Direct Amperometric Measurement versus ORP for Residual Control A Comparative Study

Transcription

1 Direct Amperometric Measurement versus ORP for Residual Control A Comparative Study The following paper was presented in part at the 1996 Water Environment Federation Specialty Conference, Disinfecting Wastewater for Discharge and Reuse, 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 satisfactory disinfection, as indicated by suitable reduction in fecal coliform concentrations, an effluent with a residual chlorine concentration of up to 0.5 milligrams per liter (mg/l) has historically been practiced. However, concerns with regard to the toxic effects of this residual level in the environment have led to the standard practice of dechlorination in wastewater treatment plants. This process, though provably important and necessary, seriously impacts the cost of wastewater treatment. Thus, precise and accurate measurement and control of chlorination in all phases of the treatment process is required in today s water treatment 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, dissolved oxygen, 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 of primary importance. Disinfection efficacy is determined as the ratio of logs of reduction of initial to final bacterial count or as a percent of destruction of initial bacteria count. Understanding and controlling the conditions under which disinfection efficacy 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 residual chlorine, 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 disinfection process (Wareham, et al). Laboratory studies, in controlled reactors have indicated that ORP can be an effective control parameter in the operation of oxidation ponds (Carberry). Additionally, real time chlorine residual control strategies through the use of ORP have been proposed (Keller). Therefore, in this experimental work data gathered using both of these measurement methods are presented, and analyzed in light of other plant process parameters. Analytical methods commonly employed in the wastewater industry are generally wet chemical, colorimetric, potentiometric or amperometric. The last two measurement methods are generally adaptable for on-line use, whereas the first two are best suited to the lab environment. Colorimetric techniques are typically based on the use of DPD ( N,N-diethyl-p-phenylene-diamine). This method can be employed in a sampled or on-line test mode; however, it is subject to interferences such as chloramines (when measuring free chlorine), chlorine dioxide and oxidized manganese, to name a few. Since minimal equipment is required, it finds wide acceptance 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). This measurement has gained consideration in the water and wastewater industry since oxidation-reduction reactions mediate the behavior of many chemical constituents in drinking, process and waste waters. The theory of operation is based on the fact 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 real systems. These factors include the presence of multiple and/or inert redox couples, irreversible reactions, small exchange currents, and electrode poisoning. For the purpose of process control, this parameter finds particular usefulness in an environment where the system is well characterized or process variables are limited. In wastewater applications, it has been used to both infer chlorine residual levels as well as indicate oxidizing potential of the solution. The latter has been said to be a better indicator of disinfection efficacy than residual chlorine, but further research is necessary to substantiate this claim

2 The amperometric measurement of residual chlorine can be accomplished in a few different ways, depending upon the skill of the operator and requirements of the plant. Amperometric titration is a standard chemical titration using an amperometric endpoint detection apparatus consisting of a cell unit, and a micro ammeter. The cell unit typically consists of a readily polarizable dual platinum electrode. When cell polarization is low, in the presence of chlorine, the microammeter reading will be comparatively high. As the titrant (commonly phenylarsine oxide or PAO ) is added, it reacts with the halogen to reduce its concentration and affect an increasing polarization of the cell, resulting in a reduced microammeter reading. The endpoint is recognized when the continued addition of titrant no longer lowers the microammeter reading. Though this is a laboratory method 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-line measurement and control systems for over thirty five years. The majority of analyzers in use today are based on the galvanic cell theory. This theory states that when two electrodes are immersed in an ion containing solution, a reduction reaction (the gaining of electrons) and an oxidation reaction (loss of electrons) occurs at the cathode and anode, respectively, resulting in a current flow between the electrodes. When the appropriate electrode configuration and materials are used, the current flow 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 Chlorination The chlorination of water serves primarily to destroy or deactivate disease producing microorganisms. The details of the mechanism of microbial destruction or inactivation are not fully understood, though factors that influence inactivation of microorganisms have been extensively studied. (Drinking Water and Health, Vol. #2 &3). The extent and rate of disinfection are influenced by the type and physiological state of the microorganism. Generally, bacteria are more susceptible than viruses, which are more susceptible than protozoan cysts. The chemical form of the disinfecting species as well as temperature are also relevant to disinfection efficacy. Hypochlorous acid is known to be 80 to 100 times more effective as a germicide than hypochlorite ion. Additionally, the microbial population can be susceptible to other chlorinated compounds such as chloramines. Thus, disinfection control at any specific site requires not only a theoretical understanding of chlorine chemistry 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, chlorine undergoes hydrolysis to form free chlorine consisting of hypochlorous acid (HOCl) and hydrochloric acid (HCl): Cl 2 + H 2 Ο 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 constant site to site, and near neutral. At this ph there exists a mixture of hypochlorite ion and hypochlorous acid. In either form, it is termed free available chlorine. Both contribute to the disinfection process though, as stated earlier, HOCl is known to be more effective. If ammonia nitrogen is present in the water being chlorinated, further reactions can occur, forming a class of compounds called chloramines. The reaction mechanism is complex, not completely understood, and the products vary with conditions such as the ph, ratio of chlorine added to ammonia present, and contact time. Formation of chloramines can be depicted as a stepwise process: NH 3 (aq) + HOCl NH 2 Cl + H 2 O (2.3) (Monochloramine) NH 2 Cl + HOCl NHCl 2 + H 2 O (2.4) (Dichloramine) NHCl 2 + HOCl NCl 3 + H 2 O (2.5) (Trichloramine)

3 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, and this 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 nitrogen compounds and phenols, Cl is substituted for a hydrogen atom, thus producing the chlorinated compound. Chlorine can also be incorporated into a molecule by addition reactions, or it may react with a compound to oxidize it without chlorinating it. The significance of these reactions in the treatment process is that they contribute to the chlorine demand, but not the disinfection 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 presented above. 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 indicator electrode and a suitable reference electrode. The indicator electrode responds to the activity of the ion or species being measured. The reference electrode potential remains constant over the range of conditions in which the cell is used. Both electrodes are joined externally through a voltmeter of a type which draws very little current due to a near infinite internal resistance. 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 reduced chemical species in solution. Ideally, the electrode system will react to changes in the solution s redox composition by a change in potential that follows the Nernst equation: E = E o RT log [red] (3.1) nf [ox] where [red] and [ox] indicate activities of the reduced and oxidized species, respectively. E o 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 reduced to a constant; V. The above equation shows that the potential of the (half) cell is a logarithmic function of ratio of the reduced to oxidized species in solution. And, for every decade change in the concentration of the subject species, a 59 mv change 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 exception of one known variable. However, if the solution contains a mixture of oxidants and reductants, the potential of the electrode will 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 as platinum, and an appropriate reference electrode such as silver/silver chloride (Ag/AgCl). Because there is no consumption of reactants in the measurement process, stirring and flow requirements are minimized. However, fouling of the sensor can inhibit 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 levels or simply to indicate the oxidizing potential of the system as it relates to disinfection efficacy. Because of the complexity of 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 a variation 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 been added

4 Furthermore, many redox reactions are found in the metabolic processes of microorganisms. Microorganisms use reduction reactions to consume electrons generated by the oxidation of an energy yielding substrate. An energy yielding substrate can 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 seen that for direct process monitoring and control, ORP values can play an integral part. However, extensive characterization of process 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 polarized condition. An electrode becomes polarized when the products of the (redox) reaction that is occurring accumulate to such an extent that they limit the rate of the reaction. Because of the reaction occurring at the electrode, it can be said that the concentration of reactant at the electrode surface is very low. When such a condition exists, the amount of current that an electrode will pass is directly related to the mass flux of reactant to the electrode surface, across the concentration gradient established by polarization. In turn, the rate of diffusion of the reactant is a function of the bulk concentration of the reacting species. Thus, the measurement of current in a cell with a polarized electrode can therefore be used to indicate the concentration 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 to each other. A potential is generated sufficient to drive the reaction at the polarizing electrode with no external voltage source added. In electrolytic cells, a potential sufficient to allow the reduction of the measured species (in this case chlorine) is applied by an external voltage source. In both cases, the current flow which is indicative of the concentration of the reacting species 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 electrode materials 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) Cu 0 Cu e - (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 form an 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 is minimized 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 hypochlorous acid (HOCl). Another parameter affecting the measured current is flow rate. Because an amperometric measurement consumes 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 a rotating striker which stirs polymer spheres between the electrodes. This action prevents fouling by removing oxide products from the anode surface. It also aids in the maintenance of a stable, accurate signal by constantly stirring to replenish the sample 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. Current designs incorporate features to avoid electrode fouling, and maximize sensitivity, accuracy, and responsiveness. This allows tight control of the chlorination process. In the following sections, the experimental sites are described, and data presented which compares these two measurement methods

5 Test Sites Instrumentation was installed at three test sites. Two were waste treatment plants, and one was a water treatment booster station. 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 was industrial. The ORP sensor and chlorine analyzer were placed in the system measuring the final effluent just prior to the dechlorination process. A ph sensor was also installed. Continuous monitoring of the chlorine analyzer and ORP sensor was accomplished through the use of chart recorders. The equipment was housed in a temperature controlled instrument room, and was maintained on a weekly basis. Reference measurements were made using the spectrophotometric DPD method. Amperometric titration with phenylarsine oxide (PAO) was used on a less frequent basis. Other parameters such as dissolved oxygen (DO), ammonia (NH 3 ), nitrates (NO 3 ) and suspended solids (SS) were also monitored by the plant, and this 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 treatment site was ~10 MGD, with 40-50% of the waste being from industrial sources. Two chlorine analyzers were placed in the same contact tank, one located at the front, and one located at the back end of the contact tank. The ORP sensor was placed in the sample line just before the chlorine analyzer at the back end of the chlorine contact tank. Continuous monitoring of both chlorine analyzers as well as the ORP sensor was accomplished through a computerized control system encompassing the entire plant. Readings were acquired and automatically plotted every five minutes. The equipment was housed in a moderately heated shed and was maintained by plant personnel, on a weekly or as needed basis As with site #1, information on 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: Figure 1 - Amperometric Chlorine Analzyer Design This site was a potable water booster facility located in the UK (United Kingdom). The main water treatment facility was located about 60 miles from the booster station. The raw water treatment process entailed initial purification by flocculation with ferric sulfate, followed by rapid gravity filtration through sand, and a finally treatment to remove organics via a granular activated carbon filter (GAC). For final disinfection, the water undergoes superchlorination to 2 ppm, followed by dechlorination with sulfur dioxide (SO 2 ), to give a final effluent of 0.5 to 0.6 ppm in free chlorine

6 Upon arrival at the booster station, the water pressure is increased and chlorine levels elevated back to 0.5 ppm. The majority of the water is sent on to a large city another miles away, but there is a small diversion shortly after the booster station to a small town. Here another dechlorination procedure is implemented to yield a 0. 4 ppm free chlorine residual. 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 the analyzer. A difference between this design and those discussed above is that three chlorine analyzers were used, as triplification is a common practice in control operations in the UK. The equipment was maintained on a weekly basis by plant personnel. The reference method for the chlorine analyzer was the DPD colorimetric technique. All signals were computer monitored, with readings taken every 15 minutes. Other parameters such as conductivity and temperature were also 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 from 40 to 75 F (22 to 39 C) are typical. The chlorine analyzers were temperature compensated, however the ORP sensor was not. 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 to residual chlorine levels for the two measurement methods. Figure 2: Chlorine Residual vs ORP (Site #1) 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 primarily to the variation of the timing of the readings. Whereas the readings plotted for the experimental equipment (ORP sensor and chlorine analyzer) were taken at the same time, those plotted for the plant equipment were recorded daily, but at times not specified exactly in the data. However, major concentration fluctuations tended to be slow and relative trends for each analyzer do correspond. At times, where major differences were seen (e.g. around 900 hours), it can be attributed to flow differences in the sample line due to equipment maintenance or blockages. Of greater interest is the ORP response versus the experimental analyzer readings. These signals should track closely if the potential exists for accurate and precise

7 Figure 3 - Disinfection Efficacy vs. Chlorine Residual (Site #1) Figure 4 - Disinfection Efficacy vs. ORP (Site #1) control of the chlorination process via ORP. This is seen in the beginning of the test. However, at times even during the initial half of the test period (e.g. at 420 hours), opposing signals were seen with each instrument. An opposite reaction of the ORP sensor could be explained by the presence of other active redox species in the wastewater, which is a common occurrence. Another aspect of the data which should be noted is the response level of the monitors. Both chlorine analyzers fluctuated over a range of 0.2 to 1 ppm Cl2, which is less than one decade of change in concentration. Corresponding ORP variations should have been over a range 59 mv or less, yet over 300 mv variations were seen. This could not be attributed to ph either, 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 by the fecal coliform count/100 ml

8 The data reflect information provided by the plant for a month prior to the initiation of the test. Thus, in both figures, ORP and experimental chlorine residual values are not shown. In both cases, the relationship expected is that when chlorine and ORP levels are level, efficient disinfection should occur. The interest lies in the periods when decreased disinfection levels are seen (i.e. high coliform counts). In these instances, the plant had experienced flooding after heavy rainfall. Throughput at this time is typically high and contact times are short. Chlorine levels should be elevated to accomplish as much disinfection as possible during this shortened contact period. These high levels may occur prior to or at the same time as the high coliform count, however, not typically after it since contact times have then returned to normal, as should disinfection efficacy. This trend is illustrated with the chlorine residual measurements, however not with ORP. This is particularly evident during the time period at 1700 hours, where the ORP signal correlated neither with chlorine residual values, nor disinfection efficacy 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 was acquired by a computer automated process, with signal sampling every five minutes, the timing ambiguity which existed at site #1 is eliminated here. In each set of data, it can be seen that the two chlorine analyzers correlate completely. It is interesting to note the lower values as well as slight offset in the variations seen in chlorine residual from one end of the contact tank to the other. The lower values are a consequence of the disinfection process which inherently results in a chlorine demand. The offset correlates with the time required for the chlorine, added at the front of the tank, to mix and migrate to the back. ORP signals, are not consistently indicative of the chlorine residual, and tend to be very slow in responding to solution variations. However, the 25 mv range of response is more within the expected value than that of the sensor at site #1, since a 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 the temperate climate of the location, temperature variations during this time were similar to the other sites. As versus the other two locations, the test equipment was housed in a temperature controlled trailer. The data presented in Figure 7 covers three separate weeks during the test period. In addition to chlorine residual and ORP, corresponding ph information is also included. The test results show little correlation of the ORP readings with chlorine residual. Qualitatively, it can be said that the ph variations seen in weeks one and three may have been indicated by the ORP signal variations as well. However, it appears as if other (unknown) factors have also contributed. This is further supported by the fact that during week two, when the ph varied very little, and the chlorine concentration varied by ±0.1 ppm, the ORP signal varied up to 45 mv

9 Figure 5 - Chlorine Residual vs. ORP, Week 2 (Site #2) Figure 6 - Chlorine Residual vs. ORP, Week 3 (Site #3)

10 Figure 7 - Chlorine Residual (ppm), ORP (mv), and ph (Site #3)

11 Conclusions Chlorine residual concentrations and ORP measurements were studied at three different sites to assess the correlation between these measurement techniques for chlorine residual control and disinfection efficacy. In some cases, a correlation was seen. However, due to other effects, such as ph changes, and the presence of other redox species, etc., the ORP reading was found to be less reliable than direct amperometric measurement of chlorine residual for wastewater disinfection and 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 , Keller, J., Changing Control Strategy Solves Chlorine Residual Problems, Water Engineering & Management, pp , June, Carberry, J. B., Options for the Rational Design and Operation of Oxidation Ponds, Wat. Sci. Tech., 24 (5), pp , Sakakibara, Y., Flora, J.R., Suidan, M.T., Biswas, P. And Kuroda, M., Measurement of Mass Transfer Coefficients with an Electrochemical Method Using Dilute Electrolyte Solutions, Water Research, 28 (1), pp. 9-16, 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 ER CHLORINATION, Environmental Impacts and Health Effects, Vol. #4, Ann Arbor Science, Ann Arbor, MI 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, Sawyer, D.T. and Roberts, J.L., Experimental Electrochemistry for Chemists, Wiley Interscience, New York, NY, Snoeyink, V.L. and Jenkins, D., Water Chemistry, Wiley & Sons, New York, N.Y., 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, Drinking Water and Health, Vol. #3, National Academy Press, Washington, DC,

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