Electrochemical Wastewater Disinfection: Identification of Its Principal Germicidal Actions X. Y. Li 1 ; H. F. Diao 2 ; F. X. J. Fan 3 ; J. D. Gu 4 ; F. Ding 5 ; and A. S. F. Tong 6 Abstract: Laboratory experiments were carried out to investigate the mechanisms of electrochemical (EC) wastewater disinfection. Artificial wastewater contaminated by Escherichia coli (E. coli) culture, and which contained different salts of NaCl, Na 2 SO 4 , and NaNO 3 , was used as the test medium. The experimental results do not favor the hypotheses that the EC bactericidal action was due to cell destruction by the electric field and the production of persulfate. In comparison to direct chlorination, the EC process displayed a much stronger disinfecting capability than that of electrochlorination assumed for EC disinfection. Observations with scanning electron micros- copy on the E. coli bacteria of wastewater treated by different means of disinfection suggested that the cells were likely killed during the EC treatment by chemical products with oxidizing and germicidal powers similar to that of ozone and much stronger than that of chlorine. All of the findings support the theory that the major killing function of EC disinfection is provided by short-lived and high-energy intermediate EC products, such as free radicals. DOI: 10.1061/(ASCE)0733-9372(2004)130:10(1217) CE Database subject headings: Chlorination; Disinfection; Ozonization; Wastewater treatment. Introduction Electrochemical (EC) disinfection has been found to be highly effective in killing a wide spectrum of microorganisms (Stoner et al. 1982; Patermarakis and Fountoukidis 1990; Matsunaga et al. 1994, Grahl and Markl 1996; Butterfield et al. 1997; Matsunaga et al. 2000). During the EC disinfection process, water is forced to pass through a disinfector equipped with electrodes on which current is charged. Our recent experimental study demonstrated the great effectiveness of EC disinfection for saline wastewater effluents collected from wastewater treatment plants (Li et al. 2002). A killing efficiency of 99.9% on total coliform bacteria was achieved for the secondary effluent with a contact time of less than 10 s and a power consumption of no more than 0.01 kWh/m 3 . Despite its great effectiveness and potential, the killing mecha- nisms of EC disinfection are not fully understood. The high bac- tericidal capacity of EC treatment has been attributed to various functions, including electrochlorination (Stoner et al. 1982), gen- eration of lethal oxidants, such as persulfate (Patermarakis and Fountoukidis 1990), destruction caused by the electric field (Shi- nohara et al. 1989; Matsunaga et al. 1994; Grahl and Markl 1996; Matsunaga et al. 2000), and inactivation by energy rich interme- diate products. Free radicals formed during electrolysis, such as O 2 - ·, ·OH - , and ClO 2 - ·, may play a critical role in exerting strong germicidal actions (Patermarakis and Fountoukidis 1990; Johnson et al. 1999; Oturan 2000; Li et al. 2002). In the present study, artificial wastewater that was contaminated by E. coli culture and contained different salts such as NaCl, Na 2 SO 4 , and NaNO 3 was used to investigate the killing mechanisms of EC disinfection in comparison with conventional chlorination and ozonation. Materials and Methods Model Wastewater Artificial wastewater was used to ensure a stable and reproducible influent condition for the investigation of EC disinfection mecha- nisms. The E. coli culture was cultivated by inoculating seed of E. coli JM109 into a 250 mL flask filled with 130 mL of growth media (m Endo broth MF, DIFCO). The culture was grown on a shaker in a water bath at 35°C for 24 h to reach its stationary growth phase with a cell density of around 10 9 / mL. Using the fresh culture as the seed, the cultivation of the E. coli culture was repeated once to assure its purity and activity. Three different salt solutions—NaCl, NaNO 3 , and Na 2 SO 4 —at different concentra- tions were used. The NaCl water was made at concentrations of 0.01, 0.025, 0.05, 0.075, and 0.1 M, while the NaNO 3 and Na 2 SO 4 water was prepared with deionized water at concentra- 1 Associate Professor, Dept. of Civil Engineering, Univ. of Hong Kong, Pokfulam Rd., Hong Kong, China (corresponding author). E-mail: xlia@hkucc.hku.hk 2 PhD Student, Dept. of Environmental Science & Engineering, Tsin- ghua Univ., Beijing, China. 3 Deputy Manager, The Macao Water Supply Co., Ltd., SAAM, Macau, China; formerly, Research Associate, Dept. of Civil Engineering, Univ. of Hong Kong, Pokfulam Rd., Hong Kong, China. 4 Assistant Professor, Dept. of Ecology and Bio-diversity, Univ. of Hong Kong, Pokfulam Rd., Hong Kong, China. 5 Research Associate, Dept. of Civil Engineering, Univ. of Hong Kong, Pokfulam Rd., Hong Kong, China. 6 Assistant Engineer, Environmental Protection Department, Hong Kong SAR Government, Hong Kong, China; formerly, Final-Year Stu- dent, Dept. of Civil Engineering, Univ. of Hong Kong, Pokfulam Rd., Hong Kong, China. Note. Associate Editor: Mark J. Rood. Discussion open until March 1, 2005. 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 technical note was submitted for review and possible publication on August 27, 2002; ap- proved on September 23, 2003. This technical note is part of the Journal of Environmental Engineering, Vol. 130, No. 10, October 1, 2004. ©ASCE, ISSN 0733-9372/2004/10-1217–1221/$18.00. JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / OCTOBER 2004 / 1217