reaction time. The initial concentrations and removal rate at different times are listed in Tab. 3. It is shown that chlorine dioxide has a very strong oxidation capability including the break down of the benzene ring. There are no other commonly used oxidants to do like this except for ozone.
Tab. 3 Removal rate of tested organic compounds at different operating time (at pH7) Compounds Initial concern Removal rate/ % / mg·L – 1 2 min 10 min 15 min Oil 18. 12 45. 8 46. 9 47. 3 Benzene 41. 25 14. 1 41. 2 60. 7 Toluene 31. 75 17. 5 54. 9 86. 8 Ethylbenzene 16. 15 24. 7 63. 5 89. 8 p-phenylmethane 10. 75 25. 9 84. 9 100 o-phenylmethane 30. 25 20. 9 79. 1 100 m-phenylmethane 33. 20 28. 6 100 100 Styrene 62. 40 100 100 100 The injecting method for chlorine dioxide gas into the solution also has an apparent influence on the removal rate. With the indirect method, the gas firstly was dissolved in a certain amount of distilled water, and then added to the tested organic solutions, as a result, removal rates appear lower than for the direct blowing method. The main reason for the difference is due to the conversion and decomposition of chlorine dioxide in the dissolving process before the reaction. It is confirmed from Tab. 3 that the removal rate was proportional to operating time. Since chlorine dioxide showed very strong oxidation capability for organic chemicals but was reduced to chlorite anion according to Equation (4), and the removal rate initially appeared quite high. Then, chlorite keeps the oxidation capacity at a level, which allows decomposition of the organic compounds to continue even though the oxidation reaction gradually became weaker with reaction time. The experiment indicated that pH values significantly influenced the removal rate of the organic compounds. The differences of degradation rates in a variety of pH through indirect input way are
shown in Tab. 4.
Tab. 4 Degradation rate of benzene homologous compounds with indirect method at different pH (after 15 min)
Compounds Initial concern Removal rate/ % / mg·L – 1 pH5 pH7 pH10 Benzene 5 54. 0 48. 2 24. 1 Toluene 5 71. 7 63. 5 52. 6 Ethylbenzene 5 84. 2 78. 7 73. 5 p–phenylmethane 5 84. 4 77. 6 73. 0 o-phenylmethane 5 84. 1 80. 3 73. 7 m–phenylmethane 5 84. 0 9. 8 70. 4 Styrene 5 100. 0 98. 7 90. 5 There are, however, some disadvantages with ClO2, such as easy loss from solution due to volatilization, and disproportionation above pH 10 into chlorate and chlorite ions that are of certain oxidation capacity, but reported to be harmful to health if the concentration is too high. Chlorine dioxide was unstable in the solution even though it has a stronger oxidation capability than chlorite and chlorate as the two resulted in anions being dominant in the oxidation processes. The actual concentration of chlorine dioxide depended on the existence of chlorine, chlorite and chlorate whose concentrations were determined by pH values of the solution according to Equations (6) and (8) respectively. Consequently, the pH is the critical controlling factor in the concentrations of chlorine dioxide, chlorite and chlorate. The latter two harmful ions can be removed quite quickly by treatment with a reducing agent such as sulfur dioxide - sulfite ion at pH values of 5～7[10 ,12]. Fe (II) can be used to eliminate chlorite from the water , and the redox reaction is kinetically more rapid at pH 5～7 as well[13]. It was evident that the decomposition in acidic conditions was much better than that in alkaline conditions because a disproportional amount of chlorine dioxide was consumed by the reactions under alkaline conditions. For drinking water treatment, it has been suggested that the mixture of chlorine 0.8 mg/L and chlorine
dioxide 0.5 mg/L will achieve disinfection and control THMs formation in preference to use of pure chlorine dioxide[14]. According to USEPA drinking water standard, the residue of ClO2 is limited as 0.8 mg/L that tends to the goal of 0.4 mg/L. 4 Control of pathogens with disinfectants
Human pathogens that are transmitted by water including bacteria, viruses and protozoa. Organisms transmitted by water usually grow in the intestinal tract and leave the body in the feces. Thus, they are infections. Fecal pollution of water supplies may then occur, and if the water is not properly treated, the pathogens enter a new host when the water is consumed, therefore, it may be infectious even if it contains only a small number of pathogenic organisms. Most outbreaks of waterborne diseases are due to breakdowns in treatment systems or are a result of post contamination in pipelines.
The microorganisms of concern are those which can cause human discomfort, illness or diseases. These microbes are comprised of numerous pathogenic bacteria, viruses, certain algae and protozoa etc. The disinfection efficiency is typically measured as a specific level of cyst inactivation. Protozoan cysts are the most difficult to destroy. Bacteria and viral inactivation are considered adequate if the requirement for cyst inactivation is met. Therefore, water quality standard for the disinfection of water have been set at microorganisms, usually take the protozoan cysts as indicator, so viruses will be adequately controlled under the same operation conditions required for inactivation of protozoan cysts. The widely found drinking water contamination is caused by protozoan that is a significant intestinal pathogens in diary cattle, likely a source of this outbreak.
There are two of the most important protozoa - Cryptosporidium and Giardia cysts those are known to outbreak diseases, frequently are found in nature and drinking water storage ponds. Protozoa form protective stages like oocysts that allow them to survive for long periods in water while waiting to be ingested by a host. Protozoa cysts are not effectively removed by storing water because of their small size and density. Cryptosporidium oocysts have a setting velocity of 0.5 um/s. Therefore, if the water tank is 2 m deep, it will take the oocyst 46 days to settle to the
bottom. Giardia cysts are much large and have a great settling velocity of 5.5um/s. It was evident that chlorine and chloramines were ineffective against Cryptosporidium oocysts, which was discovered to be amazingly resistant to chlorine, and only ozone and chlorine dioxide may be suitable disinfectants [15]. The investigations have verified that Cryptosporidium is highly resistant to chorine, even up 14 times as resistant as the chlorine resistant Giardia, therefore methods for removing it in past rely on sedimentation and filtration. Watson′s Law to study protozoan disinfection, reads as follows:
K = Cηt (9)
In the formula:
K ——constant for a given microorganism exposed to a disinfectant under a fixed set of pH and temperature conditions; C ——disinfectant concentration (mg/ L); η——empirical coefficient of dilution ;
t ——time required to achieve the fixed percentage inactivation.
For the preoxidation and reduction of organic pollutants , the recommended dosages are between 0. 5～2. 0 mg/ L with contact time as 15～30 min depending on the pollutants characteristics in the water. In the case of post - disinfection , the safe dosages of ClO2 are 0. 2～0.4 mg/L. At these dosages, the potential by - products chlorite and chlorate do not constitute any health hazard [16]. The relation between disinfectant concentration and contact time can be established by using Ct products based on the experimental data. From this the effectiveness of disinfectants can be evaluated based on temperature, pH value and contact time. Since Cryptosporidium has become a focus of regulatory agencies in the United States and United Kingdom, the prospects of controlling this pathogen show more considerable. The comparison of the Ct values by using ozone , chlorine dioxide , chlorine and chloramines for Giardia and Cryptosporidium cyst s are listed in Tab. 5[17 ,18 ] , and for some microorganisms disinfection are displayed in Tab. 6[19 ] .
Tab. 5 Ct values (mg·min/ L.) for disinfection of Giardia and Cryptosporidium cysts by using 4 disinfectants
Protozoa Ozone Chlorine dioxide Chlorine Chloramines Giardia (pH6. 9 , 5 ℃) 0. 3 4. 3 16～47 365 Cryptosporidium (pH7 , 25 ℃) 5～10 78 7200 7200 Tab. 6 Comparison of value intervals for the product Ct (mg·min/ L) for the inactivation of various microorganisms by using 4 disinfectants
Microorganisms Ozone Chlorine dioxide Chlorine Chloramines (pH6～7) (pH6～7) (pH6～7) (pH8～9)
E. Coli 0. 02 0. 4～0. 75 0. 034～0. 05 95～180 Polio 1 0. 1～0. 2 0. 2～6. 7 1. 1～2. 5 768～3740 Rotavirus 0. 006～0. 06 0. 2～2. 1 0. 01～0. 05 3806～6476 Cysts of G. Lamblia 0. 5～0. 6 26 47～150 2200 Cysts of G. muris 1. 8～2. 0 7. 2～18. 5 30～630 1400 The mean Ct value for ClO2 at pH 7 and 5 ℃was 11. 9 mg·min/ L, and dropped to 5.2 at pH 7 and 25 ℃. High temperatures normally enhance the efficiency of disinfectants while lower temperatures have opposite effects requiring additional contact time or extra quantity of disinfectants. The best performance for ClO2 is at pH 9 and 25 ℃, which yields a Ct product of 2.8 mg·min/ L [20]. Chlorine dioxide appears to be more efficient for Cryptosporidium oocysts than either chlorine or monochloramine. Exposure of oocysts to 1.3 mg·min/ L at pH 7 reduces excystation from 87 % to 5 % in a hour at 25 ℃. Based on this result, Ct product of 78 mg·min/ L was calculated. However, the Ct product for ozone to do this work was examined as 5 - 10 mg·min/ L from observation that excystation decreased from 84 % to 0 % after 5 minutes with the ozone concentration of 1 mg/ L [15]. As with other disinfectants, increasing temperature decreased the Ct values and improved the cysticidal action. Increasing temperature unexpectedly reduced the Ct values from a high of 6.35 mg·min/ L at pH5 to a low of 2.91 mg·min/ L at pH 9[20]. It is generally the rule, that for protozoa ozone is the best cysticide, chlorine dioxide is superior to chlorine and