iodine, but chlorine, in overall, is much superior to chloramines [21].
Although disinfection efficiency of ozone is higher than chlorine dioxide, this difference can be compensated by the contact time. The experiment indicated that chlorine dioxide could reach the same results for disinfection of coliform bacteria as ozone did if time lasted long enough, which can be seen in Fig. 1. The added concentrations of both of ozone and chlorine dioxide were 2 mg/ L.
Control of Cryptosporidium oocysts in potable water requires an integrated multiple barrier approach. Coagulation is critical in the effective control of Cryptosporidium by clarification and filtration. Dissolved air floatation can achieve oocysts removal of 3 logs compared to about 1 log by sedimentation. Dissolved air floatation and filtration provide two effective barriers to Cryptosporidium oocysts with cumulative log removal of 4 to 5 compared to log removals of 3 to 4 by sedimentation and filtration [22].
Fig. 1 Comparison of disinfection efficiency between ozone and chlorine dioxide on coliform bacteria
5 Tendency of disinfection for drinking water
In the future, the burden of producing water with low pathogen level and low tastes and odor will be allocated to a combination of steps, including source water protection, coagulation - flocculation - sedimentation, filtration, floatation, membrane processes and adsorption. Some form of terminal treatment with chlorine, chlorine dioxide, ozone, UV, or other agents will also be required. No single step can or should be expected to shoulder the entire burden to controlling a given contaminant. With the development of techniques, new chemical and physical agents will meet tests of practicability for use in water treatment and will reduce pathogens. These may include electromagnetic fields and other forms of treatment with light or sonic energy [23].
In light of availability, efficacy, operability and costs, the priority should be given to ultraviolet method among all of the currently utilized disinfection technologies, particularly in developing countries. The medium and low - pressure UV extends tremendous potential promise for adaptation into various scale water supply plants. The researches have validated that extremely low dosage of UV can be
highly effective for inactivate oocysts [24]. Furthermore, comparison of medium and low - pressure lamps demonstrated no significant differences. By using low - pressure UV at the dosage of 3 , 6 and 9 mJ/ cm2 , oocyst inactivation levels were yielded between 3.4 and 3.7 log. In the trials of UV in water with turbidity of more than 1 NTU, the ability of medium – pressure was not affected, and high level of oocysts inactivation could still be achieved. 6 Conclusions
To purify drinking water, chlorine dioxide can be chosen instead of chlorine, ozone and other disinfectants because of it s advantages of high efficiency of disinfection, competent stability, low cost and simple utilizing way etc. Both ozone and ClO2 are absolutely vital to ensure that any microorganisms arising from fecal contamination of water storage are destroyed. The utilization of chlorine dioxide has been found to efficiently restrict protozoa growth, to disinfect from bacteria and viruses. Taking the protozoan cysts as indicator in which Cryptosporidium oocysts were solidly resistant to chlorine, but chlorine dioxide may be suitable disinfectants to mutilate. Thus, viruses will be adequately controlled by chlorine dioxide under the same operation conditions required for inactivation of protozoan cysts. The experiment indicated that chlorine dioxide could reach the same results for disinfection of coliform bacteria as ozone did if time lasted long enough although disinfection efficiency of ozone is higher than chlorine dioxide.
It is an obvious preference for chlorine dioxide to pragmatically remove oil and benzene homologous compounds in water treatment meanwhile the formation of mutagenic and toxic substances is limited. The degradation rate was proportional to input amount of oxidants and increase of operating time. The dosage input , in overall , is suggested to range between 0. 5～2.0 mg/ L. The effective pH at which reactions occur is in the slightly acid range of 5 to 7 at which formation of chlorite and chlorate is minimized. The chlorine dioxide gas should be injected directly into the treated water body, so that high concentrations of ClO2 can be kept in the solution. Under these conditions, the elimination rate for organic pollutants will be much higher. For the storage system, input dosage of chlorine dioxide concentration should be higher
than that in laboratory studies due to complex pollutants in treated water. References:
[1 ] Katz J . Ozone and chlorine dioxide technology for disinfection of drinking water [M]. Noyes New Jersey: Data Corporation, 1980.
[2] Fiessinger F. Organic micropollutants in drinking water and health [M] . Publisher, N. Y., U. S. A: Elsevier Sci., 1985.
[3 ] Hoff J C , Geldreich E E. Comparison of the biocidal efficiency of alternative disinfectants [C] . In Proceedings AWWA seminar, Atlanta, Georgia, 1980.
[4 ] APHA , American Public Health Association. American Water Works Association and Water Pollution Control Federation. Standard Methods for the Examination of Water and Wastewater. (16th Edition) [M]. Washington D. C., 1989. [5] Dietrich A M. Determination of chlorite and chlorate in chlorinated and chloraminated drinking water by flow injection analysis and ion chromatography[J ] . A nal. Chem., 1992, 64:496 - 502.
[6] Monarca S. Mutagenicity of extracts of lake drinking water treated with different disinfectants in bacterial and plant tests[J ] . Water Res, 1998, (32):2 689 - 2 695.
[7] Vikesland P , Ozekin K, Valentine R L. Effect of natural organic matter on monochloramine decomposition : pathway elucidation through the use of mass and redox balance[J ] . Envi ron. Sci. Tech., 1998, 32 (10):1 409 - 1 416.
[8] Schulz S , Hahn H H. Generation of halogenated organic compounds in municipal wastewater [M] . Proc. 2nd Int. Assoc. Water Qual. Int. Conf. Sewer Phys. Chem. Bio. Reactor, Aalborg, Denmark, 1998.
[9 ] Aieta E M. A review of chlorine dioxide in drinking water treatment [J]. J. A WWA, 1986, 78 (6): 62 - 72.
[10 ] Gordon G Minimizing chlorine ion and chlorate ion in water treatment with chlorine dioxide[J ] . J. A WWA, 1990, 82 (4):160 - 165.
[11] Kmorita J D , Snoeyink V L. Monochloramine removal from water by activated carbon[J ] . J. A WWA, 1985, (1):62 - 64.
[12] Gordon G, Adam I , Bubnis B. Minimizing chlorate information[J ] . J. A
WWA, 1995, 87, (6): 97 - 106.
[13] Iatrou A. Removing chlorite by the addition of ferrous iron[J ] . J. A WWA, 1992, 84 (11): 63 - 68.
[14 ] Schalekamp Maarten. Pre - and intermediate oxidation of drinking water with ozone, chlorine and chlorine dioxide [J]. J. Ozone Science and Engineering, 1986, 8: 151 - 186
[15 ] Korich D G, Mead J R , Madore M S , et al . Effects of ozone, chlorine dioxide, chlorine and monochramine on Cryptosporidium parvum oosyst viability [J]. Applied and Environmental Microbiology, 1990, 56: 1 423 - 1 428.
[16 ] AWWA Research Foundation. Chlorine dioxide; drinking water issues, 2nd International Symposium [R]. Houston, TX, 1992.
[17] Lykins B W, Griese H G. Using chlorine dioxide for trihalomethane control[J ] . J, A WWA, 1986, 71 (6): 88 - 93.
[18] Regli S. Chlorine dioxide , drinking water issues , 2nd International Symposium [ R ] . Houston, TX, AWWA Research Foundation, 1992.
[19] Hoff J C. Inactivation of microbial agents by chemical disinfectants[J] . US EPA, 1986.
[ 20 ] Rubin A , Evers D , Eyman C , et al . Interaction of gerbil - cultured Giardia lamblia cysts by free chlorine dioxide [J]. Applied and Envi ronmental Microbiology, 1989, 55: 2 592 - 2 594.
[ 21 ] Rusell A D , Hugo WB , Ayliffe GA J . Principes and Practice of Disinfection [M]. Preservation and Sterilization. Blackwell Scientific Publications, Oxford, U K, 1992.
[22 ] Edzwald J K, Kelley M B. Control of Cryptosporidium from reservoirs to clarifiers to filters [C] . Proc. 1st IAWQ – IWSA Joint Specialist Conf. Reservoir Manage. Water Supply, Prague, Czech, 1998.
[23] Haas Charles N. Disinfection in the Twenty - first century[J ] . J. A WWA, 2000, 92 (2): 72 - 73.
[24 ] Clancy L , Jenneifer , Bukhari Z , et al , Using UV to Inactivate Gryptosporidium[J ] . J. A WWA, 2000, 92: 97 - 104.
饮用水的消毒及杀菌技术研究
朱琨 伏小勇
(兰州铁道学院环境工程系, 甘肃兰州 730070)
摘 要:饮用水处理消毒过程中可产生一系列致癌物质,主要是氯与水中的有机物和其它化学成分反应的结果,其中典型产物有三氯甲烷. 通过对常用消毒剂液氯,臭氧,二氧化氯,高锰酸钾,氯胺及过氧化氢的实验对比,证明二氧化氯是高效,方便,廉价的消毒剂. 它不仅对一般病原菌类有明显的抑制和杀菌作用,对清除难以灭杀的潜原性病毒也有理想的效果. 在净化水中石油类有机物时,二氧化氯的效果受到反应时间,注入方式和pH 值的影响. 关键词:消毒;氧化剂;水处理;病原菌;二氧化氯 中图分类号:X523 文献标识码:A