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Direct Amperometric Measurementversus ORP for Residual ControlA Comparative Study

The following paper was presented in part at the 1996 Water EnvironmentFederation Specialty Conference, Disinfecting Wastewater for Discharge andReuse, Portland, Oregon, by Dianne M. Phelan, Severn Trent Services.

Introduction:Chlorine is widely used for partial disinfection of municipal wastewater treatment plant effluents. To achieve satisfactorydisinfection, as indicated by suitable reduction in fecal coliform concentrations, an effluent with a residual chlorine concentrationof up to 0.5 milligrams per liter (mg/l) has historically been practiced. However, concerns with regard to the toxic effects ofthis residual level in the environment have led to the standard practice of dechlorination in wastewater treatment plants. Thisprocess, though provably important and necessary, seriously impacts the cost of wastewater treatment. Thus, precise andaccurate measurement and control of chlorination in all phases of the treatment process is required in today’s watertreatment operations to maintain the delicate balance of cost effective water disinfection and environmental protection.

Many indicators are used for water treatment process control including: disinfection efficacy, residual chlorine, pH, dissolvedoxygen, ammonia-nitrogen, COD (Chemical Oxygen Demand), BOD (Biological Oxygen Demand), Total Organic Carbon(TOC) and Oxidation-Reduction Potential (ORP). The first two, disinfection efficacy and residual chlorine are clearly ofprimary importance. Disinfection efficacy is determined as the ratio of logs of reduction of initial to final bacterial count or asa percent of destruction of initial bacteria count. Understanding and controlling the conditions under which disinfectionefficacy is maximized, economically yet with the least impact on the environmentis the ultimate goal in plant management.Though all measured parameters mentioned above play a part in this process, the determination and control of residualchlorine, the primary method of disinfection, is key.

The subject of this paper focuses first on how well direct chlorine residual measurements correlate with disinfection.However, it has been suggested that a different measurement such as ORP could be more accurate in disinfection control,because this technique inherently accounts for other solution conditions which could seriously affect the disinfectionprocess (Wareham, et al). Laboratory studies, in controlled reactors have indicated that ORP can be an effective controlparameter in the operation of oxidation ponds (Carberry). Additionally, real time chlorine residual control strategies throughthe use of ORP have been proposed (Keller). Therefore, in this experimental work data gathered using both of thesemeasurement methods are presented, and analyzed in light of other plant processparameters.

Analytical methods commonly employed in the wastewater industry are generally wet chemical, colorimetric, potentiometricor amperometric. The last two measurement methods are generally adaptable for on-line use, whereas the first two are bestsuited to the lab environment.

Colorimetric techniques are typically based on the use of DPD ( N,N-diethyl-p-phenylene-diamine). This method can beemployed in a sampled or on-line test mode; however, it is subject to interferences such as chloramines (when measuringfree chlorine), chlorine dioxide and oxidized manganese, to name a few. Since minimal equipment is required, it finds wideacceptance in a field environment for screening or reference, where semi-qualitative information is sufficient.

A potentiometric method commonly employed in on-line measurements is ORP (oxidation reduction potential). Thismeasurement has gained consideration in the water and wastewater industry since oxidation-reduction reactions mediatethe behavior of many chemical constituents in drinking, process and waste waters. The theory of operation is based on thefact that a potential will form at an inert electrode in a solution containing electrochemically active (i.e. oxidizing or reducing)ions. This potential will vary with the ratio of oxidized to reduced species according to the Nernst equation (see eq. 3.1).Although this measurement is theoretically very straightforward, many factors limit the interpretation of ORP values in realsystems. These factors include the presence of multiple and/or inert redox couples, irreversible reactions, small exchangecurrents, and electrode poisoning. For the purpose of process control, this parameter finds particular usefulness in anenvironment where the system is well characterized or process variables are limited. In wastewater applications, it has beenused to both infer chlorine residual levels as well as indicate “oxidizing potential” of the solution. The latter has been said tobe a better indicator of disinfection efficacy than residual chlorine, but further research is necessary to substantiate thisclaim.

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The amperometric measurement of residual chlorine can be accomplished in a few different ways, depending upon the skillof the operator and requirements of the plant. Amperometric titration is a standard chemical titration using an amperometricendpoint detection apparatus consisting of a cell unit, and a micro ammeter. The cell unit typically consists of a readilypolarizable dual platinum electrode. When cell polarization is low, in the presence of chlorine, the microammeter reading willbe comparatively high. As the titrant (commonly phenylarsine oxide or PAO ) is added, it reacts with the halogen to reduceits concentration and affect an increasing polarization of the cell, resulting in a reduced microammeter reading. The endpointis recognized when the continued addition of titrant no longer lowers the microammeter reading. Though this is a laboratorymethod only, it is the recommended referee method for verification of all other techniques.

Another type of amperometric measurement exists which, in some variation, has been successfully used in on-linemeasurement and control systems for over thirty five years. The majority of analyzers in use today are based on the galvaniccell theory. This theory states that when two electrodes are immersed in an ion containing solution, a reduction reaction (thegaining of electrons) and an oxidation reaction (loss of electrons) occurs at the cathode and anode, respectively, resultingin a current flow between the electrodes. When the appropriate electrode configuration and materials are used, the currentflow can directly indicate the chlorine content of the solution. Analyzers of this design are recognized as the most precise,accurate and yet rugged instruments in use today.

Chemistry and Effects of ChlorinationThe chlorination of water serves primarily to destroy or deactivate disease producing microorganisms. The details of themechanism of microbial destruction or inactivation are not fully understood, though factors that influence inactivation ofmicroorganisms have been extensively studied. (Drinking Water and Health, Vol. #2 &3). The extent and rate of disinfectionare influenced by the type and physiological state of the microorganism. Generally, bacteria are more susceptible thanviruses, which are more susceptible than protozoan cysts. The chemical form of the disinfecting species as well astemperature are also relevant to disinfection efficacy. Hypochlorous acid is known to be 80 to 100 times more effective as agermicide than hypochlorite ion. Additionally, the microbial population can be susceptible to other chlorinated compoundssuch as chloramines. Thus, disinfection control at any specific site requires not only a theoretical understanding of chlorinechemistry but also extensive characterization of many other interacting variables.

In the treatment process, chlorine is applied to water in its molecular (Cl2) or hypochlorite ion (OCl - ) form. Initially, chlorineundergoes hydrolysis to form “free” chlorine consisting of hypochlorous acid (HOCl) and hydrochloric acid (HCl):

Cl2 + H2Ο↔ HOCl + HCl (2.1)

Depending upon the pH and temperature, hypochlorous acid can further dissociate to hypochlorite ion and hydrogen ion.

HOCl ↔ OCl- + H+ (2.2)

As the pH decreases, hypochlorous acid dissociation decreases such that below pH 5.0 none of the acid is dissociated.That is, it is all in the form of HOCl. The opposite is true as the pH increases, such that, at values greater than 9.5, 100 %of the chlorine is in the form of OCl - . pH values normally encountered in wastewater treatment tend to be relatively constantsite to site, and near neutral. At this pH there exists a mixture of hypochlorite ion and hypochlorous acid. In either form, itis termed “free” available chlorine. Both contribute to the disinfection process though, as stated earlier, HOCl is known to bemore effective.

If ammonia nitrogen is present in the water being chlorinated, further reactions can occur, forming a class of compoundscalled “chloramines”. The reaction mechanism is complex, not completely understood, and the products vary with conditionssuch as the pH, ratio of chlorine added to ammonia present, and contact time. Formation of chloramines can be depictedas a stepwise process:

NH3(aq) + HOCl ↔ NH2Cl + H 2O (2.3)

(Monochloramine)

NH2Cl + HOCl ↔ NHCl2 + H 2O (2.4)

(Dichloramine)

NHCl 2 + HOCl ↔ NCl3 + H2O (2.5)

(Trichloramine)

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The relative formation of mono, di and trichloramine is strongly influenced by pH. At neutral pH values and above,monochloramines predominate. Below pH 7, significant amounts of dichloramine are found. Chloramines are referred to as“combined chlorine residual”. The sum of “free residual” and “combined” chlorine is defined as “total available chlorine”, andthis represents all forms of chlorine which contribute to the disinfection process.

Chlorine also readily reacts with organic compounds in the water. In some reactions, such as those with organic nitrogencompounds and phenols, Cl is substituted for a hydrogen atom, thus producing the chlorinated compound. Chlorine canalso be incorporated into a molecule by addition reactions, or it may react with a compound to oxidize it without chlorinatingit.

The significance of these reactions in the treatment process is that they contribute to the chlorine demand, but not thedisinfection process. Additionally, chlorinated by-products, a potential health concern, might be formed.

Measurement of Chlorine:The various methods of measuring chlorine (potentiometric, colorimetric and amperometric) have been briefly presentedabove. The details of the methodology will be confined to the electrochemical methods used in the actual experimentation;that is potentiometric (ORP) and on-line amperometric.

Potentiometric Measurements:Electrometric measurements are made by potentiometric determination of electron activity (intensity) with an inert indicatorelectrode and a suitable reference electrode. The indicator electrode responds to the activity of the ion or species beingmeasured. The reference electrode potential remains constant over the range of conditions in which the cell is used. Bothelectrodes are joined externally through a voltmeter of a type which draws very little current due to a near infinite internalresistance. Thus, the measurement is made at “zero” current.

The indicator electrode serves as a either an electron donor or acceptor with respect to electroactive oxidized or reducedchemical species in solution. Ideally, the electrode system will react to changes in the solution’s redox composition by achange in potential that follows the Nernst equation:

E = Eo- 2.303 RT log [red] (3.1) nF [ox]

where [red] and [ox] indicate activities of the reduced and oxidized species, respectively.

Eo represents the potential for the standard half-cell reaction, and RT/nF is a term containing thermodynamic constants.When the unit number of electrons exchanged in the reaction is one, and the temperature is 25°C, this term can be reducedto a constant; 0.059 V. The above equation shows that the potential of the (half) cell is a logarithmic function of ratio of thereduced to oxidized species in solution. And, for every decade change in the concentration of the subject species, a 59 mVchange in the potential of the electrode will be seen .

ORP can be a very useful measurement in a system where all the component activities are kept constant with the exceptionof one known variable. However, if the solution contains a mixture of oxidants and reductants, the potential of the electrodewill be a combination of the effect of each redox couple including, for instance, that of H + (hydrogen ion).

Typical ORP sensors (and the type used in this experimentation) consist of an indicating electrode of inert material such asplatinum, and an appropriate reference electrode such as silver/silver chloride (Ag/AgCl). Because there is no consumptionof reactants in the measurement process, stirring and flow requirements are minimized. However, fouling of the sensor caninhibit the accuracy of the reading as well as the response time.

In wastewater applications, this measurement is typically used in two ways. It may be used to infer residual chlorine levelsor simply to indicate the “oxidizing potential” of the system as it relates to disinfection efficacy. Because of the complexityof the system in question, there are several considerations which must be accounted for in interpreting these values.

First, the fact that more than one redox couple can exist adds ambiguity since changes in the signal can be the result of avariation in any one or more of the redox species. However, with the exception of pH which is typically measured independently,there is no way to further identify which redox species has varied, or even if a new one has beenadded.

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Furthermore, many redox reactions are found in the metabolic processes of microorganisms. Microorganisms use reductionreactions to consume electrons generated by the oxidation of an energy yielding substrate. An energy yielding substratecan be a variety of substances such as oxygen, formaldehyde, nitrates, sulfates or carbon dioxide.

These side reactions can all affect the ORP reading yet have little to do with the disinfection process. Thus, it can be seenthat for direct process monitoring and control, ORP values can play an integral part. However, extensive characterization ofprocess trends as well as other peripheral measurements such as pH are required for efficient operation in this regard.

Amperometric (Polarographic) Measurement:As the name implies amperometric techniques involve the measurement of current.

The term polarography is derived from the fact that the electrode at which the reaction of interest occurs is in a polarizedcondition. An electrode becomes polarized when the products of the (redox) reaction that is occurring accumulate to suchan extent that they limit the rate of the reaction. Because of the reaction occurring at the electrode, it can be said that theconcentration of reactant at the electrode surface is very low. When such a condition exists, the amount of current that anelectrode will pass is directly related to the mass flux of reactant to the electrode surface, across the concentration gradientestablished by polarization. In turn, the rate of diffusion of the reactant is a function of the bulk concentration of the reactingspecies. Thus, the measurement of current in a cell with a polarized electrode can therefore be used to indicate theconcentration of the reactant.

Such polarographic or amperometric measurements can be made with either galvanic or electrolytic cells. In galvanic cells,two electrodes of the appropriate dissimilar metals are immersed in an ion containing solution, and externally connected toeach other. A potential is generated sufficient to drive the reaction at the polarizing electrode with no external voltage sourceadded. In electrolytic cells, a potential sufficient to allow the reduction of the measured species (in this case chlorine) isapplied by an external voltage source. In both cases, the current flow which is indicative of the concentration of the reactingspecies responds linearly to changes in its concentration.

The data presented in this paper was obtained using an amperometric chlorine analyzer of the galvanic type. The electrodematerials are a copper anode, and a gold (indicating) cathode. The reactions occurring at the electrodes are shown below:

HOCl + 2e- ↔ Cl- + OH- (Reduction at the cathode) (3.2)

Cu0 ↔ Cu++ + 2e- (Oxidation at the anode) (3.3)

Since copper is consumed in the reaction, it is termed a sacrificial anode. Copper ions in solution can further react to forman oxide product, copper chloride or some other salt, depending upon the solution composition.

The signal level (current flow) in the amperometric cell is pH dependent. The current is most stable, and response time isminimized when the pH of the solution is 4.0 to 4.5. Therefore, the sample is buffered at this pH during the measurement.The chemistry of the system as previously described, shows that at this pH all of the chlorine is in the form of hypochlorousacid (HOCl). Another parameter affecting the measured current is flow rate. Because an amperometric measurementconsumes the sample, it is imperative that the sample be replenished at the electrode surface as rapidly as it is consumed.

Figure 1 shows the design of the analyzer used. An accurate signal is maintained and the cell kept clean by the use of arotating striker which stirs polymer spheres between the electrodes. This action prevents fouling by removing oxide productsfrom the anode surface. It also aids in the maintenance of a stable, accurate signal by constantly stirring to replenish thesample at the cathode.

Whether electrolytic or galvanic, chlorine analyzers are a key component in the disinfection process. Unlike ORP sensors,they measure chlorine directly. Thus, other redox type process variables do not interfere with this measurement. Currentdesigns incorporate features to avoid electrode fouling, and maximize sensitivity, accuracy, and responsiveness. Thisallows tight control of the chlorination process. In the following sections, the experimental sites are described, and datapresented which compares these two measurement methods.

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Test SitesInstrumentation was installed at three test sites. Two were waste treatment plants, and one was a water treatment boosterstation.

Test Site #1:

This site was a local treatment plant with a capacity of 4.5 MGD (Million Gallons per Day). About 20% of the waste wasindustrial. The ORP sensor and chlorine analyzer were placed in the system measuring the final effluent just prior to thedechlorination process. A pH sensor was also installed. Continuous monitoring of the chlorine analyzer and ORP sensorwas accomplished through the use of chart recorders. The equipment was housed in a temperature controlled instrumentroom, and was maintained on a weekly basis. Reference measurements were made using the spectrophotometric DPDmethod. Amperometric titration with phenylarsine oxide (PAO) was used on a less frequent basis. Other parameters suchas dissolved oxygen (DO), ammonia (NH3), nitrates (NO3) and suspended solids (SS) were also monitored by the plant, andthis data was available for use in the analysis of the experimental results.

Test site #2

Test site #2 was a waste treatment plant located in a well-populated suburb of a major city. The capacity of this treatmentsite was ~10 MGD, with 40-50% of the waste being from industrial sources. Two chlorine analyzers were placed in the samecontact tank, one located at the front, and one located at the back end of the contact tank. The ORP sensor was placed inthe sample line just before the chlorine analyzer at the back end of the chlorine contact tank. Continuous monitoring of bothchlorine analyzers as well as the ORP sensor was accomplished through a computerized control system encompassingthe entire plant. Readings were acquired and automatically plotted every five minutes. The equipment was housed in amoderately heated shed and was maintained by plant personnel, on a weekly or as needed basis As with site #1, informationon other plant conditions, such as pH, etc. was available for use in our analysis.

However, no other auxiliary measurements were made at the exact location of the experimental equipment.

Test Site #3:

This site was a potable water booster facility located in the UK (United Kingdom). The main water treatment facility waslocated about 60 miles from the booster station. The raw water treatment process entailed initial purification by flocculationwith ferric sulfate, followed by rapid gravity filtration through sand, and a finally treatment to remove organics via a granularactivated carbon filter (GAC). For final disinfection, the water undergoes superchlorination to 2 ppm, followed by dechlorinationwith sulfur dioxide (SO2), to give a final effluent of 0.5 to 0.6 ppm in free chlorine.

Figure 1 - Amperometric Chlorine Analzyer Design

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Upon arrival at the booster station, the water pressure is increased and chlorine levels elevated back to 0.5 ppm. Themajority of the water is sent on to a large city another 50-60 miles away, but there is a small diversion shortly after thebooster station to a small town. Here another dechlorination procedure is implemented to yield a 0. 4 ppm free chlorineresidual. This was also the location of the experimental setup.

As with sites 1 and 2, an ORP and pH sensor were placed in line with the chlorine analyzer sample line, just in front of theanalyzer. A difference between this design and those discussed above is that three chlorine analyzers were used, astriplification is a common practice in control operations in the UK. The equipment was maintained on a weekly basis byplant personnel. The reference method for the chlorine analyzer was the DPD colorimetric technique. All signals werecomputer monitored, with readings taken every 15 minutes. Other parameters such as conductivity and temperature werealso monitored.

Results and Discussion

Test Site #1:

Figures 2, 3 and 4 summarize data taken over a 50 day time period in autumn, where moderate temperatures ranging from40° to 75°F (22° to 39°C) are typical. The chlorine analyzers were temperature compensated, however the ORP sensor wasnot. Fluctuations due to temperature however, would be negligible compared to other influences, since the Nernst equation(3.1) predicts a 2 mV change in signal for every 10°C change seen by the sensor. Figure 2 compares the response toresidual chlorine levels for the two measurement methods.

Figure 2: Chlorine Residual vs ORP (Site #1)

The data for the two chlorine analyzers correspond within the limits of experimental error, the differences being due primarilyto the variation of the timing of the readings. Whereas the readings plotted for the experimental equipment (ORP sensor andchlorine analyzer) were taken at the same time, those plotted for the plant equipment were recorded daily, but at times notspecified exactly in the data. However, major concentration fluctuations tended to be slow and relative trends for eachanalyzer do correspond.

At times, where major differences were seen (e.g. around 900 hours), it can be attributed to flow differences in the sampleline due to equipment maintenance or blockages. Of greater interest is the ORP response versus the experimental analyzerreadings. These signals should track closely if the potential exists for accurate and precise

Figure 2 - Chlorine Residual vs. ORP (Site #1)

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control of the chlorination process via ORP. This is seen in the beginning of the test. However, at times even during the initialhalf of the test period (e.g. at 420 hours), opposing signals were seen with each instrument. An opposite reaction of theORP sensor could be explained by the presence of other active redox species in the wastewater, which is a commonoccurrence.

Another aspect of the data which should be noted is the response level of the monitors. Both chlorine analyzers fluctuatedover a range of 0.2 to 1 ppm Cl2, which is less than one decade of change in concentration. Corresponding ORP variationsshould have been over a range 59 mV or less, yet over 300 mV variations were seen. This could not be attributed to pHeither, since it varied only by 0.5 (7.5 to 8.0) over the entire test period.

In Figures 3 and 4, the signal data for each experimental method is presented versus disinfection efficacy, as determined bythe fecal coliform count/100 ml.

Figure 3 - Disinfection Efficacy vs. Chlorine Residual (Site #1)

Figure 4 - Disinfection Efficacy vs. ORP (Site #1)

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The data reflect information provided by the plant for a month prior to the initiation of the test. Thus, in both figures, ORP andexperimental chlorine residual values are not shown. In both cases, the relationship expected is that when chlorine andORP levels are level, efficient disinfection should occur. The interest lies in the periods when decreased disinfection levelsare seen (i.e. high coliform counts). In these instances, the plant had experienced flooding after heavy rainfall. Throughputat this time is typically high and contact times are short. Chlorine levels should be elevated to accomplish as muchdisinfection as possible during this shortened contact period. These high levels may occur prior to or at the same time asthe high coliform count, however, not typically after it since contact times have then returned to normal, as should disinfectionefficacy. This trend is illustrated with the chlorine residual measurements, however not with ORP. This is particularly evidentduring the time period at 1700 hours, where the ORP signal correlated neither with chlorine residual values, nor disinfectionefficacy results.

Test Site #2:

Figures 5 and 6 show two weeks of data taken during the one month test period.

The test period was again in autumn, with temperature variations being similar to those at site #1. Since this data wasacquired by a computer automated process, with signal sampling every five minutes, the “timing” ambiguity which existedat site #1 is eliminated here. In each set of data, it can be seen that the two chlorine analyzers correlate completely. It isinteresting to note the lower values as well as slight offset in the variations seen in chlorine residual from one end of thecontact tank to the other. The lower values are a consequence of the disinfection process which inherently results in achlorine demand. The offset correlates with the time required for the chlorine, added at the front of the tank, to mix andmigrate to the back.

ORP signals, are not consistently indicative of the chlorine residual, and tend to be very slow in responding to solutionvariations. However, the 25 mV range of response is more within the expected value than that of the sensor at site #1, sincea chlorine residual of 0.5 to 1.5 ppm was seen during the test period.

Test Site #3:

The test period at site #3 was six months. It began in autumn, and ended in spring of the following year. Because of thetemperate climate of the location, temperature variations during this time were similar to the other sites. As versus the othertwo locations, the test equipment was housed in a temperature controlled trailer. The data presented in Figure 7 coversthree separate weeks during the test period. In addition to chlorine residual and ORP, corresponding pH information is alsoincluded. The test results show little correlation of the ORP readings with chlorine residual. Qualitatively, it can be said thatthe pH variations seen in weeks one and three may have been indicated by the ORP signal variations as well. However, itappears as if other (unknown) factors have also contributed. This is further supported by the fact that during week two, whenthe pH varied very little, and the chlorine concentration varied by ±0.1 ppm, the ORP signal varied up to 45 mV.

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Figure 5 - Chlorine Residual vs. ORP, Week 2 (Site #2)

Figure 6 - Chlorine Residual vs. ORP, Week 3 (Site #3)

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Figure 7 - Chlorine Residual (ppm), ORP (mV), and pH (Site #3)

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Conclusions

Chlorine residual concentrations and ORP measurements were studied at three different sites to assess the correlationbetween these measurement techniques for chlorine residual control and disinfection efficacy. In some cases, a correlationwas seen. However, due to other effects, such as pH changes, and the presence of other redox species, etc., the ORPreading was found to be less reliable than direct amperometric measurement of chlorine residual for wastewater disinfectionand potable water treatment and process control.

References

Wareham, D.G., Hall, K.J. and Mavinic, “Real Time Control of Wastewater Treatment Systems Using ORP”, Wat. Sci.Tech, 28 (11), pp. 273-282, 1983.

Keller, J., “Changing Control Strategy Solves Chlorine Residual Problems”, Water Engineering & Management, pp.23-24, June, 1993.

Carberry, J. B., Options for the Rational Design and Operation of Oxidation Ponds, Wat. Sci. Tech., 24 (5), pp. 21-32,1991.

Sakakibara, Y., Flora, J.R., Suidan, M.T., Biswas, P. And Kuroda, M., Measurement of Mass Transfer Coefficientswith an Electrochemical Method Using Dilute Electrolyte Solutions, Water Research, 28 (1), pp. 9-16 , 1994.

White, G.C., Handbook of Chlorination, Van Nostrand Reinhold Co., Inc., New York, NY, 1986

Jolley, R.L., Brungs, W.A., Cotruvo, J.A., Cumming, R.B., Mattice, J, S. And Jacobs, V.A., eds. WAT ERCHLORINATION, Environmental Impacts and Health Effects, Vol. #4, Ann Arbor Science, Ann Arbor, MI 1983.

Jolley, R.L., Bull, R.J., Davis, W.P., Katz, S., Roberts, M.H., and Jacobs, V.A., WATER CHLORINATION, Chemistry,Environmental Impact and Health Effects, Vol. #5, Lewis Publishers, Inc., Chelsea, MI, 1985.

Sawyer, D.T. and Roberts, J.L., Experimental Electrochemistry for Chemists, Wiley Interscience, New York, NY, 1974.

Snoeyink, V.L. and Jenkins, D., Water Chemistry, Wiley & Sons, New York, N.Y., 1980.

Standard Methods for Examination of Water and Wastewater, 18th Ed., 1992, American Public Health Association,Washington, DC.

Drinking Water and Health, Vol. #2, National Academy Press, Washington, DC, 1980.

Drinking Water and Health, Vol. #3, National Academy Press, Washington, DC, 1982.

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