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Iodine used as a water-disinfectant in turbid waters
Wat. Res. Vol. 23, No. 6, pp. 671-676, 1989 Printed in Great Britain.All rights reserved
0043-1354/89$3.00+ 0.00 Copyright © 1989MaxwellPergamonMacmillanplc
I O D I N E USED AS A WATER-DISINFECTANT IN TURBID WATERS K. V. ELLIS1. and H. B. R. J. V A N V R E E 2 ~Civil Engineering, Loughborough University of Technology, Loughborough, Leics. LE11 3TU, England and 2Department of Water Pollution Control, Wageningen Agricultural University, P.O. Box 8129, NL-6700 EV Wageningen, The Netherlands
(First received August 1988; accepted in revisedform December 1988) Abstract--Various concentrations of iodine were employed to inactivate faecal indicator bacteria in low quality water at different turbidities in three pH ranges. The results obtained were compared with those of an arbitrarily selected standard of 1.0 mg/1 chlorine. Under all the conditions, for which dosages of 4.0 or 8.0 mg/1 iodine were used, a water of virtual potable quality was obtained within a 30 min contact period. Under none of the conditions investigated was a dosage of 1.0 mg/1 iodine found to be an effective disinfectant while both 1.0 mg/1chlorine and 2.0 rag/1 iodine were generally effectivein the lowest pH range investigated and at low turbidity but the disinfecting effect in both cases decreased with increasing pH and increasing turbidity.
Key words--iodine, disinfection, faecal indicator organisms, turbidity
INTRODUCTION Iodine is a blue-black element with a melting point of 113.5°C and a boiling point of 184.4°C which is normally obtainable in the form of crystals. It will volatilize at ordinary temperatures to produce a blue-violet gas with an irritating odour. It is the heaviest of the halogens. It has a specific gravity of 4.93 at 20°C an atomic weight of 126.9 and an atomic number of 53. Its oxidation potential (12 + 2~. . . . . > 21') is only 0.54 compared with that of chlorine of 1.39. Care should be taken in handling iodine as contact with the skin can produce lesions (Handbook of Chemistry and Physics, 1984; Lange's Handbook of Chemistry, 1985). Apparently iodine was first used in water treatment for the disinfection of drinking water for troops in France during the First World War (Vergnoux, 1915) and following work carried out by Fair and others (Fair et al., 1945) was adopted for use by the U.S. Army during the Second World War, when it was applied in the form of globaline tablets. Iodine is soluble in water to the extent of 0.029 g/100 g at 20°C which rises to 0.445 g/100 g at 100°C. Depending on the pH of the solution the dissolved iodine will hydrolyse to varying extents to form hypoidous acid (HOI). Neither this hydrolysis nor any subsequent dissociation of the produced oxy-acid (HOI) is as pronounced as with chlorine; as is illustrated from the following table adapted from Chang (1958).
*Author to whom correspondence should be addressed.
The effect of pH on the hydrolysisof iodineand chlorine(0.5mg/I titratable iodinein the presenceof iodide) Cl 2 HOCI OCI' I2 HOI OI' pn (%) (%) (%) (%) (%) (%) 5 99 1 0 0.5 99.5 0 6 90 10 0 0 99.5 0.5 7 52 48 0 0 96.5 3.5 8 12 88 0.005 0 21.5 78.5 9 0 1.0 99.0
The amount of iodine hydrolysed decreases with increasing concentrations of titratable iodine. At a pH of 8.0, or over, the HOI is unstable and decomposes slowly to form iodate and iodide; neither of which possess significant germicidal properties. In the presence of added iodide the iodine present in water will tend to form the tri-iodide. The proportion of this tri-iodide present increases both with the concentration of iodide and with that of titratable iodine. The 1-3 ion is only approximately 1/8 as cysticidal as elemental iodine and possesses no significant viricidal properties. In low concentrations of iodine, however, such as are used in water treatment the presence of the tri-iodide can be effectively ignored (Chang, 1958). Both the undissociated iodine, I2, and the hypoidous acid, HOI, possess appreciable germicidal properties but of the two the molecular iodine is apparently more effective against cysts and spores while the hypoiodous acid is a more effective viricide (Chang, 1958). Taylor and Butler (1982) found that iodine was most effective against poliovirus at pH 9.0. In work with six different significant bacterial genera Karalekas and others (Karalekas et al., 1970) found little significant difference in the bactericidal effects of 1.0 rag/1 iodine at pHs 5 and 7 but a pronounced reduction in effect at pH 9.0.
671
672
K.V. ELL1Sand H. B. R. J. VANVgEE
The possible physiological effects resulting from a continued intake of iodinated water has always created some appreciable resistance to its employment in water treatment although it has been suggested that a single serving of m a n y sea-foods would provide more iodine than the daily intake from water (Whitehead, 1981). Although, two extended investigations into the effect of long-term ingestion of iodated water, one with service personnel in the Marshall Islands (Morgan and Karpen, 1953) and the other a m o n g the prison population in Florida (Black et al., 1965), failed to elicit any harmful effects there must remain some appreciable apprehension as to the physiological results in the long term. This is an area which calls for more research. One of the principal attractions for the employment of iodine in disinfection of water is its low reactivity compared with other halogens. Iodine in water reacts only slowly with organic material present and does not react with ammonia to form iodamines. Consequently there is the possibility that it may be a superior disinfectant to chlorine for partially polluted waters and as a result might be a more suitable disinfectant in disaster situations where only low quality source waters may be available. Hence this present project was designed to determine how well, compared with an arbitrarily selected standard dosage of 1.0 mg/l chlorine, various concentrations of iodine would serve as a disinfectant against standard faecal indicator organisms with different levels of polluted water; the pollution in each case being present in the form of a fine suspension of solids. EXPERIMENTAL Test waters of various turbidities were employed (< 1, 20, 50, 75, t00 NTU). In each case the required turbidity was achieved by adding a known quantity of stock river sediment to deionized water. The sediment employed was derived from the River Soar downstream of Loughborough and was sterilized before use. A particle-size distribution for the sediment employed was determined using a Coultercounter and it is of interest that nearly 70% of the suspension was made up of particles of I0/~m or less in size, i.e. of bacterial size or smaller. 38.5% of the sediment was organic matter. Throughout, the investigation an adequate number of indicator bacteria within the test samples was assured by the addition of a sufficient aliquot of fresh treatment-works final effluent. This effluent was obtained from the completely nitrifying activated-sludge plant of the Loughborough Water Reclamation Works of the Severn Trent Water Authority. The mean concentration of free-ammonia (as N) in the effluent samples employed was less than 0.5 mg/l and the maximum 0.5 mg/1. The indicator microorganisms determined during the investigation were the faecal coliforms and the faecal streptocci. The samples of the Loughborough effluent used contained between 230 and 2100 FS/100ml and between 15 x I 0 3 and 51 x 103 FC/100ml. For all the bacteriological determinations the membrane filtration technique, according to the Standard Methods for the Examination of Water and l,Vastewater (APHA 1985), was employed. Three different pH ranges were investigated. These were pH 7.0-7.4, 7.9-8.3 and 8.3-8.5. The pH of each test sample was adjusted as required by the careful addition of 0.1 N sodium hydroxide solution. All the investigations were
carried out at a standard temperature of 20°C ( + 0.5°C). In each section of the project test batches of 1, 20, 50, 75 and 100 NTU were made up and at each turbidity five separate test samples were used to which were added either 1.0, 2.0, 4.0 or 8.0 mg/l iodine or 1.0 mg/1 chlorine. For each test run a 51. batch of test water was prepared at the required turbidity by adding the necessary amount of river sediment to deionized water, then spiking it with a suitable quantity of treatment works effluent and, finally, the pH was adjusted as required. The batch was then divided into 5 by one litre test samples and each placed in a water bath, to raise the temperature to 20°C, and slowly stirred for 30 min. At this point the pH was again checked, a sample removed for bacteriological analysis, the disinfectant added and a stop watch started. The gentle stirring was continued throughout the experiment ensuring a 30 min test period, at the end of which an aliquot was removed for checking for the presence of any residual disinfectant. Then immediately between 2.0 and 2.5 g/l sodium thiosulphate was added to arrest the disinfection action and a further sample taken for bacteriological examination.
RESULTS AND DISCUSSION The results of each of the tests are given (Tables 1, 2 and 3) and have been plotted (Figs 141) with the bacterial concentrations reported in terms of - l o g (N/No), in which No was the count of the particular bacteria under consideration immediately before the addition of the disinfectant dose (t = 0) and N the count at the end of the 30 min contact period. The important fact that initially becomes evident from the results is that in all the p H ranges investigated and at all the turbidity levels examined the dosages of either 4.0 or 8.0 mg/l iodine were nearly entirely effective for the removal of both faecal coliform bacteria and faecal streptococci. In all the tests the number of indicator organisms remaining at the end of the 30 min contact period was < 5 and frequently < 1.0. As a result information relating to the varying efficacity of iodine under the different test conditions must be gathered from the results obtained from the tests in which either 1 or 2 mg/l of iodine were employed. The comparisons of these results to those obtained with 1.0 mg/1 chlorine is also informative. In none of the tests carried out using 1.0 mg/1 chlorine or 1.0 or 2.0 mg/l iodine was any residual disinfectant apparent in the test suspension at the end of the 30 min contact period. The fact that on four occasions a minimal number of indicator organisms were found to be present after the 30 min period with, initially, either 4 or 8 mg/1 iodine might suggest a rather slower disinfection rate for iodine than suspected and could warrant a check after perhaps 45 min. On examining the results for the removal of faecal coliform (Figs 1-3) it becomes evident that an initial addition of 1.0 mg/l chlorine was appreciably effective in terms of - l o g (N/No) at low turbidities in all the pH ranges investigated, although the effect did decrease with increasing p H as had been expected (Fig. 7). The effectiveness of 1.0 mg/1 chlorine against faecal coliform organisms was also found to decline
Iodine used as a water-disinfectant in turbid waters
673
Table 1 Disinfectant dosage (mg/l) Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0
pH
Turbidity (NTU)
7.0-7.2 7.0-7.2 7.0-7.2 7.0-7.2 7.0-7.2 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4 7.1-7.2 7.1-7.2 7.1-7.2 7.1-7.2 7.1-7.2 7.2-7.3 7.2-7.3 7.2-7.3 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4
1.0 1.0 1.0 1.0 1.0 21 21 21 21 21 50 50 50 50 50 75 75 75 100 100 100 100 100
Faecal coliforms
Faecal streptococci
t= 0 t = 30 min (count/100 ml)
t= 0 t = 30 min (count/100 ml)
2075 6050 1775 5400 5400 5200 6050 5200 5200 5200 1775 6050 1775 1820 1820 6700 6700 6700 2100 14,500 1775 2100 2100
<5 155 <5 0 0 0 305 40 0 0 45 4500 15 <5 <5 260 925 15 200 715 15 <5 <5
127 300 65 500 500 500 300 500 500 500 190 300 65 190 190 335 335 335 205 205 65 205 205
0 135 <5 0 0 5 155 50 0 0 <5 180 5 <5 <5 15 190 150 10 115 5 <5 <5
Residual disinfectant (rag/I) Nil Nil Nil 1.4 5.0 Nil Nil Nil 1.4 5.5 Nil Nil Nil 0.35 3.5 Nil Nil Nil Nil Nil Nil Nil Nil
Table 2 Disinfectant dosage (mg/l)
pH
Turbidity (NTU)
Chlorine 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0
7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.1 7.9-8.1 7.9-8.1 7.9-8.1 7.9-8.1 8.0-8.2 8.0-8.2 8.0-8.2 8.0-8.2 8.0--8.2 8.0-8.2 8.0-8.2 8.0-8.2 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3
1.0 1.0 1.0 1.0 1.0 20 20 20 20 20 50 50 50 50 50 75 75 75 100 100 100 100 100
Faecal coliforms
Faecal streptococci
t= 0 t = 30 min (count/100 ml)
t= 0 t = 30 min (count/100 ml)
5100 6050 5100 5100 5100 3200 6050 3200 3200 3200 4600 4600 1775 4600 4600 6700 6700 6700 6100 6100 6100 6100 6100
525 300 525 525 525 219 210 210 210 210 240 240 58 240 240 335 335 335 300 300 300 300 300
15 155 <5 <5 <5 50 58 10 <5 <5 240 140 30 <5 <5 100 560 55 260 900 50 <5 <5
5 145 < 5 <5 <5 15 80 10 - 5 <5 55 220 5 <5 <5 45 255 20 50 225 10 <5 <5
Residual disinfectant (mg/l) Nil Nil Nil Nil Nil Nil Nil Nil 2.8 6.5 Nil Nil Nil 1.2 6.2 Nil Nil Nil Nil Nil Nil 0.3 4.2
Table 3 Disinfectant dosage (mg/l)
pH
Turbidity (NTU)
Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0
8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8,6 8.5 8.5 8.5
1.0 1.0 1.0 21 21 21 50 50 50 75 75 75 100 100 100
Faecal coliforms
Faecal streptococci
t= 0 t = 30 rain (count/100 rnl)
t= 0 t = 30 min (count/100 ml)
3200 3200 3200 3200 3200 3200 17,500 17,500 17,500 17,500 17,500 17,500 14,500 14,500 14,500
170 170 170 170 170 170 630 630 630 630 630 630 603 603 603
10 50 5 105 120 5 2000 5650 83 3500 5900 400 5575 7950 600
<5 130 5 930 155 5 83 584 205 208 595 250 243 555 360
Residual disinfectant Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil
674
K.V. ELLISand H. B. R. J. VANVREE :5.5 3.0
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~
3.0 .... 2.5
2,5 Z Z
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with increasing turbidity and this trend was evident in all the pH ranges although it was more noticeable at the highest pH. However, from the results illustrated in Fig. 2 it would be hard to say whether or not the effect of turbidity was statistically really significant. In the light of the variability of these results it might be that the true line of best fit is a horizontal straight line. One mg/l initial dosage of iodine was generally less effective at killing faecal coliform than the equivalent amount of chlorine. This effect also declined generally with increasing pH and increasing turbidity. Only in the pH range 7.2-7.4 (Fig. l) did the effectiveness of 1.0 mg/l iodine not decline continually with increasing turbidity. A dosage of 1.0 mg/1 iodine was only occasionally found to be more effective, in terms of - l o g (N/No), against faecal coliforms than 1.0 mg/l chlorine. This occurred in the pH range 7.2-7.4 with a turbidity of 100 NTU and also in the pH range 7.9-8.3 for the turbidities of 20 and 50 NTU. An initial disinfecting dose of 2.0 mg/l iodine was always more effective than against faecal coliforms than that of the reference 1.0 mg/1 chlorine, except for somewhat unusual results in the pH range 7.2-7.4 and at 20 NU. Only rarely however, could it be suggested that under the test conditions employed that 2.0 mg/1 iodine was capable of producing a water of potable quality in terms of bacterial removal. The variation in effectiveness of 2.0 mg/1 iodine with turbidity and pH against faecal coliforms is
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rather confused (Fig. 8). Only with the highest pH range was there a straightforward decline with increasing turbidity. With the lowest pH range (7.0-7.4) increasing turbidity had little or no influence while with the intermediate pH range (7.9-8.3) an initial strong reduction in effectiveness with increasing turbidity was reversed at the levels of 75-100 NTU. It is not evident whether these latter two results were merely aberrant from some unknown reason or were indicative of some specific alteration of disinfecting action. No similar effect in this pH range (7.9-8.5) was recorded with faecal streptococci. As would have been expected the results for the removal of the faecal streptococci were very similar, under all the varying conditions, to those for the removal of faecal coliforms. There was again under all the conditions of pH and turbidity a virtual elimination of the faecal streptococci when employing iodine at the concentrations of both 4.0 and 8.0 mg/1. With 1.0 mg/l chlorine, 1.0 mg/l iodine and also with 2.0 mg/l iodine the general trend was again a reduction in disinfecting action both with increasing pH and increasing turbidities. In the lowest pH range investigated (pH 7.0-7.4) it was found that an addition of 1.0 mg/l chlorine was more effective against faecal streptococci than either 1.0 or 2.0 mg/l iodine. The disinfecting ability of the chlorine in this pH range was again strongly reduced with increasing turbidity but this tendency was not so pronounced as with the faecal coliforms. The dosage
o l p p m chlorine FC D1 ppm iodine FC • 2 pprn iodine FC
"..
I I I I I I 50 60 70 80 90 100
Fig. 3. FC removal, pH range 8.3-8.6.
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I I I I 10 20 30 40
Turbidity
Fig. 1. FC removal, pH range 7.~7.4.
o
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1.5
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0.5
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1 I [ I I I I I I 20 30 40 50 60 70 80 90 100 Turbidity
(NTU)
Fig. 2. FC removal, pH range 7.9-8.3.
0
I I I I I I 10 20 30 40 50 60 Turbidity
I I I I 70 80 90 100
(NTU)
Fig. 4. FS removal, pH range 7.0-7.4.
Iodine used as a water-disinfectant in turbid waters 3.5
2.5
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Turbidity (NTU)
(NTU)
Fig. 5. FS removal, pH range 7.9-8.3.
Fig. 7. FC removal, I ppm chlorine, all pH ranges.
of 1.0mg/l iodine proved to have only a weak disinfecting action in this pH range which was not significantly affected by increasing turbidity. At the higher pH range of 7.9-8.3 the application of 2.0mg/l iodine proved to be definitely more effective than 1.0 mg/l chlorine and the reduction in disinfection efficiency with increasing turbidity was not so pronounced with the 2.0 mg/l iodine as with the chlorine. An addition of 1.0 mg/l iodine again demonstrated only a weak disinfecting action which was not materially altered by increasing turbidity. The lowest concentration of iodine (1.0 rag/l) again proved to possess only a low disinfecting efficiency against faecal streptococci in the highest pH range investigated of 8.4-8.6 and was again little altered by increasing turbidity. Also in this highest pH range the 2.0 mg/l iodine dose was found to be more effective at the low turbidities than the reference 1.0mg/l chlorine but was more greatly affected by increasing turbidity in this pH range so that its effect decreased in relation to that of the chlorine to be less effective between 50 and 100 NTU. The decline in effectiveness of !.0 mg/l chlorine with increasing pH at low turbidities was more pronounced with the faecal streptococci bacteria than with the faecal coliforms (Figs 7 and 9).
CONCLUSIONS
(1) Despite the poor quality of the waters employed in the investigation dosages of either 4.0 or 8.0 mg/l iodine were adequate to produce a water of an approximate potable quality under all the conditions employed. (2) Under none of the conditions investigated was a dose of 1.0 mg/l iodine effective at reducing the counts of indication bacteria to an acceptable level. (3) At a low pH and at low turbidities a dosage of 1.0 rag/1 chlorine was largely effective at removing the faecal indicator bateria but the effect declined generally both with increasing pH and increasing turbidity. These observations support already recognized characteristics of chlorine disinfection. (4) With 2.0 rag/1 iodine there was an effective, but rarely total, removal of faecal indicator organisms in the lowest pH range investigated and at low turbidities but the effect again declined with increasing pH and increasing turbidity. (5) A dosage of 2.0 mg/l iodine was always more effective in the removal of faecal coliform organisms than 1.0mg/l chlorine with the exception of the lowest turbidities employed in the lowest pH range. (6) In the lowest pH range (7.-7.4) 1.0 mg/l chlorine was more effective at removing faecal streptococci
1.6
• pH range 7 . 0 - 7 . 4 3.0 = ~)H range 7.9-8,3 2.8 i-'-~.--,,,, • pH range 8 , 4 - 8 . 6
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o.8
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• I ppm chlorine FS o i ppm iodine FS ~,\\ • 2 p p m iodine FS ot
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0.4
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1.6
0.2
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2.4
1.4 1,2 10 20 30 40 50 60 70 80 90 100 Turbidity
(NTU)
Fig, 6. FS removal, pH range 8,4-8.6.
0
I I I I I I I I I I lo 20 30 40 50 60 70 8o 9o loo Turbidity (NTU)
Fig. 8. FC removal 2 ppm iodine, different pH ranges.
K. V. ELLISand H. B. R. J. VAN VREE
676
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Fig. 9. FS removal, 1 ppm chlorine, different pH ranges. than 2.0 mg/1 iodine at all turbidities. The situation was totally reversed in the middle p H range (7.9-8.3) with the 2.0 mg/l iodine being the better disinfectant; while in the higher pH range (8.4-8.6) 2.0 mg/1 was found to be superior at the low turbidities with the 1.0 mg/l chlorine proving more effective at the higher turbidities. REFERENCES
APHA (1985) Standard Methods for the Examination of Water and Wastewater, 16th edition. American Public
Health Association, American Water Works Association, Water Pollution Control Federation, Washington, D.C. Black A. P., Kinman R. N., Thomas W. C., Freund G. and Bird E. D. (1965) Use of iodine for disinfection. J. Am. Wat. Wks Ass. 57, 1401-1421. Chang S. L. (1958) The use of active iodine as a water disinfectant. J. Am. pharmac Ass. 47, 417-423. Fair G. M., Chang S. L. and Morris J. C. (1945) Disinfection of water and related substances. Final Report to the Committee on Medical Research, Harvard University, Cambridge, Mass. Handbook of Chemistry and Physics, 65th edition. CRC Press, 1984. Karalekas P. C., Kuzminski L N. and Feng T. H. (1970) Recent developments in the use of iodine for water disinfection. J. N. Engl. Wat. Wks Ass 84, 152-188. Lange's Handbook of Chemistry, 13th edition. McGrawHill, New York, 1985 Morgan D. P. and Karpen R. J. (1953) Test of chronic toxicity of iodine as related to the purification of water. U.S. Armed Forces med. J. 4, 725-728. Taylor G. R. and Butler M. (1982) A comparison of the viracidal properties of chlorine, chlorine dioxide, bromine chloride and iodine. J. Hyg., Camb. 89, 321-328. Vergnoux (1915) Examen rapide et sterilization des eaux pour les troupes en campagne. L'Union Pharmaceutique, 194-201. Whitehead B. R. (1981) Focus on iodine as a disinfection agent. Wat. Pollut. Control 1981, 10-12.
0043-1354/89$3.00+ 0.00 Copyright © 1989MaxwellPergamonMacmillanplc
I O D I N E USED AS A WATER-DISINFECTANT IN TURBID WATERS K. V. ELLIS1. and H. B. R. J. V A N V R E E 2 ~Civil Engineering, Loughborough University of Technology, Loughborough, Leics. LE11 3TU, England and 2Department of Water Pollution Control, Wageningen Agricultural University, P.O. Box 8129, NL-6700 EV Wageningen, The Netherlands
(First received August 1988; accepted in revisedform December 1988) Abstract--Various concentrations of iodine were employed to inactivate faecal indicator bacteria in low quality water at different turbidities in three pH ranges. The results obtained were compared with those of an arbitrarily selected standard of 1.0 mg/1 chlorine. Under all the conditions, for which dosages of 4.0 or 8.0 mg/1 iodine were used, a water of virtual potable quality was obtained within a 30 min contact period. Under none of the conditions investigated was a dosage of 1.0 mg/1 iodine found to be an effective disinfectant while both 1.0 mg/1chlorine and 2.0 rag/1 iodine were generally effectivein the lowest pH range investigated and at low turbidity but the disinfecting effect in both cases decreased with increasing pH and increasing turbidity.
Key words--iodine, disinfection, faecal indicator organisms, turbidity
INTRODUCTION Iodine is a blue-black element with a melting point of 113.5°C and a boiling point of 184.4°C which is normally obtainable in the form of crystals. It will volatilize at ordinary temperatures to produce a blue-violet gas with an irritating odour. It is the heaviest of the halogens. It has a specific gravity of 4.93 at 20°C an atomic weight of 126.9 and an atomic number of 53. Its oxidation potential (12 + 2~. . . . . > 21') is only 0.54 compared with that of chlorine of 1.39. Care should be taken in handling iodine as contact with the skin can produce lesions (Handbook of Chemistry and Physics, 1984; Lange's Handbook of Chemistry, 1985). Apparently iodine was first used in water treatment for the disinfection of drinking water for troops in France during the First World War (Vergnoux, 1915) and following work carried out by Fair and others (Fair et al., 1945) was adopted for use by the U.S. Army during the Second World War, when it was applied in the form of globaline tablets. Iodine is soluble in water to the extent of 0.029 g/100 g at 20°C which rises to 0.445 g/100 g at 100°C. Depending on the pH of the solution the dissolved iodine will hydrolyse to varying extents to form hypoidous acid (HOI). Neither this hydrolysis nor any subsequent dissociation of the produced oxy-acid (HOI) is as pronounced as with chlorine; as is illustrated from the following table adapted from Chang (1958).
*Author to whom correspondence should be addressed.
The effect of pH on the hydrolysisof iodineand chlorine(0.5mg/I titratable iodinein the presenceof iodide) Cl 2 HOCI OCI' I2 HOI OI' pn (%) (%) (%) (%) (%) (%) 5 99 1 0 0.5 99.5 0 6 90 10 0 0 99.5 0.5 7 52 48 0 0 96.5 3.5 8 12 88 0.005 0 21.5 78.5 9 0 1.0 99.0
The amount of iodine hydrolysed decreases with increasing concentrations of titratable iodine. At a pH of 8.0, or over, the HOI is unstable and decomposes slowly to form iodate and iodide; neither of which possess significant germicidal properties. In the presence of added iodide the iodine present in water will tend to form the tri-iodide. The proportion of this tri-iodide present increases both with the concentration of iodide and with that of titratable iodine. The 1-3 ion is only approximately 1/8 as cysticidal as elemental iodine and possesses no significant viricidal properties. In low concentrations of iodine, however, such as are used in water treatment the presence of the tri-iodide can be effectively ignored (Chang, 1958). Both the undissociated iodine, I2, and the hypoidous acid, HOI, possess appreciable germicidal properties but of the two the molecular iodine is apparently more effective against cysts and spores while the hypoiodous acid is a more effective viricide (Chang, 1958). Taylor and Butler (1982) found that iodine was most effective against poliovirus at pH 9.0. In work with six different significant bacterial genera Karalekas and others (Karalekas et al., 1970) found little significant difference in the bactericidal effects of 1.0 rag/1 iodine at pHs 5 and 7 but a pronounced reduction in effect at pH 9.0.
671
672
K.V. ELL1Sand H. B. R. J. VANVgEE
The possible physiological effects resulting from a continued intake of iodinated water has always created some appreciable resistance to its employment in water treatment although it has been suggested that a single serving of m a n y sea-foods would provide more iodine than the daily intake from water (Whitehead, 1981). Although, two extended investigations into the effect of long-term ingestion of iodated water, one with service personnel in the Marshall Islands (Morgan and Karpen, 1953) and the other a m o n g the prison population in Florida (Black et al., 1965), failed to elicit any harmful effects there must remain some appreciable apprehension as to the physiological results in the long term. This is an area which calls for more research. One of the principal attractions for the employment of iodine in disinfection of water is its low reactivity compared with other halogens. Iodine in water reacts only slowly with organic material present and does not react with ammonia to form iodamines. Consequently there is the possibility that it may be a superior disinfectant to chlorine for partially polluted waters and as a result might be a more suitable disinfectant in disaster situations where only low quality source waters may be available. Hence this present project was designed to determine how well, compared with an arbitrarily selected standard dosage of 1.0 mg/l chlorine, various concentrations of iodine would serve as a disinfectant against standard faecal indicator organisms with different levels of polluted water; the pollution in each case being present in the form of a fine suspension of solids. EXPERIMENTAL Test waters of various turbidities were employed (< 1, 20, 50, 75, t00 NTU). In each case the required turbidity was achieved by adding a known quantity of stock river sediment to deionized water. The sediment employed was derived from the River Soar downstream of Loughborough and was sterilized before use. A particle-size distribution for the sediment employed was determined using a Coultercounter and it is of interest that nearly 70% of the suspension was made up of particles of I0/~m or less in size, i.e. of bacterial size or smaller. 38.5% of the sediment was organic matter. Throughout, the investigation an adequate number of indicator bacteria within the test samples was assured by the addition of a sufficient aliquot of fresh treatment-works final effluent. This effluent was obtained from the completely nitrifying activated-sludge plant of the Loughborough Water Reclamation Works of the Severn Trent Water Authority. The mean concentration of free-ammonia (as N) in the effluent samples employed was less than 0.5 mg/l and the maximum 0.5 mg/1. The indicator microorganisms determined during the investigation were the faecal coliforms and the faecal streptocci. The samples of the Loughborough effluent used contained between 230 and 2100 FS/100ml and between 15 x I 0 3 and 51 x 103 FC/100ml. For all the bacteriological determinations the membrane filtration technique, according to the Standard Methods for the Examination of Water and l,Vastewater (APHA 1985), was employed. Three different pH ranges were investigated. These were pH 7.0-7.4, 7.9-8.3 and 8.3-8.5. The pH of each test sample was adjusted as required by the careful addition of 0.1 N sodium hydroxide solution. All the investigations were
carried out at a standard temperature of 20°C ( + 0.5°C). In each section of the project test batches of 1, 20, 50, 75 and 100 NTU were made up and at each turbidity five separate test samples were used to which were added either 1.0, 2.0, 4.0 or 8.0 mg/l iodine or 1.0 mg/1 chlorine. For each test run a 51. batch of test water was prepared at the required turbidity by adding the necessary amount of river sediment to deionized water, then spiking it with a suitable quantity of treatment works effluent and, finally, the pH was adjusted as required. The batch was then divided into 5 by one litre test samples and each placed in a water bath, to raise the temperature to 20°C, and slowly stirred for 30 min. At this point the pH was again checked, a sample removed for bacteriological analysis, the disinfectant added and a stop watch started. The gentle stirring was continued throughout the experiment ensuring a 30 min test period, at the end of which an aliquot was removed for checking for the presence of any residual disinfectant. Then immediately between 2.0 and 2.5 g/l sodium thiosulphate was added to arrest the disinfection action and a further sample taken for bacteriological examination.
RESULTS AND DISCUSSION The results of each of the tests are given (Tables 1, 2 and 3) and have been plotted (Figs 141) with the bacterial concentrations reported in terms of - l o g (N/No), in which No was the count of the particular bacteria under consideration immediately before the addition of the disinfectant dose (t = 0) and N the count at the end of the 30 min contact period. The important fact that initially becomes evident from the results is that in all the p H ranges investigated and at all the turbidity levels examined the dosages of either 4.0 or 8.0 mg/l iodine were nearly entirely effective for the removal of both faecal coliform bacteria and faecal streptococci. In all the tests the number of indicator organisms remaining at the end of the 30 min contact period was < 5 and frequently < 1.0. As a result information relating to the varying efficacity of iodine under the different test conditions must be gathered from the results obtained from the tests in which either 1 or 2 mg/l of iodine were employed. The comparisons of these results to those obtained with 1.0 mg/1 chlorine is also informative. In none of the tests carried out using 1.0 mg/1 chlorine or 1.0 or 2.0 mg/l iodine was any residual disinfectant apparent in the test suspension at the end of the 30 min contact period. The fact that on four occasions a minimal number of indicator organisms were found to be present after the 30 min period with, initially, either 4 or 8 mg/1 iodine might suggest a rather slower disinfection rate for iodine than suspected and could warrant a check after perhaps 45 min. On examining the results for the removal of faecal coliform (Figs 1-3) it becomes evident that an initial addition of 1.0 mg/l chlorine was appreciably effective in terms of - l o g (N/No) at low turbidities in all the pH ranges investigated, although the effect did decrease with increasing p H as had been expected (Fig. 7). The effectiveness of 1.0 mg/1 chlorine against faecal coliform organisms was also found to decline
Iodine used as a water-disinfectant in turbid waters
673
Table 1 Disinfectant dosage (mg/l) Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0
pH
Turbidity (NTU)
7.0-7.2 7.0-7.2 7.0-7.2 7.0-7.2 7.0-7.2 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4 7.1-7.2 7.1-7.2 7.1-7.2 7.1-7.2 7.1-7.2 7.2-7.3 7.2-7.3 7.2-7.3 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4 7.2-7.4
1.0 1.0 1.0 1.0 1.0 21 21 21 21 21 50 50 50 50 50 75 75 75 100 100 100 100 100
Faecal coliforms
Faecal streptococci
t= 0 t = 30 min (count/100 ml)
t= 0 t = 30 min (count/100 ml)
2075 6050 1775 5400 5400 5200 6050 5200 5200 5200 1775 6050 1775 1820 1820 6700 6700 6700 2100 14,500 1775 2100 2100
<5 155 <5 0 0 0 305 40 0 0 45 4500 15 <5 <5 260 925 15 200 715 15 <5 <5
127 300 65 500 500 500 300 500 500 500 190 300 65 190 190 335 335 335 205 205 65 205 205
0 135 <5 0 0 5 155 50 0 0 <5 180 5 <5 <5 15 190 150 10 115 5 <5 <5
Residual disinfectant (rag/I) Nil Nil Nil 1.4 5.0 Nil Nil Nil 1.4 5.5 Nil Nil Nil 0.35 3.5 Nil Nil Nil Nil Nil Nil Nil Nil
Table 2 Disinfectant dosage (mg/l)
pH
Turbidity (NTU)
Chlorine 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Iodine, 4.0 Iodine, 8.0
7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.1 7.9-8.1 7.9-8.1 7.9-8.1 7.9-8.1 8.0-8.2 8.0-8.2 8.0-8.2 8.0-8.2 8.0--8.2 8.0-8.2 8.0-8.2 8.0-8.2 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3 7.9-8.3
1.0 1.0 1.0 1.0 1.0 20 20 20 20 20 50 50 50 50 50 75 75 75 100 100 100 100 100
Faecal coliforms
Faecal streptococci
t= 0 t = 30 min (count/100 ml)
t= 0 t = 30 min (count/100 ml)
5100 6050 5100 5100 5100 3200 6050 3200 3200 3200 4600 4600 1775 4600 4600 6700 6700 6700 6100 6100 6100 6100 6100
525 300 525 525 525 219 210 210 210 210 240 240 58 240 240 335 335 335 300 300 300 300 300
15 155 <5 <5 <5 50 58 10 <5 <5 240 140 30 <5 <5 100 560 55 260 900 50 <5 <5
5 145 < 5 <5 <5 15 80 10 - 5 <5 55 220 5 <5 <5 45 255 20 50 225 10 <5 <5
Residual disinfectant (mg/l) Nil Nil Nil Nil Nil Nil Nil Nil 2.8 6.5 Nil Nil Nil 1.2 6.2 Nil Nil Nil Nil Nil Nil 0.3 4.2
Table 3 Disinfectant dosage (mg/l)
pH
Turbidity (NTU)
Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0 Chlorine, 1.0 Iodine, 1.0 Iodine, 2.0
8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8.6 8.4-8,6 8.5 8.5 8.5
1.0 1.0 1.0 21 21 21 50 50 50 75 75 75 100 100 100
Faecal coliforms
Faecal streptococci
t= 0 t = 30 rain (count/100 rnl)
t= 0 t = 30 min (count/100 ml)
3200 3200 3200 3200 3200 3200 17,500 17,500 17,500 17,500 17,500 17,500 14,500 14,500 14,500
170 170 170 170 170 170 630 630 630 630 630 630 603 603 603
10 50 5 105 120 5 2000 5650 83 3500 5900 400 5575 7950 600
<5 130 5 930 155 5 83 584 205 208 595 250 243 555 360
Residual disinfectant Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil
674
K.V. ELLISand H. B. R. J. VANVREE :5.5 3.0
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with increasing turbidity and this trend was evident in all the pH ranges although it was more noticeable at the highest pH. However, from the results illustrated in Fig. 2 it would be hard to say whether or not the effect of turbidity was statistically really significant. In the light of the variability of these results it might be that the true line of best fit is a horizontal straight line. One mg/l initial dosage of iodine was generally less effective at killing faecal coliform than the equivalent amount of chlorine. This effect also declined generally with increasing pH and increasing turbidity. Only in the pH range 7.2-7.4 (Fig. l) did the effectiveness of 1.0 mg/l iodine not decline continually with increasing turbidity. A dosage of 1.0 mg/1 iodine was only occasionally found to be more effective, in terms of - l o g (N/No), against faecal coliforms than 1.0 mg/l chlorine. This occurred in the pH range 7.2-7.4 with a turbidity of 100 NTU and also in the pH range 7.9-8.3 for the turbidities of 20 and 50 NTU. An initial disinfecting dose of 2.0 mg/l iodine was always more effective than against faecal coliforms than that of the reference 1.0 mg/1 chlorine, except for somewhat unusual results in the pH range 7.2-7.4 and at 20 NU. Only rarely however, could it be suggested that under the test conditions employed that 2.0 mg/1 iodine was capable of producing a water of potable quality in terms of bacterial removal. The variation in effectiveness of 2.0 mg/1 iodine with turbidity and pH against faecal coliforms is
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rather confused (Fig. 8). Only with the highest pH range was there a straightforward decline with increasing turbidity. With the lowest pH range (7.0-7.4) increasing turbidity had little or no influence while with the intermediate pH range (7.9-8.3) an initial strong reduction in effectiveness with increasing turbidity was reversed at the levels of 75-100 NTU. It is not evident whether these latter two results were merely aberrant from some unknown reason or were indicative of some specific alteration of disinfecting action. No similar effect in this pH range (7.9-8.5) was recorded with faecal streptococci. As would have been expected the results for the removal of the faecal streptococci were very similar, under all the varying conditions, to those for the removal of faecal coliforms. There was again under all the conditions of pH and turbidity a virtual elimination of the faecal streptococci when employing iodine at the concentrations of both 4.0 and 8.0 mg/1. With 1.0 mg/l chlorine, 1.0 mg/l iodine and also with 2.0 mg/l iodine the general trend was again a reduction in disinfecting action both with increasing pH and increasing turbidities. In the lowest pH range investigated (pH 7.0-7.4) it was found that an addition of 1.0 mg/l chlorine was more effective against faecal streptococci than either 1.0 or 2.0 mg/l iodine. The disinfecting ability of the chlorine in this pH range was again strongly reduced with increasing turbidity but this tendency was not so pronounced as with the faecal coliforms. The dosage
o l p p m chlorine FC D1 ppm iodine FC • 2 pprn iodine FC
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I I I I I I 50 60 70 80 90 100
Fig. 3. FC removal, pH range 8.3-8.6.
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Turbidity
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0
I I I I I I 10 20 30 40 50 60 Turbidity
I I I I 70 80 90 100
(NTU)
Fig. 4. FS removal, pH range 7.0-7.4.
Iodine used as a water-disinfectant in turbid waters 3.5
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Turbidity (NTU)
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Fig. 7. FC removal, I ppm chlorine, all pH ranges.
of 1.0mg/l iodine proved to have only a weak disinfecting action in this pH range which was not significantly affected by increasing turbidity. At the higher pH range of 7.9-8.3 the application of 2.0mg/l iodine proved to be definitely more effective than 1.0 mg/l chlorine and the reduction in disinfection efficiency with increasing turbidity was not so pronounced with the 2.0 mg/l iodine as with the chlorine. An addition of 1.0 mg/l iodine again demonstrated only a weak disinfecting action which was not materially altered by increasing turbidity. The lowest concentration of iodine (1.0 rag/l) again proved to possess only a low disinfecting efficiency against faecal streptococci in the highest pH range investigated of 8.4-8.6 and was again little altered by increasing turbidity. Also in this highest pH range the 2.0 mg/l iodine dose was found to be more effective at the low turbidities than the reference 1.0mg/l chlorine but was more greatly affected by increasing turbidity in this pH range so that its effect decreased in relation to that of the chlorine to be less effective between 50 and 100 NTU. The decline in effectiveness of !.0 mg/l chlorine with increasing pH at low turbidities was more pronounced with the faecal streptococci bacteria than with the faecal coliforms (Figs 7 and 9).
CONCLUSIONS
(1) Despite the poor quality of the waters employed in the investigation dosages of either 4.0 or 8.0 mg/l iodine were adequate to produce a water of an approximate potable quality under all the conditions employed. (2) Under none of the conditions investigated was a dose of 1.0 mg/l iodine effective at reducing the counts of indication bacteria to an acceptable level. (3) At a low pH and at low turbidities a dosage of 1.0 rag/1 chlorine was largely effective at removing the faecal indicator bateria but the effect declined generally both with increasing pH and increasing turbidity. These observations support already recognized characteristics of chlorine disinfection. (4) With 2.0 rag/1 iodine there was an effective, but rarely total, removal of faecal indicator organisms in the lowest pH range investigated and at low turbidities but the effect again declined with increasing pH and increasing turbidity. (5) A dosage of 2.0 mg/l iodine was always more effective in the removal of faecal coliform organisms than 1.0mg/l chlorine with the exception of the lowest turbidities employed in the lowest pH range. (6) In the lowest pH range (7.-7.4) 1.0 mg/l chlorine was more effective at removing faecal streptococci
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1.4 1,2 10 20 30 40 50 60 70 80 90 100 Turbidity
(NTU)
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0
I I I I I I I I I I lo 20 30 40 50 60 70 8o 9o loo Turbidity (NTU)
Fig. 8. FC removal 2 ppm iodine, different pH ranges.
K. V. ELLISand H. B. R. J. VAN VREE
676
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Fig. 9. FS removal, 1 ppm chlorine, different pH ranges. than 2.0 mg/1 iodine at all turbidities. The situation was totally reversed in the middle p H range (7.9-8.3) with the 2.0 mg/l iodine being the better disinfectant; while in the higher pH range (8.4-8.6) 2.0 mg/1 was found to be superior at the low turbidities with the 1.0 mg/l chlorine proving more effective at the higher turbidities. REFERENCES
APHA (1985) Standard Methods for the Examination of Water and Wastewater, 16th edition. American Public
Health Association, American Water Works Association, Water Pollution Control Federation, Washington, D.C. Black A. P., Kinman R. N., Thomas W. C., Freund G. and Bird E. D. (1965) Use of iodine for disinfection. J. Am. Wat. Wks Ass. 57, 1401-1421. Chang S. L. (1958) The use of active iodine as a water disinfectant. J. Am. pharmac Ass. 47, 417-423. Fair G. M., Chang S. L. and Morris J. C. (1945) Disinfection of water and related substances. Final Report to the Committee on Medical Research, Harvard University, Cambridge, Mass. Handbook of Chemistry and Physics, 65th edition. CRC Press, 1984. Karalekas P. C., Kuzminski L N. and Feng T. H. (1970) Recent developments in the use of iodine for water disinfection. J. N. Engl. Wat. Wks Ass 84, 152-188. Lange's Handbook of Chemistry, 13th edition. McGrawHill, New York, 1985 Morgan D. P. and Karpen R. J. (1953) Test of chronic toxicity of iodine as related to the purification of water. U.S. Armed Forces med. J. 4, 725-728. Taylor G. R. and Butler M. (1982) A comparison of the viracidal properties of chlorine, chlorine dioxide, bromine chloride and iodine. J. Hyg., Camb. 89, 321-328. Vergnoux (1915) Examen rapide et sterilization des eaux pour les troupes en campagne. L'Union Pharmaceutique, 194-201. Whitehead B. R. (1981) Focus on iodine as a disinfection agent. Wat. Pollut. Control 1981, 10-12.