Disinfection effect of chlorine dioxide on bacteria in water

Pergamon PII: S0043-1354(96)00275-8

Wat. Res. Vol. 31, No. 3, pp. 607-613, 1997 © 1997ElsevierScienceLtd Printed in Great Britain,All rights reserved 0043-1354/97$17.00+ 0.00

DISINFECTION EFFECT OF CHLORINE DIOXIDE ON BACTERIA IN WATER H U A N G JUNLI *@, W A N G LI, REN NANQI @, MA F A N G and JULI Harbin University of Architecture and Engineering, (New district) 604, Box No. 2, Hai He Road 150001, Harbin, China (First received August 1994; accepted in revised form August 1996) Abstract--In this paper, the disinfection effects of chlorine dioxide (CIO2) on some main bacteria in water and the influence of C102 on the inactivation of some microorganisms studied under various conditions such as the dose of disinfectant and the contact time, pH value, etc. were researched and reported, and it was compared with that of liquid chlorine. The results showed that the killing effect of the CIO2 on bacteria is similar to or better than that of liquid chlorine, the bacteria were effectivelykilled off by using C102 in a relatively wider range of pH value. Moreover, the investigation of the bactericidal mechanism of CIO2was tentatively undertaken. Then, we concluded that C102 is an excellentdisinfectant to substitute for the liquid chlorine. © 1997 Elsevier Science Ltd. All rights reserved Key words~isinfection, chlorine dioxide, liquid chlorine, bacteria, coliform, disinfection effect

INTRODUCTION Chlorination has long been used as a simple and economic method for disinfection. In aspects of decreasing incidence of typhoid fever, controlling intestine infectious disease and increasing the effect of water purification, it has played an important role since the mid 1970s. It is found that chlorination may cause some organic matters (fulvic acid, etc.) in the water to form halogenated hydrocarbon which is harmful to people's health. The potable water test in U.S.A. shows that halogenated hydrocarbon substances exist universally. In China, chloroform and other haloforms have been found in drinking water in 24 large and medium cities (Arguello et al., 1979; Junli et al., 1987). In view of a potential threat to human health caused by halohydrocarbon compounds presented in drinking water, countries all over the world, especially industrialized countries, lay special emphasis on these questions, and various studies for decreasing halogenated compounds are widened and deepened day by day. As a substitutive or a supplemental disinfectant for chlorination, CIO2 has caused the people to follow with interest. ' According to a report in 1977, there have been 495 waterworks using C102 in the disinfection of drinking water in Europe, 84 in the United States, l0 in Canada and some l0 in other countries in the world. The EPA of the United States considers CIO2 as the first choice of disinfectant to replace liquid chlorine *Author to whom all correspondence should be addressed, Fax: +86 451 363 5700.

(Aieta et al., 1986; Karen et al., 1987; Junli et al., 1994; Lan et al., 1987; Chen, 1988). China stipulates that the content of chloroform in drinking water is lower than 60/,g/L, total bacteria number is lower than 100/mL and total population of E. coli is lower than 3/L. In this paper, the disinfection effects of C102 on typical bacteria in water were investigated experimentally and compared with that of liquid chlorine, the disinfection effects under conditions such as various dosage of disinfectant, pH, the contacting time, were seriously experimented. The results show that C102 is an excellent disinfectant substituting for liquid chlorine. Hence this provides a theoretical base for using CIO2 as a disinfectant in waterworks in China.

MATERIALS AND METHODS Experimental equipment (1) Electrothermal portable pressed steam sterilizer. Harbin Songhuajiang Medical Apparatus and Instruments Factory. (2) PHS-3A type digital pH meter. Xia Men Second Analytical Instrument Factory. (3) Incubator. (4) Chlorine dioxide gas generator, manufactured by ourselves. Analytical methods (Li et al., 1986; Lin, 1986; Shen, 1990.) (l) E. coli were determined by colony counting method on Endo agar medium. Endo agar medium: albumin peptone l0 g, beef extract 5 g, lactose l0 g, yeast extract 5 g, agar 20 g, Na2SO3 5 g, K2HPO4 3.5 g, 5% fuchsin ethanol solution 20 mL, distilled water 1000mL, pH 7.2-7.4, sterilizing at 0.75 kg/cm2 for 20min.

607

608

H. Junli et al.

t \ 0Ki=,0 linttme:20 g !~.o1000. :C00CI ~~cl O~110 . 2.~~. Kilingtirain_ m0=20 e_ ~70~ 80/ v..in Fig. 1. Killing effect of chlorine dioxide and chlorine dosages on E. coli (A).

"~ ~ " ~ g ~ , , 7

0.0 (2) Other bacteria were determined by colony counting method on beef extract, albumin peptone and agar medium. Culture medium of beef extract, albumin peptone and agar. Beef extract 3 g, agar 15-20g, sodium chloride 5 g, albumin peptone 10 g, water 1000mL, pH 7.4-7.6, sterilizing at 121.3°C, 1 kg/cm: for 30min. (3) Chlorine dioxide and liquid chlorine are measured by iodimetry.

1.0

2.0

3.0

DJllill_fectluat doLltge ( r a g / L ) Fig. 3. Killing effect of disinfectant dosage on Staphylococcus aureus.

liquid chlorine possess fair killing effects on E. coli (A) and E. coil (B) and the killing effects were strengthened gradually as the dose of CIO2 and liquid Source of bacterial strain chlorine were increased. The killing effect of chlorine The pure strains selected in our experiments were: dioxide was better than that of liquid chlorine. For E. coli (A), E. coli (B), Staphylococcus aureus, Sarcina, Chloropseudomonas, Bacillus subtilis and Shigella dysente- example, if 99% of killing effect was attained, the riae, etc. These strains were provided by Heilongjiang required amount of CIO2 was about 1.4 mg/L, but for Microorganism Institute, Liaoning University, liquid chlorine, 1.8 mg/L was required (see Fig. 1). HarbinMedicine Inscetion Institute and Harbin shi Yi Tang 99.9% of the sterilization can be attained by 3.0 mg/L Pharmaceutical plant, of both disinfectants. RESULTS The killing effect o f dosage o f C102 on bacteria Killing effect of CIO~ on E. coli. Various dosages of

CIO2 and liquid chlorine were added into a given volume of solution contain a given bacterium reacted under conditions of even shaking, sheltered from light and at 19°C for 20 min. Then, a colony count was done on Endo agar medium. The results are shown in Figs 1 and 2. As can be seen, CIO2 and t ~70

pH = 7.

0

Killing time: 20 mia o

/

~80

r !

/

~d 90 i" /

~100 ~ 0. 0

ClOs ~ ' ~

~ ~.~ ., 1.0 2] 0 3. 0 Disinfectant d t ~ a g e ( m g / L )

Fig. 2. Killing effect of chlorine dioxide and chlorine dosages on E. coli (B).

The killing effects o f CIO2 on other bacteria

The Staphylococcus aureus, Chloropseudomonas, Bacillus subtilis and Sarcina were selected to investigate the killing effect of CIO2 on pathogenic bacteria and other typical bacteria. Equivalent strains were inoculated in water samples containing various concentrations of CIO2 and liquid chlorine, the solution was buffered by using phosphates, keeping pH = 7.0. After reaction at 20°C for a time, the same volume of solution was placed in the culture dishes and then about 10 mL of culture medium of beef paste and albumin peptone were added at about 40°C in the culture dishes, respectively. After shaking homogeneously, cooling and solidifying, the culture dishes were removed and cultured in the culture box. Three parallel samples were taken from each example. After culturing at 37°C for 24 h, the growth state of the bacterial population was observed and the results were shown in Figs 3-5. As shown in Figs 3-5, the killing effect of C102 was obviously higher than that of liquid chlorine. When 98% of killing effects on Staphylococcus aureus was attained, 1.5 mg/L of C102 was required, as for liquid chlorine, 2.5 mg/L was required (see Fig. 3). Chloropseudomonas is similar to Staphylococcus aureus in the curve. Under the same condition of 2 mg/L dose of disinfectants sterilizing for 20 min and at 19°C, 95% of killing

Effect of CIO2 on bacteria

,70t =~

~ ~

pH==7. 0 KnUn$ time=20 rain

609

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•~

~d 90

ClO

o "a

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,,"~ ~

1

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0.0

Killing minPH ti0m=e=20 7"

1.0

2.0

3:0

-

Dislafeetaut d o l m g e ( m g / L )

1

~

2

3

Disinfectant d o u g e ( m g / L ) Fig. 6. Killing effect of disinfectant dosage on domestic wastewater.

Fig. 4. Killing effect of disinfectant dosage on Bacillus subtilis.

effects on bacteria such as Staphylococcus aureus, Chloropseudomonas, Bacillus subtilis, Bacillus brevis, Sarcina may be attained by liquid chlorine, but 99.9% of that by CIO2 (see in Figs 3-5). 99.9% of killing effects on Sarcina may be attained by 1.5 mg/L of CIO2 dosage, but about 2.6 mg/L of liquid chlorine is required (see Fig. 5). Domestic wastewater of new district of Harbin Architectural and Civil Engineering Institute is also used to carry out the test of killing effects under various doses of disinfectants to explain further the killing effect of CIO2 (see Fig. 6). As can be seen, 99.9% of the killing effect may be attained by 2.5mg/L of CIO2, but same effect can only be attained by 3.0 mg/L of liquid chlorine. The killing effect of CIO~ on domestic wastewater was stronger than that of liquid chlorine.

Influence of various pH value on the killing effects of C102 To observe the influences of chlorine dioxide and

/

7(

/

influence of pH change on the killing effect of C102 was not strong, but that on liquid chlorine was very sensitive. In the range of pH 3.0-9.0, Staphylococcus aureus may be killed effectively by C102. But, only at neutral condition (pH 7.0-8.0), may bacteria be killed effectively by liquid chlorine. Killing effects of both disinfectants on E. coli (B) are identical (see Fig. 7). Similarly, the domestic wastewater of new district of Harbin Architectural and Civil Engineering Institute was used to carry out further the test of killing effects of 0 0 2 and liquid chlorine under various pH value. The result was shown in Fig. 8. As can be seen, the influence of pH value on the killing effect of C102 is very weak. At the range of pH 3.0-9.0, bacteria may be killed effectively by

pH----?.0

9c

0

liquid chlorine at various pH values on growth and reproduction of bacteria, we selected relatively acidoresistant, baseresistant colon Bacillus (E. coli (B)) and Staphylococcus aureus as the observed objects. The growth state of floras of various bacteria were observed and determined at various pH values. The results were shown in Fig. 7. As can be seen, the

6ol. /

1 Di.lnl~-tant

2 3 domtSe(mg/L)

Fig. 5. Killing effect of disinfectant dosage on Sarcina.

d o=2.

pH Fig. 7. Killing effect of disinfectant on Staphylococcus aureus at different pH value.

H. Junli et al.

610

~ 0 0

v

10C

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.

.

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6

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CIO2. But, only at near neutral conditions can bacteria be killed effectively by liquid chlorine.

Influence of contacting time on killing effects of 0 0 2 E. coli (B) and Staphylococcus aureus were taken as observed objects, the experimental procedure was the same as mentioned above; at 17°C, bactericidal time is 1, 2, 5, 10 and 20 rain, respectively. The results are shown in Figs 9 and 10. As can be seen from the results, under the same reaction conditions, bactericidal rate of 0 0 2 was obviously faster than that of liquid chlorine. 98% of E. coil (B) or Staphylococcus aureus may be killed by C102 in 1-2 min, but for liquid chlorine, 2.5-3.5 min were required to attain the same effect. Our bactericidal time was selected to be 20 rain. Bacteria have reacted fundamentally with the disinfectant, The test of killing effects of CIO2 on various bacteria in outdoor oxidation pond Bacteria in outdoor oxidation pond were used to carry out isolation of species. Bactericidal exper-

C

pH----7. 0

Disinfectant d o s a g e . 2 . 5 m g / L

-

9 1011

Fig. 8. Killing effect of disinfectant on domestic wastewater at different pH values.

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¢= 70 0

'

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1

2

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3 4 5 Time (min)

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6

7

9

Fig. 10. Killing effect of different killing times on Staphylococcus aureus.

iments were carried out further by using the obtained bacterial species. The disinfectant dose was 3.0 mg/L and contact time was 20 rain. The result was shown in Table 1. As can be seen, killing effect of 0 0 2 was better than that of liquid chlorine. DISCUSSION

The property of CIO2 and its influence on the disinfection effect (l) Under normal temperature and pressure, CIO2 is a dark green-yellow gas and resembles chlorine in appearance and odor. CIO2 is dissolved easily in water. The solubility is 107.9 g/L in water at 20°C. It is five times as high as that of chlorine gas. As contrasted to the hydrolysis of chlorine gas in water, C102 in water does not hydrolyze to any appreciable extent but remains in solution as a dissolved gas. 0 0 2 exists as a resonance structure of AB2. Although the electronic structure of the C1Oz molecule is in unsaturated state, in water it exists in a monomolecular state. This is very favorable for its rapid diffusion in water. It is decomposed easily by light and is not changed in the dark in water for several months. The following is the electrode reaction and electrode potential of chlorine dioxide: C102 + 4H ÷ + 5e = CI- + 2H20

! .fl

•- -

o

• 70 "~ 60

(21=

Cclo~ tp0(V) = 1.511 - 0.0473 pH + 0.011 'l g e c r

pH=7. 0 Digl~t~etlmt d o u g o t 2 . 5 m g / L

0

'

1

*

2

,

3

,

.

.

t,, ~i'-~'----

4 5 6 7 Time (m~)

a

Fig. 9. Killing effect of different killing times on E. coli (B).

Thus it is shown that oxidation capacity of CIO2 depends upon the acidity and basicity of the solution. The stronger the acidity of solution, the higher the oxidation capacity of CIO2. In water treatment (pH = 7.0): C102 + e = C10~-

tp0 = 0.95(It')

Effect of 002 on bacteria

611

Table I. Test of killingeffectof 002 on bacteriain outdoor oxidationpond (pH = 7.0, killingtime = 20 rain) Killingeffecton various bacteria C12 Disinfectant dosage (mg/L)

Number of bacteria (count/mL)

Shigella dysenteriae*

0 3.0

Streptococcus pyogenes

0

Dendritic zoogloea

3.0 0 3.0

Gluconosporolato bacillus

0

Bacillus megatherium

0

1400 4 620 13 610 12 520 21 630 45

Name of bacteria

3.0 3.0

CIO2 Rate of inactivation (%) 99.7 97.9 98.0 96.0 92.9

Number of bacteria (count/mL)

Rate of inactivation (%)

1400 1 620 8 610 8 520 0 630 11

99.9 98.7 98.7 100 98.3

*Shigella dysenteriae--pure strain had to be provided by Harbin shi Yi Tang pharmaceutical plant.

C10~-+ 2H20 + 4e = C I - + 4OHtp0 = 0.78(V) The extent of the oxidation-reduction reaction depends upon the nature of reductant in water, According to electrode potential, metal ions in reductive state in water, such as Fe 2*, Mn ~+, etc. may be oxidized. Therefore, when 0 0 2 is adopted as the disinfectant in water, the removal of iron and manganese may be strengthened as well. Chlorine in water is hydrolyzed very easily to form hydrogen chloride and hypochlorous acid: C12 + H20 = HOC1 + H ÷ + ClHOC1 ~,~ H + + OC1In water treatment (pH = 7.0), HOCI and OC1coexist. Generally considering, HOCI plays a main role in bactericidal and disinfecting function. The bactericidal efficiency of HOC1 is nearly 80 times as high as that of OCI-. The higher the pH is, the lower the ratio of HOC1 and the weaker the activity is, and the poorer the disinfection effects are. As can be seen from the tests, C102 may kill the bacteria within the range of pH 3.0-9.0. But, liquid chlorine can do so only under the neutral condition, (2) Owing to the strong oxidizability of C102, and existing in nearly 100% of molecular state, it can penetrate through the cell membrane of a bacterium easily, the semipermeable membrane and the permeable pressure are destroyed. The respiration in the bacterium body are inhibited also after the permeability of the membrane is destroyed and mainly, phosphotransferase is inactivated. When the Table 2. The relationship between the various structural bacteria and disinfectant dosages as the same bactericidal effect was attained

Bactericidal effect (%) 80

85 90 95

Bacteria Staphylococcusaureus E. coli (B) C12 002 02 C102 (mg/L) (mg/t) (mg/L) (mg/L) 1.18

1.50 1.75 2.00

0.50

0.65 0.98 1.30

0.16

0.4o 0.72 1.10

0.08

0.17 0.70 1.18

disinfectant of oxidizability is used to kill the bacteria, glucose oxidase, such as mercaptogroup (-SH) oxidase, is destroyed by the disinfectant and oxidized into oxidase o f - S - S - group to lose the enzyme activity and to not be able to participate in the activities of the oxidation-reduction system further. This results in the death of the bacterium (Ryuichi, 1988; Wang, 1987; Jiang, 1986). For chlorine in CIO2, oxidation number is expressed as +4. According to counting, it contains 263% of available chlorine. Therefore, it has very strong oxidizability, i.e. oxidizability of CIO2 is about 2.5 times as high as that of liquid chlorine. This makes the bactericidal effect of CIO2 obviously better than that of liquid chlorine. Our experimental data manifest that to attain the same bactericidal effect, the required amount of C102 is not only lower than that of liquid chlorine, the required contact time of C102 is also shorter than that of liquid chlorine (as shown in Tables 2 and 3). Influence o f temperature on the disinfection effects

(1) The bactericidal effects of the disinfectants were increased as the temperature increased. Under low temperature, if the temperature is raised by 10°C, the death rate of bacteria can be increased by double. The relationship between contact times under various temperatures and survival index of colon Bacillus have been shown experimentally in Fig. 11 by Benarde et al. (1967). Thus it can be seen that the bactericidal times were shortened responsively as the increase of temperature and the bactericidal effect is strengthened. (2) Van' t Hoff-Arrhenius reaction rate formula may be used to express the relationship between required disinfection time and various temperature under a certain disinfection effect. 1ogtt E(T2 -- T,) t2 -- 2 . ~ R ' / ~ T2

E ( T 2 - T,) 19.184T, 7"2

The activation energy of liquid chlorine to colon Bacillus, under the condition of 5-25°C and various pH values, may be estimated roughly according to Van' t Hoff-Arrhenius formula and Q~0 value (i.e.

H. Junli et al.

612

Table 3. The lowesttimerequiredunderthe attaineddisinfectioneffectof 99.99% on some bacteria Bacteria

E. coli

Bacillaceae

Staphylococcus aureus

Disinfectant tm,,t tram tram (2.5 mg/L) K~v~* (rain) K.v~ (min) K~, (min) C12 1.86 4.9 1.34 6.9 1.67 5.5 CIO2 3.25 2.8 1.76 5.2 2.66 3.5 *K,~ = killingconstant, which was found roughlyfrom our experimentaldata according to Chick's law, N = No e-k'. ttmm= the lowesttime required under the attaineddisinfectioneffectof 99.99% on bacteria.

ratio of requisite time for getting the same disinfection effect, as the difference of temperature is 10°C, is called the coefficient of temperature)(Jiang, 1986, 1989). By calculation, when pH = 7.0, 8.5, 9.8, 10.7, the activation energies are about 34,580(J), 24,214(J), 52,212(J) and 63,272(J), respectively. Thus it can be seen that when pH = 7.0 and pH = 8.5, the activation energies are lower. Our experiments manifest these results also, i.e. when pH = 7.0 and pH = 8.0, liquid chlorine has the better bactericidal effect on E. coli. By using Fig. 11, the activation energy of C102 for killing E. coli was given roughly as 19,000(J), according to Van' t Hoff-Arrhenius formula, lower than that of liquid chlorine to kill colon Bacillus. It is thus clear that bacteria are killed easier by CIO: in the sterilization,

The influences ofbacterialstructure on the disinfection effects (1) Due to the difference of bacteria, the structures of peptidoglycan, i.e. mucopeptide possessed by cell wall of bacterium are also different, as shown in Fig. 12. For example, colon Bacillus (Gram negative) forms the single layer scattered structure of plate

1.0 ,~ 0. 9, I~ ~ 0. 8 "

~

ClOzt 0. 7 5 m g / L

O. 7.

"d 0. 6. ~| ~

~i|| ~ ~A

~o O. 5 0. 4-11 |,,~ ,-i~ , II I !1012 ~ 512 0. 3"I 0. 2 1 ~ ~ . X% ~2012 • ~ O. 1 1 3 ~ - ~ N " %~, O. 0 0 3 1015 20 25 30 35 40 Contact t i m e ( m l n ) Fig. 11. Influence of temperature on disinfection effect.

mesh in four-peptide side chain and is contacted easily and invaded by the disinfectants to break chemical bond of the bacterium and to kill the bacteria. But, Staphylococcus aureus (Gram positive) forms a cross, three-dimensional spatial network structure of large mechanical strength in four-peptide side chain to be contacted or invaded difficultly by the disinfectants. Therefore, Gram positive Staphylococcus aureus is stronger than Gram negative colon Bacillus in drug-resistance. As can be seen in Figs 2 and 3 and Table 2, when CIO2 and liquid chlorine were used to kill E. coli and Staphylococcus aureus, to attain the same bactericidal effect, the required amount of CIO2 was lower than that of liquid chlorine and Staphylococcus aureus was more numerous than E. coli in the required dosage of disinfection. This is the result of a different structure of two kinds of bacteria and different performance of disinfectants. (2) In addition, besides basic structures, some bacteria possess some special structures. Flagellum is locomotor organ and may make the bacteria able to escape from high concentration of disinfectant region and move into the nutritious region. Low permeability of gamma may block the exchange of substances into or out of cell and strengthen the resistance to disinfectants. Horny protein contained in membrane of gamma possesses numerous dithiobonds and sulfides which can combine with free chemical group and protect the center of gemma from being invaded by toxic drugs. This has been proved by our experimental results, as shown in Fig. 4. In addition, a young bacterium is more sensitive than an old one to disinfectant. (3) Bacterial surface is more or less negatively

charged, e.g. StaphylococcusaureusandChloropseudomonas, etc. For HOC1, due to HOCI and CIOcoexisting in neutral condition, C10- and the negative electric charges on surfaces of bacteria repel each other. The bactericidal effect of HOCI is poorer than that of C102 which exists as molecular state in water. This has been proved sufficiently by our experimental results. That the microorganism in water is killed by disinfectants depends upon both structure, performance of bacteria and the disinfectant performance. That is the result of joint action.

Effect of CIO2 on bacteria

point of e n e y m e ~

l..Ala

/f L-AIa

..de If-Ala

[ L-Ala

jfl~-Ala

r~.Alo ~------

bAP __ _/ bar

lI~-Ala

P

I

ys

~ . , U-Am

~taphylo¢oceus aureu8

/L-AIa

/ i

- ~,-LOLy_I,~ -

613

-.

l~-Pkla

DAP

1 ~ D'431U

~dAPllGt . . . . . , r ~ ~-acetylmuramic acio D-AIa ( ~ N - acetylglueosamine

g. ooli (colo~ Bacillus)

Fig. 12. The structure of peptidoglycan of cell wall for colon Bacillus, Staphylococcus aureus.

CONCLUSION

REFERENCES

(1) Chlorine dioxide possesses very favorable bactericidal effect on colon Bacillus. Colon Bacillus

Aieta E.M. and Borg J. D. (1984) J. A W W A 76(1), 64-71. Aieta E. M. et al. (1986) J. A W W A 78(6), 62-73. Arguello M. D. et al. (1979) Trihalomethanes in water. J. A W W A 71(9), 504-513. Benard M. A., Snow W. B., Olivier V. B. and Avidson B. (1967)Appl. Bact. 30(1), 159-167. Fuyu W. (1987) Disinfection and Bactericidal Action 4(2),102-104. Helin L. and Ling B. (1987) Application of chlorine dioxide in water treatment. War. Treat. Technol. 15(3), 174-178 (China). Junli H., Kou G. and Li Y. (1987a) Influencesof combined and free available chlorine on formation of chloroform. Environ. Sci. 8(5), 21-26. Junli H., Kou G. and Yang B. (1987b) Effects of humic acid etc. precursors in water on the formation of haloform. Environ. Chem. 6(5), 14-22. Junli H., Fan Q., Kou G. and Liu C. (1987c) Survey of haloform in main drinking water of China. Environ. Chem. 6(4), 80-86. Junli H., Chao Y. and Wang X. (1994) Influence of chlorine dioxideon chloroform formation. Environ. Chem. 13(5), 466-473. Karen S. W. et al. (1987) J. A W W A 79(9), 107-113. Mengzhua L. (1986) Microbiology. North-east Teachers University Publishing House (China). Okun D. A. (1976) J. Public Health 66(7), 639445. Sudou R. (1988) Microbiology of Water Environmental Purfication and Waste Water Treatment. Yu Huiqun et al. (translation). Chinese Architectural Industrial Publishing House. Xingiin J. (1986) Treatment and disinfection of drinking water. Chinese Environ. Sci. Publishing House. Xingiin J. (1989) Disinfection and Bactericidal Action 6(4), 225-233. Yinglin L. et al. (1986) Clinical Microbiology and Inspection. Ji lin Scientific and Technology Publishing House (China). Yunfen S. (1990) New Technology of the Microorganism Monitoring. Chinese Architectural Industrial Publishing House. Zhiliang S. (1984) Evalution of application of chlorine dioxide in drinking water disinfection. Environ. Sci. Collet. 5(1), 47-50. Zujian C. (1988) Haloorganics in drinking water. Shanghai Environ. Sci. 7(2).

may be killed effectively by C102 within a wide range of pH 3.0-9.0. As for getting the same effect by using liquid chlorine, the range of pH value is much narrower (pH = 6.8-8.5). (2) Chlorine dioxide possesses favorable bactericidal effect on bacteria. Bacteria may be killed effectively by CIO2 within the range of pH 3.0-8.0. In addition, the bactericidal rate of CIO2 is faster than that of liquid chlorine. Therefore, the bactericidal effect of CIO2 is better than liquid chlorine. (3) The bactericidal effect of CIO2 is different from that of liquid chlorine, originated mainly in the property; property of CIO2 is different from that of liquid chlorine. After dissolving in water, 100% of CIO2 exists in the molecular state, and contacts bacterial surface easily and permeates through cell membrane into the bacterial body. Due to appearance of semipermeability of the membrane and the permeation pressure of the membrane are destroyed, the bacteria are killed. But liquid chlorine exists as the state of HOC1 or OCI- in water and OCi- contacts with bacteria which are negatively charged. Thus the bactericidal effect of liquid chlorine is poorer than that of C102. Chlorine dioxide in oxidizability is 2.5 times high as that of liquid chlorine. The bactericidal capacity of CIO2 is also higher than that of liquid chlorine. (4) Various bacteria possess different resistance to disinfectant. To sum up, CIO2 is better than or matchable to liquid chlorine in the bactericidal effects and may attain the bactericidal effects within a wider range of pH values. Therefore, chlorine dioxide is a broadspectrum disinfectant. Thus it can be seen that chlorine dioxide is an excellent disinfectant to substitute for the liquid chlorine.