MS2 Inactivation by Chloride-Assisted

MS2 Inactivation by Chloride-Assisted Electrochemical Disinfection Qian Fang1; Chii Shang2; and Guohua Chen3 Abstract: An electrochemical 共EC兲 disinfection system employing an iridium–antimony–tin-coated titanium anode and direct current was used to inactivate bacteriophage MS2 in synthetic solutions with sodium chloride addition. The inactivation data fit the modified Chick–Watson 共n ⫽ 1兲 model well. The model indicates that, although better disinfection could be achieved with increases in salt content, contact time, and applied current, these three parameters influence the EC disinfection of MS2 in distinct manners and to different degrees. Compared with chlorination, our EC disinfection system exhibited superior inactivation capability especially with a longer contact time or in the presence of ammonium. The formation of trihalomethanes and haloacetic acids in the EC system was smaller than that from chlorination but a large formation of chlorate ions was observed. These differences indicate that the EC system is likely to produce other potent oxidants that enhance inactivation and alter disinfection by-product formation. DOI: 10.1061/共ASCE兲0733-9372共2006兲132:1共13兲 CE Database subject headings: Chlorination; Disinfection; Halogen organic compounds; Oxidation; Viruses; Water treatment; Trihalomethanes.

Introduction For more than a century, chlorination has been the most prevalent disinfection method due to the fact that, among the available disinfectants, only chlorine is widely proven for achieving both primary and secondary disinfection goals: disinfection to remove pathogens and provision of disinfection residuals in the distribution system to prevent subsequent regrowth and recontamination by pathogens. However, because of the serious safety concerns associated with the use of chlorine and the formation of disinfection by-products 共DBPs兲, chlorination is considered problematic and has become less favorable. Consequently, disinfection alternatives are being sought, among which electrochemical 共EC兲 disinfection is considered as a promising method because of its primary advantage of in situ production of disinfectants in the treatment system. EC disinfection is a process during which inactivation is achieved in an electrolytic system equipped with electrodes on which electric current is applied. Many variations in the elec1 Graduate Research Assistant, Environmental Engineering Graduate Program, Hong Kong Univ. of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. E-mail: [email protected] 2 Assistant Professor, Dept. of Civil Engineering and Environmental Engineering Graduate Program, Hong Kong Univ. of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 共corresponding author兲. E-mail: [email protected] 3 Associate Professor, Dept. of Chemical Engineering and Environmental Engineering Graduate Program, Hong Kong Univ. of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. E-mail: [email protected] Note. Discussion open until June 1, 2006. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on November 15, 2004; approved on April 14, 2005. This paper is part of the Journal of Environmental Engineering, Vol. 132, No. 1, January 1, 2006. ©ASCE, ISSN 0733-9372/2006/1-13–22/$25.00.

trodes and cell configurations have been reported in the literature. For example, a variety of electrodes have been used including granular-activated carbon 共Matsunaga et al. 1994兲, activated carbon fibers 共Matsunaga et al. 1994兲, carbon-cloth 共Matsunaga et al. 1992兲, platinum 共Kirmaier et al. 1984兲, and silver 关E. Tiesler, “Method and device for treatment of water.” Deutsche Patent No. 3,527,733 共1985兲兴. Titanium-based electrodes have become popular because this metal can form a protection layer of TiO2 for the damaged coating. It has been reported that titanium nitride 共TiN兲 electrodes worked effectively for drinking water disinfection and inactivation of marine bacteria 共Matsunaga et al. 2000; Nakayama et al. 1998兲. Recently, an electrode with IrO2 – Sb2O5 – SnO2 coating was invented and displayed long service life 共Chen et al. 2001兲. Direct current 共dc兲 and alternating current 共ac兲 of low and high frequency 共0.5– 800 Hz兲 have been utilized 共Patermarakis and Fountoukidis 1990兲. Further, additives such as sodium chloride 共NaCl兲 are sometimes added to improve system performance 共Kraft et al. 1999兲. Other factors, such as the contact time, the current applied, the quality of the water or wastewater and the resistance of the pathogen are all expected to influence the EC process significantly 共Kraft et al. 1999; Li et al. 2002兲. Some studies have been conducted to evaluate the potency of EC disinfection. It has been reported that this process is capable of killing a large spectrum of microorganisms, from viruses to relatively larger groups such as algae and protozoa 共Stoner and Cahen 1982兲. However, effective control of pathogens requires not only the use of efficacious treatment technologies but also appropriate design criteria to ensure protection of public health. For this purpose, a number of kinetic models have been advocated over the years, creating the basis for the contactor system design and the disinfection performance assessment. Nevertheless, little effort has been devoted to modeling the inactivation kinetics of EC disinfection, making process control difficult and, as a consequence, the adoption of this technology has been impeded. Further, CT values 关products of disinfectant concentration, C in 共mg/L兲, and contact time, T 共min兲兴, which are widely used to provide adequate control of microorganisms, may not be appro-

priate in EC systems due to the fact that in an EC system, the disinfectant concentration 共C兲 is a function of the contact time 共T兲. Modeling the inactivation kinetics of an EC disinfection system is therefore necessary. Another knowledge gap is that the mechanisms contributing to EC disinfection are unclear. A number of articles appeared to support the claims that the electrolysis of salt brine produces a solution containing mixed oxidants such as chlorine and other oxidative species 共Cho et al. 2001兲; whereas other studies showed that chlorine is the only oxidant produced in an EC system 共Gordon et al. 2002兲. The contributions of the electric current 共Matsunaga et al. 1992; Weaver and Chizmadzhev 1996兲 and the shortterm radicals, such as hydroxyl ions and hydroperoxide ions 共Patermarakis and Fountoukidis 1990; Diao et al. 2004兲, were also proposed. Further, though it has been reported that organic DBPs, such as haloacetic acids 共HAAs兲 and brominated trihalomethanes 共THMs兲 could be effectively destroyed through EC treatment 共Korshin and Jensen 2001; Kimbrough and Suffet 2002兲, fewer studies have been conducted to investigate the formation of organic DBPs from EC disinfection. On the other hand, much controversy has been evoked over the formation of the inorganic DBPs: some studies support the conclusion that the electrolysis of salt brine also produces chlorine dioxide and ozone, thus resulting in the formation of chlorate and bromate 共Cho et al. 2001兲. On the contrary, Gordon et al. 共2002兲 stated that the formation of chlorate is not from the produced chlorine dioxide but a result of decomposition of free available chlorine 共FAC兲. Initiated by the above-mentioned unknowns, this study sought to clarify the inactivation kinetics and to explore the possible mechanisms of the chloride-assisted EC disinfection. Consequently, the study was divided into two parts: In the first part, synthetic solutions containing microorganisms were inactivated in a completely mixed electrolytic reactor under different combinations of various operation parameters, i.e., contact time, electric current, and salt content. The time-dependent inactivation performance was thus measured, from which kinetic models were established and the relative importance of the operation parameters on EC disinfection was evaluated. In the second part, the EC system was compared with chlorination in terms of inactivation performance and DBP formation, with the intention of elucidating whether the inactivation achieved by the EC disinfection could be solely attributed to chlorine generation. The surrogate used in the disinfection was the bacteriophage MS2, which bears a morphological similarity to waterborne enteric viruses and shows high resistance to disinfection. This bacteriophage has been listed as an acceptable indicator of enteric viruses by the USEPA 共2000兲. The surrogate for natural organic matter 共NOM兲 was humic acid from Aldrich. An Ir–Sb–Sn-coated Ti pellet was chosen as the anode. Ti is a harmless metal and has a high resistance to the alkalis and acids that are produced during electrolysis. The coating layer helps extend the service life of the anode and shows low overpotential for oxygen evolution 共Chen et al. 2001兲.

Materials and Methods Solution Preparation All chemical solutions were prepared from reagent grade chemicals or stock solutions. Dilution to target aqueous concentrations was accomplished by double distilled, deionized 共DDDI兲 water.

Solutions were stored in a temperature controlled room 共4 ° C兲 and were brought back to room temperature before use. A stock solution of free chlorine 共HOCl, 4,000 mg/ L兲 was prepared from 5% sodium hypochlorite 共NaOCl兲 共from Allied Signal兲 and periodically standardized by DPD/FAS titration 共APHA/AWWA/WEF 1998兲. An experimental chlorine solution was freshly prepared by diluting the stock prior to the experiments. A humic acid solution 共5 mg/ L as DOC兲, which served as the surrogate for NOM, was prepared from the stock solution 共about 250 mg/ L as DOC兲, which was obtained by dissolving an aliquot of humic acid crystals 共from Aldrich兲 into dilution water and then filtering through a 0.45 ␮m filter paper 共from Advantec MFS兲. Solutions in the reactor for the inactivation tests were made from the DDDI water, containing demand-free phosphate buffer solution 共PBS兲 共0.01 M, pH 7兲 and sodium chloride salts 共from Sigma, 98% min.兲 at predetermined concentrations. Two ranges of salt concentrations, namely, low 共0.016 and 0.032% of NaCl by mass兲 and high 共0.5 and 1% of NaCl by mass兲, were used in this study aiming at simulating the tolerable chloride concentrations of potable water and the saline wastewater in Hong Kong due to the use of seawater in the flushing system. Bacteriophage MS2 and Host Culture Preparation The broth used for the MS2 and its host culture preparation is called the ATCC broth here. After dissolving 10 g / L tryptone, 1 g / L yeast extract, and 8 g / L sodium chloride, the broth was autoclaved at 1.5 kgf/ cm2 and 121° C for 15 min, followed by aseptically adding 10 mL of 10% filter-sterilized glucose, 1 mL of 10 mg/ mL filter-sterilized thiamine and 2 mL of 1 M filtersterilized calcium chloride to create 1 L of sterilized broth. The host Escerichia coli 共E. coli兲 stock culture was prepared by dissolving dry E. coli powders 共ATCC#15597兲 into several milliliters of the ATCC broth. The fresh host E. coli culture was developed by transferring one loop of the stock to 20 mL of the ATCC broth and incubating with aeration at 37° C for 18 h. Pure bacteriophage MS2 stock culture was prepared from dry MS2 powders 共ATCC#15597-B1兲 by dissolving the powders into several milliliters of the ATCC broth. The experimental MS2 culture was prepared by transferring 0.1 mL of the MS2 culture stock and 0.1 mL of a sterilized calcium chloride solution 共1 M兲 to the freshly prepared 20 mL of the active E. coli suspension. Calcium chloride was added to promote the adsorption of MS2 on the bacterial cells 共Adams 1959兲. The infected bacteria were continuously aerated at 37° C for 24 h. After inoculation, the nutrient-rich and host-associated MS2 suspension was first centrifuged at 1,000 g for 10 min to isolate the host E. coli cells. The decanted supernatant was then centrifuged at 2,000 g for 20 min, followed by supernatant removal and resuspension with PBS 共0.01 M, pH 7兲. This washing procedure was repeated four times to minimize the concentration of non-phage constituents. Reactor Setup In this study, a Ti pellet with a thin layer of IrO2 – Sb2O5 – SnO2 coating acted as the anode and a stainless steel ring served as the cathode. The anode had an outer diameter, an inner diameter and a thickness of 11.91, 6.33, and 13.26 mm, respectively, with a corresponding surface area of 919.6 mm2. The EC reactor is shown in Fig. 1共a兲. The setup was similar to a typical dual-electrode electrolytic cell. A 250 mL beaker was used as the container, with the anode suspended in the middle and

Experimental Procedures Inactivation Tests In the inactivation tests, a bacteriophage MS2 suspension 共200 mL兲 was introduced into the well-mixed batch EC reactor at a preset salt concentration, followed by the application of a given current. The concentrations 共mg/L as Cl2兲 of the disinfectant residuals were periodically measured by DPD/FAS titration. Samples were also taken at predetermined time intervals, dechlorinated by adding sodium thiosulfate, and subjected to MS2 enumeration to determine the corresponding time-dependent inactivation behavior. Temperature was not treated as a variable but a constant in this study since the increase was insignificant 共0.2– 0.3° C兲 due to the short operation duration 共up to 2 min兲 and the small current applied 共up to 0.2 A兲. All experiments were repeated at least twice to ensure reliable results.

Fig. 1. Schematic view of the reactors used in the study: 共a兲 Electrochemical system setup: 共1兲 cathode, 共2兲 Ti plate, 共3兲 anode, 共4兲 magnetic stirrer, 共5兲 dc power supply, 共6兲 ampere meter, 共7兲 reactor, and 共8兲 synthetic solution and 共b兲 pump chlorination system setup: 共1兲 Reactor, 共2兲 synthetic solution, 共3兲 magnetic stirrer, 共4兲 pump, and 共5兲 concentrated chlorine solution

the stainless steel cathode along the edge. The wedge-shaped Ti plate affixed to the anode was for conducting the current. The electrodes were placed in such a way that a more uniform current density could be achieved. The setup was connected to a direct current 共dc兲 power supply 共IPS 2302A, ISO-Tech兲, by which currents ranging from 0 to 0.2 A, and voltages ranging from 0 to 18.0 V were applied to generate different schemes. Testing solutions were completely mixed in the reactor. The setup for chlorination is sketched in Fig. 1共b兲, which is different from the conventional chlorination setup, in that a pump is used to add continuously a concentrated hypochlorite 共NaOCl兲 solution into the reactor. This chlorination method is hereafter referred to as “pump chlorination” 共PC兲. The PC system can simulate the oxidant generation in the EC system by dosing free chlorine with same rates of the disinfectants generated in the EC system. Analytical Methods DPD/FAS titration 共APHA/AWWA/WEF 1998兲 was used to examine the quantities of the disinfectant residuals. Assays for the bacteriophage MS2 followed the double layer method as well as the single layer method described by Adams 共1959兲 and USEPA 共2001兲, respectively. A gas chromatograph 共from Finnigan兲 with an electron capture detector was used for THM and HAA determination according to USEPA Methods 551.1 and 552.2 共USEPA 1995兲 with slight modification. Chlorate and chlorite were measured by an ion chromatograph 共Dionex 500兲 equipped with an AS-9HC analytical column.

Tests for the Mechanistic Study To investigate the disinfection mechanisms, three tests were conducted, in which the EC system was compared with the PC system. The first test compared the inactivation achieved by the two systems at equivalent disinfectant concentrations and samples were handled in the same way as described previously. In the second comparison, an excessive amount of ammonium was added to both systems before applying the current. If other disinfectants, such as ozone or chlorine dioxide, were generated in the EC system, their disinfection efficiencies would become obvious because ammonium does not react with them but converts all the free chlorine to monochloramine, which possesses much weaker disinfection power. Because ammonium can be oxidized at the anode, current was also applied in the PC system but no salt was added. All other conditions were kept the same. The third test compared DBP formation, in which a humic acid solution 共5 mg/ L as DOC兲 was employed as the surrogate for NOM. Sodium bromide at 0.1 or 1 mg/ L was sometimes also added to the solution to simulate the bromide concentrations in drinking water under normal conditions and with seawater intrusion, respectively. After two min of operation, two samples were collected simultaneously, with one subjected to measurement immediately and the other after 7 days, during which the sample was kept in a tightly sealed amber bottle in the dark. Again, all experiments were repeated at least twice to ensure reliability.

Results and Discussion EC Inactivation Kinetics under Demand-Free Conditions The modeling procedures are as follows: first, express the residual concentration in the EC system as a function of the operation conditions 共contact time, current applied, and salt content兲; then, integrate this relationship into the differential expressions of various disinfection kinetic models, including the Chick–Watson 共n = 1兲 model 共Chick 1908; Watson 1908兲, the Chick–Watson 共n ⫽ 1兲 model 共Chick 1908; Watson 1908兲, the Hom model 共Hom 1972兲, the rational model 共Roy et al. 1981兲 and the Hom–Power model 共Anotai 1996兲. Model parameters were determined by means of a nonlinear regression method. The best fit model was selected according to criteria proposed in the following paragraphs based on statistical theories.

early with increasing contact time and increasing applied current. The system displayed a similar linear trend in the case of low salt content 共data not shown兲. When the oxidant generation was compared with the salt content, an exponentially rising shape of the data points was observed 关see Fig. 2共b兲兴. Thus, the following model is proposed:

冋 冉

C = p关NaCl兴 1 − exp −

q 关NaCl兴

冊册

IT , V

共2兲

where p and q = empirical constants determined by the nonlinear regression method; V = volume of water in the reactor, 0.20 L in this study, in which the best-fit values are chosen to minimize the error sum of the squares 共ESS兲 ESS =

兺 关Cpred − Cobs兴2 .

共3兲

The p and q are found to be 10.96 and 0.033, respectively, with MATLAB. Thus, the expression describing the disinfectant residual concentration and the operation conditions used in this study is

冋 冉

C = 10.96关NaCl兴 1 − exp −

0.033 关NaCl兴

冊册

IT . 0.2

共4兲

The predicted C values are plotted against the observed values in Fig. 2共c兲. The slight deviation of the data points from the 45° line demonstrates excellent agreement. When the concentration of salt is sufficiently high, the equation reduces to C = 1.808IT, giving a current yield of 0.362 mg Cl2 / A s. This value compares favorably with the theoretical value of the current yield of 0.367 mg Cl2 / A s. It is understandable that the current yield will decrease when the salt concentration is lower.

Fig. 2. Modeling the disinfectant residual concentrations with variations in 共a兲 applied current and time 共1% NaCl by mass兲; 共b兲 salt content 共all data included兲; and 共c兲 the predicted versus observed oxidant concentrations 共all data included兲

Residual Concentration Versus Operation Conditions The residual concentration in the EC system is a function of the contact time, the current applied and the salt content C = function共T,I,关NaCl兴兲,

共1兲

where C = disinfectant residual concentration; T = contact time; I = current applied; and 关NaCl兴 = initial salt content. Fig. 2共a兲 depicts the time-dependent residual concentrations generated electrochemically with high salt content 共1% NaCl by mass兲. It can be seen that the disinfectant residual concentrations increased lin-

Inactivation Kinetics Versus Operation Conditions The relationship between C and the operation conditions 共关NaCl兴, I, and T兲 has been estimated with a high degree of correlation 共R2 = 0.99兲; it is therefore valid to incorporate this equation into the disinfection kinetic models under disinfectant demand-free conditions. The demand-free assumption is based on the following reasons: 共1兲 The contact time was short; therefore, the decay of disinfectants could be neglected; 共2兲 the solution in the reactor was prepared from DDDI water, which was free of organic materials; 共3兲 the surrogate for the inactivation tests was purified bacteriophage MS2, which caused negligible disinfectant demand after pretreatment. Table 1 summarizes the kinetic models and their integrated expressions 共the last column兲, which are derived by substituting Eq. 共4兲 into the original differential expressions 共the second column兲 and then integrating. In Eq. 共4兲, “10.96关NaCl兴 兵1 − exp共−0.033/ 关NaCl兴兲其I / 0.2” is independent of the contact time, T. Therefore, this can be considered as the oxidant generation rate, R, which is used in the integrated formulas instead. Again, the model parameters 共k, m, n, and x兲 are determined according to the nonlinear regression method and the results are listed in Table 2 along with the statistical parameters. The symbol “ˆ” indicates an estimated value. According to statistical theories, a smaller ␴ˆ 共standard deviation兲 and a smaller ESS 共error sum of the squares兲 correspond to a better model fit. Therefore, the modified Chick–Watson 共n ⫽ 1兲, Hom, rational, and Hom–Power models are considered to adequately describe the EC inactivation behavior with relatively similar ␴ˆ and ESS. However, from the standpoint of process control, fewer parameters are preferred and models with many parameters can lead to overparametrization, resulting in highly cor-

Table 1. Summary of Kinetic Inactivation Models for Disinfectant Demand-Free Conditions

Model Chick–Watson 共n = 1兲 Chick–Watson 共n ⫽ 1兲

Differential expression 共general: dN / dt = −kmNxCntm−1兲

Reference

−kNC

共Chick 1908; Watson 1908兲

n

共Chick 1908; Watson 1908兲

−kNC

Integration expressiona log N / N0

Ⲑ

− 1 2 kRT2 1 kRnTn+1 n+1

−

共Hom 1972兲

−kmNCntm−1

Hom

−

Rational

共Majumdar et al. 1973; Roy et al. 1981兲

−kNxCn

冋

1 kmRnTm+n m+n

log 1 + −

Hom–Power law

共Anotai 1996兲

−kmNxCntm−1

a

册

共x − 1兲

冋

log 1 + −

1 kRn 共x − 1兲Tn+1 n + 1 N1−x 0

m kRn 共x − 1兲Tm+n m + n N1−x 0

册

共x − 1兲

R = 10.96⫻ 关NaCl兴 ⫻ 共1 − exp共−0.033/ 关NaCl兴兲兲 ⫻ I / 0.2.

related parameter estimates 共Gyurek and Finch 1998兲. Therefore, the modified Chick–Watson 共n ⫽ 1兲 model is chosen to describe MS2 inactivation by the EC system of present study log

冋 冉

0.033 N = − 0.453关NaCl兴0.53 1 − exp − 关NaCl兴 N0

冊册

0.53

I0.53T1.53 . 共5兲

The predicted log-kill values are plotted against the observed values in Fig. 3 with the 45° line and the 95% prediction interval. Points plotted to the right of the 45° line indicate less observed kill than predicted. Conversely, points to the left indicate more actual kill. As shown by the 95% prediction interval, the model tends to underestimate the actual kill in general. It should also be noted that, with data extending to greater fractional inactivations, a less satisfactory agreement to kinetic models occurs. Similar findings were observed by other investigators studying chlorination in demand-free systems 共Haas and Karra 1984兲. The trend lines based on the model prediction are plotted along with the observed data points in Figs. 4 and 5, illustrating the effects of the current and salt content, respectively, on MS2 inactivation. It is evident that, although the log-kill increased with an increase in either the applied current or the amount of salt added, the inactivation efficiency was not enhanced proportionally with either of them. In Fig. 5, the predicted tread lines clearly

reveal that doubling the salt concentration from 0.5 to 1% provided a rather marginal enhancement in the inactivation performance. Nevertheless, when the salt content was increased from the low concentration range to the high concentration range, the inactivation was significantly enhanced. The impacts of different factors on MS2 inactivation can be thoroughly analyzed from Eq. 共5兲. In this modified Chick–Watson 共n ⫽ 1兲 model, the power of the contact time 共T兲 is regressed to be 1.53, whereas the power of the current 共I兲 is smaller 共0.53兲, showing that the influences of these two parameters on EC disinfection are not even, with the contact time being more significant. In other words, EC disinfection of MS2 tends to be more sensitive to the contact time than to the applied current. This is reasonable since the contact time exerts significant influences not only on the residual generation but also on the inactivation achieved at a constant disinfectant concentration. On the other hand, the salt content affects the MS2 inactivation in a different way by controlling the disinfectant production as shown in Fig. 2共b兲. In a narrow range of low salt content, disinfectant production is controlled by the salt content and a linear relationship is displayed. When the salt content is increased to about 0.5% and higher, the disinfectant production rate reaches its maximum and is independent of the salt content. Consequently, nearly equal degrees of inactivation were achieved by 0.5 and 1% salt contents as illustrated in Fig. 5.

Table 2. Summary of Parameter Estimates for Different Inactivation Models Describing EC Disinfection of Bacteriophage MS2 Model Chick–Watson 共n = 1兲 Chick–Watson 共n ⫽ 1兲 Hom Rational Hom–Power

kˆ

ˆ m

nˆ

xˆ

␴ˆ

ESS

0.048 0.083 0.050 0.15 0.081

— — 1.12 — 1.10

— 0.53 0.56 0.53 0.55

— — — 0.96 0.98

1.27 0.57 0.56 0.57 0.56

83.69 16.93 16.22 16.49 16.08

Note: — = not applicable; ␴ˆ = standard deviation; and ESS= error sum of the squares.

Fig. 3. Predicted log-kill of bacteriophage MS2 by the modified Chick–Watson 共n ⫽ 1兲 model versus observed log-kill values 共all data included兲

Undoubtedly, the kinetic model developed in this study is a robust tool for characterizing and evaluating the EC process in a disinfectant-demand-free environment. It should be modified when it is applied to modeling real water/wastewater disinfection, however. The modification shall focus on the development of the equation that expresses the oxidant residuals as a function the controlling parameters, since the log-kill follows the conventional disinfection kinetics when the oxidant residual profile is given. We also expect that the chlorine concentration shall be correlated with operation conditions 共efficient I, T, and 关NaCl兴兲 in a similar form as Eq. 共2兲 but the disinfectant consumption for oxidation of organic and inorganic impurities shall be quantitatively included. Of course, numerical integration may need to be applied in this case since the resultant equation may not be amenable to analytic integration. Further, since the quality of the water significantly influences inactivation, the disinfection performance should be tested using the water of interest even when the model is corrected for the disinfectant demand. Mechanisms The EC and PC systems were compared in three ways to elucidate the EC disinfection mechanisms. We posited that if the two systems behaved similarly, then it is very likely that the inactivation achieved by the EC system relies primarily on chlorine generation; otherwise, species other than chlorine may be generated. As described in the section entitled “Materials and Methods,” HOCl was pumped to the PC system at a rate equivalent 共in milligrams per liter as Cl2 per minute兲 to the disinfectant production rate in the EC system. Comparison of the Inactivation Achieved by the EC and PC Systems without Ammonium Addition The inactivation results from the two systems are shown in Fig. 6. Evidently and interestingly, the EC system bore a striking resemblance to the PC system at the beginning. However, as the contact time was extended, the EC system showed a higher kill rate. Consistent with this finding, Li et al. 共2002兲 reported that in

Fig. 4. Log-kill of bacteriophage MS2 versus time at different currents at 共a兲 low salt content 共0.016% NaCl by mass兲 and 共b兲 high salt content 共1% NaCl by mass兲

achieving a similar level of inactivation of the total coliform in a saline sewage effluent, the CT value for the EC process was nearly two orders of magnitude lower than that observed in direct chlorination. These results indicate that the superior inactivation capability of the EC system cannot be explained merely from the chlorine generated. Comparison of Disinfectant Residuals and Inactivation with Ammonium Addition in the EC and PC Systems As illustrated by Fig. 7共a兲, excess ammonium did not convert all the apparent FAC in the EC system to monochloramine, and the total residual concentration in the EC system was lower than that in the PC system. The titration results can be further supported by the difference in the inactivation behaviors 关see Fig. 7共b兲兴, with the EC system being more efficient. However, after the current was shut off at 80 s, all the apparent FAC in the EC system reacted with the ammonium and was measured in the monochloramine fraction at 200 s, whereas the total residual concentration remained the same. These remarkable disagreements further indicate that the mechanisms involved in EC disinfection are complicated and cannot be completely explained by chlorination. Comparison of DBP Formation in the EC and PC Systems Fig. 8 compares the THM formation in the EC and PC systems at bromide concentrations of 0, 0.1, and 1 mg/ L. It is found that the

Fig. 5. Log-kill of bacteriophage MS2 versus time at different salt concentrations at a fixed current 共0.2 A兲

THM formation in the two systems followed approximately the same trends, e.g., the yields of total trihalomethanes after seven days of incubation were much higher than those after the first 2 min; and higher bromide concentrations shifted the THM species toward species with higher bromine incorporation. Although the 2 min THM yields in the two systems were comparable, the seven day THM formation in the PC system displayed higher values than did those in the EC system. It should be noted that volatile species such as THM could be partially lost during the evolution of oxygen and other oxidants in the EC reactor. The formation of HAAs, the second major class of organic DBPs arising from chlorination, was also investigated and similar results were obtained 共data not shown兲. Several possible reasons are proposed to explain these observations. First, the EC system generates other potent oxidants, which can also be detected as FAC by DPD/FAS titration but these oxidants react with NOM to form THMs and HAAs to a smaller degree. Another possible explanation is that the EC system may alter some functional groups on NOM that are reactive to chlorine. If this postulation is true, the

EC system is analogous to ozonation, in which a certain percentage of the precursors for THMs, TCAA, and dichloroacetonitrile 共DCAN兲 was found to be destroyed 共Reckhow and Singer 1984兲. Nevertheless, it should be emphasized that though the EC system behaved differently than the PC system, free chlorine is generated in the EC system, since the 2 min formation of THMs and HAAs in the EC and PC systems appears to be quite similar. In addition to THMs and HAAs, we also focused on the formation of inorganic DBPs including bromate 共BrO−3 兲, chlorite 共ClO−2 兲, and chlorate 共ClO−3 兲 in this study. Chlorite ions were never detected in either the EC and PC systems; bromate was detected but its concentration could not be read on the ion chromatogram due to our analytical limitation to deal with the large interference from chloride ions. Among these three inorganic DBPs, only the chlorate concentration could be determined. In the PC system, the chlorate concentration remained fairly constant 共with a slight increase兲 for up to seven days, indicating that chlorate contamination resulted mainly from the hypochlorite solution

Fig. 6. Comparison between the time-dependent log-kill of bacteriophage MS2 in the EC and PC systems at currents of 0.05 and 0.15 A

Fig. 7. Comparison between the EC 共关NaCl兴 = 1 % , I = 0.1 A兲 and PC 共关NaCl兴 = 0 % , I = 0.1 A兲 systems with ammonium addition 共关NH+4 兴 = 0.015 M兲: 共a兲 The time-dependent residual concentrations and 共b兲 the time-dependent log-kill of bacteriophage MS2

pumped into the reactor. The absence of chlorite ions indicates that chlorite is an intermediate and is being consumed rapidly. On the other hand, during EC treatment 共关NaCl兴 = 0.016% by mass, I = 0.2 A兲, a small amount of chlorate 共47 ␮g / L兲 was measured after 2 min of operation. The yield of chlorate increased markedly to 1,880 ␮g / L after seven days of incubation. The large formation of chlorate afterward 共without applying current兲 could not be explained simply by FAC decomposition, which stoichiometrically produced only 1 ␮g / L chlorate from calculations 共Adam et al. 1992兲. These results were in conflict with the statement by Gordon et al. 共2002兲 that the source of chlorate in electrolyzed salt brine was merely from FAC decomposition. Though we cannot decisively explain the higher chlorate formation, the EC system does generate much more chlorate than the guideline value 共200 ␮g / L兲 proposed by the World Health Organization 共Gunten and Nowack 1999兲. These results provide some information to elucidate the mechanisms behind the superior inactivation performance of the EC system. First of all, it should be emphasized that though the mechanisms have been the focus of much study, the EC disinfection process is likely to rely heavily on the electrochemical conversion of chloride to chlorine. This conclusion is supported by Fig. 5, which illustrates that, without the addition of salt, inactivation of the phage cannot be observed within 60 s of operation. In practice, this indicates that merely relying on electric currents

Fig. 8. THM formation in the EC 共closed square兲 and PC 共open square兲 systems 共pH= 7, 25° C兲 in humic acid solutions 共5 mg/ L as DOC兲 with low salt content 共关NaCl兴 = 0.016% 兲 after 2 min of operation with electric current of 0.2 A and 7 days incubation at 共a兲 关Br−兴 = 0; 共b兲 关Br−兴 = 0.1 mg/ L; and 共c兲 关Br−兴 = 1.0 mg/ L

at the applied levels gives, if any, negligible inactivation and EC disinfection is economically applied to saline wastewater. Drees et al. 共2003兲 investigated the current effect on EC disinfection using reduced glutathione 共GSH兲, a substance that can protect cells by scavenging chemical oxidants but cannot protect cells from oxidation due to the electric current. A considerable decrease in the inactivation rate when GSH is added indicates that EC inactivation is most likely due to the electrochemically generated oxidants rather than to the direct oxidation by electric current. However, whether there is a synergistic effect between the current and the oxidants is beyond the scope of the available results. The three comparative tests clearly indicate that besides chlorine, other potent disinfectants also contribute to the inactivation. Among the possible oxidants that might be generated in the EC system, chlorine dioxide displays superior inactivation capability and forms fewer THMs and HAAs than chlorine does 共Edward et al. 1992兲. Further, the apparent FAC measured in the second comparison 关as shown in Fig. 7共a兲兴 can also be explained by the possible generation of chlorine dioxide, since it does not react

with ammonium but can be detected as FAC by the DPD/FAS method. Last, the excessive production of chlorate also suggests that chlorine dioxide is most likely formed.

Conclusions Our results showed that, compared with chlorination, the EC process could achieve similar or even higher reduction of bacteriophage MS2. The inactivation data was regressed to fit the modified Chick–Watson 共n ⫽ 1兲 model, which clearly showed that, though better disinfection efficiency could be achieved with increases in salt content, contact time or applied current, the influences of these parameters on EC disinfection were not even. EC disinfection mechanisms were explored from various angles. This research study confirms that generated chlorine contributes to the disinfection but contradicts the argument that inactivation is attributed to the electric current itself. The results of the MS2 inactivation and DBP formation support the claim that electrolysis of salt brine produces a solution containing mixed oxidants. Though no compelling evidence can be provided to identify these oxidants at this stage, it is most likely that chlorine dioxide can be produced in the EC system.

Acknowledgment The writers are grateful to Dr. Xueming Chen in the Department of Chemical Engineering, the Hong Kong University of Science and Technology, for building the anode.

Notation The following symbols are used in this paper: C ⫽ disinfectant residual concentration 共mg/L兲; I ⫽ current 共A兲; k , m , n , x ⫽ parameters in kinetic models; N ⫽ count of bacteriophage MS2 after disinfection 关plaque-forming units 共PFU兲 per 1 mL兴; N / N0 ⫽ survival ratio of bacteriophage MS2; N0 ⫽ initial count of bacteriophage MS2 关plaque-forming units 共PFU兲 per 1 mL兴; p , q ⫽ empirical constants in equation 共2兲; R = C / T ⫽ oxidant generation rate 关mg/共L s兲兴; T ⫽ contact time 共s兲; and V ⫽ volume of liquid in the reactor 共L兲.

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