxidize thechlorinated nitrogen compounds initial-ly formed. For instantaneous chlorineresidual, the weight ratio required maybe 20:1 or more. Reaction rates arefastest at high temperatures and pH of 7to 8 (1).

Determining BreakpointA field test can lead you in the rightdirection to finding the chlorinationbreakpoint. Although the test cannotreplicate the exact conditions of thesystem, it is a starting point. The follow-ing procedure has been used with suc-cess at several locations.

Test calculation data:

● Household bleach » 5.25% NaOCl

● NaOCl molecular weight (MW) = 74.5

● Cl2 MW = 71

● 71 / 74.5 * 5.25% = 5% as equivalentfree Cl2 or 50,000 ppm in householdbleach

Test procedure:1. Test the ammonia level in the non-chlorinated water to evaluated. Recordthe result.

2. Add 1 milliliter (mL) of original bleachsolution to 99 mL distilled water. Thismakes approximately a 500-ppm solu-tion.

3. Check strength of 500-ppm solutionby adding 0.2 mL with a syringe to 100mL distilled water. This should give 1-

ppm of free chlorine. Test using theDPD-free chlorine test. Multiply the testresult by 5. Record the result. This is theamount of Cl2 per mL that will be add toa 100 mL sample in Step 5.

4. In five beakers, add 100 mL each ofthe water to be evaluated for breakpointchlorination. Do not filter.

5. Add chlorine solution to each beaker.The amount of chlorine solution addedper beaker would be dictated by thedosage (ppm) of Cl2 desired. Theamount of chlorine added per mL ofprepared solution is: mL added * (ppmCl2/mL calculated in Step 3). To achievebreakpoint chlorination, a minimum ra-tio of 8:1 of chlorine to ammonia must beachieved. It is recommend that beakers1 and 2 be at dosages less than the 8:1ratio, beaker 3 at the 8:1 ratio, andbeakers 4 and 5 be greater than the 8:1ratio. This should give a good break-point chlorination curve. If you have toadd more than 10 mL of chlorine solu-tion, make a stronger chlorine solutionand 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 thesame five beakers in the above proce-dure is monochloramine. Hach offers aMonochlor-F test procedure for mono-chloramine. Monochloramine concen-trations will be zero when breakpointchlorination is achieved (test accuracy

Figure 2. Breakpoint chlorination example.

is ±0.1 ppm as Cl2).

Analytical Test OptionsAs mentioned before, monochloraminesinterfere with the results of the DPD-freechlorine test. This also holds true for theNessler and Salicylate test methods forammonia testing. Table B summarizesthe effects of monochloramines on theavailable analytical test options.

Testing for ammonia alone using anion selective electrode (ISE) will notdetermine when breakpoint chlorinatehas been reached since the ammoniaconcentration will go to zero ppm priorto breakpoint chlorination. Point B inFigure 1 represents the point when am-monia concentrations are zero ppm.

Case Study #1A large industrial plant recovered waste-water for cooling tower makeup by us-ing RO units. Chlorine was added up-stream of the RO with a dechlorinationstep immediately before the RO. Mem-brane fouling was becoming a real prob-lem. Reverse osmosis capacity wasbeing reduced and membrane-clean-ing frequency was increasing. The plantwas under pressure to recover morewastewater via the RO system. Mem-brane biopsies revealed microbiologi-cal fouling.

One of the first steps to take whenapproaching a problem is to first deter-mine if the steps currently being takenare being done properly. The plant wasfeeding chlorine at the proper point.Chlorine was being feed into the clearwell, which was the point of lowest chlo-rine demand prior to the RO system.The test records showed a consistent

ULTRAPURE WATER® JANUARY 2003--UP20013638

TABLE ADPD Free Chlorine Interference (ppm)

Monochloramine Sample Temperature °F (°C)(NH2Cl) Level (ppm) 40 (5) 50 (10) 68 (20) 83 (30)

1.2 +0.15 0.19 0.30 0.292.5 +0.35 0.38 0.55 0.613.5 +0.38 0.56 0.69 0.735.0 +0.68 0.75 0.93 1.05

Source: Reference 3

free-chlorine residual being maintained.So far so good, but was the free-chlorineresidual they were testing using theDPD-free chlorine test method reallyshowing free chlorine or was there mono-chloramine interference?

Water tested prior to chlorine additionshowed ammonia levels that rangedfrom 2.5 ppm to 19 ppm. With watertemperatures of 80 oF and a free chlo-rine residual control range of 0.25 to 0.5ppm, you can easily see in Table A thatfree-chlorine residual results could beentirely due to monochloramine inter-ference! The plant thought they weregetting proper chlorination prior to theRO, but were getting a much weakerbiocide (monochloramine) instead.

The breakpoint chlorination test pro-cedure described earlier was conduct-ed. Figure 2 shows the results from oneround of tests. As you can see, the freechlorine residual curve closely resem-bles that in Figure 1. Ammonia was alsotested using the Salicylate method. Eventhough monochloramine is an interfer-ence for this method, Figure 2 showsthat the ammonia level as zero at thebreakpoint where all monochloroaminehad been oxidized. At the breakpointand beyond, monochloramine does notexist and is not an interference to chlo-rine or ammonia testing.

The two solutions available to the plantwere to increase chlorine feed or sup-plement with another biocide. Becauseof variation in ammonia levels and thelarge chlorine demand required to reachbreakpoint chlorination, the plant de-cided to use dibromocyanoacetamide(DBNPA) as a supplemental biocide.With a comparatively minimal DBNPAusage rate, the plant was able to signif-icantly increase membrane life and thetime between cleanings.

Case Study #2A large industrial plant used river wateras makeup to a 2,000-gallons per minute(gpm) RO system for boiler feedwatermakeup. Due to fouling problems, eachbank of membranes was cleaned twicea week. Cleaning at this frequency isnot only bad for the membranes, butrequires a lot of manpower commitment.Reverse osmosis pressure differences,permeate quality, and RO feed pres-sure were significantly affected by thefouling. The plant knew this was aproblem, but had already had many“experts” review their system over theyears with no solution. Nine separatecompanies had already tried. They were

TABLE BAnalytical Test Options with Monochloramine

Analysis Test Methods MonochloramineInterference

Free chlorine DPD yesTotal chlorine DPD noMonochloramine Monochlor-F noAmmonia salicylate yes

nessler Yesion-selective electrode no

resistant to trying anything else and didnot want the RO touched.

The advantages of solving this prob-lem were obvious: longer service runs,less damage to membranes, minimizedmembrane replacement costs, reducedmanpower costs, lower water produc-tion costs, and decreased pumpingcosts.

First, the concepts of breakpoint chlo-rination were applied. The ammoniaconcentration in the makeup water wasdetermined. The breakpoint chlorina-tion test procedure previously describedwas conducted to find the proper chlo-rine dosage to reach breakpoint chlori-nation. The plant’s current chlorine dos-age was nowhere near that required forbreakpoint chlorination. A higher dos-age was required for proper disinfec-tion of the raw water prior to being intro-duced to the RO.

Next, to prove the findings, a pilot ROunit was set up parallel with the currentRO system. Breakpoint chlorinationdosages of chlorine were added to thepilot RO pretreatment train with the wa-ter being dechlorinated prior to enteringthe RO. Once breakpoint chlorinationwas achieved and a true free chlorineresidual was maintained throughout thepretreatment train, the pilot RO perfor-mance was greatly improved over thatof the current RO system. Much longerservice runs between cleanings wereobserved. Pressure drops across the

membranes, feedwater pressure, andpermeate quality were each greatly im-proved.

With the result of the pilot study asproof, the plant implemented breakpointchlorination dosages on the current ROand experienced a similar success.

ConclusionThe application of breakpoint chlorina-tion with RO pretreatment has success-fully been used to solve baffling micro-biological fouling problems of RO mem-branes. Although the customers thoughtthey were applying sufficient chlorinefor disinfection, what they were actuallymeasuring was monochloramine inter-ference to the DPD free chlorine test.Determining the breakpoint chlorinationallowed the customers to better disin-fect their RO feedwater and resulted inlonger service runs between membranecleanings.

Reverse osmosis systems are compli-cated with many factors to consider.The application of breakpoint chlorina-tion is just one of those factors that mustbe considered. When approaching anyproblem, one of the first steps should beto ensure that the current technologyand treatments are being properly ap-plied. Taking into account breakpointchlorination, as described in this article,is a good method to determine if chlo-rine chemistry is being properlyapplied.■

ULTRAPURE WATER® JANUARY 2003--UP200136 39

References

1. Betz Handbook of Industrial Water Con-ditioning, 9th ed., Betz Laboratories, Inc.,Trevose, Pa., pp. 196-199, (1991).

2. White, G.C. The Handbook of Chlorina-tion, 2nd ed., Van Nostrand Reinhold Co.,New York, N.Y., pp. 162-167 (1986).

3. Water Analysis Handbook, 3rd ed., HachCo., Loveland, Colo., p. 351 (1992).

Author James McDonald, EIT, is a tech-nical support engineer and MIS directorwith CROWN Solutions Inc. in Dayton,Ohio. He holds a masters of engineeringdegree for chemical engineering fromthe University of Louisville J.B. SpeedScientific School.

Key words: CHLORAMINES, CHLO-RINE, MEMBRANES, MICROBIALS,PRETREATMENT, REVERSE OSMOSIS