Any comprehensive coolingwater treatment program willinclude a biocide for the controlof biological contamination. Oxi-dizing biocides, such as chlorineand bromine, and nonoxidizingbiocides may be used. Althoughthe use of chlorine has decreasedover the past several years due tosafety concerns, its use is still verywidespread. In cooling water sys-tems, the effective application ofchlorine minimizes the presenceof microorganisms that can causebiological fouling, contribute tocorrosion, and reduce heat transfer.

The amount of chlorine requiredto maintain microbiologicalcontrol in a cooling water systemwill vary depending on thechemical and physical environ-ment of the water system.

Reactions in WaterWhen chlorine is added to coolingwater, a variety of reactionsoccur. The reactions depend onthe components present in thecooling water and the systemwater characteristics.

Chlorination of cooling watermay be accomplished with theuse of a variety of materials.However, the most widely usedand cost-effective form of chlorineis gaseous chlorine.

Other chlorination methodsinclude use of materials such assodium hypochlorite, calciumhypochlorite, chlorinatedisocyanurate, and chlorinatedhydantoin. Once in solution, allthese compounds react in water toform hypochlorous acid andhypochlorite ion, which serve asdisinfectants to control micro-biological growth.

Chlorine ChemistryIn discussing chlorination chemis-try, it is necessary to first considerchlorine reactions in pure water.When added as a gas, chlorinerapidly hydrolyzes to form hydro-chloric acid and hypochlorousacid.

Cl2 + H2O ⇔ HCl + HOCl (1)

The hypochlorous acid thenpartly dissociates to give hydro-gen and hypochlorite ions.

HOCl ⇔ H+ + OCl– (2)

The three forms of chlorine involvedin these reactions, molecularchlorine (Cl2), un-ionized hypochlor-ous acid (HOCl), and the hypo-chlorite ion (OCl–), exist togetherin equilibrium. Their relativeproportions are determined bypH value (Figure 1), temperature,and dissolved solids.

Figure 1 — Relative concentrations ofchlorine compounds vs. pH

Chlorine present in water as Cl2,HOCl or OCl–, or in any mixtureof these, is defined as free availablechlorine. This free available chlo-rine is the most biocidally activeconstituent of chlorine. Clearly,at the pH values encountered inwater chlorination, free chlorineconsists of a mixture of HOCland OCl–.

As a bactericide, HOCl is a muchmore rapid acting biocide thanOCl– (Figure 2). This is due toHOCl being uncharged and thusmore easily diffusible through thecell wall. Consequently, in freeresidual chlorination, the higherthe pH value, the slower the rateof kill because of a lower propor-tion of HOCl. This is particularlyimportant in once-through cool-ing systems and in cooling sys-tems practicing slug chlorination.

Technifax TF-132

Cooling Water Chlorination

©1982, 2002 Ondeo Nalco Company All Rights Reserved Printed in U.S.A. 4-02

Ondeo Nalco Company Ondeo Nalco Center • Naperville, Illinois 60563-1198

SUBSIDIARIES AND AFFILIATES IN PRINCIPAL LOCATIONS AROUND THE WORLD

Figure 2 — Effect of HOCl/OCl– onPseudomonas

These systems have limitedcontact time (minutes-hours) forchlorine (HOCl or OCl–) to workas a biocide, hence rate of kill isan important consideration.

In distilled water, all chlorinepresent will be free availablechlorine because there is nothingpresent that will react with orconsume the free chlorine. Indus-trial cooling water, on the otherhand, has microorganisms presentas well as other compounds thatcombine with and reduce theconcentration of free availablechlorine. These micro-organismsand compounds together makeup the chlorine demand of thewater.

In addition to chlorine reactingwith various constituents inwater, large quantities of chlorine(30 to 40%) can be lost from acooling system due to mechanicalstripping, i.e., the escape of dis-solved HOCl from water into theatmosphere. Chlorine in a systemcan also be degraded due to ultra-violet light from the sun. Free avail-able chlorine in a cooling towermay disappear in less than onehour on a bright day, but remaintwo to four hours in the dark.

Free available chlorine = chlorineadded to system — chlorinedemand — chlorine stripped —chlorine degraded due to sunlight.

Chlorine ReactantsAmmoniaAmmonia is one compound thatreacts with chlorine; the reactionproducts are called chloramines(see the following reactions).Chloramines are referred to ascombined chlorine. Free availablechlorine and combined chlorineadded together make up totalresidual chlorine.

HOCl + NH3 = H2NCl + H2OHOCl + H2NCl = HNCl2 + H2OHOCl + HNCl2 = NCl3 H2O (3)

Chloramines are much lesseffective bactericides than freeHOCl or OCl–. The reactionsbetween chlorine and ammonia toform chloramines are frequentlyovercome by the technique ofbreakpoint chlorination. In thismethod, chlorine is added untilthe free ammonia is used up;additional chlorine is subse-quently added to maintain aresidual of free available chlorine.

The quantity of chlorine requiredto achieve breakpoint, then, isbased on the amount of ammoniapresent in the system. For every1 ppm of ammonia present, upto 10 ppm of chlorine may berequired to establish free availablechlorine. A preferred alternativeto the practice of breakpointchlorination for the presence ofammonia is bromine chemistry.Bromamines, in contrast tochloramines, are excellent bio-cides and are almost as biocidal asfree available bromine.

Organic NitrogenReactions with organic nitrogenare very complex and slow,particularly with some of theproteins and nucleic acids ofmicroorganisms. The slowness ofthe reaction is probably due tosteric hindrance. The slow reac-tion rate means that a long time(up to several days) can be re-quired to reach a stable freeresidual chlorine level.

InorganicsChlorine will oxidize certaininorganic molecules. It can readilyoxidize ferrous iron (Fe++), man-ganous (Mn++), nitrite (NO2

–), andH2S. The presence of these inor-ganic constituents significantlyincreases the chlorine demand ofthe water, and consequently, thequantity of chlorine required formicrobiological control. Thechlorine which reacts is convertedto chloride ions, which are notbiocidal. Examples of oxidizingreactions involving inorganicmolecules and chlorine are:

Iron — Free chlorine will oxidizeferrous ion as follows:

2Fe++ + 6HCO3– + HOCl + H+ + Cl– →

2Fe (OH)3 + 2Cl– + 6CO2 + H2O (4)

The oxidation of soluble iron ispartially dependent on the con-centration of HCO3

– (bicarbonate),and as a result, the reaction isvery rapid above pH 7.0, wherebicarbonate in natural waters isusually present in higher concen-trations. Because alkalinity is lostin this reaction, the pH may besignificantly lowered. The amountof pH reduction will depend onthe alkalinity level of the water.

Each ppm of iron oxidizedrequires 0.64 ppm of chlorine.

Manganese — Manganese isoxidized by free available chlo-rine while chloramines have littleeffect on manganous compounds.Manganous oxidation by chlorineresults in the precipitation ofmanganese dioxide:

Mn++ + Cl2 + 4OH– → MnO2 + 2Cl– + 2H2O (5)

The oxidation of manganese bychlorine is more rapid above pH7.0 because the reaction dependson hydroxyl ions.

Sulfur — Chlorine will react rapidlywith hydrogen sulfide gas toform dilute sulfuric acid andelemental sulfur. The ratio is pHdependent. At a pH of 7.0 with aweight ratio of 8:1 Cl2/H2S, about70% is oxidized to sulfate; at pH9.0 about 50% is converted to sul-fate and 50% to elemental sulfur.

H2S + 4Cl2 + 4H2O → H2SO4 + 8HCl (6)

H2S + Cl2 → S↓ + 2HCl (7)

Below the 8:1 ratio, Equation 7 ispredominant.

Some elemental sulfur is nearlyalways present in systems wherechlorine and hydrogen sulfide arein contact regardless of the ratioof chlorine to hydrogen sulfide.

Oxidizable OrganicsNon-microbiological organicmaterials present in cooling watermay also be oxidized by chlorine.Oil, grease, leaves, pollen, processchemicals, lignin, tannin, andfood products can significantlycontribute to the chlorine demandin a system. A process leak of an

organic material may necessitatean increase in chlorination rate toovercome the added chlorinedemand and to obtain a freeresidual to maintain microbio-logical control.

Most organic compounds reactslowly with chlorine. However,the rates are dependent on pHand chlorine concentration. Theoverall water quality and otherchemical species present play asignificant role in the formation ofchloroorganics. If HOCl is themain chlorinating agent, theformation of chlorinated organicswill be proportional to the HOClconcentration. However, in thepresence of high ammonia con-centrations, a lower level ofchlorinated organics will beformed because of the more rapidreaction of HOCl with NH3 toform chloramines.

Forms of ChlorineGaseous ChlorineBecause of the potential dangersassociated with handling gaseouschlorine, this form of chlorine isbeing used less and less. Whengaseous chlorine is added towater, it reacts as follows:

Cl2 + H2O → HOCl + HCl (8)

As discussed earlier, HOCl dis-sociates to form OCl–. The amountof each is dependent upon pH.HCl is a very strong acid. Whenadded to water, gaseous chlorinewill reduce M alkalinity by 0.7ppm (as CaCO3) for every 1 ppmof chlorine added to the water.

Sodium HypochloriteAddition of sodium hypochlorite(NaOCl) solution (bleach) to wateralso results in hypochlorous acidformation:

Na+OCl– + H2O → HOCl + NaOH (9)

Adding bleach to water will affectthe pH and alkalinity. About1 ppm of M alkalinity (as CaCO3)is added for every ppm of NaOCladded. Some excess NaOH ispresent in the bleach solution tokeep the pH above 11 for storagestability. However, when bleach isadded to water, this excess NaOHhas a minimal effect on alkalinity.

The pH effect of adding bleach towater depends on the original pHof the water. In low pH waters,the pH may increase significantlydue to the additions of NaOCl andthe resulting formation of NaOH(Equation 9). However, in higherpH waters where the OCl– speciesis predominant, NaOH is notreadily formed, and the pHincrease is negligible.

Calcium HypochloriteCalcium hypochlorite, Ca(OCl)2,dissociates in water to formhypochlorous acid and calciumhydroxide.

Ca(OCl)2 + 2H2O → 2HOCl + Ca(OH)2 (10)

One mole of calcium is added forevery two moles of hypochlorousacid. Note that calcium hardnessis added.

applied electric current flowingthrough the test water sample.The decrease in current occurswhen free chlorine is reduced tochloride by the addition of a phenylarsine oxide titrant. In addition tofree chlorine, the titrator can ac-curately determine both mono-chloramine and dichloramine(combined chlorine) residuals.

The FACTS (syringaldazine) methodcan be used in chromate-treatedsystems. In this test, the syringald-azine reagent is oxidized by freechlorine to give a pink-red color.

The DPD (diethyl-p-phenylenediamine) test is an accurate methodfor use in non-chromate treatedcooling water systems. In thepresence of chromate, the DPDtest will give a false-positive testresult. In this test, the free chlo-rine reacts instantly with the DPDindicator to produce a red-pinkcolor when reacted with chlorine.The color should be read immedi-ately for a true free chlorine reading.

SUMMARYThe use of chlorine is a commonmethod to control microorgan-isms in cooling water systems.Factors such as pH, temperature,and sunlight are important in theeffective use of chlorine. Organicor inorganic water contaminantscan react with chlorine and thussignificantly increase the chlorinedemand of the water. Contacttime and concentration of chlorineare important factors affectingchlorine's performance for con-trolling microbiological growth incooling water systems.

in short contact time disinfectionexperiments, the HOCl species isrequired to give fast kill. How-ever, other laboratory experi-ments as well as practical indus-trial water treatment experiencehave shown that continuouschlorination which allows a verylong contact time is effective incooling water systems havingpH's from 5.0 up to 9.5 (where OCl–

is predominant).

Free Chlorine –Common TestMethodsA variety of test methods areavailable for determining freechlorine (HOCl and OCl–) inwater. However, regardless of thetest method used, chlorine deter-minations should be startedimmediately after sampling. Thechlorine content will decreaserapidly in water solutions, par-ticularly if the samples are stored,agitated, or exposed to stronglight. These conditions should beavoided to prevent false-low testresults.

The most common and bestmethods for free chlorine deter-mination in cooling water areamperometric titration, the FACTS(syringaldazine) method, and theDPD (diethyl-p-phenylene diamine)test method.

Amperometric titration is a particu-larly accurate method and iscompatible with virtually everycooling water treatment includingchromate. This method utilizes asmall automatic titrator. With thisinstrument, the concentration ofchlorine is determined by observ-ing meter-indicated decreases in

Biological ActivityA number of theories have beenpostulated to explain the effec-tive biocidal activity of chlorine.The major biocidal activity isattributed to HOCl and OCl–. Theactive chlorine species kill themicroorganisms by destroyingcell enzymes and membranes. Afree chlorine residual (HOCl,OCl–) will provide the bestmicrobiological control. Freechlorine species are present in anequilibrium; the ratio of HOCl toOCl– is dependent on the pH.

As HOCl is utilized in killingmicroorganisms, more HOCl isproduced immediately becauseof the redistribution from themore stable OCl– to providecontinued biocidal activity.

The efficiency and disinfectionperformance of chlorine can varydepending on the concentration,contact time, and target organ-ism. Each microorganism has a“specific sensitivity” to a certainchlorine level. Therefore, suffi-cient contact time and concentra-tion will kill any microorganism.Microorganisms cannot developa resistance to chlorine regardlessof the number of times that theyare exposed to chlorine becauseall organisms are composed ofoxidizable organic matter.

Contact time is extremely impor-tant. By increasing the contacttime, effective chlorination can beachieved at lower free chlorineresiduals. In other words, longercontact times permit smallerresiduals while shorter contacttimes (minutes) require highresiduals to give equal microbio-logical kill. Widely publishedlaboratory data have shown that