NALCO CHEMICAL COMPANY ONE NALCO CENTER NAPERVILLE, ILLINOIS 60563-1198SUBSIDIARIES AND AFFILIATES IN PRINCIPAL LOCATIONS AROUND THE WORLD

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TECHNIFAX

Any comprehensive coolingwater treatment program willinclude a biocide for the controlof biological contamination.Oxidizing biocides, such aschlorine and bromine, andnonoxidizing biocides may beused. Although the use ofchlorine has decreased over thepast several years due to safetyconcerns, its use is still verywidespread. In cooling watersystems, the effective applicationof chlorine minimizes the pres-ence of microorganisms that cancause biological fouling, contrib-ute to corrosion, and reduce heattransfer.

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 tocooling water, a variety ofreactions occur. The reactionsdepend on the componentspresent in the cooling water andthe system water characteristics.

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

Cooling Water Chlorination

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 microbio-logical 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 formhydrochloric acid and hypo-chlorous acid.

Cl2 + H2O HCl + HOCl (1)

The hypochlorous acid then partlydissociates to give hydrogen andhypochlorite 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.These systems have limitedcontact time (minutes-hours) for

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Figure 2 Effect of HOCl/OCl onPseudomonas

chlorine (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 microorganismspresent as well as other com-pounds that combine with andreduce the concentration of freeavailable chlorine. These micro-organisms and compoundstogether make up the chlorinedemand of the water.

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 ofdissolved HOCl from water intothe atmosphere. Chlorine in asystem can also be degraded dueto ultraviolet light from the sun.Free available chlorine in acooling tower may disappear inless than one hour on a brightday, but remain two to four hoursin the dark.

Free available chlorine = chlorineadded to system chlorinedemand chlorine stripped chlorine degraded due to sun-light.

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 avail-able chlorine. A preferred alterna-tive to the practice of breakpointchlorination for the presence ofammonia is bromine chemistry.Bromamines, in contrast tochloramines, are excellent bio-cides and are almost as biocidalas free 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 berequired to reach a stable freeresidual chlorine level.

InorganicsChlorine will oxidize certaininorganic molecules. It canreadily oxidize ferrous iron (Fe++),manganous (Mn++), nitrite (NO2

),and H2S. The presence of theseinorganic constituents signifi-cantly increases the chlorinedemand of the water, and conse-quently, the quantity of chlorinerequired for microbiologicalcontrol. The chlorine which reactsis converted to chloride ions, whichare not biocidal. Examples ofoxidizing reactions involvinginorganic molecules and chlorineare:

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

(bicarbon-ate), and as a result, the reactionis very 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 formationof chloroorganics. If HOCl is themain chlorinating agent, theformation of chlorinated organicswill be proportional to the HOClconcentration. However, in thepresence of high ammoniaconcentrations, 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, HOCldissociates to form OCl. Theamount of each is dependentupon pH. HCl is a very strongacid. When added to water,gaseous chlorine will reduce Malkalinity by 0.7 ppm (as CaCO3)for every 1 ppm of chlorineadded to the water.

Sodium HypochloriteAddition of sodium hypochlorite(NaOCl) solution (bleach) towater also results in hypochlor-ous acid formation:

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 bleachis added to water, this excessNaOH has a minimal effect onalkalinity.

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 NaOCland the resulting formation ofNaOH (Equation 9). However, inhigher pH waters where the OCl

species is predominant, NaOH isnot readily 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.

current flowing through the testwater sample. The decrease incurrent occurs when free chlorineis reduced to chloride by theaddition of a phenyl arsine oxidetitrant. In addition to free chlo-rine, the titrator can accuratelydetermine both monochloramineand dichloramine (combinedchlorine) residuals.

The FACTS (syringaldazine)method can be used in chromate-treated systems. In this test, thesyringaldazine reagent is oxi-dized by free chlorine to give apink-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 imme-diately for a true free chlorinereading.

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 chlo-rine are important factors affect-ing chlorine's performance forcontrolling microbiologicalgrowth in cooling 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 (whereOCl is predominant).

FREE CHLORINE COMMON TEST METHODSA variety of test methods areavailable for determining freechlorine (HOCl and OCl) inwater. However, regardless ofthe test method used, chlorinedeterminations should be startedimmediately after sampling. Thechlorine content will decreaserapidly in water solutions,particularly if the samples arestored, agitated, or exposed tostrong light. These conditionsshould be avoided to preventfalse-low test results.

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 includ-ing chromate. This methodutilizes a small automatic titrator.With this instrument, the concen-tration of chlorine is determinedby observing meter-indicateddecreases in applied electric

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 aspecific 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