Elemental Iodine as a Disinfectant for Drinking Water Shih Lu Chang and J. Carrell Morris DIVISION OF APPLIED SCIENCE, HARVARD UNIVERSITY, CAMBRIDGE, MASS. T h e germicidal efficiency of dilute aqueous iodine has been evaluated to determine the usefulness of elemental iodine as an emergency disinfectant for drinking water. Iodine concentrations of 5 to 10 p.p.m. have been found effective against all types of water-borne pathogenic organisms, including the enteric bacteria, amebic cysts, cercariae, leptospira, and viruses, within 10 minutes at room temperature. High germicidal activity is maintained over the pH range 3 to 8 in the presence of a variety of water contaminants. Waters with high organic color (>50 p.p.m.) may exhibit sufficient iodine demand to require an increased dosage for satisfactory disinfection. At low temperatures the germicidal action is slowed, so that a contact time of 20 minutes is required in near-freezing waters. Iodine has a number of advantages as an emergency disinfectant for water supplies as compared with hypochlorites or chloramine-type materials. Its germicidal action is less dependent on pH, temperature, and time of contact, nitrogenous impurities do not impair its effectiveness, and side reactions leading to consumption of the germicide are less marked for iodine than for chlorinous disinfectants. Almost all waters can be made safe for drinking with a single dose of iodine of approximately 8 p.p.m.
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LTHOUGH iodine is well known as a germicide for medical application and has often been recommended for the treatment of water supplies under emergency conditions, little information has been published on its efficacy in water treatment or the concentrations required (6). An extensive program of research on the germicidal properties of dilute aqueous solutions of iodine has now shown t h a t iodine is an effective germicidal agent at concentrations of a few parts per million and that i t can be advantageously employed for the emergency treatment of natural waters of varying quality. To be useful in emergency disinfection of water a substance must be effective not only against the water-borne pathogenic bacteria-the typhoid, paratyphoid, cholera, and bacillary dysentery organisms-but also against other water-borne pathogens, including Entamoeba histolytica, Leptospira icterohaemorrhagiae, cercariae of Schistosoma sp., and viruses. All these types of infectious agents have been considered in these studies. Investigations of the effects of temperature, pH, color, turbidity, nitrogenous material, and other possible variations in water quality on the germicidal effectiveness of iodine have also been included in order to establish its general usefulness as a water disinfectant.
Studies on Bacteria
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Most of the experiments on the bactericidal qualities of aqueous iodine solutions were conducted with the common test organism, Escherichia coli. However, a sufficient number of experiments were also carried out with the enteric pathogens themselves to establish the validity of the results obtained with E. coli. The general method of testing was to add the desired numbers of organisms to continuously stirred gallon quantities of appropriate water or solution and, following the addition of suitable concentrations of iodine solution, t o withdraw samples a t specified times, adding them immediately t o sufficient sodium thiosulfate solution to reduce the iodine present. Surviving organisms were then determined either by plating or by suitable fermentation tests in tubes after appropriate dilution of the reduced samples. Attempts t o evaluate the fundamental bactericidal behavior of iodine in pure water were not too successful. A high degree of variability was encountered, probably because the concentrations of iodine required t o give measurable rates of kill were so small t h a t they could be completely neutralized by mere traces of reac-
tive impurities. I n some tests, concentrations of iodine as low as 0.02 p.p.m. exhibited rapid bactericidal action, while in other testa t the same concentration there was no measurable killing. Hows ever, when iodine concentrations of 0.05 p.p.m. or more were used, lo4E. coli per ml. were consistently reduced t o less than one per milliliter within 10 minutes a t room temperature. The fundamental bactericidal activity of iodine in water is thus about as great as that of hypochlorous acid. Results of experiments under conditions more nearly approximating those likely t o be encountered in the practical use of iodine are shown in Table I. The initial concentrations of bacteria, 106 per ml., are considerably higher than those expected even in grossly polluted waters, and so the iodine concentrations found to be bactericidal in these tests should be more than adequate in natural waters. It was considered that satisfactory disinfection was achieved under the stringent conditions of these tests and those subsequently presented when the numbers of viable bacteria had been reduced below 5 per 100 ml. At 25" C. in acid solution a free chlorine residual of about 2 p.p.m. is required t o meet this criterion. The data in Table I indicate t h a t iodine concentrations of the order of 2 t o 5 p.p.m. are adequate for the destruction of bacteria in water at 25" C. with a 10-minute contact period. Table I.
Bactericidal Efficiency of Iodine in Cambridge Tap Water
Temp., 25' C.
pH, 8.1 to 8.5.
11 Concn., P.P.M. Initial 30 min. 1.0 0 2.0 3.0 4.0 5.0
1.2 1.6 2.6 3.3
Test organism E. coli. Initial conoentration, N O , 10s per ml.' Viable Organisms per 100 MI., MPN, after: 5 min. 10 min. 20 min. 30 min. 1600 540 240 130 4.6 1 1 2.2 4.6 4.6 4.6
